Using the jt9 executable to receive FST4W signals

Note: 

Since originally posted, WSJT-X v2.3.0-rc2 was released, adding a feature to the "JT9" executable that simplifies this process (the "-F" parameter) as described below.  Note that most of this post was originally written soon after "rc1" had become available.

* * *

As a heavy user of K1JT's WSPR and operating on the 2200 and 630 meter bands, I have noted with interest the introduction of the "FST4W" mode in the recent (v2.3.0-rc1) wsjt-x release.  Operating using the same detection bandwidth as WSPR (when FST4W is operated in the 120 second mode) it offers a theoretical 1.4dB improvement in detection sensitivity.

Being involved with wsprdaemon (link to that project here ) - an open-source project that automates and optimizes reception of WSPR signals on all bands, particularly if multiple receivers/antennas are used - we have been watching this development with interest, particularly since FST4W has the likelihood of supplanting conventional WSPR operation, especially on the lowest amateur bands (2200, 630 and possibly 160 meters) where minimal of Doppler shift is expected.

Internally, WSJT-X  uses the subordinate wsprd program as the decoding (and encoding) engine.  As a stand-alone program, the wsprd executable code may be invoked with a command line to decode signals contained within a .wav file that was captured during the standard two minute interval - aligned with even UTC minutes - and produce a text file containing the decoded signals.

Why use the executable rather than the entire wsjt-x suite?  The fact is that the use of the wsjt-x suite does not lend itself easily to script-driven, bare-minimum, lightweight implementations where further processing of the decoded data (to remove duplicate decodes from multiple receivers, antennas and to use this same data for further analysis of signal/noise) is desired.

The "jt9" executable:

After a bit of digging about, it was "discovered" that FST4W - being an offshoot of the JT9 protocol - was handled not by the wsprd executable, but the jt9 executable.  Simply executing this program with no arguments will yield a list of command-line arguments which, on the face of it, made it appear that updating the wsprdaemon to include the decoding of FST4W signals would be a relatively simple matter.

Except that it didn't work.

Initial testing with strong, off-air FST4W signals that was known to be decodable (because farther-flung stations were able to decode the very same transmissions) yielded no results when the .wav file was applied to the jt9 program - but automatic execution over many hours yielded the occasional off-air decode.  Confused by this, I sought help on the WSJT-X groups.io forum.  Fortunately, Joe Taylor and several of the developers offered a clue:  The "-f" parameter of the jt9 executable, described minimally as "Receive Frequency Offset".

Apparently, the default center frequency of the jt9 executable - at least when in FST4W mode (and maybe others) is 1500 Hz - a fact implied when one gets the display of command-line arguments.  What is not so clear - and only alluded to in the available documentation - is that the apparent bandwidth of the decoding, at least in the 120 second mode, is on the order of 40 Hz (+/- 20 Hz).  Addendum:  This issue was fixed with the "-F" parameter - see below.

At a quick glance through the source code (file "jt9.f90"), this bandwidth setting appears to be hard-coded into a shared variable (apparently accessible by other programs in the WSJT-X suite) called "ntol" (likely a number referring to the "frequency tolerance" setting in the GUI) that is not available via the jt9 command line - at least, not without modification of the source code.  (The possibility of directly accessing these shared variables exists - but this would be platform-specific, a bit messy and somewhat dangerous!)

Unfortunately, this fixed +/-20Hz bandwidth does not appear to be compatible with the way that the FST4W mode has (already!) found use on 2200 and 630 meters where it is used along-side the WSPR mode in the 200 Hz subbands.

A hell of a kludge:

Update - kludge no longer needed:

As of version 2.3.0-rc2 it appears that a new parameter "-F" was added to allow something other than a +/-20Hz bandwidth (referred to as "tolerance") to be used, likely eliminating the need for multiple decodes, below.  A possible command-line for this would be:

jt9 -W -p 120 -f 1500 -F 200 <wav file to be processed> 

With the center frequency (-f) being the center of the passband (1500 Hz) and the "-F" parameter referred to as"tolerance" (e.g. detection bandwidth) being 200 Hz. 

Initial testing indicates that the -F parameter does what it's supposed to do and the kludge below is now longer required.

This fact implies that in order to use something other than the GUI version of the wsjt-x software, a work-around must be invoked.  The following is a bare-minimum example of how one might do this via the command line:

jt9 -W -p 120 -f 1420 <wav file to be processed> 

jt9 -W -p 120 -f 1460 <wav file to be processed>

jt9 -W -p 120 -f 1500 <wav file to be processed>

jt9 -W -p 120 -f 1540 <wav file to be processed>

jt9 -W -p 120 -f 1580 <wav file to be processed>

(One might include the -H, -L and -d parameters in actual practice.)

In other words, in order to cover the entire 200 Hz WSPR subband, the JT9 executable (v2.3.0-rc1) must be executed - processing the same .wav file - at least five times:  The results of the decoding will, in each case, be found in the file "decoded.txt".  If one wishes to implement an equivalent of the -w parameter of the wsprd executable (e.g. +/- 150 Hz "wideband" mode), you will need even more invocations than above.

The result from the above mess will be five different decoding results, each of which must be saved (e.g. renamed) between subsequent executions to prevent overwriting by the previous instance.  After this, the five results would be concatenated to yield a single file - but there is a catch:  It is likely - particularly if the signal is strong - that the same signal will be decoded more than once.  Apparently, the "+/- 20Hz" limit isn't the result of a "brick-wall" filter:  Signals beyond this frequency range may be decoded, but the reported S/N values will likely be reduced as distance of the received signal from the specified center frequency increases.  In short, this means that the results of the concatenated version of the "decoded" file(s) must be sorted and all but the single, "strongest" decode (e.g. best SNR) for each station must be discarded.

Comment:

It would appear that just five iterations to cover the 200 Hz bandwidth is not enough:  I received correspondence from a reader of this blog that observed that a frequency variation of less than 10 Hz from that defined by the "-f" parameter can affect the S/N reading by about 1 dB.  Make of that what you will!

* * * * * * * * *

If one wishes to integrate the FST4W decodes into the existing WSPR captures for processing, yet another step must be undertaken:  "Fixing" the formatting.  Not surprisingly, the output in the "decoded.txt" is not formatted the same as the results of the decoding from the wsprd executable meaning that one will need to do a few things, after the fact, to "fix" them - particularly if you wish to forward them to wsprnet.org, including:

• Supply the date.  The "decoded.txt" includes the time - but not the date.  Because date of the .wav file may not be the same as the system date (e.g. later processing of the .wav files - or the interval being processed occurred just before the new day) - one must use the actual date of the recording.  The obvious place to obtain this is from the name of the .wav file being processed.
  

• Frequency offset.  The information that one might send to wsprnet.org must include the carrier frequency of the received signal, but the output in the "decoded" file has only the audio frequency:  One must obtain the LO frequency of the receiver being used from "somewhere else" and calculate this on the fly.
  

• Supply missing information.  The "decoded.txt" file does not have all of the same information fields that one might supply when uploading WSPR spots, so this information must be added as necessary.
  

• Arrange the fields in the proper order.  Once the needed information is applied, one will probably want to use "awk" or similar to produce the same order as the wsprd data - assuming this wasn't already done in the process.
  

* * *

There are two outputs from the jt9 executable - one directly from the program itself to the standard console and that output to the file "decoded.txt" - and the latter is the most useful. 

Console output:

0416 -24  0.7 1515 `  KA7OEI DN40 17                                 
<DecodeFinished>   0   1

The fields are:  <time UTC> <SNR in dB> <DT?> <Audio frequency in Hz> <always "`"> <Callsign received> <Grid of received station> <Reported TX power in dBm>

From the "decoded.txt" file:

0416   0  -24   0.7   1515.   0   KA7OEI DN40 17                        FST4

 The fields are:  <time UTC> <unknown - possibly drift in Hz> <SNR in dB> <DT?> <Audio frequency in Hz> <unknown> <Callsign received> <Grid of received station> <Reported TX power in dBm> <Always "FST4">

* * *

There you have it:  The germ of what would be needed if one wishes to supplement the existing WSPR decodes with the newer FST4W mode using just the bare executables.  If one wishes to decode other than the 120 second FST4W mode, things get even more complicated!

Sample audio file:

An audio file containing both FST4W-120 and WSPR transmissions may be found HERE - right-click to download.  This file contains an FST4W-120 transmission by KA7OEI from about 116km distant and (at least) two WSPR transmissions.

* * * 

 

Note:  It appears that the "-F" parameter, above, modifies the default ntol setting as described above.

P.S.:  While it would be pretty trivial tweak the code to allow modification of the ntol variable via command line, this would complicate the ongoing maintenance of the wsprdaemon code.  We can only hope that the current authors see fit to include a means by which the entire wspr subband can be monitored with a single invocation of the jt9 executable.

 

This page stolen from ka7oei.blogspot.com

[End]

Reducing RFI from the Tesla Powerall 2

This is a follow-up of a previous article:  "Does the Tesla Powerwall 2 produce RFI (Radio Frequency Interference)?". 

There is a newer article about my experience of RF interference to a Powerwall:  Tesla Powerwall RF sensitivity to RF transmissions - and how to deal with it.

Figure 1:
A typical Powerwall 2 installation.
Left to right:  Utility meter/original load center fed from an underground
power feed, the"new" load center to which the household circuits now
connect, the Powerwall "Gateway" (with two 4G antennas on top
 - not used in my installation), AC disconnect for the
Powerwalls, sub-panel for the Powerwalls (containing
a circuit breakers for each unit) and finally, the two Powerwalls.
This type of system is typically installed outside, near the utility's
connection to the house.
Click on the image for a larger version.

This article contains a bit if information that was original in that previous article, but has been separated and updated.

The information that follows may also be useful for interference reduction of other types of power systems that have been found to generate radio frequency interference, such as grid-tie inverters and other "battery back-up" systems that function like the Tesla Powerwall.

Note that RFI that is radiating directly from solar devices such as microinverters and optimizers require different mitigation techniques - See the blog post "The solar saga - part 1: Avoiding interference (Why I did not choose microinverters!)" for links to information about reducing this type of interference.

As a follow-on to the article "Does the Tesla Powerwall 2 produce RFI (Radio Frequency Interference)?" , this post describes some of the mitigation techniques to knock down what little interference the Tesla Powerwall might produce.

A recap:  Does it produce RFI?

But first, a possible spoiler.

The answer is:  It depends - but the fact that this article exists should have been a big clue.  In short:

• When it is neither charging or discharging:  No interference at all on any band.

This means that if it is charging or discharging, you may expect the following in terms of interference: 

• On the 80 meter amateur band and higher frequencies:  Not that I can tell.
• On the 160 meter band, AM broadcast bands and lower amateur bands:  Maybe.

As the original article notes, I couldn't detect any RFI on 160 through 10 meters when I was using a wire antenna, but I did detect some minor RFI on 160 meter when using an E-field loop - but that represents almost a worst-case scenario.  In the case of the 630 and 2200 meter amateur bands - both of which are below the AM broadcast band - the interference from the Powerwall 2 (prior to mitigation) is likely to be noticed.

Although I did not explicitly in my case, in my estimation the amount of radio interference ("QRM") emitted by a Powerwall on HF is low enough that it would be undetectable at any reasonable distance in a location not directly connected to the Powerwall's circuits.  In other words:  If you have a next-door neighbor that has a Powerwall, I would be extremely surprised if it was detectable on HF at all, particularly if your HF antenna was a reasonable distance (tens of feet/meters) away from it.

Mitigating interference from the Powerwall 2:

If we were dealing with a normal switching power supply, the mitigation of interference would be quite straightforward:  Apply "brute force" L/C filters to all of the AC connections in and out of the device - a topic that has previously been discussed in great detail at this web site (see the links to related articles at the end of this blog posting.)

Applying filtering to a plug-in device that is capable of up to a kilowatt or two is one thing, but mitigating interference issues on a device that is permanently wired in to the house's electrical system and capable of tens of kilowatts is an entirely different matter!  For example, my Powerwall 2 system consists of a two battery/inverter modules that, together, are rated for 14 kW for brief periods, or over 10 kW continuously, representing over 58 and 41 amps at 240 volts, respectively.


To afford a wide safety margin any added inductive filtering would need to be capable of handling at least 100 amps with any capacitors being conservatively rated for the voltage.  Finding and installing a commercially-available AC mains filter with such ratings could be difficult, expensive and awkward, probably requiring a separate enclosure - not to mention appropriate sign-off by inspectors.  What's more is the fact that on a battery-inverter system like this, two such filters would be required:  One on the AC mains feed-in from the utility to the Powerwall and another on the AC mains feeding the house coming from it.

A more practical solution - and one that works effectively for 160 meters - is to install snap-on ferrite sleeves on these six conductors (e.g. the two "hot" phases and the neutral for each of the lines.)  It so-happens that readily-available devices that will fit over RG-8 coaxial cable will also fit nicely over power cable that is appropriately sized for 125 amp circuits.  (The dimensions of these devices is approximately 1.55" [39.4mm] long, 1.22" [31mm] diameter and are made to accommodate cables up to about 0.514" [13.05mm] - but could be modified to go over cables that are nearly 0.6" [15.24mm] diameter).

For exclusively HF, the so-called "Mix 31" ferrite material a reasonable choice, each device providing equivalent resistance as follows:

• 1 MHz:  25Ω
• 5 MHz:  71Ω
• 10 MHz:  100Ω
• 25 MHz:  156Ω
• 100 MHz:  260Ω
• 250 MHz:  260Ω

I used two of these devices on each of the leads (for a total of 12) which, at 160 meters, would provide an equivalent of about 60Ω of resistance.  Considering that there are 3 leads per feed, this parallel resistance is roughly equivalent to 20Ω per feed, so for 160 meters a bit more "help" may be required, so I also used some "Mix 75" ferrite devices of the same size - also two if each per lead.

Intended for lower-frequencies, the equivalent resistance of each of these devices is:

• 200 kHz:  20Ω
• 500 kHz:  58Ω
• 1 MHz:  102Ω
• 2 MHz:  70Ω
• 5 MHz:  50Ω

Figure 2:
Beneath many of the boxes is a raceway/channel that contains some of the
conductors, including data lines and, as depicted above, the wires coming
from the utility mains, connecting to the Powerwall's gateway.  In
my installation there are no exposed conductors in this raceway and there
is plenty of room for the installation of the ferrites.  The marked ferrite
devices are the "Mix 75" while the unmarked are the "Mix 31."  While
it probably doesn't make a difference, I placed the Mix 75 ferrites on the
end of the leads closest to the Powerwall in the unlikely event that low-level
harmonics are generated in the Mix 75 ferrites that need to be attenuated
by the Mix 31 ferrites.  Placing large ferrites over all three conductors
at once for common-mode filtering would be preferred, but doing so
is not always practical as discussed below.
Click on the image for a larger version.

As can be seen, for covering 160 meters and higher frequencies a combination of both types of devices is suggested.  At 1.8 MHz, it is estimated that total equivalent resistance on each lead of the four devices (two Mix 31 and two Mix 75) will be on the order of 220Ω, or about 73Ω for each of the three sets of wires in parallel.

Warnings:

At this point, there are a few "weasel words" that I must include:

• While it is possible to put these ferrite devices (or anything at all!) inside the Tesla Powerwall's gateway box, doing so would probably require the "official" permission of Tesla's engineering department to avoid the possibility of voiding a warranty/service agreement.  Because of this, it is better to mount them on the conductors outside the gateway.  Filtering could also be installed at the disconnect and/or circuit breaker between the Gateway and the Powerwalls, but this, too, may require appropriate approval and sign-off by Tesla engineering to avoid warranty issues.
• Placing any ferrite devices outside the Gateway box will not affect its operation and would be less intrusive than, say, installing a whole-house surge protector as no physical connections are being made.  Because of the wide difference between the mains frequencies (50/60 Hz) and the lowest RF frequencies of interest (136 kHz-1.8 MHz) for which these devices are designed, these ferrites will have no measurable effect at mains frequencies.
• The installations described below involve the exposure of high voltage, high-current circuits inside a breaker panel.  DO NOT even think of opening such a panel when it is "live", let alone installing any such devices inside it.
• DO NOT even think of installing such devices in a panel - even if it is powered down - unless you have experience working with electrical circuits.  If you do not have such experience, refer to a licensed electrician to install such devices.
• Where I live it is permitted for me (the homeowner) to make modifications to the home's electrical system, but it is up to YOU to determine the safety and legality of any sort of modification of your electrical system and determine if you are competent to work with it.  Do not presume some/any of the described modifications to be legal or in compliance of safety regulations in your (or any) jurisdiction!
• I cannot be responsible for injury or damage and no warranties as to suitability or safety should be implied related to the content of this and related pages.  You have been warned!

Installation:

First off, note that all of the units (the two Powerwalls, breaker panels, etc.) in my installation are connected together with metallic conduit and if properly installed, this conduit will quite effectively bond all of the various boxes together electrically.  This means that it is likely to be quite effective in both preventing direct radiation of RF energy from the contained conductors as well as minimizing differential RF currents between the various boxes.

What this de-facto shielding will not do is stop RF from being conducted on the wires that leave this system - notably those that go into the house or to the power utility.  In my case, mains power is fed from underground which means that the most likely source of interference from the Powerwall is likely to be conducted into it from the main breaker panel and onto the house wiring.

Visible in Figure 2 (above) is a channel that runs underneath several of the boxes and in this channel are the conductors that, in my installation, go from the utility mains panel to the Powerwall's Gateway - and I installed one set of the ferrites (a total of 12 devices) in it as depicted in Figure 2.  Because there are no exposed electrical connections in this channel, these devices can be safely installed without turning off power.

Vibration prevention:

These ferrite devices are, by their nature, quite ferromagnetic and as such the magnetic field associated with the AC current flowing through the wires over which they are slipped will cause mechanical movement.  When I installed the first of these devices I could hear them buzzing slightly, the apparent result of the two halves of the ferrite moving with respect to each other.

Figure 3:
 This is a view inside my main house's breaker panel with the "dead front"
cover removed.  In the upper-right corner is a 125 amp circuit breaker that is
the main feed-in from the Powerwall Gateway (the partially-visible box to the
right) which can carry the power from the utility and/or from the Powerwall.
The space for these ferrite devices is a bit crowded, but they do fit.
As noted in the warning, this panel has exposed, live connections and you
should not even think about working in it unless you have experience
in working on electrical systems and the power is turned completely off!
Click on the image for a larger version.

To prevent this movement - and the possible damage of the ferrite devices over time due to this constant motion - I spread an extremely thin layer of clear RTV (silicone) adhesive across the mating surfaces of the two halves to bond them gently together.  These devices have two mirrored halves of the ferrite that, when assembled, touch each other and are polished smooth, so one need only barely "wet" their surfaces with the slightest film - only the tiniest fraction of what would be used normally, an amount so small that it looks somewhat like an oil slick is sufficient for the polished surfaces.

Alternatively, a small drop of cyanoacrylate (e.g. "Super Glue") could be used, but unlike RTV, this would make removal difficult were it required in the future!  Adding anything between the two, polished halves of the ferrites will reduce their effectiveness somewhat so it is important that the two surfaces be as close to each other as is possible by using the smallest amount of RTV.

Installation in the main breaker panel:

In my installation there was another location at which these ferrites were to be installed:  On the power feed from the Powerwall to the household circuits where the majority of RF noise is likely to be conducted - but instead of being in a raceway where there are no "live", exposed connections, the only place that this wiring appears is in the main circuit-breaker panel.

It should be noted that some ferrite mixes can be slightly conductive which means that the material itself should not be allowed to touch any metal that may carry a voltage.  The "snap-on" devices have plastic covers that effectively insulate the ferrite within, but this should be noted if "bare" toroidal cores are used:  Good-quality polyester tape is likely suitable to provide good insulation and protection.

Figure 3, above, shows the installation of the ferrites on the conductors within the breaker panel.  As can be seen, there are "live" exposed connections that pose a shock hazard which means that these devices can be safely installed only if the power is turned completely off.  As was done with the other devices, an extremely thin layer of RTV was put on the mating surfaces of the ferrites' halves to prevent their buzzing.

Comments:

It would be preferable to be able to wind several turns of the large power cables together through large ferrite cores (such as toroids) to achieve much higher effective resistance at the frequencies of interest, but this is simply not possible in the available space with the existing wiring.  Because the conductors were already in place and routed, it was deemed to be too awkward to disconnect one end of the (heavy!) cable to allow ferrite devices to be slid over it, so "split" devices were used instead.

If one is starting from "scratch" - or has the ability to add it later with some rewiring - enough extra cable length added to allow the winding of multi-turn chokes through large ferrite (toroidal) "non-split" cores inside a dedicated, metal junction box would be desirable.  Doing this can greatly increase the series inductance and provide a commensurate reduction of conducted RFI.

It would also be preferable to pass all of the power cables through the center of a single ferrite (of ferrites) as a single bundle to provide a "common mode" impedance path, but this is difficult to do as I have not found a source for split ferrites of 31, 75 or 77 mix that would accommodate three cables that are about 0.5 inch (approx. 12 mm) diameter.  The obvious alternative would be to pass the conductors through a stack of adequately large ferrite beads/cylinders or toroidal cores, but doing this would require that the conductors be disconnected from one end and temporarily pulled back.  The preference would be to have this done at the time of the original installation, particularly if several turns could be passed through some large cores, but again, this is much harder to do after the fact, particularly with the limited length of wire in an already-installed system.

If you are able to put all of the wires with the ferrite cores in a single box, it is a good idea to keep the "input" and "output" wires (e.g. "before" and "after" the ferrite) away from each other - and from other conductors and ferrite devices as well.  If the "clean" and "dirty" (from an RF standpoint) wires are run together in the same conduit, it is possible that RF energy could couple from one to another and partially negate the effects of the RFI mitigation.  Note also that the wires themselves - and the ferrites - of different sets of wires should be kept apart to prevent capacitive/magnetic coupling as well - but only a few inches/cm of distance should suffice.

Finally, while there is plenty of room in the raceway to accommodate the bulk of a number of these cores, there is much less available space within the cramped confines of the breaker panel to accommodate a large stack of ferrite rings/sleeves, particularly if one were to wind several turns of wires through them.  If you are contemplating a brand new installation, or if you are willing to pull wire out and do mechanical re-work, by all means put several turns of the three wires (both "hot" and the neutral leads) through common cores to maximize common-mode impedance.

Other RF interference paths:

In addition to the power connections to/from the Powerwalls, there are two other possible egress paths for radio frequency interference.  I did not check to see if they were sources of radiated interference, but I assumed that they would be.

Figure 4:
Also contained in the raceway is the CAT 5/6 cable for the Ethernet
cable that provides the Powerwall with internet connectivity.  In my
installation there is also another data cable that goes to current/voltage
monitoring equipment (the Neurio) where the PV (solar) equipment feeds
into its sub-panel.  Multiple turns and conductors of wire were fed
through several ferrite devices to choke any RF that might egress.  The
upper device consists of three square snap-on ferrite cores while the bottom
device is the ferrite core from the yoke of a scrapped CRT computer
monitor.  Not shown are additional multi-turn chokes wound on ferrites at
the "other" end of these same cables to prevent interference to those devices.
Click on the image for a larger version.

• The Ethernet connection from the Gateway.  It is common to "hard wire" a CAT5/6 cable from the Powerwall's Gateway to an Ethernet switch (behind a firewall) to provide internet connectivity.  While an Ethernet interface is, by its nature, galvanically isolated from its support circuitry, it does have some capacitive coupling.  It is possible to wirelessly (via either WiFi or via a cellular network) connect the Powerwall to the Internet - which would avoid such cabling - so one would have to determine the nature of the specific installation.  (Note:  Some Powerwall owners report issues with connectivity when using a wireless connection so a direct, wired connection is best.
• Serial power cable to voltage/current monitoring.  A typical Powerwall 2 installation uses devices made by Neurio to monitor the voltage and current at both the connection to the power mains and at the PV (solar) electrical connection.  While a wireless connection between some of these devices is possible, there may be a (more reliable!) 2-wire (half-duplex, RS-485 serial) connection between some of these devices and RF egress could occur on this cabling as well.  (I originally had a wireless connection to the Neurio, but it was unreliable at the required distance.)

In my case I have both an Ethernet cable going to my firewall/router and a wired RS-485 connection to the Neurio monitoring the PV system.  To reduce the possibility of either of these lines conducting RF energy into a circuit that might radiate, the two cables were put together and wound through several ferrite devices as shown in Figure 4.  The upper devices are square, snap-on ferrite chokes while the lower device is the mass of ferrite from the CRT yoke of a discarded computer monitor.  The use of several devices and multiple turns greatly increases the effective inductance of this coil and its effectiveness overall.

Figure 5:
EMI capacitors in panel.  These are 4 uF at 600VAC
"pulse-rated" plastic units specifically designed for
filtering of noise in high-current AC circuits.  Each of
the wires on top go to a circuit breaker with one capacitor
for each phase of the circuit.  The bottom lead connects
to the ground bus and, eventually, the metal junction box.
Only high-current "pulse" type capacitors along with
 circuit protections should be used in this application.
Click on the image for a larger version.

While using ferrite devices on CAT5/6 cable will not normally affect the high-speed Ethernet signals within, CAT5/6 cable should not be coiled extremely tightly as doing so will distort the geometry of the twisted pairs and the integrity of the signals.  While this is unlikely to have much of an effect on 10 or 100 Megabit connections unless the cable is very tightly wound, it can degrade a "Gig-E" (1 gigabit) Ethernet connection (the Powerwall only uses a 100 Mbps connection) if the coil is smaller than 3-5 inches (about 8-12cm) in diameter or if the outer jacket of the Ethernet cable is "kinked".

Adding RF bypass capacitors:

As mentioned above, I did (later) add some RF bypass capacitors to the system:  Two capacitors (one for each phase) on the "house" panel connected to the output of the Powerwall and another pair of these same capacitors on "utility" side of the world in the original distribution panel.  These capacitors are 4uF plastic film units designed specifically for bypass and pulse service and are rated for at least 630 volts AC.  Their connection to the power bus is made by dedicated 15 amp circuit breakers and the other end of these capacitors is to the ground bus inside the panel - See figures 5 and 6.  In theory, the 4uF capacitors will present around 660 ohms at the 60 Hz mains frequency, implying a current of 180mA through each at 120VAC for a total of about 22VA, but since this is reactive-only, no heat will be generated.  This high capacitance was chosen for its ability to effectively shunt energy at 137 kHz where its reactance will be on the order of 0.3 ohms, although the overall reactance+resistance of the leads and circuit breakers will be higher than this.

Figure 6:
Added breakers to connect EMI capacitors to the panel's
power bus.  Using a separate breaker for each capacitor
is the easiest and safest way to connect the it to the
main bus bar in the panel.
Click on the image for a larger version.

Installation of these capacitors significantly reduced the amount of inverter noise present on 630 meters and "noticeably" reduced it on 2200 meters.  Eyebrows might be raised about the rather long lead lengths to connect these capacitors - both on the "ground" and the "hot" side, but this could not be helped.  At these low frequencies (e.g. <500 kHz) this isn't as critical as it might be at HF, but lead length should be minimized.

Another pair of capacitors (not shown in the attached pictures) was similarly installed in the breaker panel that is on the "unprotected" side of the isolation switch:  Whereas those in the picture are on the "house" side of the Powerwall, the others are on the "utility" side.

Comment:

In the U.S., electrical code typically requires wiring of electrical circuits inside a grounded metal enclosure with a large bus bar for carrying the current from the (usually) two phases found in residential wiring - which is likely going to be a 240 volt, center-tapped source from the utility's mains transformer.  In North America and some other places, "small" items (up to about 1800 watts) are powered from a 120 volt circuit against the common "neutral" wire (the center-tap) while larger items will operate from a 240 volt circuit in a balanced fashion.

In many other parts of the world there is a single neutral connection to the house and a 240 volt "hot" wire which means that one would need only apply interference mitigation to these two wires instead of three as depicted elsewhere on this page.  It is also possible that in some areas electrical codes may not require an enclosure that provides a convenient common power "bus" and large ground plane (e.g., metal enclosure that also acts as a shield) per se, requiring different techniques to apply the described mitigation.

The results:

While it may be a bit of overkill, the addition of the two types of snap-on ferrites (e.g. two of each type on each conductor for a total of 24 snap-on devices) has reduced the interference on 160 meters to the point of inaudibility - and pretty much the same thing on 630 meters.

On 2200 meters the interference is significantly reduced - only being "just visible" that band as the spectral display below shows.

Figure 7:
Annotated spectral display showing the harmonic (and other) energy from various devices, including the Powerwall 2 after the described RFI mitigation techniques.
Click on the above image for a larger version

This graph in Figure 7 shows the frequency range from 0 through 250 kHz, a range that is entirely below any HF or MF amateur band, but includes the 2200 meter amateur band near 137 kHz.  As can be seen from this, harmonics from the Powerwall 2 (which appears to have a fundamental frequency of 32 kHz) are pretty much gone by 250 kHz.

This spectral display also depicts the harmonics from a pair of SunnyBoy grid-tie inverters (only the first several harmonics of the 16 kHz switching frequency are even visible) plus some rather strong harmonics from a plug-in switching wall-wart that operates at a fundamental frequency of about 73 kHz:  It is this wall wart - and (potentially others like it) that is likely to cause more QRM to reception on amateur bands than the Powerwall 2!

To completely quash interference at the 2200 meter frequency (around 137 kHz) it would probably be necessary to increase the inductance in the offending leads  between each of the three conductors (ground, L1 and L2) even more than has been done with the ferrite devices.

What about RF interference to the Powerwall? 

The Powerwall itself is a computer-based system with a number of analog monitoring points and as such, it is theoretically possible for external RF to cause it to malfunction if that energy somehow "glitches" one of its computers and/or causes one or more of its many sensors to read incorrectly.  To provide protection, the Powerwall is designed very conservatively and in the event of a serious discrepancy or fault, it will shut itself down.

The question should be asked:  Is it possible for external RF to cause such a shut-down?

The answer is:  Maybe. YES

There is a newer article about my experience of RF interference to a Powerwall:  Tesla Powerwall RF sensitivity to RF transmissions - and how to deal with it.

About a week after my Powerwall was installed and running I happened to tune up on 40 meters using my 1.5kW amplifier.  While I was doing this, the power to my entire house "blinked" several times and went off with the Powerwalls indicating some sort of error condition.  Unfortunately, the isolation relay had tripped and my house was disconnected from the mains and the Powerwalls did not reset themselves even after turning them "off" for over 15 minutes.  After a bit of hassle, I was able to get the Powerwalls reset - but the question remained:  What happened?  I opened a ticket with Tesla support and they came out to investigate a few days later.

It was determined that a possible cause of this "loss of power" event wasn't due to RF, but instead due to arcing at one or more connecting clamps on the mains side of the isolation relay in the gateway that had not been properly tightened when it was installed.  The extra 2+ kW of load on the AC mains from the RF amplifier may have been enough to cause arcing in that loose connection and the Powerwall, detecting this as a potentially dangerous fault (as arcs can be!) killed all of the power for reasons of safety.

Since the clamps were tightened I have never been able to recreate this event, but being "gun shy" I immediately started installing the various ferrite devices on the power and data communications cables - not only to keep RF interference from the Powerwall from radiating, but also to prevent RF from getting in.  In other words, if it had been sensitive to external RF before, it certainly is not sensitive anymore, now that I made the above additions!

Parts sources: 

There are several sources of snap-on ferrite devices described on this page, including:

• KF7P Metalwerx - link - Supplier of a variety of Ferrite devices and many other things.  At the present time he stocks the "Mix 31" devices, but does not stock "Mix 75" snap-on cores at the time of posting.

• Mouser Electronics - link - The "Mix 31" snap-on cores - P/N:  623-0444164181  (Fair-Rite P/N:  0444164181);  "Mix 75" snap-on cores - Mouser P/N:  623-0475164181  (Fair-Rite P/N: 0475164181).  Mouser Electronics has other sizes and mixes of these various devices.  Also obtained from Mouser Electronics were the Kemet 4 uF, 600 VAC "pulse" capacitors Mouser P/N:  80-C4GAMUD4220AA1J (Kemet P/N:  C4GAMUD4220AA1J) depicted in Figure 5.

Links to other articles about power supply noise reduction found at ka7oei.blogspot.com:

• Containing RF noise from a "pure" sine wave UPS.   Even when it is not operating your sine wave UPS may be producing a lot of HF radio interference!
  
• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.
• Quieting an insanely noisy LED floodlight - link.  This describes how a constant-current LED supply that produced enough interference to quash HF reception was quieted down to the point of undetectability.

Other solar power related posts at ka7oei.blogspot.com:

• Teasing out the differences between the "AC" and "DC" Powerwalls - link.  In this post I discuss generally how "AC Battery" systems like the Powerwall and how they work as well as the general differences between the so-called "DC" Powerwall and the AC Powerwall.
•  The solar saga part 1: Avoiding Interference (Why I did not choose Microinverters) - link.  Having had first-hand experience observing a microinverter-based PV system, I discuss why I went the route of the series-string inverter. 
• The solar saga part 2: Getting the system online - link. Like most large projects something intervenes that it makes it take longer to complete - and that was the case here, but it was successfully completed... eventually!

This page stolen from blogspot.ka7oei.com

[End]

Does the Tesla Powerwall 2 produce RFI (Radio Frequency Interference)?

Follow-up article:  There is a follow-up article to this one describing how I mitigated what interference was being produced by the Powerwall 2:  Reducing RFI from the Tesla Powerall 2

Figure 1:
A typical Powerwall 2 installation.
Left to right:  Utility meter/original load center fed from an underground
power feed, the"new" load center to which the household circuits now
connect, the Powerwall "Gateway" (with two 4G antennas on top
 - not used in my installation), AC disconnect for the
Powerwalls, sub-panel for the Powerwalls (containing
a circuit breakers for each unit) and finally, the two Powerwalls.
This type of system is typically installed outside, near the utility's
connection to the house.
Click on the image for a larger version.

Now that I have an installed and operating Tesla Powerwall 2 system I've had the opportunity to answer a question that I've not seen answered elsewhere:

Does the Powerwall 2 cause radio interference?

Why I care:

Being an amateur radio operator that uses a wide range of frequencies across the electromagnetic spectrum (from below 137 kHz to at least 24 GHz) and often "listens" over wider ranges than that I'm always on the look-out for devices that unintentionally produce radio frequency energy which will be manifest as radio interference, reducing my ability to receive signals.

This sort of interference is increasingly commonplace, the incidence having accelerated with the prevalence of "switching" type "wall-warts" (a.k.a. "power cubes") that ubiquitously power nearly anything that is plugged into the wall.  As part of their power conversion, these small devices contain powerful oscillators - typically operating in the 20-100 kHz range - that have the potential to cause radio interference, even at frequencies far removed from their operating range.

What this means is that the inclusion of even more of these devices in my household - including a Tesla Powerwall 2, which is a really big switching power converter - all have the potential of adding to this sea of noise.

What is a Powerwall?

A Powerwall is the Tesla-specific name for what amounts to a "whole house UPS" (Uninterruptible Power System).  There are other manufacturers of similar systems and they have their own nomenclature, generically called an "AC Battery" because they internally perform the AC to DC conversion for charging and DC to AC inverting to provide external AC power.

As the name implies, if the mains power disappears, this system can provide electricity to the entire house (or a portion of it) during the power outage.  As you might expect, very large, high duty-cycle loads such as whole-house air conditioning, electric water heaters, electric clothes dryers and electric furnaces are typically not backed up by a system like this as they would draw down the battery very quickly.

When integrated with a PV (solar electric) system it can be charged from solar energy and if the grid remains unavailable, the house can run indefinitely from the Solar+Powerall, provided that the short-to-medium-term power budget is positive - that is, more solar power is produced than is being used and the battery is not discharged so much between charges (e.g. overnight, on cloudy days) that it reaches the point of cut-off.  My system has two Powerwall units which, working in tandem can provide at least 10kW of power with a storage capacity of a bit more than 26kWh - enough for about a day (without any solar input) with normal usage or several days (without solar input) or little/no sun at all (e.g. the dark of winter with snow-covered panels) if serious power conservation measures are taken.

In areas where there are significant electric rate (tariff) differences between "peak" and "off-peak" hours, this type of system can be used to "zero out" (or reduce) utility usage during peak hours and charge during off-peak hours from the grid and/or with solar.  In my area, this is not relevant as the power rates remain constant throughout the day and it is configured to charge only from solar - which also makes it eligible for the (current as of the time of writing) 30% federal tax credits.

Having one of these systems is a bit like having a back-up generator - except that if the sun is shining, the "gas tank" can be refilled.  Practically speaking this system is unlikely to save me any money in the same way that a back-up generator probably wouldn't, so I would consider it to be a sort of extravagance - like owning an RV, boat or some 4 wheelers - a bit like a somewhat expensive hobby, but more utilitarian.  Being an amateur radio operator I'm also interested in having back-up power in case there is some sort of event that causes the loss of the grid for a period of time, hence the concern about possible radio interference.

How it's connected:

Figure 2, below, shows how a typical "AC Battery" might be wired into a household power system and integrated with a PV inverter.

Figure 2:
A generic block diagram of an "AC Battery" type of back-up power system.
In "Tesla speak", the "Gateway" comprises the functions depicted in the box labeled "Supervisory Control" while the
Powerwall(s) themselves are depicted by the boxes labeled as "Battery-backed inverter/charger system(s)".  If one has large loads that you do not want to be backed up by the Powerwall (e.g. electric water heater, central air conditioner, etc.) these would be connected "outside" the backed-up circuits via a panel connected at the point marked "To Utility Metering/Breaker".
A manual disconnect are required to give first responders an easy way to kill power to the entire house should it be necessary - such as in the event of a fire or other disaster - as simply killing the mains circuit alone would
not do this as the Powerwall would simply restore power automatically!
Click on the image for a larger version.

Comment:  As the time of writing there are some parts of the world - many being in Europe - where, due to regulations, the "whole house back-up" during a grid failure is not available.  The radio interference potentials described below still apply in these cases.

As can be seen, in normal operation the AC battery system is in parallel with the house's power bus and the power grid.  When "charging" from the solar, the system simply monitors the output power of the PV system and adjusts its charge rate to match.  In the "Self-Powered" mode (described below) when there is a grid connection it will charge/discharge at a rate that precisely matches the house's usage, effectively zeroing-out the power going to/from the grid when charging the battery, or export excess power to the grid once the battery has been charged in the same way as a typical "net meter" installation.

If the mains power fails the "Grid Isolation Relay" opens, disconnecting the house from the grid, allowing power to the backed-up loads to be maintained without back-feeding the utility.  The process of detecting a grid failure, disconnection from the grid and full restoration of the power seems to take between 200 and 750 milliseconds but the return to a grid connection - after the mains power has returned and stabilized for a few minutes - is nearly instantaneous.

If there is sufficient power budget (e.g. enough battery storage to last until the next day's sun and there is enough PV capacity to run the house and recharge the battery) it is possible to run "off grid" entirely - theoretically, indefinitely.  The downside of running in an "island" mode is that if you have a "Net Metering" arrangement, the "excess" power that would normally be exported back to the grid would simply be lost and no credit would be available for it:  Since there's really no down-side to having a system that disconnects the power grid during a failure, anyway, why not get credit for "excess" power?

So, does the Tesla Powerwall 2 cause radio frequency interference?

Yes and no.

The "no" part:

On the HF bands I have determined that in my particular case (and prior to mitigation techniques described later) the interference potential on the HF bands to be minimal or negligible.

When the unit is idle (neither charging or discharging) no interference can be detected on any LF, MF or HF band.  No interference has been noted on VHF or UHF bands, either.

When the unit is operating (either charging or discharging) and I am using my normal HF antenna system I cannot detect any interference from it on the HF amateur radio bands of 80 through 10 meters (e.g. 3.5-30 MHz).  Additionally, I cannot detect any interference from the Powerwall 2 system on any VHF or UHF band, either.

If I walk up to the Powerwall 2 system with a portable shortwave radio while it is operating I can hear a bit of noise when I am within a foot or so (less than a meter) that is likely due to short-range magnetic fields, but this noise energy doesn't seem to be being coupled to the connecting wires outside the unit.

The "Yes" parts:

160 meters:

Prior to noise mitigation techniques (mentioned below) on the highest MF band, 160 meters (1.8-2.0 MHz), the story is a bit different:  When the unit was operating, I could just detect a bit of noise from the unit in the far background, just below the local noise floor - but whether or not I could hear this at all depended on which antenna I'm using for receive.  For example, on an active E-field whip I could just hear this noise, but it is not at all audible when using a wire antenna.

Now that I've done a bit of noise mitigation, it is no longer audible on 160 meters with any antenna.

On lower frequencies (below 160 meters):

Prior to noise mitigation techniques (mentioned below) going down lower in frequency - into and below the AM Broadcast band (e.g. below 1.7 MHz) - the RF noise being produced by the Powerwall 2 (again, when it is charging or discharging) gradually increases, was fairly obvious by the time one got to the bottom of the AM broadcast band (e.g. 530 kHz).  Below the AM broadcast band are two more amateur bands - relatively recent additions to amateur radio in the U.S. - and both of these are bands on which I operate:  The 630 meter band (472-479 kHz) and the 2200 meter band (135.7-137.8 kHz).

At these lower frequencies the interference from the Powerwall 2 (when it is operating) ranged from "significant" at 630 meters to "considerable" at 2200 meters - but this is not surprising.  It would appear that the main power converter(s) inside the Powerwall(s) operate at 32 kHz - and the 2200 meter band is at only about 4 times this frequency.  Because the 2200 meter band's frequencies are comparatively close to the operating frequency of the inverter and its 4^th harmonic at 128 kHz - and because RF interference filtering works better as frequency is increased while the harmonics of these converters (and their significant mains-frequency modulated sidebands!) also decrease in amplitude - the amount of energy at 2200 and 630 meters will naturally be higher than it would be on the HF bands.

In short:  If you do not plan to operate on the 160, 630 or 2200 meter bands, you will likely not experience any interference at all, even if no mitigation techniques are used.

I can only speak from experience with my system:  Other systems may be better or worse in terms of interference, depending on the situation.

An interference source that can be controlled - "The devil you know":

One of the biggest problems with interference is that the source is often unknown - but if you know what is causing interference, it can often be controlled - by adding some sort of filtering, replacing the device with an equivalent that is less likely to cause interference, or simply not use it during those time during which interference would be a problem.  Following this last point, if the RF interference from the Powerwall 2 were to be of great concern it's worth noting that the user has pretty good control of when this might happen as interference from the Powerwall 2 seems to occur only in two possible states:  When it is charging, or when it is discharging.  What this means is that even if you use the MF (160 or 630 meters) or LF (2200 meters) bands it will not cause interference when it is "idle."

A typical Powerwall 2 owner would operate it in one of two modes, selectable from a phone app:

• Backup-only.  In this mode the Powerwall 2 operates only as a "whole house UPS" - that is, it is not producing power except when the utility mains is offline (e.g. a power failure or the user has disconnected it from the grid).  In this configuration and in a typical installation, charging of the Powerwall 2's battery is done only with energy from the PV system (solar + inverter) when it needs to do so - and this usually occurs only if the battery has been discharged below 95% or so.

• Self-powered.  In this mode the Powerwall 2 monitors the net inflow and outflow of power from the house.  In this configuration the Powerwall will either output enough power to "zero out" the usage of the house so that there is, on average, no power going to/from the utility and/or it will take excess power from the PV system to charge its battery which will also "zero out" the power to/from the utility.  If the battery is fully-charged, excess power from the PV system will be fed back into the Grid, just as is done in a normal "Net Metering" situation.

Note:  At the time of this writing there is expected to be a "load leveling" There is now a mode offered in the near future available where the Powerwall may be configured to charge/discharge at specific times to take advantages of time-based tariffs (e.g. lower-cost power during "off" hours).  This does not apply to me and such operation is beyond the scope of this article, but the interference potential could be inferred.

In the "Backup-only" mode the Powerwall 2 system is not usually operating (charging/discharging) and will thus not typically produce any noise on any amateur band - but in the "Self Powered" mode, the only time that interference would not be being produced would be when the Powerwall 2's battery is fully-charged and the excess PV power is being exported to the utility grid.

What this means is that if there is the possibility of interference, one would typically operate in the "Backup-only" mode where it is fairly rare for the unit to operate at all.  In my case, the charging portion of the inverter will operate only for a few hours in the morning as soon as the PV system starts to produce power, one or two days a week when it "tops off" the battery.

If, for some reason one wanted to completely eliminate the possibility of the unit going active - say, during some sort of contest - the Powerwalls could simply be turned off, but this would be done at the risk of losing the power back-up capability in the event of a grid failure, but this is something that you would probably consider only if you were operating on 160 meters or lower.

"My neighbor is putting in a Powerwall system - Will I hear it on the HF bands?"

Probably not.  As noted above, I could hear it only on 160 meters and only on a small active vertical whip which is far more sensitive to such things than a typical wire antenna.  If your antenna isn't very near the Powerwall, you probably won't even hear it at all.

One thing that I have going for me is that my power feed is underground - but this is largely unimportant in my case:  My house is connected directly to the wiring of the Powerwall and if it were to conduct a lot of stray RF onto the AC wiring, I would certainly hear it as it would be conducted directly into my home's wiring and be radiated.  On that basis, I do not believe that even with overhead wiring, the Powerwall would be radiating enough "grunge" from the powerlines alone to be audible.

Another point:  If your neighbor is in the process of installing a Powerwall - or already has a system -  there are several opportunities to determine if QRM is coming from it:

• Does the QRM go away in the dark?  If so, it's not the Powerwall - more likely some microinverters or "optimizer" modules on each panel.
• Has the Solar+Powerwall system apparently causing QRM been installed, but not "commissioned" by the utility yet?  If the system in question is not currently operating in a net-metering fashion (e.g. feeding power back to the grid) then the Powerwall will, by design remain offline.  It will not be until the system is fully "on-line" with the solar power system feeding back into the grid and charging the Powerwall that the Powerwall itself is likely to be active and capable of producing any RF interference at all - if it's going to do so.
• Some microinverters are known to cause interference even at night - if the mains power is applied to them.  If interference is suspected, be sure to check, at night, with the mains power to the microinverter system shut off.
• The possibility of conincidence should not be ruled out:  Something else could have been installed, causing interference - in the house with the solar power system, a different neighbor's house or even your own!

Follow-up article:

There is a follow-up article to this one describing how I mitigated what interference was being produced by the Powerwall 2:  Reducing RFI from the Tesla Powerall 2



Other articles related to the mitigation of interference from switching power supplies:

• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
• Containing RF noise from a Sine Wave UPS - link.  There was a "pure sine" UPS that was causing a tremendous amount of RF interference across the HF spectrum whether it was active or not - but we managed to completely quiet it!
• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.
• The article "Common Mode Chokes" - link by W1HIS.  This paper describes techniques and materials used for interference mitigation. It is recommended reading for anyone who is interested in how RF interference is conducted from a device and how to prevent RF energy from causing devices to malfunction as well as practical advice as how to accomplish it.
• Quieting an insanely noisy LED floodlight - link.  This describes how a constant-current LED supply that produced enough interference to quash HF reception was quieted down to the point of undetectability. 

Some of the above articles contain additional links to other web pages on related topics.

[End]

This page stolen from ka7oei.blogspot.com

A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands

Even though the general availability of the 630 meter (472-479 kHz) and 2200 meter (135.7-137.8 kHz) bands to U.S. amateurs is a recent phenomenon, I've had interest in these frequency ranges for about as long as I can remember.  Back in the "old" days (the 1980s, for me) I would listen in these low-frequency ranges (10kHz to 530 kHz) using my modified Drake TR-7 which has an "LF input" on the back panel.

From the very beginning, I discovered a few things that did not work well for receiving these frequencies:

• Simply connecting an end-fed random wire to the "Low Frequency" input.
• Using my 40 meter dipole.
• Anything that was indoors.

Attempts to do any of the above resulted in either no audible signals other than a racket of power mains "buzz" that would drown out anything that I could hope to hear.  I quickly realized that there were a few signals that I could hear without too much trouble - mostly the very high-power VLF transmitters between 17 and 30 kHz and WWVB at 60 kHz, which is only few hundred miles/km away - and I knew that unless I could hear those signals really well that there would be little hope of hearing anything that was actually weak.

The "discoveries":

Figure 1:
The LF-400B active e-field whip on my
roof.  The antenna is about 5 feet (1.5
meters) above the roof, mounted to a vent
pipe.  The red ground wire connected to
the coaxial cable's shield at the bottom
of the antenna can be seen along with
choke.
Click on the image for a larger version.

Even as a teenager with limited experience and knowledge in such things I realized that at such long wavelengths even a rather long piece of wire as a receive antenna would be akin to putting a paper clip in the antenna connection of an HF rig and expect to "hear the world" - but I also knew that it was possible to hear low frequencies quite well on a very short antenna:  The short whip on my car could hear the entire AM broadcast band pretty well - so it was possible if done correctly.

These realizations told me several things:

• I would probably have to match the "short" antenna to the receiver input to be able to hear anything.  I determined that this could be done with a series inductor or some sort of high-impedance amplifier - or a combination of both.

• When in a car, I could be well-away from interference sources - such as power lines and noisy appliances - and could hear weak AM stations.  Somehow I had to keep the interference from things in the house from finding their way into my receiver.

Rummaging around in my junk box I found a large, variable inductor - probably from a scrapped TV - that I placed in series with my wire antenna and receiver - and over a limited frequency range (dictated by the adjustment range of this inductor) I noticed a dramatic improvement in the signal strength at about the frequencies that the combination of the coil and antenna provided a semblance of matching - although the noise was still substantial.

The next "breakthrough" was to wind a simple 1:1 transformer on a chunk of ferrite - probably the flyback transformer core of an old TV -  that allowed only magnetic coupling between the radio and its chassis, and the antenna and a connection that went directly to my kludgy system of buried ground rods.  By doing this, the "noisy" ground of my receiver - which was connected throughout the house with its noisy devices - was no longer referenced to the antenna.  Because the antenna must have a "ground" of some sort to "push" against I knew that if that "ground" was the radio itself, which was connected to the noisy house wiring, that this noise would, in effect, appear on the wire antenna.  This transformer effectively decoupled the two, using, instead, the comparatively "pristine" ground rod for the antenna to "push" against.  (For a depiction of this method see the external link to a paper by DL1DBC at the bottom of this page.)

Between the above two tricks an entirely new world opened up as I could now hear the (now defunct) Omega transmitters between about 10 and 14 kHz and a myriad of "NDBs" (non-directional beacons) and similar signals in the range from 190 through just below 530 kHz.  To be sure, I had to do most of my listening at night when TVs and lights were turned off, but that's when most of these frequencies propagated best, anyway!

The "LowFER" band:

Somewhere around this time I learned of the so-called 1750 meter "LowFER" band - a spectral slice from 160 through 190 kHz where legal, unlicensed operation (according to FCC §15.217 - read more here) could occur with some very strict limitations (e.g. an antenna that was, at most, 15 meters "long" and a maximum of 1 watt of input power.) but the challenge of both transmitting a usable signal with these limitations and receiving it via conventional techniques (e.g. CW) had its appeal.

It was at about this time - in the mid 1980s - that I purchased an LF Engineering LF-400B - a commercially-available active E-field whip antenna that seemed to have decent reviews in the various longwave-related newsletters to which I then subscribed.  This antenna, with a built-in amplifier and a strong low-pass filter to remove signals above 500 kHz, was much more convenient than trying to string a long piece of wire and matching it as it was rated from "3 kHz to 500 kHz".  One slight disadvantage of this - or any active antenna - is that it needs power, supplied in this case by a "power inserter" that ran from an external power supply or a pair of contained 9 volt batteries.

Being an E-field whip antenna it was still sensitive to the direct radiation of interference from the household and neighborhood wiring and appliances, but provided that I located it away from the house and "decoupled" its cable by winding as many turns as could fit on the core of a flyback transformer from a scrapped TV and grounding the shield at the antenna, it seemed to hear the background static very well - and if I could hear the background noise, there was hope that I could hear the weak signals buried within.

It was during this time period that I actively listened on the LowFER band, managing to hear a number of stations that were 200-700 miles (about 300-1100 km) away and, on one winter evening, hearing a station halfway across the continent - about 2000 miles (3200 km) away.   I also set up my own LowFER beacon that, although very modest, was occasionally heard, on CW, up to 700 miles (1100 km) away.

Comment:

Another antenna to consider for MF/LF/VLF reception is a shielded H-field loop.  By its nature, it is less-sensitive to nearby E-field energy - often that which emanates from electrical devices' interference radiating from wiring.

Another advantage of a loop is that it has a "figure-8" pattern with two nulls, allowing the possibility of rotating it such that one of these nulls is oriented toward an interference source.  The obvious disadvantage is that a loop should have provisions for rotation to steer it into the null for the worst interference - or take care of those instances where the desired station happens to be in the direction of the null.

Shielded loops are available and they can be constructed fairly easily, typically using a piece of coaxial cable.  Unless they are rather large and/or actively tuned to the receive frequency, they - like a short E-field whip - must have an amplifier that is externally powered.

Fast forward to the 21st century:

As it happens, I still have the L-400B and it has been outside, on a roof, for most of the time since the mid 1980s.  Other than having to repair it a time or two (usually due to condensation and related corrosion) it still works as well as it ever did.  While I had not been as active on LF as I once was, I'd been maintaining that receive antenna system and with the recent availability of the 630 and 2200 meter bands, interest has been rekindled.

To this end, I decided to document my receive antenna installation, showing what "works for me."

The antenna on the roof:

I will admit to a luxury that most others will not have:  My house has a metal roof.

Figure 2:
A close-up of the coax choke at the antenna.  This
choke consists of 10 turns wound on a large
ferrite bar.  The coax used is solid-dielectric RG-58.
The use of a solid dielectric rather than a foam
dielectric - such as that found in RG-6 - allowed
a very tight radius winding without worrying much
about the center conductor "migrating" and shorting
to the shield.  A cable like RG-174 would have also
been usable, allowing a tight radius and more turns.
At these frequencies, the loss of the coaxial cable is
insignificant.
Click on the image for a larger version.

The metal roof not only acts as an excellent ground plane, but it is also an effective barrier between what is "inside" my house and the "outside world".  This means that at VLF and LF frequencies, things in my house that generate noise (light dimmers, switching power supplies, TVs) are fairly effectively isolated from this antenna on the roof - at least in terms of direct radiation of energy from these devices.

If you are not "blessed" with a metal roof on your house - but you are willing to go through a bit of hassle - you could lay down a suitable ground plane:  Many people have been known to put down a layer of chicken wire on the roof or an interconnected grid of wires to act as an effective shield.  Practically speaking, it need not cover the entire roof, but if the radius of this plane is 1-2 times the height of the antenna over the roof, it will probably have reasonable effectiveness.

Somewhere this plane must be grounded and it is best that this is done via its very own ground system - which could be as simple as a ground rod - which is preferred over tying into the house's "noisy" electrical ground.

As can be seen from the picture in Figure 1 the whip antenna is mounted to a vent pipe at a height of approximately 5 feet (1.5 meters) above the roof - which happened to be the length of the piece of aluminum that I'd found to mount the antenna.  When experimenting with mounting this antenna I found that if I placed it just above the metal roof, it was very quiet and relatively insensitive - but much of that was due to the fact that the very E fields to which the antenna is sensitive decrease significantly with proximity to "ground".  By raising the antenna above the roof the signals increase very dramatically, but still seemed to be within the "cone of silence" afforded by the metal roof.

Decoupling the coaxial cable at the receive antenna:

At the time that I bought the LF-400B antenna it was offered only with a permanently attached RG-174 feedline, but after about a year of use, often hauling it into the wild to listen, away from the city, the coaxial cable fatigued and broke, so I carefully disassembled it and installed a BNC connector (later versions of this antenna have a choice of connectors as an option.)  This modification allowed me to connect a ground directly to the bottom of the antenna.

Figure 3:
A block diagram of the antenna and receive system showing the grounding and coax chokes.
Note that the "roof ground" - which is, in my case, the metal roof itself, but it could be a grid of wire or fencing material laid on the roof and is grounded elsewhere - is connected directly at the shield of the e-field whip itself, "before" the coaxial choke.  At the ground level, in close proximity to the building entry is another connection to a "clean" local ground such as several ground rods and/or some buried ground radials.
The "Inside coax choke" has the most inductance and does most of the isolating of common-mode noise currents that could otherwise "light up" the antenna with electrical noise that would be conducted from the radio system's ground connection to the power mains.  The DC power inserter puts DC on the coaxial cable to the antenna - but not on the coax to the receiver - to provide power.
Click on the image for a larger version.

As mentioned earlier, one of the "tricks" to a quiet E-field antenna is to prevent electrical noise from being conducted from the receiver and "lighting up the ground" of the antenna itself - a problem that is arguably worse than the antenna itself picking up noise, directly.  One of the better ways to to do this is to "decouple" the coaxial cable between the antenna and receiver using a large amount of inductance on the feedline - and I chose to do this several ways.

As can be seen from figure 1 there is a (red) wire connected directly to the antenna's connector that, in turn, connects to the "local ground" - that is, the metal roof itself.  By doing this, the "ground" of the antenna and the roof are at the same RF potential and the interception of "local" interference by the antenna is reduced.

Also visible in figure 1 - and in more detail in figure 2 - is an inductor in the form of a portion of the connecting coaxial cable being wound around a large ferrite rod from a discarded AM radio.  The location of this inductor places it between the antenna and the receiver and its inductance adds common mode impedance to signals that would be conducted along the coaxial cable, but will not affect the desired signals within the cable itself.  A better choke for this location would be like that depicted in Figure 4 (and described below) as it has higher effective resistance at the frequencies of interest, but since I'd already installed this one, I left it in place.

Figure 4 shows the other end of the cable just after it enters the house.  Just as it enters through the window there is another BNC connector, and connected to the shield at that point is a wire that goes directly to a grounding system that is located immediately outside the window.  Between this grounding point and the inside of the house where the connection to the radio is made the coaxial cable is wound around a much more substantial choke - this one consisting of as many turns of the RG-58 coaxial cable as will fit on a TV flyback transformer ferrite core that was scavanged from a discarded CRT TV or computer monitor.  The details of the locations of these chokes and the grounding points is detailed in Figure 3.

It is this second "inside" choke that does most of the work:  Consisting of about 20 turns, it has a measured inductance of about 15 millihenries.  In running the math we can see that this large amount of inductance is what is required to effectively isolate the coax at LF and VLF frequencies, as in:

Where inductive reactance is calculated using the equation:

Z = 2 * Pi * F * L

Where:

Z = Reactance in ohms
F = Frequency in Hz
L = Inductance in Henries

Because we are dealing with milliHenries and kHz, the "10s" parts cancel out, so:

At 500 kHz:


500 kHz * 15 milliHenries * 6.28 = 47100 ohms

Because this is a linear equation, we can then re-run the numbers which tells us that at 50 kHz, the reactance is 4710 ohms and that at 5 kHz it would be 471 ohms.


What this shows us is that even at very low (VLF) frequencies, the impedance of our rather substantial inductance is still effective, so it will work nicely at both 630 and 2200 meters - and everything in between!

Figure 4:
The indoor coax choke consists of 20 turns of  RG-58 wound on a TV flyback transformer core.  This choke, with a measured inductance of about 15 milliHenries, provides excellent isolation even down below 10 kHz.  If a flyback transformer core cannot be found a suitable choke can be wound on a high-permeability ferrite core using smaller (e.g. RG-174) coaxial cable as described below.  Note that to be effective at these frequencies this choke really does need to have at least several milliHenries of inductance!   See the links at the end of this page for sources of suitable ferrite devices.
Click on the image for a larger version.

Obtaining the inductance:

While "current-mode" 1:1 baluns that isolate the feedline in the manner we desire are readily available, unless they were specifically designed for LF and VLF use they do not have enough reactance to operate effectively at these low frequencies!  What this means is that unless a suitable product is offered by one of these companies that is has been designed for LF and VLF use, they will not work well!  This means is that you will probably need to construct your own coaxial choke.

Using flyback transformer cores:

Many years ago it was pretty easy to scavange flyback transformer cores from old CRT-based TVs or computer monitors, but these are getting harder to find - but this is mostly a good thing since these transformers were part of the very device that caused a lot of interference at VLF and LF frequencies!  Once one manages to get the core out of an old flyback transformer in the first place (sometimes a trick in and of itself!) the fact that these cores are in two pieces makes it easy to wind the coaxial cable over one half and then assemble it.  When I come across a flyback transformer, I often resort to putting it in a toaster oven and heating it so that the glue softens.  Often, the core breaks - but ferrite typically breaks very cleanly and the two pieces can be rejoined using a drop of cyanoacrylate (e.g. "super") glue with little change in performance.  Before using the ferrite core, make sure that any plastic or fiber shims between the two halves are removed:  These were important in the role as a flyback transformer, but they will reduce the inductance of our choke and aren't helpful to us in our quest to quash QRM.

If a flyback transformer core is not available, what can be used, instead?

Using high-permeability toroidal cores:

While not as convenient as a flyback transformer core - which can be disassembled during winding - a ferrite toroidal core can be used, instead.  To maximize the number of turns, smaller coax such as RG-174 would be used and the connectors installed/connected after winding was complete.

Take, for example, a common ferrite material designed for low frequencies - "Mix 75" (sometimes called "Mix J") with a typical permeability of about 5000.  A reasonably large toroidal core would be the FT-240 (the complete part number would be either "FT-240-75" or "F240-75").  Note that the ferrite mixes that one would normally use for things like HF baluns aren't ideal for this purpose as they have lower permeability.

Extrapolating from a data sheet and rewriting the equation we can see that if we can manage to wind 30 turns on this particular toroidal core, we can expect:

L = Al * (T/1000)²

Where:

L = Inductance in mH
T = Turns
Al = mH per 1000 turns from the spec. sheet - 6850 for an FT-240-75

So,

6850 * (30/1000)² = 6.165mH

Clearly, this is a bit less than half as much as I'd measured on my discarded TV flyback, but if we use the equation above we still get 194 ohms at 5 kHz and over 5 kohms at 2200 meters - a respectable amount of reactance!  Using this size of core (an inside diameter of 1.4 inches/3.5cm) it is likely that more than 30 turns of RG-174 could be wound on it - and if you make this type of core, by all means, put as many turns on at as you can!

Unfortunately, the "Mix 75" toroids are not as easy to find as typical toroids designed for higher (HF) frequencies and if we use a more common type such as Mix 31 the result will be between a quarter and a fifth of the inductance for the same number of turns whereas "Mix 77" will, for the same number of turns, yield about 1/3 of the inductance as Mix 75, but this would still imply between 1 and 2 kohms at 2200 meters - still quite good.

Where does one get this sort of toroid?   Toroids can be found at a number of places, including:

• Palomar Engineers (link) 
• Amidon Associates (link). 
• Another distributor of some of these devices is the web site kf7p.com - link.

Again, while "Mix 75" is preferred, "Mix 77" is the second choice - and cores may be stacked to increase the inductance for a given number of turns.

Another possibility - Common-mode chokes:

While a coaxial-based choke is preferred, there are other devices - possibly in your junk box - that may be suitable:  A common-mode choke used for power supply filtering.  The best place to find these is from scrapped switching power supplies - such as those used in computers.

Figure 5:
An assortment of power line filtering chokes and devices.  In the upper-left
is a self-contained AC line filter, but it is not suitable for this purpose as it
is designed to block all RF - both differential and common-mode.  All of
the other devices are dual-winding common-mode chokes that allow
differential currents to pass, but will block common-mode currents - but
not all of these devices are suitable for our purpose - see text.
Click on the image for a larger version. 

Figure 5 shows an assortment of typical devices - but not all of them are suitable.   As noted in the caption, the self-contained power line filter (upper-left) blocks all RF and wouldn't work, but the other devices allow differential currents to flow while blocking common-mode currents - which is what we want.

In order for these devices to be suitable for our purpose, they need to have:

• Adequate inductance.  As we noted above, we need milliHenries of inductance to effectively choke out interference at LF and VLF frequencies.  The smaller toroidal chokes shown - typically those wound on toroidal cores - have hundreds of microHenries of inductance which may be suitable at 630 meters, but could be marginal at 2200 meters.  For example, a choke with 100 microHenries per winding will offer about 295 ohms of reactance at 630 meters, but only 86 ohms at 2200 meters.  Because we want as much reactance as possible - at least in the many hundreds of ohms - we would hope to do better!
• Good balance.  All of these chokes consists of two identical windings and the idea is that if a common mode signal appears across both windings, they will be suppressed.  If, however, the two windings are not identical, this suppression will be incomplete.  It is likely that the "transformer-looking" chokes (e.g. those that do NOT look like toroids) will have reasonable suppression at 2200 meters - and maybe even 630 meters - but as one goes up in frequency even more, the imbalance will grow.
• Low loss to differential signals.  The reason that we can pass a signal through a coaxial cable wound on a large piece of ferrite without affecting the signal being carried by that cable is that the coaxial cable, by its very nature, is fairly low loss to the signals carried within where the signal on the inside conductor of the coax is precisely equal and opposite to that carried on the shield.  If one has separate windings, each carrying an equal and opposite signals, imperfections in these two windings - sometimes the same as those that cause imbalance - can cause degradation of those signals.  As one goes up in frequency these ferrite cores - which are formulated to block low frequencies - can start to get lossy - and this doesn't include the self-capacitance of the windings which can cause other things to happen, such as strange resonances or coupling.  In other words, they may work find at low frequencies, but "fall apart" at higher frequencies such as 160 meters (1.8 MHz) and up.

Figure 6: 
An example of how a bifilar (or similar) choke would be
connected to a coaxial cable.
The diagram above depicts how the two windings would be
connected, keeping straight which is the "center", and that
which is the shield of the coaxial cable.  The dots indicate "phasing" -
that is, same ends of the two windings connect to the antenna side and
the other ends connect to the receiver side.  On the "antenna" side's
coax shield would be connected our "quiet ground".
Click on the image for a larger version.

In short, the suitability these devices for our purpose is best determined experimentally.

How it is would be connected:

Figure 6 shows how such a device would be connected to coaxial connectors.  Note that the winding for the shield on one side of the choke connects to the same shield on the other side.  In theory, this wouldn't matter at RF, but because we may need to conduct DC to power the active antenna, we would also need to preserve the polarity.

Not also that both sides of the input and output coaxes connect to the same "side" of the dual winding choke as indicated by the dots - in other words, the two windings are in phase with each other:  Were either one of the windings (ground or center conductor) "flipped", this choke would do exactly opposite that which we desire - that is, the signal on the coax would be blocked, leaving only noise!

For an inductor such as that depicted in Figures 5 and 6 that is not wound with coax, it doesn't matter which side is the shield and which is the center - just as long as the windings are "shield-to-shield" and "center-to-center".

Another example of feedline choking and grounding:

Figure 7:
RG-174 coaxial cable wound on a TV flyback core.  About
55 turns fit on this core yielding a measured inductance
of a bit over 1.5 milliHenries.  If this core
had equal gapping in both of its "legs" (see text)
the inductance would have been higher.
Click on the image for a larger version.

Figure 7 shows another example of how an E-field antenna's feedline (not mine) was isolated.

In this case - which just happened to be another LF-400B - RG-174 coaxial cable was permanently attached to the antenna.  This cable was cut, leaving about 20 feet (approx. 6 meters) of it still attached to the antenna and, leaving a "service" loop of about 1.5 feet (35cm) the remainder was wrapped on the flyback core of a discarded computer monitor.

It is worth noting (again) that these flyback cores are usually "gapped" - that is, a small - usually plastic - insulator is placed between the two halves of a core to prevent it's being saturated.  On some of these cores there are two equal gaps - one on each of the two mating surfaces and if these are removed, the two ferrite surfaces mate closely.  In the case of the core in Figure 7, only one of these mating surfaces had a gap, meaning that one side mated closely while there was a gap on the other side:  Sometimes the cores are symmetrical and one can be "flipped" to eliminate this problem, but that was not the case with the core in Figure 4, so a slight gap was inevitable and this has the inevitable result of reducing the total inductance of the core.

In the case of the flyback pictured in Figure 7 the plastic gapping material was carefully retained and a very thin layer of epoxy was put on the two sets of mating surfaces and the metal bail holding the two together was reinstalled, the cores being worked back-and-forth to squeeze out extra epoxy.  Once this was done epoxy was applied to the wire bail itself to keep it in place.  After the epoxy was allowed to cure, the remaining RG-174 coaxial cable was wound on it, filling it up.

Figure 8:
A wire attached to the shield to permit grounding, necessary
because the coaxial cable was permanently attached to
 the antenna, preventing a connection from being made
at then antenna, between it and the choke.  See text.
Note that the ground wire emerges from the "downstream"
side of the coax.  When installed, this ground wire will
face down to reduce the probability of moisture ingress.

Click on the image for a larger version.

Figure 7 shows this core mounted in a plastic "pull" box intended for non-metallic electrical conduit.  On one end of the box is mounted an "F" connector to which the end of the RG-174 is soldered while the other end - connected to the whip antenna - emerges through a plug:  If you do it this way you will surely want to pull the coax through the plug before winding it on the core!  After it was assembled, it occurred to us that we should have put the "F" connector on the same side as the plug so that they could both be faced downwards.  Between the receiver and the choke box, ordinary RG-6 TV coax will work:  The impedance mismatch/loss is unimportant at this frequency, in this application.

Figure 8 shows a bit of detail about the grounding of the antenna.  This particular antenna has a permanently attached cable and the owner didn't wish to modify the antenna to add a coaxial connector to it.  That which follows was done before the coaxial cable was wound on the ferrite core.

The ground connection needed to be made directly to the cable's shield and this was done by carefully baring a bit of the shield by removing a small amount of the outer jacket and then using a hot soldering iron to quickly make the connection without melting the inner dielectric - a bit of a trick to do if one isn't skilled in the art of soldering!  To make this weather proof the connection was covered with a thin layer of thermoset (e.g. "hot melt") glue and a small piece of heat shrink tubing was slid over the joint and shrunk.  Over the top of this a thin layer of RTV ("Silicone") sealant was spread over the entirety of the connection and another, slightly longer piece of heat shrink tubing was installed and shrunk - and then another thin layer of RTV and slightly longer heat shrink tubing.

While this sounds like overkill, it should prevent moisture from finding its way in between the jacket of the coax and the tubing.  Finally, this connection should be oriented at the time of installation such that water runs away from it - which is to say, the part with the wire coming out from underneath the tubing should be facing down.

More information about interference reduction:

While the above techniques will go a long way to reduce the amount of noise picked up by an E-field antenna - and, to a degree, any antenna - it is too-often the case that there will be some device that simply radiates a lot of noise.  While at HF frequencies and higher it is possible to reduce this noise with the application of large ferrite devices on cables, power cords, etc. this tactic simply does not work well at VLF/LF/MF frequencies because it takes so much reactance (inductance) to introduce enough effective resistance in the wire conveying this noise and a "snap-on" choke simply cannot do this.  Even if a device contains "good" noise suppressing components (not all do!) they simply may not be very effective at VLF/LF/MF frequencies.

If you are interested in listening on the LF and MF amateur bands, the necessary first steps are outlined above:  Do what is necessary to prevent noise from being conducted out, onto the antenna in the first place.

Once that is done, you may need to "seek and destroy" devices that are particularly egregious when it comes to generation RF "grunge" - and the typical suspects are switching power supplies, light dimmers and some brands of LED lights.  Plasma TVs are notoriously bad interference generators, but since they are no longer being made, their contribution to the miasma of QRM is slowly decreasing as they die off.

The best way to find noise that you can do something about is to power the receiver from a battery (NOT including an inverter!) and turn off all of the power to the house - including shutting down any UPSs that you might have.  If the noise decreases or goes away, turn on one circuit at a time until it returns and upon finding the circuit, isolate the specific device that causes the problem.  If the noise is just the same with your power off as it is on, there may be a noisy power line nearby and/or a neighbor may have a noisy device - and how you deal with those two entities is up to you!

If you find a device (or devices) that generate lots of interference, they might either be replaced with "quieter" ones or modified to be quiet.  Unfortunately, the latter can be a challenge and the links below include techniques for doing this.  If your goal is interference reduction at VLF/LF/MF - and you are constructing better filtering - remember that the higher-inductance chokes will be best!

How well does my receive antenna system work?

In the late evening and overnight, I can easily hear the "band noise" - that is, the sounds of the ionosphere and propagated storm static.  During the day time the noise level is typically lower as it seems as though propagated noise from a wide geographical area is suppressed somewhat - possibly by the formation of the ionospheric E-layer.  During the "busy" hours - particularly from, say, 5 to 11 PM, there can be a bit of interference from other peoples' TVs, appliances and whatnot, but it is usually not severe enough to completely quash reception.

In my ham shack I have some track lighting over the workbench that is equipped with LED floodlights and is controlled by a light dimmer.  While I do not "hear" the LED's switching power supplies, I do get a significant "buzz" on 2200 meters from the dimmer itself - but I don't hear it on 630 meters.  The work-around for this is to use a smaller work light near the workbench - both of them being fluorescent - one having an iron ballast and the other electronic - but neither of them causing detectable interference on either 630 or 2200 meters.

For the past 5 years or so there have been a number of Canadian amateur stations (who have had access to the frequencies around 630 and 2200 meters for a while) plus some U.S. based "experimental" stations that have also operated on a number of other frequencies and in this time, I've been able to "receive" these stations which are typically using a digital mode like WSPR or a more analog-like mode like QRSS (slow-speed Morse code) - both typically being detected by computer.  The operational frequencies of these stations has varied from above 500 kHz to below 30 kHz, depending on the authorized frequencies of the various experimental stations and I've generally been able receive such signals including a number of stations operating in the 470-500 kHz range across the U.S. and an experimental station operating near 29 kHz (yes, 29 kHz!) from New York state to my QTH in Utah - a distance of about 2000 miles (approx. 3200km).

In the relatively short time since U.S. amateurs have been allowed to operate on the 630 and 2200 meter bands I've heard several stations on both bands - some well enough to have copied using Morse code via ear and, possibly, even SSB voice.  As the northern hemisphere descends into winter - and as more amateurs receive authorization and put their systems on the air - I expect to hear even more stations.

One device in my arsenal is a "Line Synchronous Noise Blanker" - that is, a device that will mute the antenna signal when an interfering pulse - which is usually in sync with the power mains - comes in.  This devices is adjusted manually and can go a long way to knocking out this type of noise.  This device is described on this page:  A Line-Synchronous Noise Blanker for VLF/LF/MF use - link.

Links to other articles about power supply noise reduction:

• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.

In case you get intermodulation distortion (e.g. overload) from nearby AM broadcast stations:

•  An effective low-pass filter for 500 kHz and below - link.

Other information about the use of active antennas at VLF, LF and MF frequencies:

• Discussion from the DL1DBC web site about active antennas, including their operation and installation - link.
• Construction and installation of a PA0RDT whip by VK6YSF - link.
• Discussion of E-field whip antennas by PA3FWM - link.

Once again, here are some links to suppliers of suitable ferrite devices:

• Palomar Engineers (link) 
• Amidon Associates (link). 
• Another distributor of some of these devices is the web site kf7p.com - link.

While "Mix 75" is preferred, "Mix 77" is the second choice - and cores may be stacked to increase the inductance for a given number of turns.

Final comments:

The L-400B still seems to be available - at about twice the price as it was when I bought mine in 1986-7.  The page with information on this and similar products may be found here - link.

In addition to the L-400B, there are now other active whips, including the AMRAD active whip, the PA0RDT "Mini-whip" and variants on those designs which may or may not include a low-pass filter to remove mediumwave signals.  All of these are reported to work well, but be aware that some receivers have difficulty dealing with signal from strong, local AM broadcast transmitters.

I have built my own copy of the PA0RDT mini-whip and I tried it only briefly:  When I put it up, it was badly overloaded by my own HF WSPR transmitter - so I took it down and put the LF-400B back up as its built-in low-pass filter makes it pretty immune to being overloaded nearby HF transmissions.

[End]

This page stolen from ka7oei.blogspot.com