Thursday, February 8, 2018

Un-Bricking an RTL-SDR Dongle after an EEPROM write

Figure 1:
One of the better general-purpose "RTL-SDR" receiver dongles available,
with a TCXO for better frequency stability and built in filtering/amplification
on the "Direct Sampling" path to allow HF reception - among other
things!  The entire point of this exercise was to give the RTL-SDR
dongle a unique serial number, seen in the picture, so that it could be
uniquely identified if more than one were plugged into the same computer.
The dongle shown above is sold by "".  It seems to be
no more or less susceptible to "bricking" than any other RTL-SDR dongle
using the same chipsets, so this fix may apply to other versions as well.
Click on the image for a larger version.
The other day I was using the "rtl_eeprom" utility to change the serial number of an RTL-SDR dongle (they all ship with serial number "00000001") to make it easier to identify it when it is online with other units - and I "bricked" it.

This seems to be a common occurrence - the causes (possibly) being:
  • Unplugging it too soon after programming it.  Perhaps one should give it a slow "10" count before unplugging it?
  • Changing more than one parameter at a time.
  • Changing something other than the serial number.
Figure 2:
The SOIC-8 to DIP header (no EEPROM) in the
programmer.  This particular carrier came from
"" although many different types
are available for cheap on EvilBay.  The
programmer in this case is the "MiniPro".
Click on the image for a larger version.
I probably was guilty of doing the first two.  The remedies suggested online seem to be limited variations on a theme, such as:
  • On the EEPROM chip, short the SCL and SDA pins (5 and 6) together while plugging it in - then removing the short and programming it.
  • Shorting the SDA pin (pin 5) of the EEPROM to ground (pin 4) while plugging it in - then removing the short and programming it.
Neither of the above worked for me as the device was steadfastly not being recognized on the machine that I was using.  Interestingly, it was still usable on another machine with the "SDR#" program - but it wasn't being properly identified by the operating system.

On a hunch I plugged in a "good" RTL SDR (one that I hadn't bricked) and downloaded the contents (using the "-r" option) and saved it with a .BIN extension as raw data using the same rtl_eeprom program.  In looking at this file with a HEX/ASCII editor (such as the one built into the "PonyProg" program) I could see the data (some ASCII text) which seemed to comprise a device ID and the serial number.

Using a hot-air rework station I removed the 24C02B EEPROM (256x8) chip from the dongle and soldered it to an 8-pin DIP-to-SOIC-8 header and put it into a programmer.  Interestingly, when I looked at the EEPROM's contents after having done this they were all zeroes - clearly something had gone wrong in the write process!

Importing the .BIN file into the programming software I changed the serial number to what I'd originally wanted it to be and programmed it, verifying that it "took".  I then unsoldered the EEPROM from the header and put it back in the RTL-SDR dongle, again with the hot air rework tool.

Success!  The dongle was working again - with the "new" serial number.

Figure 3:
The location of the EEPROM chip within this particular
RTL-SDR.  Using a hot-air rework tool it took only seconds
to remove the chip and plop it onto the header in Figure 2
so that it could be reprogrammed.  Many RTL-SDR
dongles have similar EEPROM chips, but if you happen to
brick yours when changing the EEPROM contents it will
be up to you to find it on the circuit board if you open it up.
Because fewer than 128 bytes are used, the EEPROM may be
of larger or smaller capacity than the 24C02 mentioned
on this page.
Click on the image for a larger version.
For your edification, the .HEX file is included below, with the serial number of "00001234" which visible when editing it in the programmer software.  In addition to this serial number, the IR receiver has been disabled, since it wasn't needed - and isn't used at all with these particular RTL-SDR dongles.  I won't include pictures and details on how to solder and unsolder or use an EEPROM programmer - or tell you which one to use (there are other sites for that) but I hope that it is useful nonetheless.  Having said that, the "PonyProg" programming software is popular and one can homebrew a quick programmer with it.

Anyway, it saved me $20 in not getting another dongle - not counting my time!

* * *

This is the contents of a .HEX file that can be programmed into the EEPROM on an RTL-SDR dongle to "un-brick" it.  As noted above, you will need to be able to program the chip - probably out of circuit - to effect the repair.

Simply copy and past the hex data below into a plain text file and rename it with a .HEX extension:


In the above file, the serial number is in plain ASCII as "00001234" starting at address 0x37 with the last character at 0x45, skipping every other byte with a null (0x00) between each character.  Again, I'd recommend using a .HEX editor with an ASCII pane such as the one in the "PonyProg" program to better-visualize its contents.  There are clearly fewer than 256 bytes used which means the smaller (128x8) 24C01 EEPROM chip could have been used, so be sure to check which device is being used before programming.


This page stolen from

Saturday, January 13, 2018

A simple crystal oven/heater that uses no power resistors

There are times where one needs to elevate a component to a consistent temperature to better-maintain its characteristics, the most common being the need to heat a quartz crystal to attain better temperature stability.

I had such a need when I was constructing my 24 GHz transverter - link - and needed to have the 99 MHz crystal oscillator (multiplied by 240 to yield 23.76 GHz for a 432 MHz IF at 24.192 GHz) that was locked to a high-stability 10 MHz reference.  This 99 MHz oscillator uses an overtone crystal oscillator, but these are notoriously difficult to electronically tune over much of a range so I needed to maintain the crystal and oscillator at a constant temperature to keep it "close enough" to frequency to be within its narrow tuning capability to allow it to be locked precisely at frequency.

To do this I needed to construct a "crystal oven" - a circuit/device that holds the critical components at a (fairly) constant temperature to accomplish this.

How it works:

Typically, one uses several power resistors to heat a crystal, but there are ways that this can be done using no power resistors at all:  Consider the circuit in Figure 1, below.

Figure 1:
Crystal oven using a power MOSFET as the heating element.
R320/Q306 form a constant-current limiter circuit so that the FET can never be fully "on", keeping its resistance higher so that it can function as a heater.  The FET itself, along with thermistor R316 are "thermally coupled" to the crystal being heated.  The (nominal) 0.6 volt turn-on threshold of Q306 and the value of R320 limit the maximum current to about 600mA, but this could be adjusted by selecting appropriate values for R320.
Click on the image for a larger version.
In this circuit Q305, a generic N-channel MOSFET power transistor, is used as the sole heating element:  Its tab could be soldered or bolted directly to the device that needed to be heated such as the crystal or soldered/bolted to a substrate to which the crystal and other components are mounted, etc.  Because the tab of  typical power FET is the drain lead, make sure that it has DC isolation from ground.

The heart of this circuit is the Q305/Q306/R320 combination.  Assuming that voltage has been applied to the gate of the FET via R319, when the current through R320 exceeds that which is required to effect an (approximately) 0.6 volt drop, Q306 turns on, pinching off the gate drive and regulating the current through R305 to that level.  With the components shown, the maximum amount of current that will flow is that which causes 0.6 volts to appear across R320, which, in this case is (0.6 volts * 1 ohm = 0.6 amps) 600 milliamps.  By limiting the current to a reasonable level the FET's "on" resistance is regulated, allowing it to act as a heater.  Without this current limiting U303 would simply turn FET Q305 "on" and its resistance would be a fraction of an ohm, effectively shorting out the power supply - but more likely it would dissipate a lot of power and blow up if there was sufficient power supply current.

Assuming that there are 9 volts available across Q305 (e.g. the 10 volt supply shown, the drop across R320 and a few other, miscellaneous losses) Q305 will produce about (9 volts * 0.6 amps) 5.4 watts of heat, conducted from its metal tab into whatever it is that is being heated.

To regulate the temperature a simple controller is built around an op amp - in this case, a generic 741 (U303) and a thermistor (R316):  Because of the typical 2-4 volt threshold voltage of typical power FETs, the fact that the output voltage of a common op amp like the 741 doesn't get very close to its negative rail isn't a problem - but this should be kept in mind if you happen to use a "logic threshold" FET that starts to turn on at about a volt.  R316 is "thermally coupled" to the device(s) being heated - but not coupled too closely to the heat-generating component, Q305 that there is constant over/undershoot when the heater is active.

The actual temperature at which the oven will stabilize is determined by matching the value of fixed resistor R315 that of thermistor R316 at the desired temperature - a resistance that can be determined from the thermistor's 'spec sheet or by experimentation.  The actual value of the thermistor at the operating temperature is not particularly important but it is recommended that it be in the range of 5k-100k for practical reasons.  The thermistor that I happened to use had a nominal resistance of 30k at 25C, decreasing to about 11k at 50C, the target temperature, so I used a value of 11k for R315.

When the oven is cold the resistance of thermistor R316 is going to be higher than that of fixed resistance R315 which causes the non-inverting (+) terminal of the op-amp to be higher than that of the inverting (-) terminal which is biased at mid-supply by two equal resistors.  When this happens the output of the op amp goes high, providing gate voltage to Q305 via resistor R319, allowing it to heat up.

Figure 2:
The oven and its controller.  The tab of Q305 is soldered directly to
a large, electrically-isolated island of circuit board material.  As can be
seen from the picture, the board to which the heater is mounted is actually
smaller than the surrounding enclosure, mechanically "floating" in the center
via four pieces of small-gauge wire that provide both a DC return and RF
ground connections as well as allowing a gap that is filled with an air
space and insulating foam.  The circuit is wired "dead bug" with the op
amp being "leads-up", just to the right of Q305.  At the bottom of the
picture is a 3-terminal 10 volt regulator (a 7810 - not shown in figure 1) that
provides a stable 10 volt source for both the oven and the crystal oscillator.
Click on the image for a larger version.
When the oven comes up to the design temperature (e.g. that which the resistance of thermistor R316 is the same as fixed resistor R315) the voltages at the inverting and non-inverting terminal of the op amp are equal and the voltage being output by the op amp drops, removing the bias voltage from the gate of Q305 and preventing the oven from heating further.  In reality, there isn't an "on/off" action by the oven, but a more gradual "power up/power down" caused by the inclusion of R317 between the output of the op amp and the noninverting (-) input of the op amp.

It's worth noting that using the turn-on voltage of a transistor as a current reference means that the actual current will vary depending on that transistor's temperature (e.g. that of Q306), but because Q306 is located within the oven chamber it, too, will be heated and the maximum "oven" current (e.g. that through Q305) will be quite stable.  What can affect the dynamics of this oven system is a variation in the voltage applied to Q305:  The higher the voltage, the more power (in watts) will be produced in heat.  While the oven controller will help to maintain temperature, if the power supply voltage is quite variable - as could happen when this oven is run from a "12 volt" battery (anywhere from 11.5-13.5 volts, depending on state-of-charge) the thermal input power can change and cause a slight instability in the closed-loop temperature control due to this change in available thermal power.  In this case - with the crystal being externally locked to frequency, anyway, this wasn't too important.

There are a few other component sprinkled about in the diagram as well:
  • C318 and R321 are used to prevent the Q305/Q306 circuit from oscillating.
  • R322 is an optional test point to measure the oven current.  FT304 is an (optional) feedthrough capacitor used to prevent RF ingress/egress along this monitor point.
  • C316 and C317 are power supply bypass capacitors - always a good practice to include.
  • R318 and LED D303 provide an optional "oven on" indication.  The cycling of this LED between full brightness and dim/off after being powered up indicates that the oven is heating/stabilizing.  If it cycles on and off continually this can indicate that there is too much thermal resistance between the heater (Q305) and the thermistor, causing the circuit to overshoot.
  • The values of R313 and R314 are not critical - but they should be equal.
  • The value of the thermistor is not critical, but it should probably be between 1k and 100k at the desired operating temperature.  Select R315 to have the same resistance as the thermistor at the desired oven operating temperature.
  • To provide a temperature adjustment, R315 may be made variable with a good-quality multi-turn potentiometer.  Alternatively, resistors R313/R314 can be replaced with a single 20k-50k multi-turn potentiometer.
What to use this for?
Figure 3:
The "oscillator side" of the circuit shown in Figure 2.  The oscillator is a 5th-
overtone "Butler" type build "dead bug" on a piece of double-sided
copper-clad epoxy board.  The crystal is located directly opposite the
location of Q305, the heater (e.g. the board is rotated 1/4 turn counter-
clockwise from Figure 2).
Click on the image for a larger version. 

I devised this circuit when constructing a homebrew 24 GHz amateur radio transverter (transmit/receive frequency converter) and needed a "fairly stable" source of a 99 MHz signal to be multiplied upwards and to be locked to an outboard, stable 10 MHz reference (e.g. high-stability crystal or rubidium source.)

Because this crystal oscillator was to be externally locked, it didn't need to be ultra-stable - just stable enough to keep its temperature close enough to the rather limited frequency-pulling range afforded by high-frequency overtone crystal oscillators.  With this relaxed requirement, the crystal could actually vary a few degrees about the set point with no ill effects whatsoever.


I didn't need to optimize this circuit for ultimate frequency stability as the 99 MHz oscillator is locked to an external 10 MHz reference:  All that is necessary is that the frequency be "close enough" - which is to say, within the rather narrow frequency tuning range afforded via the VCXO (Voltage Controlled Crystal Oscillator) circuit.

If this circuit is to be used for a "stand-alone" oscillator where the frequency is directly affected by the temperature, additional care will be required to appropriately thermally couple the thermistor and heater (Q305) - and possibly tweak the value of resistor R317 - to prevent the temperature from oscillating about its set point.

Finally, even though the temperature controller is entirely ratiometric - that is, power supply voltage variations will not affect the temperature set point to a significant degree -  remember that at higher voltages the power going into the heating element (e.g. the power FET Q305) will also increase.  This change in thermal input can cause the dynamics of the oven to change somewhat and slightly change the rate-of-change and potentially alter the stability of the feedback loop.  Because this oven was intended to keep an oscillator "close enough to" rather than "dead on" frequency this factor wasn't important.

Additional resources:
  • W6PQL Crystal Oven Controller - link - This discusses a more conventional "heater-resistor" circuit for maintain constant crystal/component temperature.
  • The OCXO/Si5351A synthesizer - link - QRP Labs sells a version of their Si5351A synthesizer board with a built-in crystal oven controller providing a stability of 1ppm or better.  The assembly manual for this kit (linked on the referred page) uses low-power FETs as heaters and a bipolar transistor as the temperature sensor.  This manual discusses the operation of the circuit and is an interesting read.


This page stolen from

Thursday, January 4, 2018

A quick fix for a Yaesu FT-757GXII blank display

A couple weeks ago I was contacted by an old friend of mine, having obtained his amateur license in mid-2017, who has a Yaesu FT-775GXII - a synthesized, all-mode HF transceiver from the the mid-late 1980s which had been working well until, one day, there was no display.  Clearly the main processor was fine as the front panel buttons would work, he could transmit and receive and he could even "see" and control the radio's frequency and mode on his computer via the serial CAT interface.
Figure 1:
The front panel of an FT-757GXII with a working display!

One clue was that when power cycled, the display would occasionally flash very briefly, a possible indication that something was almost doing what it was supposed to.  Via email (he lives across the country from me) I had him do some preliminary troubleshooting such as the checking of voltages - but based on the rather sparse information available in the service manual and the difficulty in accessing some of the test points:  Even a couple key capacitors in circuits that often cause problems with displays in some radios - namely the switching supplies that provide the odd voltages for the vacuum fluorescent display - were swapped out, but the display remained blank.

I offered to look at it, so he packaged it up and sent it to me.  When I put it on my workbench I started probing the various lines on the display processor with an oscilloscope:  I could see many of the signals that I was expecting - namely the 500 kHz signal from the display processor's clock, the data coming from the radio's main processor that changed as I pushed buttons and turned the main tuning knob and another signal that appeared to be an acknowledgment pulse from the display processor to the main processor.  What I seemed to be missing were half of the multiplexing signals that drove the display:  It appeared that I was seeing the "common" signal lines for the display, but the signals on the pins that appeared to carry information as to which display segment was to be illuminated were missing as if the display was supposed to be blank.  Without both sets of signals activated appropriately, a multiplexed display will remain forever dark.

I'd already consulted the internet and determined, based on postings in various forums, that at least for its predecessor, the 757GX, the failure of the display processor wasn't terribly uncommon - but not surprisingly this part was long gone from the spare parts inventories of Yaesu and other means of repair/replacement such as getting displays from scrapped radios or even the construction of an "alternate" display unit using a different processor and driver transistors was discussed.  What was interesting was that the "important" signals - namely those for data, acknowledgement, scanning and synchronization - seemed to be present, so the display processor clearly wasn't completely dead.
Figure 2:
Annotated picture showing the two buttons that, when both are set to their
"in" position will disconnect the radio's internal memory back-up battery.
If both buttons are in when the power is removed the processor will be
reset to its "factory" state.
Click on the image for a larger version.

At about that point the old adage drilled into me from the early days of computers and Windows came back to me - although it probably should have been one of the first steps to be taken when the display went blank:  "When in doubt, reboot!"  Perusing the user's manual I determined that a complete "memory reset" was done on the FT-757GXII by setting both the "Linear" and "Marker" switches on the back panel (see Figure 2) to the "in" position at the same time and turning off the radio for 30 seconds - and then turning it back on and restoring the two rear switches to their normal position:  It would appear that these two switches have a second, "non-intuitive" function that when used together, disconnects the internal battery.

The result?  The display came back to life!

What had apparently happened was that somehow, the data stream between the display and main processor wasn't what it should be and the main processor was apparently sending some sort of garbage that the display processor didn't understand - probably due to something in the main processor's static RAM.  It would appear that in the absence of sensible data, the display processor remains blank, relying on the main processor to send the various bits and bytes that display frequency, mode, etc. rather than reverting to some sort of static display.  Clearing the battery-backed RAM of the main processor and resetting it apparently cleared whatever junk had gotten into the memory that had caused it to work improperly.

I checked the back-up battery - an innocuous-looking 2-cell NiCd pack that was near the rear of the main synthesizer board - and it read 2.8 volts with the radio having been disconnected from power for over 24 hours indicating about 1.4 volts/cell, which was appropriate for a properly-charged NiCd.  Visually, this small battery pack looked OK in that there were no signs of corrosion, so it is probably OK, despite its age - longevity being one of the virtues of a properly cared-for, high-quality NiCd cell.

How did the main processor's memory get scrambled?  Who knows - it could have been an entirely random event, due to static from a finger touching the front panel, the back-up battery's voltage having sagged below the point of memory retention while the radio was turned off or the results of some sort of spike - perhaps lightning - intercepted by the antenna that found its way into other circuits.  This sort of "display failure" - apparently caused by the processor's memory being scrambled - doesn't seem to be too common, so my friend considers himself very lucky!

After restoring the radio's operation I did a few tests and found that everything seemed to be working as it should, so it will be packed up and returned to its (very fortunate!) owner very soon.


This page stolen from

Wednesday, December 20, 2017

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

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), AC
disconnect for the Powerwalls, sub-panel for the Powerwalls (containing
two circuit breakers), 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 at frequencies far removed from that.

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 - 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, 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) 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.

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)".  Both circuit
a manual disconnect and circuit breakers 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!
Click on the image for a larger version.
Comment:  As the time of writing there are some parts of the world - mostly in Europe - where, due to regulations, "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 and the power grid.  When "charging" from the solar, it simply monitors the output power of the PV system and the power to/from the grid and adjusts its charge rate to match.  Likewise, 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 or export 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 power 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 power has returned and stabilized for a few minutes is nearly instantaneous.

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

On lower frequencies:

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, being fairly obvious by the time one gets to the bottom of the AM broadcast band (e.g. 530 kHz).  Below the AM broadcast band are two more 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 frequencies the interference from the Powerwall 2 (when it is operating) ranges 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 4th 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:

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" mode offered in the near future 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.

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

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 many 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 short 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 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.

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 3:
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.  As can be seen from the pictures, I used two of these "Mix 75" devices on each of the leads.


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 as described here 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 installation described below involves 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 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 related to anything described on this page.  You have been warned!

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 which 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, utility 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 1 (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 3.  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 magnetic 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 4:
 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 these devices due to this constant motion over time - 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.

Figure 4, 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.

It would be preferable to be able to wind several turns of the large power cables through non-split ferrite cores 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 - particularly in the preferred common-mode fashion (e.g. all conductors going through the same core(s)).  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 or 75 mix that would accommodate three cables that are about 0.5 inch (approx. 1cm) 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.  If this were done at the time of the original installation, it would be the preference - particularly if several turns could be passed through some large cores - but this is much harder to do after the fact, particularly with the limited length of wire in an already-installed system.
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:
Figure 5:
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 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.
Note shown are additional multi-turn chokes wound on ferrites at
the "other" end of these same cables.
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 4G 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.
  • 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 2-wire (half-duplex, RS-485 serial) connection between some of these devices and RF egress could occur on this cabling as well.
In my case I have both an Ethernet cable going to my firewall 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 5.  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 multiple devices and multiple turns greatly increases the effective inductance of this coil and its effectiveness overall.

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".

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 greatly reduced it on 630 meters.

On 2200 meters the interference is reduced, but still significant:  To completely quash interference at this frequency it would probably be necessary to, at the very least, install pulse-rated bypass capacitors (perhaps 1uF or greater) between each of the three conductors (ground, L1 and L2).  If I do this I'll do so using a low-current (15 amp) circuit breaker to provide the connection between L1 and L2 and the ground as a safe and simple way to make the connection.

If adding such capacitors were found to be insufficient to reduce the interference to inaudibility and working "around" the operation of the Powerwall were not practical (e.g. when it was not active) the next step would be a rather awkward and potentially expensive one:  The addition of the aforementioned extra junction box and re-running of the cables to allow the installation of multi-turn common-mode chokes.

What about RF interference to the Powerall? 

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 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.

About a week after my Powerwall was installed 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 was 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!

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.
  • 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. 

Other solar power related posts at this site:
Other articles related to the mitigation of interference from switching power supplies:
Some of the above articles contain additional links to other web pages on related topics.


This page stolen from

Wednesday, December 6, 2017

KA7OEI now QRV on 630 and 2200 meters

Figure 1:
The LF/MF transmit station, configured for 630 meter operation.
At the time this picture was taken I had not yet completed the gear and
put the various pieces in their boxes, hence the mess of clip-leaded-
together modules sitting on my workbench.  Not visible is
the low-pass filter in the power amplifier box or the thermocouple-type
RF ammeter.  The pictured 630 meter variometer has been supplanted
with a "new" one wound with 660/42 Litz wire for lower loss (see Figure 6).
Click on the image for a larger version.
It so-happened that I had a few days off around Thanksgiving and I took this time to throw together a fairly simple transmit converter for the "new" amateur LF and MF bands - notably 2200 and 630 meters.  Having had already obtained my approval from the UTC to operate on both of these bands, I was "good to go".

It took only two evenings to put together the transmit converter and power amplifier as I had the parts on hand - and none of them were particularly exotic.  While the transmit converter will be described in greater detail in a future post, the signal path for the transmitter is approximately thus:

(See the block diagram in Figure 2, below.)
  • A 5 MHz IF is used, allowing a "broadbanded" FT-817 (with TCXO) to serve as the exciter.
  • The 5+ MHz signal (about 5137 kHz for 2200 meters, 5475 for 630 meters) is mixed (using a 74HC4066) with a 5 MHz local oscillator (a 10 MHz OCXO divided-by-two) to yield frequency-stable LF/MF signals.
  • A low-power post-mixer amplifier boosts this signal to a level capable of driving the power amplifier.
  • A single-ended MOSFET-based broadband power amplifier, running on 12-30 volts, provides between 10 and 50 watts of RF at either 630 or 2200 meters.  Because the transmit converter is broadband, it is agnostic to the operating frequency meaning that one needs only use the appropriate low-pass filter to change bands.  (The 630 meter low-pass filter is always in line - another filter is added for 2200 meter operation.)  This power amplifier is designed to be driven by either the transmit converter or another device, such as a QRP Labs Ultimate 3S beacon transmitter configured for these bands.
  • The 50 ohm output of the power amplifier goes to a tapped autotransformer wound on what is probably an FT-240-61 toroidal ferrite core and is used to match the transmitter's output to input resistance of the loading coil.
  • Also in the drawing is a relay the disconnects the loading coil from the autotransformer when not transmitting.  This was necessary to prevent the transmit antenna from "sucking out" some of the receive signal being intercepted by my E-field whip and also to prevent the transmit antenna from conducting "house noise" from the transmitter onto the transmit antenna which gets coupled into the receive antenna, reducing ultimate sensitivity. 
  • The loading coil, placed in series, cancels out the capacitive reactance of the antenna system.  For 630 meters my antenna requires about 230uH while about 2.5mH is needed to resonate the same antenna at 2200 meters.
Figure 2:
Block diagram of the 630 and 2200 meter transmit chain.  The transmit converter is broadband, capable of operating
from below 100 kHz to at least 500 kHz which means that one need only provide appropriate matching and low-pass filters to operate on either band.
Click on the image for a larger version.

When I made my first-ever transmission I had not yet constructed the variometer, but I fished around in my "box-o-inductors" and found several Litz-wound ferrite inductors that were probably rescued from some scrapped TVs or computer monitors and wiring enough of these in series I was able to achieve  resonance with about 750mA of antenna current.  On the very first WSPR transmission I managed to be "heard" by several stations (See Figure 3, below.)
Figure 3:
A screen shot (from of the very first 630 meter WSPR transmission that I made with the badly-kludged loading coil.
Not too bad for a temporary lash-up!
On the next night, after observing a few stations engaging in JT-9 QSOs, I answered a CQ by VE7SL and he replied, giving me a signal report of -22dB while I gave him -19dB.  This was quickly followed by two other QSOs as both W7IUV and NC0B noticed the "new guy" on the band!

Over the next several days I got around to constructing the "new" variometer depicted in Figure 4 and this boosted my antenna current to about 1.25 amps - a theoretical improvement of about 4.4dB with more QSOs to follow - including 2.5 (one "partial") CW contacts on the band.  After operating for a while it became apparent that, for the most part, I could work anyone that I could "hear".

A few days later I constructed yet another variometer for 630 meters - this time using some 660/42 (e.g. 660 strands of 42 AWG) Litz wire which reduced the skin-effect losses by a significant amount and this, along with minor improvements of the ground system, decreased losses and resulted in a further increase of antenna current to a bit over 2 amps - a theoretical ERP improvement of more than 8.5dB as compared to my original configuration. The measured resistance at the input of the 630 meter Litz coil is about 13.5 ohms, implying an overall antenna system efficiency roughly 1% - but still enough to work quite a few stations with a few 10s of watts of RF.

Figure 4:
 The "Mark 1 version of the 630 meter variometer.  This device is wound
on "4 inch" ABS triple-wall sewer pipe using 22 AWG insulated hookup
wire.  Inside is "3/4-inch" ABS waste pipe (actual O.D. about 1-1/8")
that forms the rotatable portion of the variometer.  This unit has an
adjustment range of approximately 175-235 uH.
Click on the image for a larger version.
The "Q" of the antenna system+Litz wire coil is now such that if I QSY from 475.75 kHz for WSPR operations down to about 475.0 kHz for JT-9 I actually see noticeable drop in antenna current until I readjust the variometer, but if I QSY from 475 kHz down to 473 kHz for CW operation the antenna current plummets to a few hundred milliamps and I absolutely must retune!

As is the custom on both the LF and MF bands, my WSPR signal reports not the transmitter power, but rather the estimated EIRP.  I've typically been reporting 0.5 watts (+27dBm) which, assuming about 25 watts of RF power, implies an antenna efficiency of about 2% which, while in the general ballpark, may still be a bit optimistic.  With the recent changes/improvements in my system (mostly improving the grounding, radials and counterpoise network) I will have to re-analyze my estimated system efficiency.

Operation on 2200 meters:
Figure 5:
Antenna and ground system of my LF/MF TX antenna system.  The
yellow line represents the outline of the "Lazy Loop" - a horizontal HF
antenna fed with 450 ohm window line with both conductors of the
feedline being tied together and fed as a tophatted vertical on LF/MF.
The total circumference of this antenna is about 215 feet (65 meters) -
dimensions mostly dictated by the locations of trees at an average
height of roughly 30 feet (9 meters).
The red lines show the extent of my ground/radial system showing
extra wires, including sections of chain-link fences with electrically-
bonded sections and wires buried in the ground, including an
abandoned CATV line.  The roofs of both the house and garage are
metal which are ultimately tied into the ground/radial network.  There
are several ground rods near the feedpoint of the antenna to which
all of the grounds/radials are connected.
Click on the image for a larger version.

I have since wound yet another variometer (visible in Figure 6, below) - also on 4" ABS pipe - for 2200 meters.  This coil, adjustable from about 1.7-2.0mH, uses the same 22 AWG hook-up wire as my original 630 meter loading coil.  As it turned out this coil, by itself, doesn't have quite enough inductance to resonate my antenna at 137 kHz so I place the other two 630 meter coils in series with it.  As compared to the 630 meter loading coils, it is somewhat lossy, but I am able to obtain about 900mA of antenna current:  Not surprisingly, this coil runs slightly warm in operation due to the losses - but these are, no doubt, minor in comparison with the ground losses.

Update - 12 December, 2017:  After improving the ground system my antenna current is now around 1.1 amps on 2200 meters, implying an improvement of at least 1.7dB from current alone.  The actual far-field improvement, based on readings seen from monitoring stations on WSPR, appears to be in the area of 2-3dB.

The measured resistance at the input of this loading coil is about 43 ohms implying an overall antenna system efficiency of well under 0.1%.   Based on estimated antenna efficiency, I've configured WSPR to report my ERP as 50mW, which assuming a transmitter output power of about 25 watts implies an actual antenna efficiency of about 0.2% which is probably very optimistic!

Not surprisingly, operation on 2200 meters - even at this power level - can be a bit hazardous.  With the rather low antenna capacitance the voltages on the feed are quite high - an estimated 5000-8000 peak volts!  What this means is that the feed wire has to be kept well clear of other conductors or else corona will occur, sapping transmit power, filling the room with ozone and becoming a potential fire hazard.  Fortunately, at this modest power level - and with the current-regulated power supply that I'm using - almost any sort of fault will detune the antenna system to the point that the high voltage will all but disappear and/or the power supply will go into current limiting and effectively shut down the transmitter.

Figure 6:
Left to right:  The original 630 meter variometer (seen in figure 4
wound with 22 AWG stranded wire , the new 630 meter wound with
660/42 Litz wire and the 2200 meter variometer, wound with the
same 22 AWG stranded wire and insulated with PET tape to allow it
to withstand the high voltages.  In the lower right corner is the
autotransformer wound on an FT-240 ferrite core.  With my current
antenna I must put all three of these variometers in series to
resonate the system at 2200 meters.
Click on the image for a larger version.

Despite this simple arrangement I've managed to be "heard" by at least seven other stations in the western U.S. and Canada using WSPR to date, but I've not yet made any 2-way contacts.  The relative scarcity of stations that listen or transmit on 2200 meters - coupled with my rather weak signal - means that a contact will probably have to be arranged and conducted using a weak signal mode like JT-9 or QRSS.


There are plenty of improvements to be made, most notably getting the feed of my antenna a bit higher, laying out a few additional ground wires to further-reduce losses and improving the variometer for 2200 meters - but there are only so many things that I can do on my relatively small city lot.  This entire arrangement has so far been precariously sitting on my workbench meaning that the high RF voltages are also also nearby, just waiting to leap out at me when I reach over to tweak a variometer.

At some point I'll "remote" the matching network outside, but I need to get/build a few other items first, namely some stepper motors, control circuity, more vacuum relays and a means of remotely monitoring the antenna current.

Comment:  Despite having the feedpoint in my shack, I've not had any problems at all with transmit RF getting into computer speakers or other devices in my house.

* * * * * * * *

My recent operation, as of the date of this post, seems to be the only actively transmitting station on either 630 or 2200 meters in Utah.  I have been running WSPR on 2200 meters most of the time, occasionally switching to 630 meters in the local evenings when the activity level on that band is highest.

If you are QRV on 2200 or 630 meters and would like to arrange a CW, JT-9 or QRSS contact with me, or if you are interested in just "hearing" my signal (via your ears or with a computer+sound card) drop me a line using my callsign at arrl dot net.

Other entries on related topics found at this site:
Other web sites that have information on 630 and 2200 meters: 

This list is by no means comprehensive.  Peruse the "links" sections on the sites below for even more information.
  • NJD Technologies - link  - This web page has a wealth of information related to 630 meter operation, propagation and reports of activity, plus lists of known-active operators on both 630 and 2200 meters.  This web site also has many links to others that have credible information on LF and MF band topics.
  • W1TAG's web site - link  - John, W1TAG, has long been an experimenter and operator on the MF and LF bands.  This site has details on equipment both for operating and measuring performance at these frequencies.
  • W1VD's web site - link - Jay, W1VD, has long been an experimenter on the LF/MF bands and this page offers a lot of information on equipment for transmitting and receiving on these bands.
  • Antennas by N6LF - link - The callsign gives  you the clue that this guy likes LF/MF operation.  This page includes detailed information on LF/MF antennas and how to characterize/improve them.


This post stolen from

Friday, December 1, 2017

Containing RF noise from a sine wave UPS

An amateur radio friend of mine (WA7X) has a cabin in the mountains.  It is not a particularly "rustic" cabin as it is festooned with radios, antennas, propagation beacons, computers and cameras and has an internet connection, but because it is in a remote location it occasionally has 3-6 hour power outages and thus it also has a UPS (Uninterruptible Power System) to keep many of these things online in the interim.

This "sine wave" UPS is a 1.5kVa unit that was cast off from by someone for the same reason most UPSs are cast off:  Its internal battery went bad.  Rather than simply replacing the battery, its previous owner simply got another UPS and asked the question "Do you want this?"

Instead of replacing the internal battery, the DC connections for the batteries were brought out and a bank of six 12 volt lead-acid batteries was wired up (two sets of three parallel batteries connected in series amounting to a nominal 200-ish amp-hours at 24 volts) to provide the needed 24 volts and a DC circuit breaker was added for safety, this battery capacity allowing the unit to run for far longer than it could have on the original battery.

While this UPS was more efficient than the previous unit and produces a fairly nice sine wave rather than the typical, ugly "modified sine wave" there was a price to pay:  RF Interference (e.g. RFI) that was present whether the unit was active or on standby.

But first, a few weasel words:
  • This project involves high voltages and/or currents:  Do not attempt to construct a similar device unless you are thoroughly familiar with electrical safety and the wiring of such devices.
  • If you use an external battery bank with a UPS it is imperative that you include some sort of current liming, such as a fuse or circuit breaker rated for both the expected current and battery voltage.  Such devices are available from auto-parts stores.
  • If you use an external battery bank with a UPS you must determine if this battery bank is "mains referenced" internally by the UPS or not.  If it is mains referenced (e.g. connected directly or indirectly to the mains AC voltage) then the low-voltage DC terminals will pose a line voltage safety hazard and care must be taken - the least of which is enclosing the battery and any exposed DC terminals to prevent accidental contact.  This UPS's battery terminals were isolated from the mains and given that the room in which the UPS has restricted access, the low-voltage battery connections themselves were deemed to be "adequately safe" left exposed.
  • YOU are responsible for the safety and any liability if you choose to do something similar to what is described on this page.  You have been warned!
    Figure 1:
    The completed AC/DC filter.  This
    box contains a "brute" force L/C filter
    for the battery (DC) leads as well as
    separate filters for the AC mains in/out
    power connections.
    Click on the image for a larger version.


The radio frequency "grunge" from the UPS manifested itself on the HF ("High Frequency" or shortwave) bands in several ways.  Most obvious was a loud "buzz" every 40-50 kHz on the lower (80 and 40 meter) bands, but there was also more subtle interference that pervaded these and higher bands:  A background "hiss" that might initially escape the notice of the casual listener until one realizes that this noise masked signals that should have still been perfectly audible.  If one switched the receiver to AM it would be observed that this "hiss" was subtly modulated at twice the mains frequency, 120 Hz.

To determine the extent of this sort of interference the typical procedure is to start turning things in the house off one-at-a-time (or, more reliably, the reverse:  Turn everything off at the breaker panel and then turn on one thing at a time) until the culprit is found.  This was done with the UPS and the magnitude of its "radio interfering" nature was determined.  Clearly, this "grunge" was being conducted from the power leads going in and out of the UPS.

Further experimentation revealed the true extent of the noise:  Even with everything disconnected from the UPS - that's to say, with it running on its battery, unplugged from the mains and the load disconnected - there was still detectable noise getting to the antenna and a quick check with a portable shortwave receiver proved that the remainder of this noise seemed to be being radiated by the physically-large battery bank and the wires that connected it.

While the proper application of a "brute force" AC line filter would likely quash the noise conducted in and out of the UPS on the mains power leads, something else would be required to minimize/eliminate the noise emanating via the DC lines.

"Brute force" line filters:
Figure 2:
A typical "brute force" L/C line filter typically found on devices
to minimize conduction of extraneous RF energy on the
AC mains.  For a 1500kVa UPS the filter would need an
appropriate current rating - particularly the fuse!
In some filters, two sections are used, with the components
L1, C1, C2 and C3 repeating.  This same filter
topology is used for the described DC filter.
Click on the image for a larger version.

One of the best ways to eliminate or minimize the amount of RF energy that might be conducted out of a potentially interference-generating device is to apply a combination of inductance and capacitance to that line as depicted schematically in Figure 2.

With the noise coming from the power supply (the UPS in our case) capacitor "C4" effectively "shorts" this RF noise to both sides of the power line, leaving the AC signal (pretty much) unchanged.  Inductor L1 consists of two equal windings on a ferrite core and it is practically invisible to signals that are equal and opposite, but it acts as a series choke for signals that are "common mode" - that is to say, equal on both sides of the power supply's mains leads - such as the RF noise energy.

On the "mains" side of the filter capacitor C3 reinforces the common mode again while capacitors C1 and C2 shunt any remaining RF energy from the power supply - its impedance now made much higher by inductor L1 - to ground - which would be the metal enclosure in which everything was mounted.

As it happens, these filters are available on the surplus market, and we would need three of them:
  • One for the AC mains connection to the UPS.
  • Another for the UPS's AC output
  • A third one for the battery connection to the UPS.
With the UPS being rated for 1.5kVa, some surplus AC mains filters, rated for 16 amps, were obtained while a filter designed specifically to filter DC lines rated for at least 50 amps at 60 volts was found (at The Electronic Goldmine - item G21652 - link) to filter the power connection between the UPS and the battery:  50 amps at 24 volts is not quite 1.5kVa, but there was nowhere near this amount of power being drawn from the UPS and the added circuit breaker/disconnect would provide the required safety - and the filter should be able to handle a brief overload, anyway.

Putting the filters in a box:
Figure 3:
Inside the filter box.  Along the top edge is the main AC input.  On the bottom edge - and just below the outlet on the left side - are the short leads that conduct the "dirty" AC power to/from the UPS, each through its own, separate filter - the two black boxes.
On the far right is the high-current DC filter with stud (bolt) connections being used to make the connections between the battery and the DC input of the UPS.  Barely visible along the bottom, one of the three studs is used to connect a piece of heavy wire or braid to bond this box to the chassis of the UPS to minimize conducted/radiated RF.
In the middle is a circuit board that contains a mains transformer, a high-current mechanical relay and a small solid-state relay.  This board - added later in the design - allows one pair of the outlets to be turned on and off with a simple contact closure of an internet-connected remote power switch.  On the control cable for the relay (the thin white wire) is a ferrite device which minimizes the amount of RF energy that might possibly be conducted in or out of the box on that control lead.
Click on the image for a larger version.

Ideally, one would have put the UPS and the batteries in a large metal box and passed the power leads in and out of this box only via the filters, but this simply wasn't practical, so the next best thing had to be done:  Put the filters in a single, metal box that would be electrically bonded to the UPS chassis and make the connections in and out of the UPS using short leads.  By keeping the leads short and bonding our new filter box to the UPS, we'd do our best to limit the number and length of conductors that carried the RF interference and, most importantly, preventing RF circulating currents from finding their way on external connections.
Figure 4:
The end of the box with the mounted outlets.  As noted,
one pair of these outlets is connected to a relay to allow the
connected devices to be remotely controlled.
Click on the image for a larger version.
  To that end a power distribution box was found at a home improvement store (Lowes Depot, I think) for less than $25 and the "guts" removed (the box with "guts" was cheaper than just an empty box by itself!) and the filters mounted inside.

With the short-as-possible conductors between the UPS and the filter, they will have minimal ability to directly radiate RF while the RF conducted on these lines will be filtered by the circuitry in the box itself with the bonding of the two boxes minimizing differential RF currents.  The power "to" the UPS was made with a short length of "SO" cord with a female connector on it while the power "from" the UPS is via a short cable with a male connector, plugged into one of the UPS's outlets - and being the ONLY thing plugged into the "dirty" AC output of the UPS.


Figure 5:
Perhaps a bit cluttered, this is the UPS sitting atop the filter, installed
and working.  In the lower-left corner of the picture, above two
batteries may be seen the DC circuit breaker/disconnect that protects
the DC circuit for short circuits.
Click on the image for a larger version.
Being that this is a remote location, the filter unit was installed a few weeks after it was constructed after having been tested (as best as could be done) on the workbench:  Power flowed through the various filters and the load control relay worked properly.

When installed, the filter box was placed underneath the UPS as it had a slightly larger footprint - and to minimize the length of the "noisy" DC and AC power leads from the UPS, along with the lead used to bond the two cases together.

To connect the DC, the cable coming from the batteries was effectively cut so that there was just enough of it emerging from the UPS to connected to the "output" side of the filter:  On both sides of this cable heavy "ring" lugs were attached and these were connected  to the studs of the DC filter.  To eliminate the probability of accidental shorting, the ground stud on the "input" side of the filter was removed and near both connectors a plastic wire clamp (one may be seen in Figure 6, below) to keep the positive wire in a fixed position and rotating into and shorting to the other stud.

Figure 6:
The back side of the UPS, showing the coiled power cord and the
(green) bonding wire that connects the chassis of the UPS and filter
box firmly together.  At the bottom of the picture can just be seen the
plastic clamp the keeps the positive wire lifted and away from the
the negative wire, to prevent shorting.
The ONLY thing plugged directly into the "dirty" AC output
of the UPS is the cord going to the filter:  Plugging anything else
directly into the UPS would at least partially negate the filtering! 
Click on the image for a larger version.
The AC input and output of the UPS was simply "plugged in" to the cables and the excess cordage was neatly coiled and stowed at the back end of the UPS using plastic "zip" ties:  It was important to do this because these cables are unfiltered and are "noisy" with RF meaning that they should be kept as small and as close to the metal of the cabinet as possible and kept away from any other conductors to minimize cross-coupling which would defeat the purpose of this filter.

To the remaining ground stud of the DC filter was attached a short piece of heavy (8 AWG) green wire with a ring lug on each end, this wire being visible in Figure 6.  The other end of this wire was attached to a marked grounding screw on the UPS chassis to bond the two boxes together and minimize RF circulating currents and to prevent the UPS chassis itself from being a source of radio frequency interference by direct radiation.

The result:

Taking the 20 meter (14 MHz) amateur band as an example, the UPS caused an extra 2 "S" units or so of noise above that of the typical ionospheric noise floor when it was powered up, before the filter was installed - this, being detected on a Carolina Windom antenna lofted between two trees high above the cabin's roof.  After this filter was installed the noise from the UPS was completely undetectable on any HF band, revealing other weaker low-level noises from other devices - the quieting of some of these will be discussed in later installments.

Figure 7:
A general block diagram of how the parts are interconnected.
The external UPS battery bank is protected with a DC-rated circuit breaker/
disconnect switch.  This particular UPS operates from 24 volts, hence the two
series-connected 12 volt batteries.  If the battery is inside the UPS's metal
cabinet, the DC filtering and connections are not needed.
Note that the "output" side of both AC line filters are both connected to
the UPS:  This is done because many filters are designed such that the
"output" side is that best-suited for connecting to RF-noisy circuits.
This diagram does not include the remote relay described.
Click on the image for a larger version.
Before the installation of the filter I'd placed my FT-817 (a small, portable HF transceiver) across the small room from the UPS, receiving with a short antenna and tuned to the 20 meter band.  When the UPS was operating and connected to its loads its noise was clearly audible, causing several S-units of indication on the signal meter.

After the filter was installed and the unit was powered up again with the loads connected, the noise was barely audible in the FT-817 and from across the room, it went away completely when the green wire was connected, bonding the UPS and filter chassis together.  If the radio was moved to within a foot or two of the UPS I could start hearing the "hash", but it seemed to be emanating only from the coils of AC cables "zip"-tied to the back end of the unit.   Because of the short length of the wires - and their being close to the metal case and not near any other wires into which this noise could be coupled it is unlikely that these short conductors will radiate any detectable noise at a distance greater than a few feet.

Links to other articles about power supply noise reduction found at this site:


This page stolen from