The Morrow CM-1 receiver - A relic of the cold war's effect on amateur radio

Figure 1:
The front panel of the Morrow CM-1 CONELRAD monitor.
The meter indicates relative signal strength and below it is the
on/off/volume control.  The dial - with a pair of "CD"
markings, is a smooth-tuning reduction drive - which is also
very accurate.  Shown here, it's on and tuned to a local station.
Click on the image for a larger version.

If you were an amateur (Ham) radio operator in the late 1950s and early 1960s, you  had a broadcast radio turned on in the background whenever you were on the air - or at least you should have!

The reason for this was spelled out in the FCC rules:  Beginning January 1, 1957, Section 12.192 required all amateur radio stations to monitor a broadcast station - at a minimum of ten minute intervals - to see if it transmitting, while they (the amateur) was on the air.  If the station had gone off the air, it was then required that the amateur determine if this was because of a CONELRAD alert - and if so, immediately cease transmitting.

What was CONELRAD?

CONELRAD, short for "CONtrol of ELectromagnetic RADiation", was a system - part of the U.S. Civil Defense - where, if an imminent attack of Soviet bombers was detected, ALL radio stations - commercial (AM , FM and TV) and amateur - were to go off the air to prevent their signals from being used as directional signals for navigation by the enemy.  The exception to this was that some of the AM broadcast stations were to ultimately occupy either 640 or 1240 kHz - but only for a few minutes at a time, the frequencies to be used by several different stations during that period - in a "round-robin" sort of system.  The "musical chairs" of transmitters, frequencies and locations was intended to make it difficult for the Soviet bomber to use them for navigation.

If you have seen an AM broadcast receiver intended for the U.S. market from the time period when the CONELRAD system was in existence (from about 1951 to 1963) you may have noted that at 640 and 1240 kHz there are triangular marks - often with the letters "CD" in them (the Civil Defense logo) - on the dial signifying the two CONELRAD frequencies - and these symbols are clearly visible on the tuning dial in Figure 1.  The intent of this is that civilians would tune to those frequencies to receive civil defense instructions during the "on" periods of the broadcast station near them.

Figure 2:
Rear of the CM-1.  The terminals connect to normally-open
contacts of the relay (if fitted) and the wire antenna can be
seen emerging from the chassis.  Between the two is a
potentiometer for adjusting the S-meter and relay sensitivity.
Click on the image for a larger version.

How it worked

As you might imagine, this system was cumbersome.  All stations had to be alerted in some way and "key" stations - perhaps notified via a wire service - would turn their transmitters off-on-off and on again in five second intervals and then transmit a 1 kHz tone for 15 seconds:  As transmitters of the day weren't designed to be "short-cycled" in this way, it was hard on the equipment - sometimes leading to failures.  There was also the requirement that some transmitters - and their antenna systems - change from their normal frequency to either the 640 kHz or 1240 kHz CONELRAD channel - a further complication - to participate in the on/off "round robin".

Needless to say, this system had several well-publicized false alarms as well as failures when tested over the years that it was extant.  Eventually, because of the implementation of ballistic missiles rather than piloted bombers, its reason for existing ceased in 1963.

How it affected amateur radio operators

As noted earlier, if you were an amateur radio operator, you were required to monitor a local broadcast station, while you were on the air, at least every ten minutes:  If the radio station went off the air, the idea was that you were to check for a CONELRAD alert and if it was happening, immediately stop transmitting.

Figure 3:
Top of CM-1 chassis as seen from its left side.  In the fore-
ground is the tuning capacitor, IF transformers and amplifier
tube and main filter capacitor.  The 6C4, if installed, would
be plugged in the empty socket just visible behind the tuning
capacitor.  It's well laid-out - even the audio transformer is at
an angle to minimize coupling to the AC power transformer.
The antenna wire is lightly coupled to the preselector circuit.
Click on the image for a larger version.

It's difficult to know these days how well the average amateur radio operator adhered to this rule, and for those that did, doing so certainly added a bit of complexity and awkwardness to their operating style.  A number of article appeared in the amateur radio magazines at the time describing how to "automate" the detection of the local radio station going off the air - typically by monitoring the AGC voltage of an inexpensive radio and sounding a buzzer or turning on a light.

There were also some receivers designed for just this purpose.

The Morrow Radio Mfg. CM-1

The Morrow CM-1 was one such receiver.  The receiver pictured was found among the effects of a good friend - and fellow amateur radio operator - that passed away several years ago, sitting dusty on a shelf in his basement.  After careful inspection, I was surprised to see that it had no bad capacitors:  The components used in its construction - particularly the capacitors - are all good-quality Cornell Dubilier (all are ceramic disk types other than the main filter) - and they seemed to be a cut above those found in a typical, cheap domestic radio of the time and it worked perfectly the first time it was turned on - and I haven't even bothered checking the tubes!  Even the main electrolytic filter capacitors are still in excellent shape - likely a result of the receiver having spent the past six-plus decades in a cool, dry basement.

Figure 4:
Schematic of the Morrow CM-1.  Despite the somewhat unusual tube line-up, it functions
the same as a typical "All American Five" superheterodyne receiver of the era, complete
with a 455 kHz IF.  The main difference is that it has provisions for connecting to an external
alarm or light based on the presence/absence of a signal using the (optional) 6C4 tube and the relay.
Click on the image for a larger version.

A "brief" circuit description  (Refer to Figure 4)   While many "AA5" receivers use a resonant loop - either a ferrite stick or a coil wound on the back panel of the chassis for an antenna - the CM-1 uses just a short piece of wire, depicted in the upper-left corner of the diagram connected to C1, one half of the tuning capacitor and inductor "L1".  This and the 50-100pF series capacitor "lightly" couple a short piece of wire to the resonant circuit.  This works, but its effects can be swamped if a long piece of wire (more than a few feet/meters) is used which can spoil image rejection an cause the radio to overload on strong signals. The 6EA6 converter doubles as both the local oscillator - its frequency determined by the other half of the tuning capacitor C2 and L2 - and mixer as it converts and amplifies the received signal to the Intermediate Frequency (IF) of 455 kHz via its plate through transformer T1 which forms a selective band-pass filter.  Following T1 is a 6BJ6 which functions as an amplifier at 455 kHz and this goes to T2, another 455 kHz IF transformer, and then to the 6AV6 diode/detector/amplifier tube.   The secondary output of transformer T2 applies the 455 kHz signal to a diode section and since its cathode is grounded, it causes that part of the transformer to go negative with the rectified DC voltage as well as the detected audio:  The received audio is then coupled via a 0.01uF capacitor and 10 MegOhm potentiometer to the triode section of this tube for amplification where the audio is then coupled to the plate of the 6AS5 - its grid biased slightly negative by the 220 Ohm resistor in the cathode - and amplified.  Functioning as a "Class A" single-ended audio amplifier, transformer T3 couples its high-impedance plate to the low-impedance speaker. Referring back to the bottom of the secondary of T2 where the audio is tapped, the negative DC voltage that is also produced there is in proportion to the amount of signal getting to the 6AV6 detector tube and ultimately, the strength of the signal to which the radio is tuned.  This voltage is smoothed to remove the 455 kHz IF by a 250pF capacitor and then sent to one of the grids of the 6EA6 converter tube through the antenna-tuning inductor, L1 and also to one of the grids of the 6JB6 IF amplifier via the secondary of T1 and when the voltage gets more negative, its sensitivity is reduced.  In this way the "AVC" (Automatic Volume Control) is formed so that both weak and strong stations produce similar amounts of audio. This same AVC voltage is also sent to the (optional) 6C4 tube:  If this voltage is very negative - as it would be with a moderately strong signal - this tube is "cut off", but if the signal disappears and the AVC voltage is less negative, it will conduct and thus turn on the (optional) relay. For the signal level meter, a voltage divider using a 15k resistor  and 2k potentiometer (seen at the bottom of T1) sets the voltage threshold on one side of the 1 mA meter - the other side of which is connected to the cathode of the 6JB6 IF amplifier tube along with a 1k resistor to ground.  There are two mechanisms of action here for driving the meter:  If there is more signal into this tube, it conducts more current and the voltage goes up across the 1k resistor, but a higher signal will also result in a more negative AVC voltage which will negatively bias the tube and reduce the current.  It's this latter aspect that prevails:  A more-negative AVC will reduce the tube's conduction and also the voltage across the 1k resistor and when this voltage is lower than that across the 15k-2k voltage divider, the meter will move up-scale.   The final section is the power supply which consists of a transformer to isolate the circuit from the mains.  The high voltage is half-wave rectified by a "modern" (for the time) selenium diode and then filtered by one section of a two-part capacitor which is then decoupled by a 750 ohm resistor in series which then goes to another capacitor.  In this way, the voltage is reduced somewhat and better-filtered than it would be with just a single stage of smoothing.

The CM-1 is a compact, tube (valve) type superheterodyne receiver from this time period that was designed to indicate when a station went off the air.  As can be seen from the photo, it looks more like a piece of ham gear than the AM broadcast band receiver in that it has a "nicer" geared (and accurate) tuning dial and a meter than indicates the relative strength of the signal to which it is tuned.

Internally, he CM-1 itself is mostly an unremarkable receiver:  Electrically, it's a variant of the "All American Five" (AA5) superhet that was produced by the millions over about four decades, but with a few interesting differences.  Sporting a power transformer, all of the tubes use six volt filaments and with the isolation, it - unlike most later, cheaper versions of the "AA5" - it does not have a "hot" chassis (e.g. one that is connected to one side of the line cord) - but with a solid-state (selenium) rectifier rather than the typical 35W4 and a rather different six volt tube line-up than a typical AA5.  A 6BE6 as the converter, 6BJ6 as the IF amplifier, 6AV6 as the detector/amp and interestingly, a 6AS5 (a tube typically used in car radios) for the audio output.  As can just be seen in Figure 5, the 6AS5's "getter" shows discoloration and Figure 3 reveals that there is a slight stain around the audio transformer where some of its wax wicked onto the chassis due to heat, both indicating that this receiver has quite a few "on" hours!

An interesting departure from a typical AA5 is its antenna connection.  Most AA5s had either a ferrite loopstick or a large coil of wire wound on the cardboard/masonite/phenolic back cover of the receiver that doubled as an RF pre-selector, but this receiver simply has a piece of wire capactively coupled to the tuning network connected to one of the capacitor's gangs.

According to the manual, only 2-3 feet (50-95cm) of wire is necessary for reception and testing of this CM-1 indicated that this was true - but it also means that one must resist the temptation to connect it to a longer wire:  Doing so simply overloads the receiver and wrecks image rejection, particularly since the front-end preselector tuning is overcoupled to the antenna itself, effectively bypassing it altogether. 

I suspect that having a really sensitive receiver was not required in this application.  When monitoring for CONELRAD, one would probably use a local station with a strong signal:  Doing so would not only help with the fact that many AM broadcast stations had to reduce their power at night, but a weak/distant station would be interfered with with the onset of nightly skywave propagation which could, at times, cause signal levels to fluctuate wildly, resulting in false alarms.

During testing - with only the 2' (50cm) wire antenna - when placed outside, I found that the receiver was capable of picking up even the weaker signals across the broadcast band, but since it is a wire rather than a coil, it's non-directional and is far more prone to pick up local "E-field" noise energy than the typical "H-field" loop which is not only directional, but offers a degree of rejection of such noise.  Practically speaking, one could probably add a loop antenna, connecting to the wire and the grounded screw (one of the two used for the relay) on the back panel to improve reception in today's modern electrical environment.

Figure 5:
The other side of the top of the chassis.  In the foreground is
the power transformer and next to it is the 6AS5 audio amp.
The socket next to the 6AS5 is for the (optional) relay, driven
by the (also optional) 6C4.  The antenna tuning coil is visible
next to the tuning capacitor, almost against the front panel.
Click on the image for a larger version.

When I first removed the receiver from its chassis I thought that it was missing two tubes as evidenced by empty sockets, but I then realized that it had a selenium rectifier accounting for there being only four tubes and immediately wondered why there were two extra sockets.  A bit of "Google-Fu" found the manual online and I learned why:   This receiver could optionally be fitted with a relay in one of the sockets (this also required another tube - a 6C4 to detect the loss of signal and drive the relay) that was intended to operate a sounder, a light or some other means of indicating loss of signal.  I find it interesting that this relay and its driver tube were omitted as shipped from the manufacturer, but I suspect that this was a cost-saving measure - and it may have been enough for most amateur radio operators using this receiver to simply glance at the front-panel meter occasionally to see if a signal was still there.

Is it still useful?

The reason raison d'etre for this device ceased to be when, in 1963, the FCC rule requiring the amateur radio operator to monitor broadcast stations while operating was rescinded and removed from FCC Section 12.192, but since it's rather compact and has a speaker that dominates the top of the case means that it still works as a pretty nice AM broadcast receiver.  Whether or not one might think an AM-only receiver is still useful overall is another matter altogether!

Figure 6:
Bottom of chassis of the CM-1.  High-quality components -
including all Cornell Dubilier capacitors - are used
throughout.  The orange object left of center is the selenium
rectifier and the oscillator coil is just right of center.
Click on the image for a larger version.

The usefulness and practicality of this receiver is probably on par with most other vintage radios that one might collect and own:  They are not likely to be "everyday drivers" and more likely to sit on a shelf with other old radios - but this one, at least, has a particularly interesting bit of cold-war history behind its existence.

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 This page stolen from ka7oei.blogspot.com

[END]

DXing distant SolarEdge PV optimizer modules (or long-distance propagation of PV system QRM)

From how far away can you hear the spurious emissions from a known-noisy PV system?

Quite a racket!

Figure 1:
The spectrum of a SolarEdge PV system from several meters
away across the 6-8 MHz range showing "spurs" (clumps of
low-level carriers) at 200 kHz intervals and other places.
In the above plot the true nature of the individual peak -
the fact that each contain many carriers - is not apparent.
Click on the image for a larger version.

In a previous post (linked HERE) I described the interference produced by a SolarEdge PV (photovoltaic) system to an amateur from installations on neighboring houses.

The "take-away" from this analysis is that the current version of SolarEdge systems produce rather strong signals at 200 kHz intervals - each module on the back side of a solar panel producing its own carrier at its own frequency as depicted.  The peaks in Figure 1 show these groupings of carriers every 200 kHz (plus some additional frequencies) while the image in Figure 2 shows, in extremely high spectral resolution, many individual, narrow carriers that comprise each of these peaks.

In driving around with an HF mobile station in my vehicle I can hear these 200 kHz-spaced carrier groups almost everywhere around town during daylight hours - the roar getting much stronger in/near residential areas as you would expect.  If driving through a residential neighborhood, it is very easy to tell when you drive past a house equipped with a SolarEdge PV system - and it is easily audible from a block or two away.  Knowing the "fingerprint" of this PV system allows it to be identified uniquely - even at some distance.

Figure 2:
A "zoomed in" view of the spectrum of local SolarEdge
carriers recorded just below 7.4 MHz from my home.
See Footnote #3, below for detailed information.
Click on the image for a larger version.

Are they DX? ¹

A question arose in my mind:  Does this "grunge" produced by the SolarEdge PV systems propagate long distances?

To answer this question I checked a KiwiSDR at the Northern Utah WebSDR (link) - a site with which I am very familiar ².  This receive system is located about 3 miles (5km) from any residential area, bounded on three sides with mosquito-laden bird refuges (wetlands) and on the fourth side - the same as the closest houses - by a mountain.  Additionally, the antenna used for the reception in Figures 3 and 4 below was the TCI-530 omnidirectional log-periodic (with circular polarization) - which does not have good gain at very low radiation angles, further precluding the reception of "nearby" PV systems via "ground wave".

The quick answer to the above question is YES - the roar of SolarEdge systems is propagated when conditions are "reasonable" ⁴ as shown in the screen capture below:

Figure 3:
Propagated noise from myriad SolarEdge PV systems from the remote Northern Utah WebSDR's
remote HF receive site.  The "hump" in the middle is the combined energy of likely thousands of
SolarEdge PV systems that are being ionospherically propagated.  Amateur signals are
visible at 14.200 MHz and above.
Click on the image for a larger version.

The signals represented by the "hump" in the highlighted portion in the center of the analyzer plot in the top part of the image - and the "band" of noise on the waterfall display - between 14.199 and 14.200 MHz are the sum of the propagated low-level PV system carriers from... who knows where?  To be clear, this energy is not likely to be from just one SolarEdge PV system and its individual optimizers (one for each panel) but more likely from the many thousands of such devices that are each, individually, radiating energy.  What we are seeing is the total energy of the propagated systems, the frequency spread being centered around 14.1993 MHz.

It's worth noting that the fact that these signals do not land on exactly the same frequency ⁵ - hence the Gaussian-like distribution of energy - and this has interesting implications.  Even though the signal from each, individual optimizer is (more or less) a CW (unmodulated) carrier, the fact that there are so many of them clustered together means that, for statistical purposes, they might as well be a distribution of noise energy:  Unlike with a single coherent CW signal, the DSP filtering on modern radios will be able to do little/nothing to reduce their effects if they were to cause interference due to its similarity to white noise.

A quick power and spectral analysis of the signal above showed that if the signals above were a single, coherent CW signal, the total amount of energy contained in the "hump" in Figure 3 would have easily been at least 15-20dB above the noise in a 50 Hz detection bandwidth:  A CW signal of this strength would certainly be cause for complaints!

I also looked at other 200 kHz multiples around 14.000 and 14.400 and the same, exact types of signals were present on those frequencies - and similar bunches of energy fitting this profile were noted at least as low as 10.200 and as high as around 18.200 MHz as well (probably higher) and every (otherwise) clear frequency in between - this range being related to current ionospheric propagation at the moment that I checked (e.g. around 1845 on September 10 UTC, 2025).  ⁶

To verify that these signals were propagated and were likely from SolarEdge systems, several things were done:

• The presence at many 200 kHz multiples/intervals across the HF spectrum is telling!  Their being slightly below exact 200 kHz multiples as mentioned in Footnote 5 adds to their "uniqueness".
  
• On days with poor propagation overall, these signals were absent - or limited to frequencies commensurate with the MUF (Maximum Useable Frequency).

• These signals disappear at night.  (This test is somewhat complicated by the fact that propagation on these bands also changes at night - but sunlight is still illuminating the ionosphere well after sunset on the ground.) 
  

• An "S-meter" plot was run over the period of several minutes:  A propagated signal(s) would show variations in signal strength - but this can be foiled to a degree by the fact that many, many individual point sources would each be propagated differently and unlike a single source, would not experience as deep a fading as the plot below shows:

Figure 4:
Propagated signal strength variations caused by ionospheric variations.  This would seem to indicate
that the signals are propagated - but the magnitude of the fading would be mitigated by the large
number of point sources, each being affected individually along the signal path.
The top/bottom of this chart represents 10dB.
Click on the image for a larger verion.

As noted in the original article analyzing a system close-up (linked above) the SolarEdge optimizers produce other signals 6-10 dB weaker at various points above each 200 kHz interval - these are visible in Figure 1.  When the above plots were made these signals weren't readily apparent - but I suspect that they will be visible during "excellent" propagation conditions rather than the "mediocre-to-average" conditions that were present when Figures 3 and 4 were produced.

Conclusion:  They do get propagated!

So yes, you can DX SolarEdge PV systems - it's just that there are so many of them each doing their own radiating that you probably won't know from where those signals originate, so it's hard to know from how far away you might actually be hearing them!  To be clear, it's difficult to determine if a the radiated RF from a single optimizer would be audible via ionospheric propagation, and with many thousands of them out there this may be impossible to determine - but it is clear that the summation of many thousands of them does produce an audible signal.

Do these signals actually cause QRM ⁷ ?  As noted in the earlier post (liked above) they most certainly do if you live within a city block or two of one of the SolarEdge PV systems and operate on or near any of the frequencies occupied by the spurious radiation represented in Figure 1.  If your receive system is located well away from a SolarEdge installation, the above shows that you may still experience interference from these systems - even from a significant distance.

Figure 3 also shows that the emissions do propagate over long distances:  The 20 meter band's optimal "single skip" distance would likely place the majority of these signals in a 700-1500 mile (1100-2400 km) radius of Northern Utah - and this includes quite a few populated areas in parts of the U.S. where the number of solar installations is quite high. 

You, too, can check for QRM at your station

If you have an HF station with a receiver with a waterfall display you might want to check the various amateur bands just below the 200 kHz multiples ⁸ during daylight hours:  If there is a SolarEdge PV system within a couple city blocks of you ⁹ you will most likely see and hear it - but don't blame me if, after finding that you can see those signals, you can't "un-see" them!

* * *

Links to related pages (about solar power) on this blog:

• Analysis of a SolarEdge system (link) - This is the article linked at the top of the page where careful measurement was done to characterize the interference created by a SolarEdge system neighboring a local amateur.
  

• It *is* possible to have an RF quiet home PV system (link) -  On this page I describe the SunnyBoy based system at my home QTH - and the fact that it does NOT produce QRM.
  

• Reducing QRM from a Renogy 200 watt (or any other) portable solar panel system (link) - Here I describe a portable solar system that I take "car camping" and how I reduce its interference to the point of inaudibility.

• The Solar Saga Part 1: Avoiding Interference - (Why I did not choose Microinverters) - link

• The Solar Saga Part 2:  Getting the system online - link
  

• Does the Tesla Powerwall 2 produce Radio Frequency Interference? - link
  

• Reducing RFI from the Tesla Powerwall 2 - link 
  

• Tesla Powerwall susceptibility to RF Transmissions - and how to deal with it - link 

Footnotes:

1. The term "DX" means distance.  Generally speaking, if a signal is "DX" it is understood that it must be being propagated over much more than a line-of-sight distance - in this case, via ionospheric propagation at distances of hundreds or thousands of miles/km.
2. The author of this post is one of the original founders and current maintainers of the Northern Utah WebSDR which has a remote HF receive site about 80 miles (94km) north of Salt Lake City.
3. Figure 2 shows a "close-up" spectral view of the signals emitted by several SolarEdge PV systems within a mile/kilometer or two of my house - the closest system being about a block away.  The center frequency of this cluster of signals was approximately 7.39965 MHz and a 256k-point FFT with a bin width of 183 mHz (milliHertz) - along with some averaging - was used to create this plot.  Clearly visible are a large number of individual carriers along with a background "roar" of many more weaker carriers that are not individually distinguishable in this plot.  This plot was purposely done on a frequency above the 40 meter amateur band during daylight hours (the local time is visible in the image) and during this time there is no strong, long distance propagation (a fact verified by the absence of a similar set of signals on the remote Northern Utah WebSDR site) indicating that this energy is, in fact, originating from systems proximate to my own receive site.  At sunset, these carriers will gradually disappear - often "blinking" out - as the solar panels lose their light and will reappear the next morning:  This "blinking" can be heard as individual tones flicker on/off during the day<>night transition by listening on an ordinary SSB-capable receiver at one of the frequencies noted above.
4. The frequencies mentioned have also been checked when ionospheric propagation is poor (comparatively few strong signals) and the characteristic SolarEdge carriers were absent at the remote receive site.  This further illustrates the fact that the signals described above are not local to the remote receive site and reinforces the likelihood that they are, in fact, being propagated. 
5. Observation of a SolarEdge PV system at very close distance (less than 50 feet/15 meters) indicates that each, individual optimizer - a device attached to the back of every individual solar panel - will radiate the signals at 200 kHz intervals.  Due to the slight variations in oscillator frequencies (e.g. quartz crystals or MEMs devices) the precise frequencies of these signals - and their harmonics - will vary, but the mean frequency separation appears to be around 199.9901 kHz which puts them slightly below a precise 200 kHz multiple which is why the peak of the distribution shows up around 14.1993 MHz on 20 meters, 7.19965 MHz on 40 meters and so on.  As noted in the text, the actual frequency spread of the individual modules is such that it has a Gaussian-like distribution above and below the mean frequency.
6. I also checked several remote receive systems around the world during their local daylight hours and could see the same "humps" of energy at frequencies just below the aforementioned 200 kHz multiples on some of them.  One such system was that located at the University of Twente in the Netherlands:  It is not known to what degree the signals that were radiated (likely) from PV systems were propagated and which might be within a few kilometers of this receive site, but they are certainly "there".
   
7. "QRM" is a "Q" signal referring to "Man Made Interference" and the magnitude of this interference in comparison to the desired signals determines if this is harmful interference.  If QRM makes it difficult/impossible to receive a signal on frequency, that would fit the definition of harmful interference.
8. The frequencies on which the radiated signals from a SolarEdge PV system (every 199.9901 kHz) will likely land within an HF amateur band are clustered around the following:  3.5998, 3.7998, 3.9998,  7.1996, 14.1993, 21.1990, 21.3990, 28.1986, 28.3986, 28.5986, 28.7986, 28.9986, 29.1986, 29.3986 and 29.5986 MHz plus similar frequencies in the 6 meter band:  They can also be heard on non-amateur frequencies at the same 199.9901 kHz intervals as well.  As the above frequencies are the actual frequencies, you will need to tune above or below the frequencies (using LSB or USB, respectively) by 1.5 kHz or so to hear the "roar".  Of course, you will only hear these signals during daylight hours when the PV systems are active.  Note that the combination of naturally-higher noise levels on the lower bands (80, 40 meters) and the likely lower efficiency of the PV system's component ability to radiate RF there - plus the tendency for nighttime propagation on those bands (when the PV systems are inactive) - means that observing this phenomenon on those frequencies via the ionosphere is much less likely.
9. If you do remote operation like POTA or SOTA at a significant distance from any likely PV system, you might want to take a look at some of the 200kHz-interval frequencies mentioned above during daylight hours and good propagation:  You'll probably see the propagated PV signals there, too.
   

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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.

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This page stolen from ka7oei.blogspot.com