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Analysis of interference from a SolarEdge PV (solar) electric system.

By: KA7OEI
1 May 2024 at 04:04

Comment:

This article - while it centers about the investigation of a SolarEdge PV (PhotoVoltaic) system - the discussions of techniques and strategies should be generally useful when investigating interference from any make or model of PV system - or even interference from other sources.

* * *

Several months ago I got a call from a local amateur who was very concerned about a sudden rise in his noise floor across the HF spectrum (3-30 MHz).  This increase in noise seemed to be coincident with the installation and commission of a 5 kW PV (Photovoltaic, or "Solar") electrical system on the house of an adjacent neighbor.  I suggested that he talk to the manufacturer of the PV system to discuss the situation - and to request from them possible solutions.

A few weeks ago, he got back to me and he had, in fact, talked to the manufacturer and an online meeting was arranged in which they would remotely idle the neighbor's system while we were monitoring via the amateur's receive antenna.

Out of curiosity - and as sort of a practice run - I went over the weekend before the online meeting to get a better idea as to the spectral signature of this system - a SolarEdge series string system with optimizers - when it was operating normally.  The amateur had obtained permission from the neighbor to allow us to enter their yard to make very "close in" measurements (e.g. within a few inches/cm of the equipment, conductors) to obtain spectral "samples" of the system, thereby excluding external signals.

For these measurements, I used an amplified, shielded magnetic ("H") loop antenna (about 18"/50cm) in diameter as the "sense" antenna and an HP/Agilent/Keysight spectrum analyzer, recording the plots electronically - although a Tiny SA "Ultra" would likely have sufficed as well.  None of the readings were to represent "absolute" signal levels as all that we were really interested in were relative measurements, and as such all that we needed to do was keep our measurements consistent - that is, being able to precisely repeat the conditions between subsequent measurements.  These readings would allow us to understand the nature of the RF energy that it was creating - its "signature", if you will.

Note:  For information about the H-field loop used for this testing - and using an inexpensive spectrum analyzer such as the "Tiny SA" or, better yet, the "Tiny SA Ultra", see a previous blog entry "RFI Sleuthing with the Tiny SA" - Link.

The nature of this QRM:

The interference observed by this amateur - now known for certain to be signature of this model of SolarEdge PV system - was evident as two general types of signals:

  • Moderate to strong clusters of carriers every "even" 200 kHz.  At 200 kHz intervals (e.g. 7.0, 7.2, 7.4 and 14.0, 14.2 and 14.4 MHz) from below 3 MHz through at least 30 MHz could be heard a melange of closely-spaced carriers within about 500 Hz of each other on the lower bands.  While these carriers sounded like mostly CW (unmodulated) signals, there was also evidence of low rate data signalling buried in the cacophony as well as additional lower-level carriers.
  • Background "white" noise amplitude-modulated at the mains frequency.  If you were just casually listening at this amateur's QTH on the HF bands - say 40 meters - you might be forgiven in the short term for presuming that nothing was wrong.  In reality the noise floor had been elevated several "S" units by the PV system - the result sounding superficially like plain, old white noise:  Switching from SSB to AM reveals the loud "hum" that is riding on the noise - modulation that is almost "invisible" if one is listening only using SSB.

While the appearance of the above interference coincided with the activation of the neighbor's system, that fact that it disappeared at night further pointed to a PV system as the source of the QRM.

"Close-in" measurements

Placing the sense antenna right at the main inverter on the back of the neighbor's house, we wanted to take a snapshot of the spectrum at that location:

Figure 1:  0-30 MHz sampled right at the main inverter

With each horizontal division representing 3 MHz, this 0-30 MHz plot shows a high concentration of noise in the 3-9 MHz area from a location right at the inverter.

Because Figure 1 represents the spectrum at the inverter, we wondered what it would look like at one of the solar panels so we placed the sense antenna right against one of the solar panels:

Figure 2:  0-30 MHz sweep with sense antenna placed next to a solar panel

Figure 2 is in the same frequency and amplitude scale as Figure 1 - but with the "reference level" adjusted by 20 dB to move the trace "up" a bit - and we can see that the spectrum next to the panel looks quite different from that sampled right at the inverter.  This isn't unexpected as Figure 2 would likely represent more of the noise that is emitted from the DC (input) side of the optimizer whereas the spectrum represented in Figure 1 would be more likely to show that of the DC output of the optimizer plus whatever noise was riding on the conductors carrying the DC input and AC output of the main string inverter.

Although it is difficult to be sure, the 0-30 MHz plots taken from a greater distance (10 meters or more) had the general appearance of the noise spectra shown in Figure 2 more than that of Figure 1 leading me to believe that a significant portion of the QRM may be being radiated from the panels themselves rather than just the conductors going from the optimizers  to the main inverter - but certainly, both are likely involved.

Note:  For both of these plots, the RF energy from the PV system was many 10s of dB above the typical background noise floor - in this case, 40-50dB for Figure 1 and at least 30dB for Figure 2 in the area around 7 MHz.

As the 0-30 MHz sweep does not have enough resolution to visualize the narrower 200 kHz signals, the analyzer was readjusted as depicted in Figure 3 - again with the sense antenna next to the panel:

Figure 3:  From near the panel, a "zoomed in" spectral sweep showing narrowband birdies.

In this spectrum plot we can see not only the "white" noise on the floor of the sweep representing the "hummy hiss", but also the much stronger signals every 200 kHz - plus a number of weaker signals in between:  It is these signals that are the most obvious to the casual operator and appear to be unique to a SolarEdge system of this model/type.

On this same day we waited until after sunset - monitoring the groups of carriers at 7.2 MHz and hearing them "flicker" out of existence as it got dark and we re-did the "next to the panel" measurements - this time the spectrum was devoid of the 200 kHz-spaced carriers (they were no longer audible on the amateur receiver, either) and the 0-30 MHz plots were 10s of dB lower than in the daylight. 

Important:  The 2 MHz sweeps in Figures 3-7 use a resolution bandwidth of 10 kHz which is almost exactly 4 times wider than the typical SSB bandwidth of an amateur receiver of about 2.5 kHz making their apparent level above the background noise appear lower than it is actually is.

What this means is the coherent signals - such as the 200 kHz carriers - appear to be another 6 dB farther above the noise floor in an SSB bandwidth than what the analyzer plots show.

Plots from a distance

Having captured some "close-in" plots, we now had an idea as to what the signals emitted by the PV system looked like.

A few days after we made the above plots we were in a virtual meeting with the manufacturer of the PV system (SolarEdge) from the ham's shack.  Having reconfigured the feed to his main radio, we could quickly switch the feedline from the antenna feeding the radio and the spectrum analyzer.

At this time we also learned that there was a second SolarEdge system south of this amateur's QTH - about 150 feet (45 meters) away across the cul de sac - and that the neighboring system and the one across the street would but remotely shut down, in that order, to determine how much QRM was emanating from each.

While we captured 0-30 MHz plots of each stage of system shutdown, for the purposes of this article we'll show just the "narrow" plots in 2 MHz sweeps as depicted in Figure 3 as the presence of the 200 kHz signal are generally representative of the presence of the broadband noise as well and these signals were easily identifiable and now known to be indicators of QRM from this type of PV system.

First, here's the plot from the amateur's 40 meter inverted Vee antenna with both systems on:

Figure 4:  6-8 MHz plot from the 40 meter antenna showing the 200 kHz peaks - and a bit of broadband noise as well.
 
The next plot shows the effects when the neighboring system was turned off, but the one across the street still on:

Figure 5:  The neighboring system off - but the one across the street still on.

As can be seen, the broadband noise floor around the 40 meter band (approximately one horizontal division below and above the marker) has dropped visibly - around 3-4 dB - and the amplitude of the carrier at 7.2 MHz has dropped about 6 dB - and the 200 kHz signals have disappeared almost entirely below about 6.5 MHz.  The system across the street was then shut off and the only remaining signals were those that happened to be on the 40 meter band.  (No trace is available for this configuration, unfortunately.)

As the 40 meter inverted Vee is oriented to favor east-west signals it was not necessarily the best candidate to test the effects of the PV system across the street, so we switched to a 20 meter antenna which was responsive in that direction and this trace shows the plot between 13 and 15 MHz:

Figure 6:  This plot of the 20 meter band and surrounding frequencies shows only propagated signals, with no sign of PV system QRM.

As both systems were off, the trace was quite clear - only showing signals that actually were on or near the 20 meter band, propagated from elsewhere in the world.  The folks at SolarEdge then turned on the system across the street with the following result:

Figure 7:  Same as Figure 6, but with only the PV system across the street activated.

The effect is clear:  In the vicinity of the 20 meter band, the appearance of rather strong signals every 200 kHz is apparent - and there is an obvious 2-4 dB increase in the noise floor indicating that this system, too, is causing harmful interference.

Readings on the radio:

It would seem that the folks at SolarEdge had worked with more than one amateur radio operator on similar issues and I was pleasantly surprised when they asked for some "S-Meter" reading comparisons with the neighbor's system on and off.  Using a calibrated signal generator, I'd already determined the signal level (in dBm) that correlated with the S-meter readings for the Icom radio - and here are the results for 40 meters:

Both systems off:

S1 (<= -84 dBm) - no carrier groups every 200 kHz.

Neighbor system on:

S4 (-78 dBm) - white noise between 200 kHz carriers.

S9 (-67 dBm) - carriers at 7.2 MHz.

This shows that at 40 meters, the degradation to noise alone was on the order of 6 dB (most Japanese radios are calibrated for 3dB per S-unit) and that the cluster of carriers on 200 kHz intervals was far more destructive, rising a bit short of 20dB out of the noise floor.

As our time with the SolarEdge folks in the virtual meeting was limited, we were not able to do similar "S-meter" tests on 20 meters, but we can use the 40 meter results along with the relative strength of the 200 kHz-spacing carriers  and correlate them with the 40 and 20 meter spectrum analyzer traces and determine that the severity of QRM from the PV system on 20 meters on the receiver would have been roughly comparable to that on 40.

Analysis of these readings and implications:

As mentioned earlier, there are two types of interfering signals produced by these SolarEdge PV systems:

  • Moderate to strong clusters of carriers every "even" 200 kHz.  These are very obvious, easy to identify, and quite strong compared to the noise with a few weaker signals in-between that were also clearly audible.
  • Background "white" noise amplitude-modulated at the mains frequency.  This is also present, but it borders on insidious as the average amateur may not be able to quantify its existence - let alone its effects - as its effects may be obscured if one only listens for it using SSB modes.

Will my radio's DSP help?

The quick answer is "No".

While you might think that modern receivers' ability to "notch out" tones might help alleviate the effects of the signals every 200 kHz, you would be wrong.

It appears that each, individual optimizer module (there is one for every solar panel) produces these signals - and being based on individual oscillators, their frequencies will be slightly different from each other meaning that instead of needing to notch just one tone, your DSP would have to notch out dozens emanating from a single PV system - and it just cannot do that!  What's worse, these carriers are also modulated by the low-rate data used to communicate to/from each, individual module which broadens their spectrum as well.

As for the "white" noise, it is unlikely that noise reduction would help much, either:  The source of this appears to be an artifact of the actual voltage converters themselves and as it is random, it is as difficult to reduce in its effects as the normal background noise of the bands.

As each optimizer module contains is own switch-mode power converter to maximize panel efficiency, they, too - like any switch-mode supply - will produce harmonic energy.  It would appear that SolarEdge uses switch-mode controllers that employ "spread spectrum" clocking so that instead of having a myriad of harmonics and birdies all throughout the RF spectrum, that energy is "smeared" all over the place making it somewhat less obtrusive.

The use of spread-spectrum clocking is very widely used these days for the reasons noted above - and for the fact that it also enables the exploitation of a quirk when a device is subjected to testing for regulatory (FCC) compliance:  Aspects of that testing specify the maximum amount of signal energy that may be present in a given bandwidth - but by "spreading" it over a much wider bandwidth, that same amount of energy would be diluted and make the readings obtained during the testing appear lower.  This is perfectly legal and commonly done - but this technique does nothing to reduce the total amount of energy radiated - only filtering can do that!

It is apparent that in this particular case, both the neighboring system and the one across the street contribute a magnitude of interference that would be considered to be "harmful" in that it is perfectly capable of submerging weak-to-moderate signals into locally-generated noise - and if such signals happened to land near a 200 kHz harmonic rather than the elevated noise floor in between the effects are >10dB more destructive.

It is also apparent that the radiated noise extends - at the very least - from the 40 meter to 20 meter bands (7-14 MHz) but the 30 MHz plots imply a significant amount of RF energy above and below this:  The limited time permitted a semi-detailed analysis of only the interference around the 40 and 20 meter bands.

After the meeting:

At the conclusion of these tests, the analyzer readings that took were forwarded to the folks at SolarEdge for their analysis - and it is still too soon to know of any conclusions that would indicate what sort of actions that they might take.  We were, however, heartened to know that they seemed to understand and were sympathetic to the plights of amateurs affected by neighboring systems that might be adversely affect amateur radio operation.

The folks at SolarEdge themselves offered the best hope of resolution:  They noted that they have a special version of PV hardware (e.g. optimizers) that has additional filtering that could be retrofitted into an existing system to reduce the potential for interference.  As this retrofit would be done on their "dime" - and it would be rather expensive - they understandably want to be sure that they have identified only systems that are of their manufacture that are causing interference.

Is a system near you?  You can listen for yourself!

Somewhat ominously, I have since tuned to 14.2 and 14.4 MHz on my mobile HF station while driving around residential and interstate roads in my local area (Salt Lake City, Utah) during my normal commute/business:  I can, in many places, hear the characteristic "roar" of narrow carriers every 200 kHz - likely from SolarEdge PV - systems as these carriers seem to disappear during the hours of darkness.

I have heard this characteristic signal even in locations that appear to be several city blocks from any structure that might be equipped with a PV system.  They may also be heard on other bands - including 40 meters - but the signals emitted on the higher bands (e.g. 20 meters) seem to be emitted with greater efficiency.

It would seem that these 200 kHz-spaced groups of carriers really get out!

"I have interference from a PV system - what should I do?"

At this point I will not reiterate in detail remediation methods that might be undertaken by a radio amateur affected by this type of PV system:  The June, 2016 QST article (link) discusses attempted mitigation using ferrite devices in detail. (Note:  This article also refers to experiences with a SolarEdge system - but the spectra of the system described there is different from what I found on the systems described here likely due to it being a now-older system.)

 I will only mention in passing that there's the possibility that a degree of mitigation may be possible with the use of "noise cancelling" antennas of the sort offered by Timewave, MFJ and others - but their utility is also somewhat limited owing to practical concerns:   Such techniques work best on distant "point sources" of interference rather than very nearby, spread-out (in terms of subtended agle) radiators in the near field.

If you have interference from a PV system, it is up to YOU to do your due diligence to determine that it is, in fact, a PV system that is causing the issues and NOT other devices in your house or those of your neighbors that is causing the problem.  If you own a PV system - or have one installed on your house - that you suspect is causing a problem, making detailed measurements with it on and off on various frequencies would be a suggested first step.

As this article relates only to the SolarEdge PV system that I investigated, I cannot possibly offer advice to another brand of system that uses other brands of equipment in regards to interference potential - but if you suspect that you or your neighbor(s) have this brand of PV system that is causing interference, I would suggest the following checks during daylight and hours of darkness as appropriate:

  • Are there signals every 200 kHz?  Common frequencies where this would be observed include 3.6, 3.8, 4.0, 7.0, 7.2, 14.0, and 14.2 MHz.  This is definitely one of the hallmarks of a SolarEdge system of the same/similar model - but it similar artifacts may be produced by others.
  • Does the "hiss" that is elevating your noise floor have an obvious "hum" to it when you switch to AM?  You can't easily hear this when you are in SSB mode.  Listen for this on frequencies in the vicinity of 60, 40, 30 and 20 meters on a frequency devoid of other signals.
  • Does the "hummy hiss" greatly reduce when it gets very cloudy?  The "hummy hiss" - which appears to be a property of the voltage converters - seems to become more intense with increased output from the PV system.
  • Do the 200 kHz signals and the "hummy hiss" go away after sunset and return only after sunrise?  Not unexpectedly, this is hallmark of many PV systems' noise generation.
    • Be aware that some models/brands (although not the one discussed in this article) can produce RF interference if either solar illumination OR mains voltage is present and that it takes the removal of BOTH to silence them (e.g. turning of the mains breaker feeding the system at night.)

If you believe that you are being affected by a PV system, it is up to YOU to be prepared to take all appropriate measures to document the interference, do your own testing, and make repeated observations prior to reporting them to the manufacturer, a regulatory agency, club or national organization.  A few things to consider:

  • Treat this as if you were causing interference to someone else.  Just as if a neighbor complained that you were causing problems to their equipment, it is incumbent on YOU to determine if the problem is on your end.  There are likely many, many devices in your house that can cause similar types of interference so be sure that you have ruled those out - and DO NOT forget that you may have devices running on UPSs or battery back-up that may still make noise even if you shut off your power.  (Many UPSs are known to be noisy in their own right!)  In other words, be certain that your house is clean before involving them as this will not only make determining the magnitude/nature of interference from a PV system easier, but it shows good will and competence on your part.
  • Document the issue over the period of days, weeks or even months.  Many sources of interference come and go - but if it's a PV system, it will be there day in and day out.  Noting over time the consistency of the noise may give you a clue if it's some other type of device - and if it, in fact, related to a PV system: GOOD documentation will only help your case.
  • Once you have ruled out everything else, go ahead and contact the manufacturer - but be nice!  If you are confident that your own house is in order (e.g. you have ruled out other devices) then contact the manufacturer.
    • If you have been following the above steps, you will already have some documentation which makes your specific case more solid.
    • The manufacturer may schedule an online meeting to discuss the issue and run tests.  Be sure that you have the ability to use Zoom or Google Groups - or find someone who does.
    • If the manufacturer runs tests, they will likely turn on/off suspected systems so YOU should be ready to document changes in noise floor - and of the signals every 200 kHz (in the case of a SolarEdge system of the type investigated here).  If you have already been taking notes/documenting, you should be already familiar with your local signal environment and be able to expedite the running of these tests - and have a basis of comparison as well.
    • If the manufacturer decides that they wish to help remedy your situation, remember that they may be doing it at their own expense:  It is incumbent on YOU to be cooperative, competent, courteous, accurate and honest when you are dealing with them and their requests.
    • If you feel the need to do so, you may wish to enlist the help of one or more friends to help you with these tasks that may be more experienced - and having a second or even third pair of eyes on the problem is always a good idea.  If you are not comfortable doing so, I would suggest have someone else - familiar with your problem - who can talk "nerd" be your spokesperson when dealing with the manufacturer!
    • You should be clear to the manufacturer to define "interference" differently from "harmful interference".  If you can just hear weak birdies that don't really cause any issues, this could be considered just plain, old "interference" and you may not get as much sympathy or action as you like.  "Harmful interference" is that which - when present - obliterates even moderately strong signals that would otherwise be quite usable and thus, they should be taken more seriously.

While this article is rather specific to the SolarEdge PV system as described, this is likely be applicable to other manufacturers and models in more general ways.

Good luck!

* * * * *

P.S.  Overall, I was pleased with the knowledge and responsiveness of the SolarEdge folks with respect to interference to amateur radio stations.  After they have had time to digest the information supplied and executed their plan of action I hope to do a follow-up to ascertain the results of their mitigation efforts.

Myself and several other local amateur radio friends have PV (solar) at our own QTHs and experience ZERO interference.  As we had chosen to take an active part in our PV system design, we had installed SunnyBoy series-string systems which are known (and proven!) to have zero interference potential on any LF, MF or HF amateur band as described in the link(s) below.  Unfortunately, some installers will not entertain the use of this type of system if it is not in the suite of products that they offer.

Other local amateurs that I know have microinverter-based PV systems using Enphase IQ modules and have reported minimal or no interference.  As I have not (yet) had the opportunity to carefully analyze the spectral signature of this product, I can only go by their assertion that their own system has not caused them obvious problems.  I hope to do a careful analysis of a modern Enphase system and if so, I'll report the findings on this blog.

Please post in your comments your experiences with PV systems - but please do so in the context of having fully read this article and at least perused the articles linked below.

  * * * * *

Other articles at this blog on related topics:


Other articles related to this topic:

 

Stolen from ka7oei.blogspot.com


[END]

 

Remote (POTA) operation from the Conger Mountain BLM Wilderness Area (K-6085)

By: KA7OEI
27 December 2023 at 07:03

It is likely that - almost no matter where you were - you were aware that a solar eclipse occurred in the Western U.S. in the middle of October, 2023.  Wanting to go somewhere away from the crowds - but along the middle of the eclipse path - we went to an area in remote west-central Utah in the little-known Conger Mountains.

Clint, KA7OEI operating CW in K-6085 with Conger
mountain and the JPC-7 loaded dipole in the background.
Click on the image for a larger version.

Having lived in Utah most of my life, I hadn't even heard of this mountain range even through I knew of the several (nearly as obscure) ranges surrounding it.  This range - which is pretty low altitude compared to many nearby - peaks out at only about 8069 feet (2460 Meters) ASL and is roughly 20 miles (32km) long.  With no incorporated communities or paved roads anywhere nearby we were, in fact, alone during the eclipse, never seeing any other sign of civilization:  Even at night it was difficult to spot the glow of cities on the horizon.

For the eclipse we set up on BLM (Bureau of Land Management) land which is public:  As long as we didn't make a mess, we were free to be there - in the same place - for up to 14 days, far more than the three days that we planned.  Our location turned out to be very nice for both camping and our other intended purposes:  It was a flat area which lent itself to setting up several antennas for an (Amateur) radio propagation experiment, it was located south and west of the main part of the weather front that threatened clouds, and its excellent dark skies and seeing conditions were amenable to setting up and using my old 8" Celestron "Orange tube" C-8 reflector telescope.

(Discussion of the amateur radio operations during the eclipse are a part of another series of blog entries - the first of which is here:  Multi-band transmitter and monitoring system for Eclipse monitoring (Part 1) - LINK)

Activating K-6085

Just a few miles away, however, was Conger Mountain itself - invisible to us at our camp site owing to a local ridge - surrounded by the Conger Mountain BLM Wilderness Area, which happens to be POTA (Parks On The Air) entity K-6085 - and it had never been activated before.  Owing to the obscurity and relative remoteness of this location, this is not surprising.

Even though the border of the wilderness area was less than a mile away from camp as a crow files, the maze of roads - which generally follow drainages - meant that it was several miles driving distance, down one canyon and up another:  I'd spotted the sign for this area on the first day as we our group had split apart, looking for good camping spots, keeping in touch via radio.

Just a few weeks prior to this event I spent a week in the Needles District of Canyonlands National Park where I could grab a few hours of POTA operation on most days, racking up hundreds of SSB and CW contacts - the majority of being the latter mode (you can read about that activation HERE).  Since I had already "figured it out" I was itching to spend some time activating this "new" entity and operating CW.  Among those others in our group - all of which but one are also amateur radio operators - was Bret, KG7RDR - who was also game for this and his plan was to operate SSB at the same time, on a different band.  As we had satellite Internet at camp (via Starlink) we were able to schedule our operation on the POTA web site an hour or so before we were to begin operation.

In the late afternoon of the day of the eclipse both Bret and I wandered over, placing our stations just beyond the signs designating the wilderness study area (we read the signs - and previously, the BLM web site - to make sure that there weren't restrictions against what we were about to do:  There weren't.) and several hundred feet apart to minimize the probability of QRM.  While Bret set up a vertical, non-resonant end-fed wire fed with a 9:1 balun suspended from a pole anchored to a Juniper, I was content using my JPC-7 loaded dipole antenna on a 10' tall studio light stand/tripod.

Bret, KG7RDR, operating 17 Meter SSB - the mast and
vertical wire antenna visible in the distance.
Click on the image for a larger version.
Initially, I called CQ on 30 meters but I got no takers:  The band seemed to be "open", but the cluster of people sending out just their callsign near the bottom of the band indicated to me that attention was being paid to a rare station, instead.  QSYing up to 20 meters I called CQ a few times before being spotted and reported by the Reverse Beacon Network (RBN) and being pounced upon by a cacophony of stations calling me.

Meanwhile, Bret cast his lot on 17 meters and was having a bit more difficulty getting stations - likely due in part to the less-energetic nature of 17 meter propagation at that instant, but also due to the fact that unlike CW POTA operation where you can be automatically detected and "spotted" on the POTA web site, SSB requires that someone spot your signal for you if you can't do it yourself:  Since we had no phone or Internet coverage at this site, he had to rely on someone else to do this for him.  Despite these challenges, he was able to make several dozen contacts.

Back at my station I was kept pretty busy most of the time, rarely needing to call CQ - except, perhaps, to refresh the spotting on the RBN and to do a legal ID every 10 minutes - all the while making good use of the narrow CW filter on my radio.

As it turned out, our choice to wait until the late afternoon to operate meant that our activity spanned two UTC days:  We started operating at the end of October 14 and finished after the beginning of October 15th meaning that with a single sitting, each of us accomplished two activations over the course of about 2.5 hours.  All in all I made 85 CW contacts (66 of which were made on the 14th) while Bret made a total of 33 phone contacts.

We finally called it quits at about the time the sun set behind a local ridge:  It had been very cool during the day and the disappearance of the sun caused it to get cold very quickly.  Anyway, by that time we were getting hungry so we returned to our base camp.

Back at camp - my brother and Bret sitting around
the fake fire in the cold, autumn evening after dinner.
Click on the image for a larger version.

My station

My gear was the same as that used a few weeks prior when I operated from Canyonlands National Park (K-0010):  An old Yaesu FT-100 equipped with a Collins mechanical CW filter feeding a JPC-7 loaded dipole, powered from a 100 amp-hour Lithium-Iron-Phosphate battery.  This power source allowed me to run a fair bit of power (I set it to 70 watts) to give others the best-possible chance of hearing me.

As you would expect, there was absolutely no man-made noise detectable from this location as any noise that we would have heard would have been generated by gear that we brought, ourselves.  I placed the antenna about 25' (8 meters) away from my operating position, using a length of RG-8X as the feedline, placing it far enough away to eliminate any possibility of RFI - not that I've ever had a problem with this antenna/radio combination.

I did have one mishap during this operation.  Soon after setting up the antenna, I needed to re-route the cable which was laying on the ground, among the dirt and rocks, and I instinctively gave it a "flip" to try to get it to move rather than trying to drag it.  The first couple of "flips" worked OK, but every time I did so the cable at the far end was dragged toward me:  Initially, the coax was dropping parallel with the mast, but after a couple flips it was at an angle, pulling with a horizontal vector on the antenna and the final flip caused the tripod and antenna to topple, the entire assembly crashing to the ground before I could run over and catch it.

The result of this was minor carnage in that only the (fragile!) telescoping rods were mangled.  At first I thought that this would put an end to my operation, but I remembered that I also had my JPC-12 vertical with me which uses the same telescoping rods - and I had a spare rod with that antenna as well.  Upon a bit of inspection I realized, however, that I could push an inch or so of the bent telescoping rod back in and make it work OK for the time-being and I did so, knowing that this would be the last time that I could use them.

The rest of the operating was without incident, but this experience caused me to resolve to do several things:

  • Order more telescoping rods.  These cost about $8 each, so I later got plenty of spares to keep with the antenna.
  • Do a better job of ballasting the tripod.  I actually had a "ballast bag" with me for this very purpose, but since our location was completely windless, I wasn't worried about it blowing over.
  • If I need to re-orient the coax cable, I need to walk over to the antenna and carefully do so rather than trying to "flip" it get it to comply with my wishes.

* * *

Epilogue:  I later checked the Reverse Beacon Network to see if I was actually getting out during my initial attempt to operate on 30 meters:  I was, having been copied over much of the Continental U.S. with reasonably good signals.  I guess that everyone there was more interested in the DX!

P.S.  I really need to take more pictures during these operations!


This page stolen from ka7oei.blogspot.com

[END]

It *is* possible to have an RF-quiet home PV (solar) electric system!

By: KA7OEI
30 June 2023 at 15:55

Figure 1:  Half of the array on my garage - the other half is
on the west-facing aspect.
There's a bit of shade in the morning around the end of June,
but it detracts little during the peak solar production
of the day - the hours on either side of "local" noon.
Click on the image for a larger version
For the past several years an incremental nemesis of amateur radio operation on the HF bands is solar power and the cover article of the April 2016 issue of QST magazine, "Can Home Solar Power and Ham Radio Coexist?" (available online HERE) brings this point home.

Personally, I thought that the article was a bit narrow in its scope, with an unsatisfying conclusion (e.g. "The QRM is still there after a lot of effort and expense, but I guess that it's OK") - but this impression is understandable owing to the constraints of the medium (magazine article) and the specific situation faced by the author.

Solar power need not cause QRM:

I can't help but wonder if others that read the article presumed that amateur radio and home solar were incompatible - but I know from personal experience that this is NOT necessarily the case:  There are configurations that will not produce detectable QRM on amateur bands from 160 meters and higher.

Before I continue, let me state a few things important to the context of this article:

Expertise in HF radio interference and home solar installations seems to mutually exclusive - which is to say that you will be hard-pressed to find anyone who is familiar with aspects of both.  This means that in the solar industry itself, you will not likely find anyone who can offer useful advice in putting together a system that will not contribute to the crescendo of electrical noise.
I have heard that many installers (at least in my area) will strongly pressure their potential customers to use microinverter-based systems - and this my experience as well:  From the very start of the process, I was adamant that the design of my system would be series string using SunnyBoy inverters which were known to me to be RF-quiet.  If your installer will not work with you toward your goals, consider a different company.
Designing an "RF-quiet" system as described here may incur a trade-off in available solar production as the use of microinverters can eke out additional efficiencies when faced with issues such as shading and complicated roofs that present a large number of aspects with respect to insolation (e.g. amount of light energy that can be converted to electricity).  Only in the analysis of proposed systems appropriate for your case can you reasonably predict the magnitude of this and whether or not you find it to be acceptable.
What is presented here is my own experience and that of other amateur radio operators with similar PV (PhotoVoltaic) system.  The scope of this experience is necessarily limited owing to the fact that when spending tens of thousands of dollars, one will understandably "play it safe" and pick a known-good configuration.
I will be the first to admit that there are likely other "safe" (low RF noise) combinations of PV equipment that can be demonstrated to be "clean" in terms of radio frequency interference.  I have anecdotally heard of other configurations and systems, but since I have not looked at them first-hand, I am not willing to make any recommendations that could result in the outlay of a large amount of money.  For this reason, please don't ask me a question like "What about inverter model 'X' - does it cause RFI?" as I simply cannot answer from direct experience.

An example system:

The system at my house consists of two series-string Sunny Boy grid-tie inverters:  I can unequivocally state that this system, which has both a SB 5000TL-US-22 (5 kW) and an SB3.8-1SP-US-40 (3.8kW) does not cause any detectable RF interference on any HF frequency or 160 meters - and I have yet to detect any interference on 6 meters, 2 meters or 70cm.  Near the LF and lower MF band (2200 and 630 meters, respectively) some emissions from these inverters can be detected - but none of the switching harmonics (about 16 kHz) land within either of these bands.  

Figure 2: 
One of two inverters in the garage. 
The Ethernet switch (upper right) produces
more RF noise than the inverter!
Click on the image for a larger version
This PV system is very simple:  I have a detached garage with a north-south ridge line meaning that the roof faces east and west.  While this orientation may seem to be less than ideal compared to a south-facing roof, it actually produces equal or greater power during the summer than a south-facing roof - and there are two usable surfaces onto which one can place panels (east and west) whereas one would typically not place any panels on a north-facing roof.  This means that one may be able to put twice as many panels on a symmetrical east-west facing roof than a south-facing roof.


Simple roof configuration can equal low noise:

The "simple" roof also has another advantage:  All panels on the faces are oriented the same and a larger number of panels may simply be wired in series.

This simple fact means that known-quiet series-string inverters may be used and known noise-generating components may be omitted from the system - namely, many models of "microinverters" and optimizers.  Both of these devices - despite being very different in their operation - are installed on a "per panel" basis and able to adjust the overall contribution of each panel to maximize the energy input of the entire solar power system.

Having each panel individually optimized for output power sounds like a good idea - and in most cases it is - but this nicety should be taken in context with the goals in mind - but considering that the panels themselves represent a rather small portion of the overall system cost, efficiency losses from not having optimizers can often be offset with the addition of more panels.  To be fair, it is not always possible to simply "add more panels" to make up for loss of production - but this must be carefully weighed against a major goal, which is to produce a "noise free" PV system.

The options have changed:

Since the 2016 article was written, the number of options for series-string inverters has significantly increased and the prices have gone down, allowing options to be considered now that may have been dismissed at that time.  Take the article as an example.

From the photographs accompanying the article, there appear to be two different aspects of panels:  A large array consisting of 30 panels, all seeming to face the same direction;  a smaller array of 8(?) panels:  There appears to be an array of 4 panels, but let us presume that this is an independent energy system.

Assuming that each panel is rated for 300 watts (likely higher than a circa-2016 panel) and that one would wish to limit the maximum open-circuit potential to about 450 volts, this implies the use of at least four MPPT circuits:  The 8 panel array and three arrays consisting of 10 series panels, each.  The maximum output of this system would theoretically be about 11.4 kW - but since one can optimistically expect to attain only about 80% of this value in a typical installation the use of an inverter system capable of 10 kW, as stated in the article, is quite reasonable.

Back in 2016, it would be reasonable to have a 10kW series string inverter with two MPPT inputs representing two separate inputs that could be independently optimized.  If such an inverter were used, this would mean that one input would have just 8 panels and the other would have all 30 panels on the main array - not particularly desirable in terms of balancing.  While all 30 of the panels in the larger array would ostensibly be producing the same output, snow, leaves and shading might cause the loss of efficiency should certain parts be thus impaired.

Having already ruled out the optimizing of each panel independently in the interest of having a "known-quiet" system, we might want to split things up a bit.  As an example, a single 10kW inverter with two MPPT inputs could be replaced with a pair of 5 kW inverters, each with 3 MPPT inputs and having a total of six independent DC inputs allowing the 8 panels of the isolated roof to be optimized together and the remaining 30 panels being divided into 5 arrays of about 6 panels, each.

The 2016 article did not mention the price the system, but a reasonable estimate for that time would be around US$35000 - and it was mentioned, in passing, that the cost of RFI mitigation might have been about 10% of the total system cost, implying about $3500 - about the cost of two Sunny Boy  SB5.0 5 kW series-string inverters, each with three MPPT inputs.

Replicating success:

At least two other local amateur radio operators used the same recipe for low-noise PV systems:  Series-string SunnyBoy grid-tie inverters - specifically the SB 3800TL, SB 5000TL and SB3.8s.  In none of these cases could RFI be detected that could be attributed to the inverter - and the only noise to be detected was with a portable shortwave receiver held within a few inches of the display.

What is known not to be quiet:

From personal experience I know for certain that microinverters such as the older Enphase M190 can be disastrous for HF, VHF and UHF reception.  As noted in the QST article, the Enphase power optimizers (model number not mentioned) also caused QRM.

Figure 3:
The two Tesla Powerwalls, gateway and electrical sub-
panels for the system located remotely on the east wall
of the house.
Click on the image for a larger version

Additionally, it has been observed that the Solaredge inverters - particularly coupled with optimizers - have caused tremendous radio frequency interference:  The aforementioned April, 2016 QST article about solar RFI deals with this very combination.

It probably won't work in all cases.

Compared to some installations that I have seen, my system - or the one in the 2016 article - are very simple cases - and there are a number of practical limitations, which include:

  • A "minimum" array size limitation.  Taking the Sunny boy SB5.0 as an example, there is a 90 volt minimum input which means that one would (very conservatively) want at least four 60-cell panels on each circuit.  This limitation may affect what areas on a roof may be candidates for placement of solar panels, reducing the total system capacity as compared to what might be possible with individually-optimized panels.
  • Systems with complicated shading.  If there are a number of trees - or even antennas and structures - portions of sub-strings may be shaded, causing reduction in output and compared to individually-optimized panels, series-strings are at a disadvantage, but careful selection of sub-string geometry can help.  For example, if a tower shades a series of panels during the period of highest production, placing all of those panels on one particular string can help isolate the degradation - but this sort of design consideration will require careful analysis of each situation.

Final words:

The design, configuration and layout of a home (or any) PV system is more complicated than depicted here and any system to be considered would have to take into account.  While I am certain that there are other ways to make an "RF Quiet" PV system, this article was intended to be limited to configurations and equipment with which I have direct experience.

Again, the likelihood of finding a "solar professional" who thoroughly understands RFI issues and knows which type of equipment is RF-quiet is unlikely, so it is up to you as the potential recipient of QRM to do the research.

Other articles at this blog on related topics:


This page stolen from ka7oei.blogspot.com


[End]

The case of the Clicky Carrier - Likely high-frequency trading (that can sometimes clobber the upper part of 20 meters)

By: Unknown
3 December 2021 at 21:34

Note:  As of 9 February, 2022, this signal is still there, doing what it was doing when this post was originally written.

* * *

Listening on 20 meters, as I sometimes to, I occasionally noticed a loud "click" that seemed to pervade the upper portion of the band.  Initially dismissing it as static or some sort of nearby electrical discharge, my attention was brought to it again when I also noticed it while listening on the Northern Utah WebSDR - and then, other WebSDRs and KiwiSDRs across the Western U.S.  Setting a wide waterfall, I determined that the source of this occasional noise was not too far above the 20 meter band, occasionally being wide/strong enough to be heard near the top of the 20 meter band itself.

Figure 1:
The carrier in question - with a few "clicks".  In this case,
the signal in question was at 14.390 MHz.
Click on the image for a larger version.

During the mornings in Western North America, this signal is audible in Colorado, Alberta, Utah, Oregon, Idaho, Washington - and occasionally in Southern California.  It is only weakly heard at some of the quieter receive sites on the eastern seaboard and the deep southeast, indicating that its source is likely in the midwest of the U.S. or Canada, putting much of the continent inside the shadow of the first "skip" zone. 

From central Utah, a remote station with a beam indicates that the bearing at which this carrier peaks is somewhere around northeast to east-northeast, but it's hard to tell for certain because of the normal QSB (fading) and the fact that the antenna's beamwidth is, as are almost all HF beams, 10s of degrees wide.  Attempts were made to use the KiwiSDR "ARDF" system, but because it is effectively unmodulated, the results were inconclusive.

What is it?

The frequency of this signal appears to vary, but it has been spotted on 14.378 and 14.390 kHz (other frequencies noted - see the end of this article) - although your mileage may vary.  If you listen to this signal sounds perfectly stable at any given instant - with the occasional loud "click" that results in what looks like a "splat" of noise across the waterfall display (see Figure 1), with it at the epicenter

Comment:   If you go looking for this signal, remember that it will be mostly unmodulated - and that it will be subject to the vagaries of HF propagation. 

When a weird signal appears in/near the amateur bands - particularly 20 meters - the first inclination is to presume that it is an "HFT" transmitter - that is, "High Frequency Trading", a name that refers not to the fact that they are on the HF bands, but that it's a signal that conveys market trades over a medium (the ionosphere) that has less latency/delay than conventional data circuits, taking advantage of this fact to eke margins out of certain types of financial transactions.  Typically, the signals conveying this information appear to be rather conventional digital signals with obvious modulation - but this particular signal does not fit that profile.  Why blame HFT?  Such signals have, in the past, encroached in the 20 meter band and distrupted communications - see the previous blog post "Intruder at the top of the 20 meter amateur band?" - link.

Why might someone transmit a (mostly) unmodulated carrier?  The first thing that comes to mind would be to monitor propagation:  The amplitude and phase of a test carrier could tell something about the path being taken, but an unmodulated signal isn't terribly useful in determining the actual path length as there is nothing about it that would allow correlation between when it was transmitted, and when it was received.

Except, that this signal isn't unmodulated:  It has those very wideband "clicks" could help toward providing a reference to make such a measurement.

What else could it be?  A few random thoughts:

  • Something being tested.  It could be a facility testing some sort of HF link - but if so, why the frequency change from day to day?  The "clicks"?  Perhaps some sort of transmitter/antenna malfunction (e.g. arcing)?
  • Trigger for high-frequency trading (HFT).  Many high-frequency trading type signals are fairly wide (10 kHz or so) - possibly being some sort of OFDM - but any sort of coding imposes serialization delays which can negate some of the minimization of propagation delay being attained via the use of HF as compared to other means of conveying data over long distances.  Likely far-fetched, but perhaps the "clicks" represent some sort of trigger for a transaction, perhaps arranged beforehand by more "conventional" means.  After all, what possible means of conveying a trigger that "something should happen" exists than a wide-bandwidth "click" over HF?  Again, unlikely - but seemingly so did something like HFT in the first place!  Additionally, it would seem that the "other" HFT signals that had been present have mostly disappeared - to be replaced by, what?  I suspect that they haven't just gone away!

A bit of analysis:

A bit of audio of this carrier, complete with "clicks" was recorded via a KiwiSDR.  To do this, the AGC and audio compression were disabled, the receiver set to "I/Q" mode and tuned 1 kHz below the carrier and the bandwidth set to maximum (+/- 6 kHz) and the gain manually set to be 25 dB or so below where the AGC would have been.  Doing this assures that we capture a reference level from the signal itself (the 1 kHz tone from the carrier) at a low enough level to allow for a very much stronger burst of energy (the "click") to be detected without worrying too much about clipping of the receive signal path.

The result of this is the audio file (12 kHz stereo .WAV) that you may download from HERE.

Importing this file into Audacity, we can zoom in on the waveform and at time index 13.340, we can see this:

Figure 2:
Zoomed-in view of the waveform from the off-air recording linked above.
These "clicks" seem to come in pairs, approximately 1 msec apart, and have an apparent
amplitude hundreds of times higher than the carrier itself.
Click on the image for a larger version.

Near the baseline (amplitude zero) we see the 1 kHz tone at a level of approximately 0.03 (full-scale being normalized to 1.0) but we can see the "clicks" represented by large single-sample incidents, one of which is at about 0.83.  Ignoring the fact that the true amplitude and rise-time of this "click" is likely to be higher than indicated owing to band-pass filtering and the limited sample rate, we see that the ratio between the peak of the "click" and the sine wave is a factor of 27.7:1 or, converted to a power relationship, almost 29dB higher than the CW carrier.

This method of measuring the peak power is not likely to be very accurate, but it is, if anything, under-representing the amplitude of the peak power of this signal.  It's interesting to note that these clicks seem to come in pairs, separated by 12-13 samples (approximately 1 millisecond - about the distance that it takes a radio signal 300 km/186 miles) - and this "double pulse" has been observed over several days.  This double pulse might possibly an echo (ionospheric, ground reflection), but it seems to be too consistent.  Perhaps - related to the theoretical possibility of this being some sort of HFT transmission - it may be a means of validation/identification that this pulse is not just some random, ionospheric event.

Listening to it yourself:

Again, if you wish to listen for it, remember that it is an unmodulated CW carrier (except for the "clicks") and that you should turn all noise blanking OFF.  Using an SSB filter, these clicks are so fast that they may be difficult to hear, particularly if the signal is weak.  So far, it has been spotted on 14.378 and 14.390 MHz (try both frequencies) which means that in USB, you should tune 1 kHz lower than this (e.g. 14.377 and 14.389) hear a 1 kHz tone.  Once you have spotted this signal, switching to AM may make hearing the occasional "click" easier. 

Remember that depending on propagation, your location - and your local noise floor - you might not be able to hear this signal at all.  Keep in mind that the HF bands are pretty busy, and there are other signals near these two frequencies with other types of signals (data, RTTY, etc.) - but the one in question seems to be an (almost!) unmodulated carrier.

It's likely that this carrier really isn't several hundred kHz wide, so it may not actually be getting into the top of 20 meters, but the peak-to-average power is so high that it may be audible on software-defined radios:  Because the total signal power across 20 meters may be quite low, the "front end AGC" may increase the RF signal level to the A/D converter and when the "click" from this transmitter occurs, it may cause a brief episode of clipping, disrupting the entire passband.

* * * * *

If anyone has any ideas as to what this might be, I'd be interested in them.  If you have heard this signal and have other observations - particularly if you can obtain a beam heading for this signal, please report them as well in the comments section, below.

Updates:

  • November, 2022:   As a follow-up, it would seem that the nature of this "clicky carrier" has changed very slightly.  It appears as though the bandwidth of the "click" is now better-contained and is only a few 10s of kHz wide rather than around 100 kHz wide.

    It also appears that other frequencies are being use - including 14.372 MHz.   More frequencies may be used routinely, but I don't monitor this signal frequently.

  • December, 2022:  This type of signal was noted on 14.380 MHz - and possibly 14.413 MHz simultaneously, making for a total of at least four frequencies where this type of signal has been observed.
  • July, 2023:  This type of signal was noted at 14.413 and 14.446 MHz - "clicks" and all.  Since the previous update, other frequencies have been noted - singly and simultaneously in the same general area.
  • Related to the above: A proposal to modify FCC Part 90 was made by a group with an interest in High-Frequency trading via the 2-25 MHz frequency range using ionospheric propagation.  This proposal may be read here:  https://www.fcc.gov/ecfs/document/1042840187330/1

 

This page stolen from ka7oei.blogspot.com.


[End]

A "portable", high power, high-sensitivity remote repeater covering deep river gorges in Utah

By: Unknown
30 June 2021 at 20:31

From the late 1950s until about 2012 there was a (mostly) annual event held in southeastern Utah that was unique to the local geography:  The Friendship Cruise.

The origins are approximately thus:  In the late 1950s, an airboat owner - probably from the town of Green River, Utah - decided to go down the Green River, through the confluence of the Green and Colorado rivers, and back up to the town of Moab.  Somehow, that ballooned into a flotilla in later years - with as many as 700 boats - in the 60s and 70s.  By the mid 90s, interest in this unique event seemed to have waned and by about 2012, it finally petered out.

Communications is important:

Figure 1:
A high-Q 80 meter magnetic loop
on one of the rescue boats
Click on the image for a larger version
From the beginning it was realized that there was a need for the boats and support crews to be able to communicate with each other - but the initial attempts using CB and/or public safety VHF radios were unsuccessful, reaching only a few miles up and down the river - not too surprising considering that most of the course runs through winding, deep (1200 foot deep, 365 meter) gorges.  In later years, cell phones - and even satellite phones - were tried, but due to the remoteness and narrowness of the gorges (and limited view of the sky) they were of extremely limited use.

At some point, probably in the mid 1960s, amateur radio operators got involved, successfully closing the communications link using the 80 meter amateur band.  This tactic worked owing to the nature of 80 meters:  During the daytime, coverage is via skywave over a radius of about 200 miles (300km) and this high angle of radiation allowed coverage into and out of the deep canyons.  Furthermore, the same antennas that were small enough to be usable on boats, vehicles and temporary stations on this band were well-suited for radiation of RF energy at these steep angles.

For (literally!) decades, this system worked well, providing coverage not only anywhere on the river, but also to the nearby population centers (e.g. Salt Lake City) where other amateur radio operators could monitor and relay traffic as necessary and summon assistance via land line (telephone) if needed.  Because the boats were typically on the river only during the day, this seemed to be a good fit for the extant propagation.

While it worked well, it was subject to the vagaries of solar activity:  An unfortunately-timed solar flare would wipe out communications for hours at a time, and powering and installing a 100 watt class HF transceiver and antenna was rather awkward.  Occasionally, there was need to communicate after dark, and this was made difficult by the fact that 80 meters will go "long" after sunset - often requiring stations much farther away (e.g. in California or Nebraska) to relay to stations just a few 10s of miles away on the river!  Finally, it was a bit fatiguing to the radio and boat operators to have to listen to HF static all day long!

Enter VHF communications:

Figure 2:
General coverage map of the course
showing coverage of various sites.
Click on the image for a larger version
While VHF communications had been tried early on - and had been available in the intervening years - the biggest problem was that these signals could not make their way along the river for more than a few miles between twists and bends in the deep river gorges.  While useful for short-range communications, it simply wasn't suitable for direct boat-to-boat communications along the vast majority of the river's course.

By the time that the 1990s had come along, there was renewed interest in seeing if we could make use of VHF, on the boats, on the river.  The twist was that instead of direct communications between boats, we would try to relay signals from far above, on the plateaus farther away, and a few experiments were tried.  It 1996, I was on a boat on the river and took notes on what sites covered and where, trying nearby mountaintop repeaters and temporary stations set up at places near-ish the river courses themselves - the resulting map being presented in Figure 2.

Using the color-coded legend across the top and the markings on the map itself, one can see what sites covered where.  Included in this was the coverage from the 147.14 repeater near-ish Green River, Utah, the 146.76 repeater near Moab, and several other temporary sites atop the plateaus surrounding the river.  As can be seen, coverage was spotty and inconsistent over much of the route - with the exception of a site referred to as "Canyonlands Overlook" (abbreviated "Cyn Ovlk") which commanded a good view of the Colorado River side of the river course.  Clearly missing was reasonable coverage in the depths of the gorges along the lower parts of the Green River side - which started, more or less, where the coverage of the "Spring Canyon" (abbreviated "Spring Cyn") stopped.

Figure 3:
The two TacTecs used for 2 meter reception,
the voting controller (blue box) and the FT-470 used
as the UHF link radio.
Click on the image for a larger version.
As it happened, there were amateur radio operators camping at a site called Panorama Point when I was on the lower Green River and because we were using the Utah ARES simplex frequency, they just happened to hear the simplex activity on the river.  At that moment, I happened to be in areas that were not well-covered by any of the other sites and while their signals weren't extremely strong, it made me wonder what could be accomplished should I wield both gain antennas on the receiver and high power and gain antennas on the transmitter of a 2 meter repeater.

The birth of a repeater:

During the next year I put together a system that I'd hoped would make the most of the situation.  Because of the remoteness of the site, accessible via a high-clearance Jeep road - and that we had to bring everything to live for a few days, it had to be relatively lightweight and compact - and I also wanted to avoid the use of any duplexers (large cavity filters) that would add bulk and - more importantly - losses to the system.  Taking advantage of a weekend to visit Panorama Point the next spring we determined that we could split the transmit and receive portions by about 0.56 miles (0.9km) apart, placing the receive antennas behind some local geographical features and using local topography to improve isolation.  The back-of-the-envelope calculations indicated that this amount of separation - and the rejection off the backs and sides of the beam antennas - would likely be sufficient to keep the receiver out of the transmitter.  The receive site - surrounded by three sides by vertical cliffs - also provided a commanding view of the terrain as can be seen in Figure 5, below.

Figure 4:
GaAsFET preamplifier mounted right at the
receive antenna to minimize losses.
Click on the image for a larger version.

In addition to site separation and gain antennas, I decided to go overboard, adding mast-mounted GaAsFET preamplifiers, right at each antenna (Figure 4) and implementing a voting receiver scheme - something made much easier with the acquisition of two, identical RCA TacTec "high band" VHF transceivers.  These receivers were modified - clipping the power lead to the transmitter and adding a 3.5mm stereo plug to each radio to bring out both discriminator audio and the detector voltage from the squelch circuit.

A relatively simple PIC-based repeater controller was constructed, using a simple comparator to determine which receiver had the "best" signal, based on the detector voltage from the squelch circuit, and also using another set of comparators and onboard potentiometers to set the COS (squelch) setting for the receivers.  As it turned out, the front-panel squelch control adjusted the gain in front of the squelch detectors in the radios themselves, allowing each receiver to be "calibrated" from that control, allowing easy fine-tuning in the field.

To link the receiver site to the transmitter site, a single UHF channel was used and I modified my old Yaesu FT-470 handie-talkie to this task.  The mysterious rubber plug on the side of this radio was replaced with a 3.5mm jack, providing a direct connection to the modulation line of the UHF VCO while using the top panel 2.5mm external microphone jack for transmitter keying.  As it turns out, not only did this transmitter provide linking to the nearby transmitter site, but its UHF beam was pointed across the way, to another 2 meter repeater at Canyonland's Overlook that provided coverage on the Colorado River - providing what amounted to a linked repeater system.  A later addition was a CdS photocell on a grommet and a piece of "Velcro" strap allowed the detection receiver activity by "looking" at the front-panel LED to prevent the link transmitter from "doubling" (transmitting at the same time) and clobbering an ongoing transmission from the other repeater site.

Figure 5:
The remote RX site, surrounded on
3 sides by sheer cliffs.  The mast
has two 2 meter and one UHF link
beam antenna.  The solar panels are
just visible along the far right edge.
Click on the image for a larger version.
One of my goals was to minimally process the audio, causing as little "coloration" as possible to maintain quality, and to this end I took the receivers' discriminator audio from the voter and put it directly into the modulator of the UHF link radio, completely avoiding the need for de-emphasis and pre-emphasis.  This worked pretty well - but I noticed during the first year that it was used that when weak signals were present on the input, the noise and hiss from weak signals would sometimes cause "squelch clamping" on the receivers being used by us and others owing to the fact that such noise was being passed along the link without alteration:  For the next year I added a 3.5 kHz low-pass filter in the transmit audio line to remedy this.

The receive site itself was solar-powered, using lead-acid batteries to provide the energy when insufficient sun was available (e.g. heavy clouds, night).  In later years, the PIC controller was modified to not only read the battery voltage, but to regulate the solar panels' charging of the battery bank using a "bang-bang" type charger (See note 1) but also to report the battery voltage when it did its legal identification.  In this way, we could keep an "eye" on things without having to walk out to the receive site.

The two 2 meter and the 70cm link antennas were mounted on a single mast, the VHF antennas pointed in different directions to take advantage of the slight difference in physical location and in the hopes of providing diversity for  the weak signals from the depth of the canyons - which were all reflections and refractions.  As it turns out, despite the close proximity of the antennas, this worked quite well:  At the site, one could monitor the speakers on the receivers and watch the voting controller's LED and see and hear that this simple, compact arrangement was, in fact, very effective in reducing the number of weak-signal drop-outs caused by the myriad multipath.

In testing on the work bench, the measured 12dB SINAD sensitivity of each of the receivers (plus GaAsFET preamps) was on the order of 0.9 microvolts - far and away better than a typical receiver.  Later, I did the math (and wrote about it - see the link at the bottom of this article) and determined that it was likely that the absolute sensitivity of this receiver was limited by the thermal noise of the Earth itself and that it could not, in fact, be made any more sensitive.  This notion would appear to be borne out by a careful listening to the repeater in the presence of weak signals:  Very weak signals - near the receive system's noise floor - sounded quite different than what one might hear on a typical FM receive system near it's noise floor.  Instead of a "popcorn" type noise, signals seemed to gradually disappear into an aural cloud of steam.

The transmitter site:

Figure 6:
The transmit site.  The tall (30 foot) mast and 2 meter transmit
antenna is visible in the background with the UHF link
antenna and the VHF "backup" TX antenna in the foreground.
Click on the image for a larger version.

With so much effort having gone into maximizing receiver performance I decided to do the same on the transmit site in the years that this system was used.  For the first year, the transmitter was modest:  A Kenwood TM-733, on low power, driving a 50 watt RF amplifier into a vertical on a short mast.

The next year I decided to erect a taller mast and place atop it a 5 element beam, pointed in the general direction of "up river".  To boost my RF output power, I scavenged a pair of 110 watt RF amplifiers from some ancient Motorola Mocom 70 mobile radios (with some DC fans for cooling) and used two Wilkinson Power divider - one to split the input power and another to combine the outputs of the amplifier, yielding a bit over 200 watts of RF and about 1500 watts of ERP (Effective Radiated Power) - all without causing any measurable desensitization of the receiver system.  After a few days, one of these amplifiers failed, but the remaining 110 watt amplifier, now operating without the output combiner, happily chugged along.

The next year I acquired a 300 watt Vocomm amplifier and was able to use it for the remainder of the times that the Friendship Cruise was held.  Requiring 50 watts of drive, I still had to use the 50 watt amplifier, driven by 5 watts from the TM-733 to attain the full RF output.  When keyed down, the entire transmitter system drew about 60 amps at 12 volts from the battery bank, requiring frequent topping-off by a generator and DC power supply that were brought along. (See note 2)

With that much transmit power, the antenna was held aloft by a 30 foot (9 meter) mast to keep it away from people - and to help clear the local terrain and its effects.  As can be seen in Figure 6, there was a second mast with the UHF link antenna and a "back-up" 2 meter antenna.  When we arrived at the site, the first order of business was usually to set up the receive site, but once back at camp, we used a radio in cross-band mode and the two antennas on this short mast to get it on the air, providing "reasonable" transmit coverage.  Because of the effort required to set up the tall mast, battery bank and power amplifier, we often waited until the next morning to complete the setup, bringing our radiated transmit power up to its full glory!

"Listening" on the link frequency, this transmitter not only relayed my own, nearby receive site, but also the "other" repeater at Canyonland's Overlook. 

How well did it work?

The Panorama Point repeater itself worked better than we could have hoped:  It was "reachable" nearly everywhere on either the Green or Colorado River - although some sections of the upper Green and Colorado had somewhat weaker signals, requiring a good antenna and 50 watt radio - comparable to a typical car mobile installation - for reliable coverage.  Unexpectedly, it also provided coverage into the town of Moab, as far north as Price, Utah and even down near Hite, Utah - both well outside its expected coverage range and well outside the expected pattern of the beam antennas.

I'm confident that if I'd simply plopped down a "store bought" repeater with a single antenna and cavities, its performance - particularly on receive - would have been very much inferior as the signals from the depths of the gorges on the upper Green River were very weak and "multipathy". (See note 3)

With about 2.5kW of ERP one would expect that this repeater would have been an "alligator" (all mouth, no ears) but this was not the case:  When users were operating from the more extreme fringe areas - as in a deep river gorge, using a 50 watt mobile radio - the transmitter and receiver seemed to be more or less evenly matched, and despite running this much power, we did not experience any detectable "desense" where the strong transmit signal would overload the receiver.  At least part of this was attributed to the receivers themselves:  The RCA TacTec receivers used only modest amounts of RF gain in their front ends and a passive diode-ring mixer.  I have little doubt that if we had used more "modern" receivers we would have experienced overloading and would have had to place notch cavities, tuned for the transmit frequency, between the GaAsFET preamps and the receivers.

As a system, the Panorama Point and Canyonlands Overlook repeaters completely replaced the need for HF gear on the boats in the last decade or so that the Friendship Cruise was held, providing nearly seamless coverage from start to finish.

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Note 1:   A "bang-bang" solar regulator simply connects the solar panels directly to the battery when the voltage is too low - say, 13.2 volts - and disconnects them again when it rises above about 13.7 volts.  The PIC software implemented a timer so that after a disconnect from the panel when the voltage was high, it would not reconnect for at least 30 seconds, preventing rapid cycling.  With an open-circuit voltage of around 15 volts for the panels used, this was a simple, safe and reasonably efficient approach that could simply not cause radio-frequency interference in the way many modern "MPPT" solar chargers (with their PWM switching) might.

Note 2:  In the later years, a pair of 40 amp switching power supplies were used at the transmitter site to charge the battery as quickly as possible.  Not unexpectedly, we could load the generator to only about 60% of its rated output, owing to the terrible power factor of these supplies caused by their simple capacitor inputs:  Power-factor corrected supplies were not cheap and readily available at that time.  Also in later years, a very low power (1 milliwatt) 2 meter transmitter was constructed, connected to the battery bank, that telemetered the battery voltage using MCW (Morse Code).  If the battery voltage got too low, this transmitter would activate a subaudible tone and a receiver that had been parked on this frequency, configured to detect that tone, would remain silent unless/until the voltage dropped below the threshold, alerting us to the need to start the generator.

Note 3:  "Multipath" is when a signal - likely due to obstructions - finds more than one way to the other end of the communications path via reflection and refraction - a condition that is the rule rather than the exception when trying to get signals in/out of the deep gorges along these rivers.  While these multiple signals can reinforce each other, they are equally likely to cancel each other out.  By having multiple receivers and antennas - even two antennas very close to each other - the probability is significantly higher that at least one of the receiver/antenna combinations will be able to hear such a signal.  Because of the nature of FM signals, one can generally infer its quality by analyzing the amount of noise on it:  By comparing the amount of noise on the same signal, from two different receiver/antenna combinations - and always selected the "better" signal - the probability is increased that the received transmission will suffer less degradation.

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Additional (related) articles:

This page stolen from ka7oei.blogspot.com

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