RF Harmonics Accompanied by Broadband Upconverted 120 Hz

[kzerozr]

I have a situation wherein RF transmissions on one frequency, 2 MHz for example, not only show up at 4 MHz as one would anticipate, BUT have additional broadband "hum" (harmonics of 120 Hz) all around the 4 MHz signal. The noise around the 4 MHz RF pedestal can extend perhaps +/- 100 kHz. The problem is that other equipment needs to operate near this harmonic and the broadband noise is a tremendous impediment to this function. Application of RFI filters has diminished the problem, however the problem moves to other breakers because the impedance and "antenna effects" have been altered by introduction of these filters. Any suggestions?

 

[mpoulton]

Sounds like more careful filtering or shielding is required. What is the source of the 2MHz signal, what is the power level, and how it that signal being coupled to the wiring?

 

[GoldDigger]

Sounds like more careful filtering or shielding is required. What is the source of the 2MHz signal, what is the power level, and how it that signal being coupled to the wiring?

First try to identify the source of the harmonic and intermodulation distortion in the first place.
To get 4MHz with 120Hz sidebands requires that high levels of both 2MHz and 120Hz be present at some non-linear impedance. That impedance could be anything from an actual diode or complex circuit to a corroded connection in a wire, raceway EGC, or even structural members. Surge protection devices may also be involved and may need parallel capacitors to bypass RF around their non-linear elements.

Shield or correct that non-linear element and you will stop the stray frequencies at their source instead of chasing filtering and shielding of high level spurious signals.
You can probably get close to identifying the source with a signal strength meter.

Local radio amateurs as well as RF professionals may be well practiced at locating this sort of problem.

 

[kzerozr]

This phenomenon occurs at literally any frequency up to perhaps 50 MHz and the X2 harmonic. Power levels range from 1 W to 100's of W.

If you are familiar with Manley-Rowe equations, this appears to be what is happening. When the impedance at one point is changed by adding a filter, other parts of the electrical distribution are impacted. By placing RFI filters on what are believed to be, from test, the "nonlinear culprits", those circuits do not affect the interference further, but other circuits then have heightened sensitivity because the unintended antenna formed by the other wiring is now altered.

The X2 harmonic energy accompanied by the myriad of 120 Hz sidebands is "radiated", so going around with a field strength meter sniffing different wires, etc is really an impossibility. Placing a sharp-skirt bandpass filter around the transmit frequency and accompanying that with a sharp bandpass filter around the receive frequency ( X2 harmonic ) has NO impact on the interference. The interference is radiated and at 2X the fundamental. The actual transmitter is exceedingly "clean" as shown on a spectrum analyzer and monitored with another receiver.

To date I have found sensitivities in electrical devices that have, for example, 120V to 24 V step-down transformers. I have pulled all the GFIs out.

 

[GoldDigger]

This phenomenon occurs at literally any frequency up to perhaps 50 MHz and the X2 harmonic. Power levels range from 1 W to 100's of W.

If you are familiar with Manley-Rowe equations, this appears to be what is happening. When the impedance at one point is changed by adding a filter, other parts of the electrical distribution are impacted. By placing RFI filters on what are believed to be, from test, the "nonlinear culprits", those circuits do not affect the interference further, but other circuits then have heightened sensitivity because the unintended antenna formed by the other wiring is now altered.

The X2 harmonic energy accompanied by the myriad of 120 Hz sidebands is "radiated", so going around with a field strength meter sniffing different wires, etc is really an impossibility. Placing a sharp-skirt bandpass filter around the transmit frequency and accompanying that with a sharp bandpass filter around the receive frequency ( X2 harmonic ) has NO impact on the interference. The interference is radiated and at 2X the fundamental. The actual transmitter is exceedingly "clean" as shown on a spectrum analyzer and monitored with another receiver.

To date I have found sensitivities in electrical devices that have, for example, 120V to 24 V step-down transformers. I have pulled all the GFIs out.

Unfortunately, to me this just means that you have not yet found the specific place where the X2 harmonic is being generated. It may be caused by radiated RF energy (necessary since you have a transmitter!) is encountering the combination of an incidental antenna and a non-linear element.
The non-linear element needs to also be exposed to 60Hz voltage or current, but other than that need not be part of any AC circuitry directly associated with the transmitter.
The non-linear element must be exposed in some way to the high RF field from the transmitter output, so identifying it and shielding it may be the best way to resolve the problem. The same metal that allows the transmitter RF to reach the modulating element will also directly radiate the harmonic(s), so just filtering the wires will not often help. That radiated energy is then being coupled by other incidental antennas into unrelated circuits, making it hard to trace.
Looking specifically for radiated second harmonic and NOT trying to find it on AC wiring may be possible. But difficult I am sure.
Off the wall idea: temporarily connect the transmitter output to a directional antenna and find out which orientation of that antenna results in the highest harmonic radiation level and then look in that direction from the transmitter.

In the case of radio stations, the culprit is often a bad joint or leaking insulator in the guy wires supporting the transmit antenna.

 

[mpoulton]

I assume since you're referring to the 2MHz signal being transmitted that this is some kind of actual transmitter facility, not just a location where 2MHz equipment is being used and leakage is causing a problem. I also assume the field strength in the facility is acceptably low and that the antenna location or radiation pattern cannot be changed to reduce the irradiation of the building's electrical system. Some details about the facility and equipment would be pretty helpful here.

The 120Hz component to the interference points to accidental junctions in power system wiring as the likely culprit. The nonlinearity of the accidental junction is only "in play" when the RF signal is able to reverse the potential across the junction (the starting and stopping of conduction is what matters). When the junction is biased by a current in one direction in excess of the RF signal current, the junction will see no zero-crossing at the RF frequency and thus will not create harmonic interference. In a junction that's part of the building's power system and carries current at 60Hz, the junction will be biased one way or the other by that current flow most of the time. It can only act as a source of harmonic interference during the brief period near the 60Hz zero-crossing (120 instances per second) when the 60Hz power is not significantly biasing the junction and so the RF signal can do so. This will result in a series of short pulses of harmonic interference 120 times per second, with some spectral broadening of the interference at the beginning and end of each pulse. This is more easily seen in time domain than frequency domain (oscilloscope rather than spectrum analyzer) but your description of the spectrum matches perfectly.

Now the bad news: I doubt there's much of anything that can be done to fix this permanently in this installation. Without more information about the facility and equipment it's nearly impossible to say what solutions might be viable, but what you're describing sounds a lot like the result of pretty high field strength applied to a facility that was not designed for it. Virtually every single dissimilar junction in that place (whether an intentional PN junction or just metal-on-metal) will radiate somewhat, and that's going to be a LOT of junctions. You're not going to be able to get rid of all the junctions. Judging from your description, you're not going to be able to get a grip on the RF absorption/re-radiation by the power system either. There's good reason people do not often co-locate two transceivers at the same facility that operate on exact harmonics of one another - this happens.

You may be able to determine if there are specific junctions that are causing most of the problem. If you can make the 120Hz component reduce significantly by turning off power to any specific circuits or devices (leaving transmitter power output constant), then you've isolated the location of one or more of the problematic junctions. You should expect to see a sudden reduction in 120Hz component, but an overall increase in harmonic interference level, when 60Hz power is removed from an offending junction. Filters at or near any connections (especially dissimilar ones) on those circuits may help more than just putting them everywhere.

 

[GoldDigger]

...

You may be able to determine if there are specific junctions that are causing most of the problem. If you can make the 120Hz component reduce significantly by turning off any specific circuits or devices, then you've isolated the location of one or more of the problematic junctions. Filters at or near any connections (especially dissimilar ones) on those circuits may help more than just putting them everywhere.

:thumbsup::thumbsup:

Note also that the RF has to be getting to those junctions somehow, so wiring done in NM or using other unshielded wiring methods would be a prime suspect.

 

[kzerozr]

RF Harmonics Accompanied by Broadband Upconverted 120 Hz

Thank you to mpoulton and GoldDigger for your comments. I know this takes time and I appreciate your endeavors on my behalf.

More details:
- this IS an amateur radio installation wherein multiple transmitters are on the air at the same time
- the frequency"bands" are 3.5, 7, 14, 21, and 28 MHz
- the "band pairs" where the problem is more egregious are 3.5/7, 7/14, and 14/28 MHz
- up to 1,500 watts of RF is radiated WITH direction gain (yagi) antennas, further compounding things.
- the interference is ONLY when the yagis are pointed at the house. Moving the antennas in azimuith 45 degrees or more greatly diminishes the interference, and if rotated farther the interference is absolutely "gone"
- While transmitting with a kilowatt, a dummy load can be placed on the coax end where the receive antenna would be normally located and NO RF is practically noticed at the fundamental OR the second harmonic. In other words, the system appears "closed". All the interference is only coming in via the antennas intended for transmission/reception.

I agree that if the RF were causing zero crossings, one would have serious harmonic issues. However, any pn junction even if it remains forward biased is non linear due to the exponential characteristic of the junction itself. So this nonlinearity will be less than the "on/off" situation, but there is still a nonlinearity without any zero crossings. The signal level creating all this trouble is on the order of only a few hundred microvolts across 50 ohms, which is not much.

Yes, the field strength is quite high when the antennas are pointed in the direction of the house.

"Everything" in the house is exposed to the RF unquestionably. Except for the cable run up the side of the towers and inside the house, all cables are buried. The cables used are heliax - extremely RF "tight".

While the directivity of the yagi's allows me to ascertain that the troubling interference is coming directly from the house, the overall size of any one of these antennas, due to the low frequency, precludes relocating any antenna and doing any type of "probing". It is just physically not possible. I have transmitted around 29 MHz and looked around the house with a 146 MHz 3 element yagi in hopes of finding a "hot spot" at the 5th harmonic but this has proven to be unsuccessful. Having installed Corcom filters on a) all the garage door openers, b) upstairs and downstairs ACs, things have improved, however the problem is extremely bad on the 14/28 MHz pair. I will likely try sniffing with the 146 MHz yagi again since I have had some success after installing the Corcom filters.

I have had a friend listen to my signal(s) from a distance of 2-3 miles and he reports "extremely clean". No harmonics, no spurs, and no elevated phase noise.

As was mentioned, clearly the high power fundamental RF is impinging on something nonlinear while simultaneously connected to AC power, with the two signals mixing and developing this "hash" at the harmonics of the fundamental.

And finally, after introducing the RFI filters on all the garage door openers, that circuit breaker ceased to change the interference had ben the case before. Then the breakers for the AC units seemed to be the greatest contributor to the interference, thus the introduction of RFI filters there. Now those breakers cease to change the interference and another breaker is the major player. I will play this game a little longer, then probably have to concede defeat and at my next home, keep all the antennas far outside the near field of any antenna.

Again, thanks for your help.

 

[GoldDigger]

Sounds like you have the basic principles well in hand.
Just remember that although most semiconductor diodes will not pass noticeable current below around 200mv (for Schottky or germanium diodes, higher voltage for silicon) the imposition of even a small 60Hz current through the element (well within the magnitude of ground to neutral voltages among other things) will allow the low RF voltage to intermodulate.
The garage door openers had their own antennas (although short) and may have been wired with NM. It makes sense that they would be a good potential source.
The A/C units were coupled to a lot of copper and aluminum piping, which could provide contact junctions.
If very low voltages are applied, a copper to copper junction with an oxide layer can also work as a diode with a forward voltage of about 200mv.

 

[kzerozr]

Your comments are appreciated. If you think of anything further, please let me know. I have expended far too much time on this problem, but serious problems generally are not solved right away as we all know.

Are you able to convey any ideas about voltage differentials between the neutral and utility ground? In some reading I have done the claim has been made that this is the most likely area of common-mode problems rather than between "hot" and the "neutral". Pretty tough to clean up any delta voltages between the neutral and ground, I think.

 

[mpoulton]

Now it makes sense - still no easy solutions, but I see the problem. Every junction in your house is radiating somewhat when you light it up with a huge ERP from that beam. You're right that even biased junctions will radiate due to their nonlinearity, but they won't radiate nearly as much as an unbiased junction and they won't exhibit the 120Hz modulation behavior (or the temporally varying spectral broadening of the noise pulses). It sounds like you've found some specific culprits and filtering helps. Maybe just continue the hunt. Chances are that installing the filters near the equipment rather than the panel will be most effective, since the wiring acts as the antenna.

As far as neutral-ground voltage and common mode interference, I think that's a red herring in this situation. Neutral and ground are bonded at your service entrance. Any voltage differential between them is caused by voltage drop along feeder and branch circuit conductors in your house, and it should never be more than a couple volts. There would be no plausible situation where a higher differential exists except as a transient during a ground fault. The neutral-ground voltage matters to poorly designed audio equipment but not for much else. From an RF perspective, neutral and ground have to be treated as if they are completely separate because the inductance of the bonding path is likely to be quite high even at "low" frequencies. This doesn't really pose any problem though: you just handle RF filtering of the neutral the same as you handle the hot, with lots of bypass caps and some series inductance. All three power conductors supplying equipment (hot, neutral, and ground) each have the same potential for common mode interference. They're all just long antennas - what they're connected to at the other end doesn't matter much. RF bypassing of all three conductors to an excellent RF ground is about all you can do to fix common mode problems - but that RF ground better be good!

73 DE KC0LLX

 

[kzerozr]

Now it makes sense - still no easy solutions, but I see the problem. Every junction in your house is radiating somewhat when you light it up with a huge ERP from that beam. You're right that even biased junctions will radiate due to their nonlinearity, but they won't radiate nearly as much as an unbiased junction and they won't exhibit the 120Hz modulation behavior (or the temporally varying spectral broadening of the noise pulses). It sounds like you've found some specific culprits and filtering helps. Maybe just continue the hunt. Chances are that installing the filters near the equipment rather than the panel will be most effective, since the wiring acts as the antenna.

As far as neutral-ground voltage and common mode interference, I think that's a red herring in this situation. Neutral and ground are bonded at your service entrance. Any voltage differential between them is caused by voltage drop along feeder and branch circuit conductors in your house, and it should never be more than a couple volts. There would be no plausible situation where a higher differential exists except as a transient during a ground fault. The neutral-ground voltage matters to poorly designed audio equipment but not for much else. From an RF perspective, neutral and ground have to be treated as if they are completely separate because the inductance of the bonding path is likely to be quite high even at "low" frequencies. This doesn't really pose any problem though: you just handle RF filtering of the neutral the same as you handle the hot, with lots of bypass caps and some series inductance. All three power conductors supplying equipment (hot, neutral, and ground) each have the same potential for common mode interference. They're all just long antennas - what they're connected to at the other end doesn't matter much. RF bypassing of all three conductors to an excellent RF ground is about all you can do to fix common mode problems - but that RF ground better be good!

73 DE KC0LLX

Yes, the hunt will continue, at least for a time. I have located each Corcom filter inside the unit it is to protect; garage door opener, AC unit, etc. I have used a Dremel tool to ensure good bonding. Don't know if you have ever dealt with varactor frequency multipliers? This problem is akin to that in that the location of the harmonic is so dependent on the impedance seen by the junction causing the problem. Every "improvement" I make alters the antenna circuit, thus the impedance, so some other bad actor steps in to take the place. So far it does seem that the most notorious elements have had internal step-down transformers which are connected to the line all the time. The AC units, the microwave, the oven, the refrigerators, etc are all sitting in a holding pattern waiting for a command from someone, thus vulnerable each time I transmit. What is rather mind boggling is that this effect on 14/28 MHz is very noticeable at only ONE watt of RF on 14 MHz. The yagi does have around 10 dB of gain, but still, just one watt.

BTW I have taken three different radios, using two at a time, put them in the basement directly within reach of the RF antenna cables. Nothing connected except antenna and power. I will run each radio separately from a marine gel cell and see this effect. So it seems rather convincing nothing in "the station" itself is responsible.

The hunt and the education continue.