Unknown source of local interference
edited October 2021 in Installation, Antennas and Interference
I had to move one of my Kiwi antennas to another point around the house and it picks up a new source of strong wideband interference, making reception in the range from 1350- 4900 now nearly impossible.
The signal is horizontally polarized ( it is not seen using a vertical loop ), shows peaks every 29 KHz and has a distinct left edge.
I include some screenshots in the hope that somebody does recognize the sort of equipment that can cause this. Having already powered down all electricity in my own house without success, I suspect the source is in the house of my less than cooperative neighbour.
I recently bought a cheap power supply for a PC, now I have a very similar muck on the waterfall when I turn it on from 0 to 900kHz, before connecting an additional strip with a filter + a few ferrite clamps, it was more up to 1700kHz. The same mess was generated by my old plasma tv up to 12MHz
Thanks ArturPL for your comment. The strange thing is that in this case the signal begins around 1350 KHz and it does not show up at lower frequencies. VLF thru MW below 1350 are rather clean.
My guess is that the source is sharp edges on a bipolar SMPS in some piece of equipment. These edges are likely producing a spectrum of 29 kHz terms but the coupling mechanism is really the question and there are many possibilities. It would be helpful to rule out common-mode mechanisms which might have significant shaping vs. frequency. That is, even if there is somewhat more energy in the lower frequency terms, those near the fundamental, typical sources and sinks required to push common mode current through a Kiwi have 1/lambda-squared taper to the real part of their impedances so shorter waves are preferred even though the stimulus may be stronger at longer wave. The combination of these effects may meet in the middle, as you are seeing.
A portable Kiwi, one completely independent from mains, networks and with short antenna connection to small antenna - a system with high CM immunity - and one with a rotatable antenna, either loop or dipole probe, would be ideal. This would let you eliminate CM as a coupling mechanism while it also offered you the ability to point a null in direction and polarization to better understand the (almost certainly near-field) coupling.
Knowing this you might be able to arrange your normal antenna such that it has a null in offending direction and polarity, is physically as far as possible from the source (falling at D^-N, N likely 4,6 or even higher) as well as to identify the particular source causing it.
IMO Killing the coupling mechanism(s) is better than quenching a particular source since it can apply to future troublemakers and not only the one you've identified already.
for HF (above MW) I find that a Belka DX works well for such sniffing
Thank you Glenn, for your analysis and suggestions.
Interesting, I had not considered CM coupling mechanisms that would result in this type of interference only showing up at higher frequencies. Most SMPS effects that I have seen in the past already started at VLF/LF.
This particular antenna is an active horizontal loop about 9 meter above a roof and performed quite well on HF when positioned 10 meters away horizontally. With the loop mounted vertically the surrounding nearby MW transmitters cause considerable IMD products on HF and rotating only gives marginal improvements.
I would have to lash up a portable Kiwi again to check in detail. This will have to wait for a break in the wet monsoon, hi
Best regards, Ben
@WA2ZKD Thanks, that is a good idea to use a small portable hf radio for sniffing on a wet and steep roof. Certainly quicker than trying to repack the Kiwi as a portable.