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Notes On SSB RFI In Homebrew SO2R Box

Chasing down the RFI caused by inserting all of the home-brewed SO2R components into the station set up was a useful hands-on experiment. Annoying, but certainly educational.  I verged on ordering the ARRL RFI tome, but now the thing is fixed, owning the reference seems less urgent.  Might be worth reading though…..probably quicker than re-inventing each technique personally.

To start, the shack layout resulted in a few less than ideal situations. Both radios are side-by-side, separated by about 300mm. The computer that logs and controls both radios is on a rolling cart normally kept close to the station desk. The computer was also being introduced into the audio chain as the DVK, and I was also working towards routing the mic audio through the sound card full time. The cramped space on the desk is further reduced by the antenna switching controls and an antenna tuner. One set of bandpass filters is built into a relatively large computer case, and that occupies much of the top shelf.

No RF problems were noticed on CW, but on SSB the audio was terrible, and I got many reports to that effect. Apologies to those who were exposed to it.

The unshielded plastic enclosures used may have contributed to the problem, but so far most of the trouble has been corrected by applying the normal RFI kludge, clamp on ferrites.  Shielded enclosures probably can’t make the problem any WORSE though.

The audio stream for the Yaesu FT-920 was relatively easy to clear up. Three or four turns of cable through one or two ferrites seemed to do the trick. The K2 was more difficult to tame, but it was also the furthest from the computer. I expect that the longer audio cables needed to reach the K2 made better antennas for picking up the stray RF. The cables used are a mix of CAT-5 and shielded RCA audio cables. The CAT-5 cables carried the mic audio, PTT and CW from the So2R box to either radio.

In addition to the ferrites, I also routed the audio from the computer through an isolation transformer. That step alone almost completely solved issues with the FT-920.  Using a separate power supply for the SO2R box resulted in acceptable audio –  better, but not BEST.

Two of the issues were a big surprise – and I only discovered them as RFI ingress points because I reached the point where I was determined to cover every base.  The separate power supply was an issue that was unexpected, but should not have been.  NT4D has made that point to me several times over the years.  Unfortunately good advice often falls on deaf ears.  I have heard the gospel now….

The one that I really dd not expect was that the PTT line might be an RFI source.  It became obvious this was a source when I methodically disconnected various cables on the K2 end of the chain.  Low and behold, once the PTT line was disconnected from the SO2R box – no more RFI.

I may now re-visit the entire chain, substituting a better quality cable to see if there is any difference or if fewer ferrites might be required.  It took three ferrites with about five turns on each to subdue the RFI ingress from the PTT line.

Here’s a summary of the mitigation steps taken to end the RFI issues.

Problem #1:
Power supply – One source of RFI problems was sharing the radio power supply with the SO2R box. Putting the SO2R box on a separate wall wart helped a lot.

Problem #2:
SO2R box cabling. Because of the shack layout, the SO2R cables are all pretty long. Putting ferrites on all cables more or less solved the problem with the FT-920. Still had RFI on the K2.

Problem #3:
Added .01 bypass caps across all of the relay coils and DC connections in all switch boxes.

Problem #4:
Didn’t really have ferrites on ALL of the cables. I really didn’t expect the PTT line to be an RFI issue, but solving the RFI problem on the K2 required ferrites on both ends of the PTT line(at the radio mike jack, and at the relay output in the SO2R box), as well as on the foot switch itself(at the SO2R box). While I was at it I added ferrites to the DC power cords too. It took three ferrites right at the mike jack on the radio end, so that may have been the real source of ingress.

Problem #5: Add isolation transformer in line with audio from the computer before going into the SO2R box.  Putting the transformer at the input to the SO2R box was an arbitrary choice – I don’t know if its location in the audio stream is of great consequence in mitigating the RFI.  Its placement there ensured only one transformer was required, since all audio is routed through that location, not being split until later in the SO2R box.  The location was convenient – perhaps not ideal.

Caveat: It is possible I also did something else inadvertently which helped solve the problem, but after a couple of weeks of head scratching and trial and error, I can say that UNdoing any one of #1 thru #5 will re-introduce some amount of RFI.

The problem with the PTT line really has me perplexed, but I guess it is in close proximity to the audio cable at the radio mike jack, so any RF on the PTT line is probably getting into the audio there.

After all of the trial and error, my impulse in the future will be to add ferrites on all control cables on both ends, and immediately after they enter/exit each device.

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