RFI is Bad

de KM5FA

Club members have begun to learn a new HF radio, the Kenwood TS-850SAT donated recently to the club station by very generous alumni and faculty members. It is in many ways a superior radio to the Kenwood TS-830S that has served the club well for the past 12 years. In one respect, however, the TS-850SAT is less capable than the TS-830S. It doesn't handle RF very well when used to operate single sideband phone. In all the time that I used the TS-830S, I never had a problem with RF getting into the sideband audio. On the new radio, it's a serious problem. Hopefully, the problem has now been fixed.

The RFI problem was first noticed when the packet cable was left plugged into the rear panel of the radio. Trying to communicate with SSB phone on high power, RF would get into the packet cable. The TS-850SAT audio input connectors probably have some sort of RF choke on them, but the engineers at Kenwood skimped on this part of the design. The RF would enter the mixing stages of the radio along with the intended transmit audio signal, and the resulting modulation resulted in something that could be described as anywhere from a little bit wrong to completely unintelligable trash, depending on the band and where in the band you were transmitting. This was not a problem that the TS-830S ever suffered, probably because RF chokes in that radio handled the problem all by themselves. Unplugging the packet cable from the back of the TS850SAT solved the RFI problem - but only temporarily.

The RFI problem was next noticed when I tried to wire up an interface cable for the W9XT contest card digital voice keyer (DVK) that we have. The W9XT card sits in the computer and can be used from inside contest logging programs like TR. When the computer is turned off, relays on the card make the interface cable act essentially as an extension cable. When recording or playing back a message, the card makes sure that the microphone $\rightarrow$card and card $\rightarrow$radio lines are switched in the appropriate manner. Even with the computer off, just having the cable in line caused the RFI problem on SSB phone.

The W9XT Contest Card.

I spent at least a couple dozen hours over several weeks trying all sorts of solutions to the problem. I focused on fixing the DVK cable first, because I knew it would be useful to have in the Collegiate Championship, and because I knew that the solution for the DVK cable would likely work for the packet interface cable as well. Much of the experimentation involved using split ferrite ``beads'' (hollow rectangles, really.) Theory says that if you wrap the cable through the bead a few times, it acts as an RF choke, preventing RF from travelling further down the cable. One of the things I learned in all of this experimentation, is that there are different mixes of the ferrite material used for these beads, and that some work better at certain frequencies than do other mixes.

It turns out that the beads I was using (from Radio Shack) are type 43 mix. This mix works best at frequencies from 30 to 400 MHz. Of course, the RF that was causing the problem was all below 30 MHz. The Radio Shack beads worked better at stopping the RFI at 28 MHz than they did at 3.8 MHz, which makes sense, but they never solved the problem completely, and at 3.8MHz, they didn't appear to have any effect whatsoever. At one point, I had six beads in a row, and it still didn't have any effect.

Robert K5PI stopped by one afternoon and brought a large ferrite ``donut'' with him. It was 2.5'' OD, 1.5'' ID, and 0.5'' thick, and was probably of type 77 mix. This mix is most effective from 2 to 30 MHz. We wrapped the ends of the DVK cable around the donut four or five times, and plugged it in. There was no RFI whatsoever, even at 3.8 MHz high power. The solution has probably less to do with the size of the bead or donut, but more with the type of mix. Type 77 mix is the key.