An Unwelcome Guest: Self-RFI

The modern world is full of devices which generate RFI. Few people notice because hardly anyone listens to OTA (over the air) broadcasts these days, or when they do it's local rather than fringe reception. As hams we notice. On the bright side, we can now operate a kilowatt in a densely populated area and not have neighbours pounding on our doors during a major contest. How times have changed.

Unfortunately the RFI we suffer is often self-imposed. We use the same devices as every other household and we experience their negative effects. Even though we can turn our devices off, add filters or carefully screen our purchases, problems will persist, either because of what devices our family members have and use or because amateur radio equipment also generates RFI or is not immune to its effects. We can be our own worst enemies.

It is especially annoying when the guilty equipment is found in our shacks. Transceivers, computers, audio peripherals and more can be the source or victim of RFI. It occurs more often than you might expect. When I was preparing the station for CQ WW SSB there were several issues that I spent time tracking down and resolving; some are so intermittent that they might still be lurking to show up later.

In this article I'll mention what I'm calling "self-RFI" problems that I dealt with leading up to the contest. I had allowed some to fester since I was the only one affected. I didn't want guest ops to have to be taught the how and why of working around the issues. It should be my problem, not theirs.

Audio rectification is a common form of RFI. With two kilowatt transmitters active at the same time there are many opportunities for it to manifest. It happens even though, for the most part, the audio connections to our rigs are designed to block RF or bypass it to ground. Headsets have two audio paths, mic and headphones, at risk.

I keep a bag of ferrites near at hand for dealing with these types of issues. It takes less than a minute to wrap the cord around a ferrite RFI suppressor. When it happened with the Koss SB45 headset a few years ago, the problem was resolved in less than a minute. I kept it there ever since. It's a little heavy so I try to keep it on the desk rather than dangling in midair, where its weight can damage the cord and pull down on one ear. 

Thankfully its mate on the other radio and those brought by my guest ops didn't experience the same problem. But I made sure everyone knew where to find the bag of ferrites.

USB and video cables can radiate RFI due to the high data transfer speeds they operate at. They are similarly susceptible to a high RF environment such as our shacks -- if it can transmit it can receive. Good quality cables are fully shielded and grounded at both ends, and protected from common mode with ferrite cores at both ends. Quality is such that they don't always work or work well enough in a high RF environment.

This is the back of a small Windows PC running one of the stations. There are commercial USB and video cables with a built-in cylindrical ferrite core. Between them is the USB cable to the CAT connector on the transceiver with a ferrite that I installed. All have a matching ferrite at the other end. I added the cores to the CAT cable when I previously experienced intermittent drop outs.

I did the same for the rather long USB cable to my home brew Arduino-based antenna selector. Again, there were problems before I made the addition. 

You'll notice that I selected rectangular (split) cores designed for HF and cheap import cylindrical snap-on ferrites with unknown specifications. The latter work pretty well so I keep using them. Perhaps I got lucky since their uncertain source can make for a risky online purchase. 

The USB ferrites did not fully protect the antenna selector. Therefore I added another to the 12 VDC power cord the day before the contest. There were no RFI incidents all weekend. It can be difficult to say whether that was the work of the ferrite suppressors or just luck. Regardless, they will remain. Fully encasing the antenna selector would be even better but that would be difficult due to the multitude of connectors. Perfection is nice but not always necessary.

I saved the most curious case for last. Ever since acquiring the Acom 1200S I experienced occasional problems with the control screen and buttons due to RFI. It happened on most bands and antennas but not all. There was no obvious single cause of RF ingress. 

The first symptom was that the buttons would fail to work. However the amp continued to operate normally. Band changes had to be by RF detection (I don't use external band control at present) and there was no ability to switch the amp offline (OPR/STB). Although it's annoying there is no real need to do anything, just keep operating.

The second symptom, always preceded by the first, is that the screen would freeze. Although the amp continued to operate normally, there was no way to know the power output, reflected power, or PA temperature. That's a bigger problem. You can probably continue with the same band and antenna but eventually the lack of feedback becomes unacceptable.

The third symptom, always preceded by the first two, is that the screen goes blank. Now you're flying blind even though the amp continues to operate normally. That is, for a very liberal interprettation of "normally".

The cure is simple: cycle the power, since that button at least still works. That can take up to 30 seconds, which is okay most of the time but not while running in a contest. You'll lose contacts, and your run frequency.

There are only 4 connections to the amp: RF in, RF out, PTT in and mains power. Putting a ferrite core on the PTT line had no effect. The day before the contest I added two cylindrical and one rectangular core to the power cable. I included the cylindrical cores because only a few turns of the thick power cord fit onto the rectangular core.

I added all three at once rather than test each addition since I was running out of time to troubleshoot the problem. Luckily that did the job. There were no amp glitches due to RFI for the entire weekend. 

There was one more issue that caused a headache during the contest. Whether that is considered RFI is a judgment call. Let's see what you think.

With the 15 meter stack pointed to Europe and/or Asia, those antennas and the south pointing TH6 on the north tower point at each other -- they're 60 meters apart. Of course the TH6 isn't used on 15 meters when that happens since the other radio must be on 10 or 20 meters to use the TH6, however it's a tri-band yagi that will happily accept 15 meter signals. The 1200S sees a few watts of RF at the output connector and decides it's a fault. The protection circuit takes the amp offline.

That was really irritating. We could not use the TH6 when that happened. Had the TH6 been on the other radio (band selections reversed) there would have been no problem since the Drake L7 has no protection circuits and in any case doesn't care.

Unfortunately there is no good solution to the problem other than upgrading my station to use high power BPF (between the amp and antenna) rather than the 6-band switchable low power BPF that I use. That's an expensive solution to an intermittent problem. There are alternatives that I will have to explore. This new problem and other ongoing issues may spell the end of the line for the TH6 in my station.

I fully expect that I have not seen the last of self-RFI issues in my station.