ARRLWeb: If You Hear Hoof Beats, Don’t Assume They’re Horses

As hams, we learn that if X happens, then Y may not always be the solution. When interference strikes a repeater, the repeater association’s members take charge to discover just what is causing the problem by looking at the situation from a few new angles.

Joe Nunemaker, KD3VR (seated), works on the faulty repeater component via a telephone conversation with another club techrep while I look on. Roy Ashlin Jr, N3NGA, is partially visible on the left. [Courtesy Michael Gregory, KB3IYQ]

The repeater, fully repaired and back in operation without interference. [Courtesy C. T. David, K3GXF]

Joe Nunmaker, KD3VR (ARRL Atlantic Division’s 1997 Ham of the Year), and Jim Cross, WI3N (ARRL MDC Section Manger), on the repeater’s 4 element vertical Cellwave dipole antenna. [Courtesy Joe Nunmaker, KD3VR]

If you hear hoof beats, don’t assume they’re horses. My doctor said this to me once; he himself had heard it from one of his professors at medical school. It’s not a bad analogy for a ham story that I would like to share with you.

Our repeater association, the Green Mountain Repeater Association, operates two local repeaters. One of them was being held up by an unknown signal one evening. It was constant, somewhat noisy and tended to drop out for a second or two every few minutes. The signal coming into the repeater’s receiver showed half scale from full quieting. Having had our share of interference over the years from faulty commercial and amateur transmitters of all types, it was natural to assume this was another faulty transmitter or spur. The odd part was that not one of our 250 members, even those with large 2 meter beams at 65 feet, could hear the incoming signal.

As I tuned through the 2 meter band, I heard a similar steady carrier signal on 145.250 MHz. In one or two previous interferences cases, finding a carrier away from the repeater’s input frequency turned out to be the root cause of a spur coming into our repeater. The signal’s characteristics seemed to match that of the one holding up the repeater. We took some beam headings, but we received a number of different and conflicting bearings. Naturally so!

Why? Because 145.250 MHz is one of many problem channels for hams. In this case, CATV video signals were leaking through faulty cable connectors. As members drove around town, they could find the signal in many areas, but none were coming through on the repeater’s input frequency. It was time to rethink the problem. Video cable leakage it was not.

The “key” here was that no one could hear the signal on the repeater’s input frequency; in retrospect, we probably should have thought of that sooner. Some of us made a second visit to the repeater site. We disconnected the repeater’s receiver cable to the antenna and reconnected it to a handheld radio. The interfering signal disappeared! We then reconnected it back to the repeater and the signal returned.

We removed the power amplifier’s cover and made a few gentle taps to several circuit board components. The first few taps didn’t do anything, but when we tapped a component at the top of the board, the signal stopped. The “hoof beats” were definitely not horses.

As it turned out, faulty components created an RF signal or spur that fed into the repeater’s receiver. This created a false impression that an outside signal was the source of the interference. Live and learn.

Repeaters have, and continue to be a boon to VHF FM communications, enabling hams to communicate mobile-to-mobile over long distances. That’s the upside. On the downside are the interference problems that operators, licensees and control operators have to put up with, and eventually correct if it is continuous and extended.

In larger cities and towns, I suspect the problem is more prevalent than in smaller ones. Our system has operated and maintained two 2 meter repeaters in the Washington, DC metropolitan area since 1971. We have had more than our share of interference. The computer frequency printouts of all transmitters in the nation’s Capital is staggering.

While other local repeaters have taken the route of employing Continuous Tone-Coded Squelch System (CTCSS), our repeater association has resisted the temptation, although not without serious consideration. The mindset, while arguable, is that CTCSS masks the source, permitting it to go uncorrected. (This rationale does not apply to repeaters using the same frequency where users can bring up both repeaters.) Additionally, if the interference has noise to it, as opposed to only RF, once the CTCSS opens the repeater, the noise is there. If it is just RF, it could deteriorate weak signals from handheld radios and mobiles, depending on the strength of the interfering signal.

I will be the first to agree that CTCSS reduces a user’s, and especially a control operator’s, level of aggravation. And, for example, if 80 percent of the interference is only RF related, then the CTCSS can be a very effective deterrent in masking the interference, still permitting the repeater owners to track down the interference. But do they? My guess is that they do not unless there is noise associated with the RF. Simply said, “out of sound, out of mind,” and the dirty transmitter or associated antenna continues to operate undetected and uncorrected.

Fortunately, our association has been 100 percent effective in tracking down every single source of interference that has found its way into the input of its repeaters over the past 30 years. In each and every instance, the owner amicably corrected the problem, in addition to establishing a rapport between the respective technical representatives. As a result, the repeater association’s technical representatives have gained a lot of respect, not only from other techrep repeater peers, but from those FCC representatives who unofficially were made aware of some of the many problems. It also cemented a lot of excellent relationships between hams and commercial communication companies.

The day may come when interference becomes rampant enough that our repeaters may go the CTCSS course. Until then, it is the association’s policy to continue to track, isolate and notify owners of defective transmitters. It’s the technically correct thing to do.

Murray Green, K3BEQ, has been licensed for 53 years and is one of the founders and control operators of the Green Mountain Repeater Association (GMRA), operating in the Washington, DC area. He wishes to express appreciation to Joe Nunemaker, KD3VR; Paul Freirich, W3HFA; Michael Gregory, KB3IYQ, and GMRA members who helped to correct the problem described in this article.