Winter 2006 Issue

“Sherlock” in the XP Age

The author first introduced his system of capturing the turn-on and
turn-off characteristics of a transmitter in the September 1996 issue of
CQ VHF. Now, nine years later, operating systems have changed
considerably. As a result, WA9BVS has developed a new system for
detecting these characteristics.

By Malcolm C. Mallette, WA9BVS


Amateur radio repeaters are in a new age. The internet has made it possible to link repeaters across the world. Ever smaller HTs appear with more features than you can learn to use. From coordinating the choice of a restaurant for a group of hams’ Friday lunch to the disaster operations of the Red Cross, VHF and UHF repeaters are very important. They must not be shut down by jammers.

Unintentional Jamming
of a Repeater Input

A common problem is a transmitter unintentionally being left on. A microphone is stuffed down between the seats in a car and the microphone button is pushed in. An HT has an electronic failure that results in a signal being transmitted on the repeater input. Those problems generally are solved by direction finding, which is simple if the offending transmitter on the repeater input stays on for long periods.

Intentional Jamming

Another problem is more difficult. That is the intentional jammer who kerchunks the repeater continually, uses obscene language, or otherwise interferes with the operation of the repeater, all without identifying. Jammers make life miserable with their unidentified, sometimes obscene, transmissions.

The easiest way to stop a jammer is to ignore him. Do not mention or threaten a jammer on the air. He wants to hear how much you hate him. Keep your mouth shut. Do not mention that a direction-finding team is after him or any other means is being used to locate him.

DFing is one way to find the jammer. It is more difficult than DFing a stuck transmitter, as the jammer usually does not transmit all the time. The jammer may even be in a moving car.

 

The 1996 “Sherlock” System and Transmitter “Turn-On” and “Turn-Off” Fingerprints

There is a different approach to finding the identity of a jammer. In the September 1996 issue of CQ VHF the author presented “Sherlock,” a hardware-software system that captured the turn-on and turn-off characteristics of a transmitter. Modern 2-meter and 440-MHz rigs have hundreds of channels available, with a microprocessor-controlled PLL (phase-locked loop) determining the transmit frequency.

Figure 1. The Virtins Oscilloscope display showing 10 seconds of reception. The first three seconds are noise, followed by a few seconds of a received transmission, then noise again. (All screen shots courtesy of WA9BVS)

 

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