There is one interest I had never thought about before until one of my ham friends asked me one day, "Want to help us catch a spy?" What are you talking about, I asked, and what does this have to do with ham radio. As it turns out, this activity is probably pretty rare among hams, but it combines the activities of radio direction finding (RDF), antenna design, CW, home brew receiver and antenna building, and maybe even public service, of a sort (if they catch him).
Hap and Frank (names are changed to protect the innocent?) have been following a station operating within a designated ham band that sends coded groups for about a half hour the same time once a week. Of course their immediate concern was this was obviously not an authorized amateur radio operator because the station never identifies with any call letters.
Coded groups are simply groups of alpha-numeric characters that are usually five characters in length. In the simplest of terms, if you could break the code and knew what each number or letter stood for, you would know what the message said. Hap and Frank aren't trying to break the code, which would be quite difficult for the untrained, they just want to locate the guy through direction finding techniques such as triangulation. By rotating or otherwise moving the antenna(s) to find a null (lowest signal reception) you then know that the signal is coming from one of two points plus and minus 90 degrees from their compass reading. I find this whole activity fascinating in that is reminiscent in some ways to what I did forty-some years ago during my military service with the United States Army Security Agency (USASA).
They are experimenting with different types of antennas to get compass readings that are consistent from week to week. The first time I visited their site I was Hap's helper -- Frank couldn't make it. That morning we were using a huge Adcock antenna they had made out of wood and plastic paired with a commercial amateur radio receiver. I think the Adcock probably stood twenty or twenty-five feet in the air. It was transported to the site each week on a car-top carrier and probably consisted of 20 or 30 pieces all together. It took maybe a half hour for them to set this thing up each week, if you knew what you were doing; a lot longer if you didn't. By rotating the Adcock slowly by hand and listening to the signal rise and fall they hoped to get a good reading. The morning I visited we set up in a thick morning fog that lasted throughout the spy's operating schedule. We couldn't get an agreed upon null. Was it the fog? Was it the QSB? Was the antenna not set up properly? Were we not far enough away from from metal objects like the cars in the parking lot? These are some of the things that DFers have to contend with.
The second time I visited they were trying a two vertical array coupled to a home brew crystal controlled receiver with a signal strength meter mounted to the top. This time I was only an observer. Each antenna was mounted on a 2 foot by 2 foot piece of plywood with a rope attached. One member of the team would pull one of the verticals across the grass in a rough arc from the stationary antenna until a null was reached. Then a compass reading was taken. After several attempts at this, each time trying to mentally eliminate the effect of the QSB that was happening on the signal, they had several different but close readings. Later at home Hap compared the latest readings to ones from previous attempts and realized that they could now statistically arrive at an average which led to a true bearing to the spy. Success!
Now that they have a bearing they believe is reliable, they plan to move to another location one or two hundred miles away to take more bearings. This is phase two and may take several attempts to also arrive at a reliable bearing. If they then draw these bearings on a map, the point that they cross is where the spy is located. Of course that's not the end because that will only tell them, hopefully, what city he's in. Then it's on to phase three which might be going to that city and driving around with car-top mounted DF equipment.
Well, I'll have to leave the rest of this story for some time in the future. Needless to say, Finding the spy will be a little bit of a let down, as their project will be over. In this case the joy is in the journey. They tell me that once they locate the address of the transmitter, they will be turning over all their evidence to the proper authorities.
Until next time...
'73 OM
DE WA2KKG
Thanks to K6BMG for the nice graphic of an Adcock design.
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