Fishing Net Direction Finder          


Ok, this is yet another example of necessity, combined with a lack of funds, being the incentive to create that which we needed.

It was during the 1978-1979 time frame, and I was a member of the Channel 13 group. During this timeframe, we had been experiencing a period of increased deliberate interference.  Much of it could be traced to a rival group on Channel 11, who didn't care for the bleed over that they got from us. It didn't help that the Channel 11 folks were clustered right in the path between the two biggest geographical clusters of our group, and that we normally pointed our beams at each other right through the Channel 11 group. This almost constant friction is what precipitated this audio clip. There were other incidents as well, and our guys were starting to get fed up with it. We decided that the solution to the problem was for us to actively track down those who threw carriers, or caused other interference to our channel, and make examples of them.  Since most jammers are cowardly in nature, our hope was that once the agitators realized that their anonymity was now at risk, the problem would hopefully go away, or at least become more manageable.

Sounds like a plan!  Now how do we facilitate this seemingly daunting task? Jimmy, myself, and two other guys had directional beam antennas at our base stations. Triangulation of a carrier thrower was fairly simple. Using the incredibly sensitive meter on the Hy-Gain 623, I could resolve a beam heading down to less than +/- 10 degrees. Then add in the directional headings from the other guys, and you could reduce the search field down to less than a 1/2 mile radius.  It was then only a matter of transferring to the car and finishing the job. Usually it took longer to drive to the area, then the time spent actually searching.  It was quick and efficient in most cases, but there were sometimes problems. One of the biggest problems occurred when the jammer was found to be within the town of Norristown itself, the inner sections of which were made up primarily of row homes.  In the "fad" days of CB radio, there was usually more than one CB antenna on a typical street block.  Sometimes they were as close as next door in a row house (Talk about bleed over!).  So just finding the street and then looking for a lone antenna, was often not enough to positively identify a jammer.  We had to figure out which antenna, out of several potential candidates, belonged to the jammer.  At the time, I had a commercially made DF (Direction Finding) loop antenna to help pinpoint the exact location of a signal.  The sharp null of the DF loop could often resolve the difference, even from a signal as close as two houses.  This proved to be an effective tool in our campaign against jammers.  It was only natural then, that the other guys on the channel would want a similar device.  But no one wanted to pay the $40 that it cost to mail order them back then. We were after all, still young and not earning gobs of money yet.

I decided to try making one.  A DF loop is a fairly simple and basic device.  A piece of aluminum tubing, bent into an approximately 10" diameter loop, with an impedance matching capacitor mounted at the top, and a shunt fed coax connection near the bottom of the loop. This assembly is then connected to a mast which can be rotated 360 degrees. Determining direction was accomplished by turning the loop until the lowest signal (called a null) is obtained, which occurs when the loop is broadside to the direction of the signal. The design was fairly straight forward, but we needed materials to make the 2 loops. While looking around in the garage for suitable material, I happened upon an old fishing net. The "loop" was roughly the same size as the commercial DF loop, but is was not perfectly circular. I thought "What the heck", and took it back to the bench. I cut a section out of the "top" of the loop to mount the tuning capacitor. I attached the coaxial shunt feed, using a hose clamp for the center contact of the coax cable, to one of the side rails at roughly the same point as on the commercial unit. Using a low power walkie-talkie, I tuned the feed point and tuning capacitor for best VSWR.  I then tested the directivity of the "net", and found the null to be nearly as sharp as that found on the commercial unit. This "fishnet DF loop" also had the advantage that it had an integral handle which allowed it to be used as a portable, instead of being restricted to vehicle mounting.  It looked like the fishnet loop was a success.  One of the other guys had a similar fishing net at home, and we converted it as well with similar results.

Now, with our network of "DF hunters" in place, we became fairly successful at quickly identifying jammers.  Usually, once their identity became known, they tended to stop. It's unfortunate that this would not be the case years later, when jammers would become more sociopathic and belligerent.