David’s Fox-Hunting article reminded me of an interesting interference chase that baffled all who searched. The interference was affecting my employer’s equipment, a multipoint telemetry radio system for monitoring and control of remote facilities. This went on for about six weeks.

 

These were the first few things we noticed about the interference.

·         It only happened after dark and it only happened exactly licensed on our frequency, with no other frequencies affected.

·         It was a very periodic and brief pulse. Imagine someone sending only the letter “e” in CW at about 5 WPM, every night, all night.

·         The signature of the interfering signal was clear and unmistakable on the IFR1200’s CRT display, as was the endless  “pop-pop-pop-pop” audio response (RX set to FM narrow band.)

·         The number of buildings in this suburban area (near Houston Memorial & Beltway 8) caused so many reflections that use of a directional antenna for azimuth or triangulation was more confusing than helpful.

 

Several of our techs searched for weeks with no real result. At one point they thought it might be an alarm system in a bank. It wasn't, but  we did get a lot of calls from the bank’s security company. The bank wanted to know why a bunch of guys were hanging around their property, technically trespassing, well after dark, pointing those funny looking antennas into the bank windows. They were not very understanding. Go figure.

 

After all that, those techs weren’t comfortable, let alone interested, in pursuing the interference anymore. So, it was my turn. I drove around that part of town and tried the Yagi. It was useful at a distance, but the signal and its reflections were so strong within a mile of the source that the Yagi had me chasing my own tail instead of the fox’s! I began wondering about the mental health requirements for “fox hunts,” and concluded “fun” would not be the description I would choose. I was getting paid for this, yet nothing about it seemed fun. In retrospect, it was lots of fun. The frustration was probably because it wasn’t a game. My employer expected a result. I even located the bank in question and stopped briefly in their parking lot. My cell phone rang about 45 minutes later. Guess who? I explained, and it seemed to be the right thing to say.

 

So, I tossed the Yagi and set the spectrum analyzer in the floorboard in front of the passenger seat, with the CRT visible from the driver's seat. I found an old “rubber-ducky” antenna that looked like it had seen better days, hooked it up with some mismatched coax and poked the antenna under the passenger seat. I must confess, this was more “frustration” than “refined technique.” With the monitor in the floor, I was able to hear the monitor's speaker and therefore still drive safely in what had become a tenacious (OK, “hell-bent”) search-and-destroy mission. I started with a  five or six mile square by choosing perpendicular streets and making a lot of left turns. I would increase the squelch slightly to quiet the popping as the signal grew stronger with proximity. After a few more multiples-of-three left turns, the search area began to shrink. Eventually, I was within 300 yards of the source and circumnavigating a very large (and empty) church parking lot.

 

The strongest signal was on a remote edge of this parking lot, and appeared to originate behind a wall of thick woods and in the direction of a small, winding gravel road that disappeared into the vegetation. I could only see fifty feet or so down that road, which had heavy brush, trees and vines along both sides—somewhat like a gauntlet. I checked my watch: 11:30 PM. There was not a soul in sight or within earshot--that I knew of. I checked my cell phone. No signal. Great. I was alone and no one knew exactly where I was. At least I hoped so. It was quiet, except for the diesel engine on the service van I had checked out. Anyone within 200 yards into the woods would have heard the engine and known I was there. I gave it a lot of thought before heading down the road. My state of awareness became what I’d call “hyper-vigilant,” not that it would’ve made much difference. I figured with the horsepower of the diesel and the reverse gear, I could get back out quickly if I needed to. Probably. Maybe.

 

As I crept down the little gravel road I was able to resolve a chain link fence from the darkness. Then I could see a metal sign on the fence. Holy Cow! This was one of our own sites! I’d never been to this site and wasn’t familiar with it. Because it was friendly territory, and I “belonged” there, I felt slightly more comfortable. A few milliseconds later it occurred to me that “whose property it was” would not be important to anyone lurking in the woods, and the comfortable feeling evaporated, immediately. I looked around, and hopped out to unlock the gate. It must’ve been a world record, because very soon I found myself inside the locked van, inside the fence, with the locked gate behind the rear bumper.

 

A familiar mast and antenna configuration came into view and it was apparent this site was one that we monitor with the same radio telemetry affected by the interference. I unlocked the building to have a look at the radio. Predictably, it was on the same frequency as the interference we’d been seeking. And it was keying up inappropriately, at what appeared to be the same pattern I’d seen and heard on the service monitor. So I picked up the house phone to call dispatch. I could hear the pop-pop-pop-pop over the dial tone. OK, so something is amiss in the local equipment. I disabled the radio and deemed my task complete. Someone else could troubleshoot it in the morning. If nothing else we knew where the problem was and that we were interfering with our own network. Usually, we just had to find the source of interference. This time we had to fix the problem. No one else to blame.

 

As I left, I saw a small, blue plasma flash near the ground, in the dark. Then I saw it again. Then again. It appeared to be the same pattern as the interference! I switched the monitor back on and watched the CRT. Nothing. Disappointment. Then, oh yeah! I’d shut the radio down! Of course there would not be any interference. I got the radio enabled again and verified the connection between the two. There it was. But what was it? A short drive across the property revealed a varmint fence along the ground. It had been installed to combat the critter-failure-factor in the equipment, being in thick woods and all. It was shorting to ground, or trying to. Trees have this bad habit of dropping leaves and sticks, and the wind will often blow the debris to an inconvenient spot. This time that inconvenient spot caused an arc to ground and an EMI pulse that radiated into wiring, thus keying the transmitter erroneously and endlessly.  Why was it only at night? They only turn this fence on at night, when the raccoons and other mischievous, nocturnally-inclined come out to party!

 

Problem solved. I felt I had been more lucky than smart. But, the positive feedback from my co-workers the next morning was overwhelming--and it almost lasted halfway until the first coffee break.