Heartwarming interference stories
Well, here it is 2002 — my 10th year of writing this column. How time flies when we’re having fun. To start the year, I am passing along a couple of stories about interference that came from readers. Stories such as these are priceless because much can be learned from the experience of others.
The first story was sent to us by David Parcigneau, a radio inspector with the DOC in Montreal. David wrote:
“We just resolved a good interference case from which I was reminded of a few basic principles. The police of Montreal called us saying they had interference on two of their five 800MHz trunking system channels. We went to the site and, at first, didn’t see anything. However, upon closer checking we did measure an average of 10dB of degradation in receiver performance on the two affected channels. Making sure that everything was well filtered, we installed an amplifier in front of our spectrum analyzer and saw a strange signal. It was weak and looked like frequency bursts.
“We then set out in an attempt to locate the origin of the interfering signal. We finally made progress on the corner of two streets that were in line with the affected police system located about one mile from the affected trunking site. We were able to receive it faintly in the truck at about 10dB above the noise level. It sounded like white noise. The key was to go with intensity using a whip antenna on the roof of the truck. We went back and forth pinpointing an area where the signal was the strongest.
“Then, using a portable receiver with a built-in signal strength meter, we went for a walk in the various buildings where we suspected the interfering signal was emanating. No success. We received something, but the signal level just wouldn’t increase. In other words, we weren’t getting any closer.
“We fetched a 10dB-gain yagi antenna from the truck and were able to get a bearing with it. However, we dismissed the indications it gave us because we just didn’t pick up any signal at ground level at the location indicated by the yagi antenna. We thought the indication was just a reflection because this would be common in this frequency band — especially in a big city where the streets become waveguides because of the buildings.
“After walking through several buildings with no luck, we decided that we had nothing to lose by going in the direction indicated by the yagi. We went to the roof of the building indicated by the yagi and the signal ‘went through the roof.’
“Eureka! We found it. The culprit was a cell enhancer for the interior of the building. It appeared that the amplifier was oscillating and generating spurious signals from 807MHz to 828MHz with bursts about every 2MHz.”
According to David, the moral of the story is: “Don’t take your theories for reality, even if they seem right. We thought it (the direction first indicated by the yagi) was from a reflection, but it wasn’t. Secondly, without blindly trusting it, trust your equipment. It might be right. We didn’t receive anything at ground level. Why? Because we were in the null of the cell enhancer’s yagi antenna. And at the point where we started, we were in the main lobe on the street corner a little farther away.”
David further opined: “While there is still nothing to substitute for an experienced technician/engineer, a little lesson in humility is a good reminder of the basics.”
Got any motor oil?
Bob Swinney once served as an FTR with Motorola. When he related this story to me via email, he was an RF engineer with Andrew. This story comes from Bob’s days as a Motorola FTR. Bob wrote:
“It was in the early 70s, not long after UHF had become the new ‘frontier’ of land mobile radio. Motorola was king and its MSY station was the unchallenged leader of the pack.
“Heavily populated antenna sites and accompanying IM interference were becoming the norm. Some UHF stations came with a (two-isolator) circulator package, and I believe Motorola provided them free of charge to selected high-IM sites. Many of the circulator packages were not installed by the shops, particularly those that were not legitimate MSSs. It’s hard to imagine now — a two-way shop that couldn’t appreciate the absolute necessity of circulators.
“Intermodulation interference can take some strange turns on large antenna sites. In the early “UHF” days, new FTRs were regaled with the story of a classic IM chase — the legendary Russ Larson. It seems that Russ had gone to Kansas City, MO, to help some poor FTR with an impossible IM problem on the largest Motorola site in town. The FTR had completely given up. All the UHF stations were equipped with circulators, yet the strong IM came and went — mostly came.
“It had to be an external mix, but where? It was quite strong and involved stations on the site. But all parties to the mix were protected. What’s a poor FTR to do?
“Russ Larson was an engineer in Dallas at the time and had been dispatched to the Kansas City problem site. Russ arrives on scene and reviews all the procedures that had been followed, to no avail, toward locating the terrible IM problem. Russ strolls over the rooftop for awhile, takes a puff on his pipe and says to the FTR: ‘Got any motor oil?’ The FTR answers: ‘Well, yes. I carry a spare quart in the company car, but why do you want motor oil?’ Russ simply replies: ‘Go get some, I think it will help.’
“When the motor oil arrives, Russ takes the can and climbs up the back of a large metal sign, maybe some 10 feet to 12 feet high and situated on the edge of the building. He proceeds to pour oil down into the sign and almost if by a miracle, the IM goes away. Russ had found the classic rusty-spot: the nonlinear IM mixing point that we all have heard of but seldom ever see in the real world of RF.”
Thanks to David Parcigneau and Bob Swinney for sharing their stories. When you’re out there looking for that interference source, keep an open mind. In some cases, finding the culprit is as much art as science. Experience doesn’t hurt, either.
Until next time — stay tuned!
Contributing editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. His email address is [email protected].
More useful sites:
Duane Vosburg recommends checking out www.hallelectronics.com. Click on “GE Tech Info” and then click on “Links.” The “Miscellaneous Links” provide good information, as well. Thanks for the tip, Duane.
A good site for lots of Motorola-related information is www.batlabs.com/index.html.
Another Web site that I recommend is www.decibelproducts.com. From the home page, click on “Support.” You will then find options for “db” tech notes, installation, software tools, FAQs and glossary.