The Prime Directive - Radio World

The Prime Directive

Tom Osenkowsky wrote a while back in Radio World about interference. Maybe somebody could start a Web page about this topic.
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Tom Osenkowsky wrote a while back in Radio World about interference. Maybe somebody could start a Web page about this topic. I can give several examples.

About 1960, interference was reported to reception in ambulances in the downtown Rochester, N.Y., area. The interference was determined to be a combination of the signals of two Rochester FM stations and a paging service. The two FM frequencies were 4 MHz apart, and the paging service was 4 MHz from the ambulance frequency.

One day somebody noticed that the amount of interference seemed to change with the position of a weather vane belonging to the city of Rochester’s smoke abatement program, located near one of the FM antennas. Eventually the weather vane blew down, and the interference disappeared.

In 1984, when I moved to Laredo, Texas, I noticed several spurs in the FM band. One day the interference had disappeared. I called the chief engineer of one of the TV stations and asked if anything had changed.

“They are working on the roof.”

“What kind of roof is it?”

“Metal.”

Apparently there had been a corroded contact between metal pieces on the roof. Problem solved.

More recently, interference was noticed to reception of a communication service near Yuma, Ariz. It appeared to be caused by a combination of an FM signal and a TV signal. Somebody noticed that shaking one of the FM guy wires changed the level of interference. That guy wire was near the FM antenna, but this was not obvious from the ground. Moving the antenna to a different leg on the tower eliminated the interference.

I know of a case in south Texas in which a spur appears in the FM band caused by two FM stations on the same tower. The problem is probably something on the tower or something in or near the transmitter building. Any volunteers to find this one?

Stan Swanson
KYRM(FM)
Laredo, Texas

Tom Osenkowsky replies:

Stan, thank you for your stories. Interference is many times caused by passive objects such as two or more pieces of metal, fences, rain gutters and everyday objects that are illuminated with sufficient RF from one or more sources.

In your last example, it can also be caused by RF ingress from one station’s antenna to the final amplifier of another station causing unwanted mixing products. In such cases, filtering is required to eliminate the problem.

It would certainly make good reading, a web page or book dedicated to such tales. There would be many and each solution is often unique onto itself.

As in many cases, fixing the problem is simple. Finding it is the hard part! A spectrum analyzer and patience are valuable tools.

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