Illustration Paul Combs

Dear Radiohead,
Recently, my agency had interference on our radio system. For a few days, it was so bad we couldn’t hear our patrol units, but we could hear units from a department 200 miles away. Then the problem just disappeared. What gives?
Signed,
Technologically Confused

Dear Techy,
Your radio system must be a VHF system, operating between 150 and 170 MHz. During certain times of the year, when the atmosphere is just right, VHF is very susceptible to this type of interference.

VHF and UHF signals operate over “line of sight,” traveling in a straight line. If something gets in the path of that radio wave, it’s blocked and won’t travel any farther. Think of the VHF radio signal as a flashlight’s beam. As long as you can see the light (the radio wave), you’ll be able to use the light (i.e., hear the signal). If you try to shine the light over a great distance, it will fade into space in a straight line rather than curve around the Earth’s surface.

Under normal conditions, the line of sight for a radio wave is about the same as your ability to see to the horizon. From the Earth’s surface, this distance is about seven miles (12 km). If you climb a tower, the horizon is farther away and the range is greater.

I’m a ham operator and once used a VHF handheld in Kansas to speak to guys in Los Angeles. (Actually, I was on a United Airlines flight about 35,000 feet over Kansas.) We were talking through a repeater (a topic for another column) situated on top of mountains east of L.A. at about 8,000 feet. The combination of altitudes made it possible to speak over almost 1,000 miles (1600 km). I digress.

Under certain conditions, the atmosphere acts like a mirror for radio waves. When weather conditions are right and the atmospheric layers are at the right height, VHF radio signals reflect off these layers and bounce back to Earth well beyond the radio horizon.

Think of that flashlight again. If you aim it at a mirror (the reflecting layer of the atmosphere), the signal will bounce back in a direction based on the mirror’s angle. With VHF signals, that bouncing can extend the range by hundreds of miles. This phenomenon peaks in the spring.

Many years ago in Miami, one incident caused by this “skip” effect was the impetus for the county to completely replace its radio system. An officer was involved in a foot pursuit and radioed in that he was taking gunfire. Unfortunately, all the dispatcher could hear was a night shift supervisor in a small town in central Florida who was out on his nightly donut run. That police department used the same frequency as the Miami department, and the skip signal was so strong that it drowned out the local officer’s calls. (Note: This officer was not injured and captured the suspect.)

Other frequency bands are also susceptible to atmospheric conditions. VHF low band (from 30 to 50 MHz) is frequently affected by atmospheric and solar activity. There are few low-band systems out there these days, but the ones that are can get interference from users as few as 100 to a few thousand miles away. Its effects are so pronounced that local units can’t talk to each other, just to these very distant agencies. If you’ve ever had a CB set, you’ve probably heard long-distance communications caused by this same effect.

A more insidious form of interference is known as intermod or intermodulation interference. Intermod is a very technical form of interference—a whole book can be written on the topic. Generally, intermod’s effects result in users hearing strange, usually garbled, transmissions on their frequency. Sometimes you may hear parts of a conversation, music or paging tones.

To understand what causes intermod, think of two or more radio signals mixing together in space and showing up on your frequency. The frequencies of other radio transmitters “mix.” Mathematically, they add or subtract from each other. The result: The added or subtracted parts of these mixed signals produce another frequency, which may be the same frequency used by your agency. If that happens, you’ll hear it.

For technicians and engineers, the frustrating part of this phenomenon is that it’s always an intermittent problem and very difficult to analyze. They have to determine the actual frequencies of the offending stations. Then, they have to find out what’s causing them to mix and retransmit on your frequency. The problem can take weeks or months to diagnose. I’ve seen signals retransmitted by filters in transmitters designed to prevent interference. I’ve also seen intermod signals retransmitted by a rusty screw on a tower.

Editor’s note: The opinions expressed herein are those of the columnist and do not necessarily reflect the views of APCO International.

Tell it to Radiohead: Got a question for Radiohead? Wanna sound off about an issue? Put it in an e-mail to [email protected].

Originally published in Public Safety Communications, Vol. 77(6):56, June 2011.