Broadcast Radio Stations and Telephone RFI

by Pete Krieger

An irate homeowner tells the engineer, "Your frequency is turned up too high. I'm hearing you on my telephone." A motel chain builds a 120-unit facility five hundred feet from a tower site and every phone is inoperable. The fax/modem at a nearby business isn't working; they're getting a radio station on the line. Does anything sound familiar here? 

While telephone RFI dates back to the early days of broadcasting, it has become a larger issue in recent years. Poorly designed telephones continue to flood the consumer market, creating interference where it didn't previously exist. Questionable installation techniques and poor quality wire often go into new construction or remodeling, defeating the interference protection designed-in to the twisted pair balanced circuit. With age and exposure to the elements, telephone wiring inside buildings and occasionally on telephone poles can deteriorate and function as radio receivers. These are typical of the factors that can heighten a station's interference profile, even in the absence of changes at a station's physical plant.

In the FCC's own words, telephone interference is not the result of a rules violation by the transmitter operator, "Telephone interference generally happens because telephones are not designed to operate near radio transmitters and the telephone improperly operates as a radio receiver." While the legal language is crystal clear, station managements realize the problem contains public relations ramifications. Bill Glasser, director of engineering at WHBC AM&FM in Canton, Ohio says management does not take a hard-line legal approach. "That's all you need is people screaming and threatening to sue you when your radio station is coming through on their telephone. Ninety percent of the problem is [solved] by being nice to the people and saying we want to take care of it." When the case warrants hands-on action, he goes to homes to install filters on phones and occasionally in the wiring at the service entry. "We do it because we want to be good neighbors," says Glasser.

WKOX in Framingham, MA needed city approval for a planned antenna reconfiguration to triplex three signals including 40 kW WMEX. According to Paul Andrews, chief engineer, "When we approached the city for a special permit, there may have been seventy-five people in attendance and the atmosphere was hostile. They extracted from the station an agreement to solve interference complaints. And of course 95 to 99 percent ... were telephones." Prior to this the station had received only occasional complaints, he recalls. "This was quite a shock. I didn't realize there were that many people who had problems with their telephones." In his role as interference guru and good will ambassador for the station, Andrews has solved problems at more than two hundred homes and businesses. "I go through a lot of filters," he says.

As for station resources to treat interference complaints, how much is enough? Broadcasters need to make that judgement based on their own circumstances. "We never suggest a station get into the practice of making purchases to satisfy a problem other than possibly filters or inexpensive trials," says Whit Adamson, President of the Tennessee Association of Broadcasters. He manages a cooperative effort to mitigate station-to-listener interference problems. "Most stations know to reference the complainant to us as a first line of defense." Should a complaint go direct to FCC, Adamson's staff acts as mediator with the blessing of the Atlanta District FCC office. "We talk with station managers to ensure that they are right on top of a problem or they'll get with their engineer to get a complete check-up and get back to us." He says sometimes the local Radio Shack or electronics store is brought into the equation. "Once everyone is convinced they are where they are supposed to be, then we can go back to the complainant. This relationship has worked great over the years to the satisfaction of the public, the Commission and the licensees." 

Telephone interference is sometimes more of a problem for the station than it is for neighbors. At WNQM / WWCR in Nashville the studios and offices are at the transmitter site consisting of four 100 kW shortwaves and one 50 kW AM. George McClintock, station owner, says telephone interference from shortwaves causes problems in the extreme, "The phone line has so much RF coming in on it, I put filters at the input of the telephone unit. It is made of plastic and of course there is no shielding and the RF is just slam-dunking on it. I put a [metal] box around it with a front door so you can open it. It seems to help." He uses rf shielding extensively throughout the plant. The floor, the ceiling and the walls are all screen mesh copper. Telephone wiring is distributed in conduit separate from station audio, control and power wiring. RF chokes are used extensively, "All the filtering in the world doesn't help you until you've found out how it is entering. That's the point where you eliminate it. You use filters up close to the phone. If that doesn't do the trick, I look at the spring cord. With the AM, I don't think I've had any problem I can recall caused by the spring cord. But at some point it runs across part of the wavelength of shortwave and in comes RF."

1.		Install a filter at the line input of an affected telephone. This is the solution in most cases. If you achieve a noticeable improvement but some interference remains, add another filter. If necessary, install a hard-wired filter on the phone's circuit where it originates at the service entry.
2.		If interference continues after trying filters, simplify the problem by unplugging all phone equipment from the jacks. Don't overlook spike suppressors and modems. One at a time, reconnect and listen. This can identify one or more defective or RF-prone devices that are the source of interference.
3.		Interference can originate at the service entry where poor connections and/or corrosion may set up rectification. In these cases, rectified audio (rather than RF current) is distributed through the wiring and tends to be of equal volume on all phones. Clean and tighten connections.
4.		Interference can enter from the phone company drop. Confirm this by disconnecting the customer side of the demark and monitoring the drop. If interference is present, they are responsible, but getting cooperation can be frustrating. Try to develop a relationship with someone on the technical side who can help you cut through the bureaucracy. WKOX's Andrews said that demonstrating ability to correctly diagnose problems yielded the direct number for the repair foreman.
5.		With business telephone systems, the best plan of attack is to filter incoming lines ahead of the switch. If required, add filters on the switch output and possibly at individual phones. Where telephone headsets are used, a filter on the spring cord may be needed.
6.		An exceptionally long run of wire in the telephone circuit can intercept massive levels of RF current. If you find that filters at the phone end and the service entry aren't enough, it may be necessary to physically cut the wire at one or more points and insert hard-wired filters to electrically break-up resonances.
7.		As an alternative to the above scenario involving a long span of wire, I prefer the following. Run a new dedicated circuit directly from the service entry. See if you can temporarily run the wire on the floor and cut it intentionally long by 50 or 60 feet. Use some of the excess to wind one or more decoupling baluns, starting at mid-point. The baluns consist of 10 to15 feet wrapped neatly at a diameter of six inches. Activate the circuit. If interference traces remain, adjust the number of turns and/or try more than one balun. Once you have the fix, run your wiring as necessary through the walls and ceiling. Every time I use this method, the interference is eliminated. Conduit is not required or recommended.
8.		If you are doing new construction or expanding your telephone system, insist on the highest possible quality wire. Category 3 or better is my choice due to its very aggressive, interference-fighting twist on both the individual pairs and the group. I find that shielded wire has no helpful effect on common-mode RF; it only changes the velocity factor.
9.		One possible interference entry point is the AC power line. Effective RF suppression devices are available from manufacturers such as Trip Lite.
10.		If you expect to be doing diagnostic work on a regular basis, invest in a good quality telephone monitoring test set. Use a Dracon TS21 or similar unit to achieve convenient access to telephone circuits and make your evaluations with an instrument that is practically RF-immune.

Pete Krieger is a telephone interference specialist and author of the telephone chapter in the RFI Book published by American Radio Relay League, Newington, CT. He retired as a customer systems engineer for Lucent Technologies in 1999 and now devotes full time as owner and chief engineer of K-COM Telephone Interference Filters, Randolph, Ohio. He can be reached at
1-877-242-4540.