Interference: Define and Conquer
Broadcasters encounter interference in two forms: that which someone may accuse the broadcaster of causing, and that which the broadcaster experiences.
The most familiar type of the former is a consumer complaint that the broadcaster’s signal is being received on a device not intended to do so. Typical devices are telephones, home sound systems and radio receivers.
Broadcasters may face complaints from other broadcasters, commercial and government entities. A broadcaster also may receive interference on its RPU, STL, EAS and off-air monitoring receivers.
Locating the source of interference often is a challenge, especially if the problem is intermittent.
Curing interference requires a combination of art and science; the same solution may not be effective in similar cases. A typical arsenal of components used to remedy consumer gear interference is shown in Fig 1. Ferrite devices, inductors, capacitors, AC power line filters, DSL filters and ground wire may be employed to reduce or eliminate unwanted reception.
Fig. 1: Curing interference is a combination of art and science; the same solution may not be effective in similar cases. A typical arsenal of components used to remedy consumer gear interference is shown.
A common consumer complaint is reception of an AM station on a stereo or public address amplifier. This can also occur in a house system where the studio and transmitter are collocated.
Typical cures involve placing a .1 µf capacitor across the speaker output terminals on the amplifier and/or the speaker itself. An inductor in series with the speaker output terminal may help, as can twisted (not zip cord type) speaker cable. Try wrapping several turns of the speaker cable through a toroidal ferrite choke.
With ferrite products, use the proper suppression material for the offending frequency band. Fair-Rite #31 and 73 are effective at AM frequencies, whereas #43 and 44 are effective at FM frequencies; #61 is effective at UHF frequencies (see sidebar for Fair-Rite link).
RF entering through the audio input terminal of an audio amplifier may be cured by placing a small capacitor across the hot lead(s) to ground, or across the balanced hot pins. A small inductor may also be used. For VHF/UHF frequencies a ferrite choke may be placed around the input terminal(s) inside the unit. A small value capacitor placed across a tape head can remedy FM/VHF/UHF interference. Take care to select the lowest value of capacitor possible to eliminate the RF while still preserving the audio frequency response.
Unwanted reception of RF in telephones often can be treated effectively by installing a DSL filter in series with the telephone line. Determine whether RFI is heard on the incoming telephone line; just plug in a Western Electric 500 or 2500 set into the demark jack with the customer’s equipment disconnected. If the incoming line has RFI on it, the matter should be referred to the telephone service provider. If the line is free of RFI, the issue lies with the customer’s equipment. If a DSL filter does not solve the problem, refer the complainant to the telephone manufacturer’s website for technical support.
Many telephones lack RFI protection; however, the manufacturer may offer external filters. Third-party vendors like K-Y Filter Co. also offer such filters.
Receivers in close proximity to transmitter sites may experience front-end overload with the resulting inability to receive other stations. A notch filter from a supplier like Tin Lee Electronics Ltd. may be helpful for FM reception where an external antenna is employed. These are also helpful in resolving television interference.
The FCC has a resource page that may be helpful to consumers, headlined “Interference: Defining the Source” (see sidebar).
It is not advisable to attempt repairs to consumer equipment; some states require licensure to do so, and you also may face liability issues. An engineer or station may stand accused of causing damage to the device or be blamed for its unrelated improper operation. Advise consumers on how to purchase filters; refer them to local licensed repair shops or the manufacturer for resolution.
Here are several cases of interference complaints and how they were resolved.
I received a call from the chief air traffic controller at a local airport claiming that our Class A FM station was interfering intermittently with the tower frequency, 119.4 MHz.
I asked if he could hear our audio and whether aircraft could hear the interference. He checked and said the problem had been heard by listeners in aircraft and separately on the ground via a portable receiver. The audio was unintelligible but our station was suspected because the antenna of the FAA’s direction finding van pointed toward our tower. Yet this also could mean that the offender was someone else, located between the van and our tower or beyond us on that bearing.
I invited the FAA technician to visit our site. Not having a spectrum analyzer, I brought along a Potomac FIM-71 and Icom R-7100 receiver.
I raised/lowered transmitter and exciter power; I mistuned the IPA and PA. But I could not produce any of our audio on 119.4 MHz. I could hear the tower and aircraft, but no evidence of our signal, other than on our licensed frequency.
Asked for a recording of the interference, the chief controller produced a GPS time-indexed DAT tape as the worst example.
We listened to 8 minutes of pure garble, akin to wideband FM detected on an AM receiver. According to our music log, our station at the time had been playing “Run” by Collective Soul. Listening to the tape, he and I agreed we could hear no change in tempo or cadence, and no voice. Also, “Run” was not an 8 minute song.
One aircraft trying to call in during this time had been diverted to a secondary frequency, and no wonder. I would not want to listen to that garble all day. This was a safety issue. I told the controller to call me if the interference reappeared.
It did so the next morning at 9:30; so I turned off our transmitter/exciter. This produced no change at all; the interference remained. Our station, it appeared, was not the offender.
I offered my assistance to the FAA not only as the chief engineer of the station but as a pilot who earned his license at that airport. I never heard back from the controller but I did meet the airport manager at a local restaurant some months later and asked if they’d ever found the cause of the interference.
The FAA technician finally had identified an SUV from which the signal was emanating. The owner had installed an FM booster to improve his reception. Removing the fuse to the booster cured the problem.
In another case, a local police department called to report that our 850 kHz station was interfering with their 45 MHz repeater.
They could not hear our audio, only beeps and tones at all hours of the day and night; but we stood accused because their radio technician had examined FCC records and identified ours as the most powerful transmitter licensed to that town.
I explained that the source probably was a paging transmitter at their multi-user site miles away. When the lieutenant told me the problem had been very bad a few days earlier at 4 a.m., I also pointed out that we were a daytimer and not on the air until 6 a.m.
I never heard back.
Playing against the house
At a Class A FM station, we received a complaint of interference to the CSPAN-2 channel on the local cable system at a nearby residence.
The resident was not home when the head-end chief technician and I visited. The center frequency of the CSPAN-2 was 800 kHz removed from our licensed frequency. The problem remained even after the drop cable and a trunk section were replaced.
I asked the technician if he could see our signal on a spectrum analyzer at various nearby tap points if the CSPAN-2 modulator was turned off. He said yes. Clearly this was a cable leakage issue.
It turns out the source was the neighbor’s house. Aluminum siding recently had been installed there, and the drop cable’s shield had been punctured and stapled in three places. The siding was acting as a receive antenna.
In another instance, this same FM station was said to be interfering with one 800 kHz up the dial. We cured this by installing double shielded coax cable between the audio processor/stereo generator located in a rack and the exciter composite input. Our collocated AM station operates on 800 kHz.
In another such case, we realized that when the station’s ATU had been installed, its ground strap had never been connected to the base ground strap at the tower. In fact we found the strap coiled up inside the ATU. When we connected it, the problem was solved.
Interference has many variations, causes and cures. Evaluate and treat each case individually. Some remedies are simple, where others may involve considerable effort to locate and cure.
Tom Osenkowsky is a broadcast engineering consultant and long-time contributor to Radio World.