ARRL

Broadcast Station

Introduction

ARRL does receive occasional reports of interference to amateur radio from AM broadcast station harmonics.  Like amateur radio, broadcasters have limits for spurious emissions that may be generated by broadcast transmitters and installations. In part because their transmitters can sometimes use very high power, those limits are more stringent than the limits for amateur transmitters.  If a transmitter meets the limits, but those harmonics or other spurious emissions cause harmful interference to a licensed radio service, the broadcast-transmitter operator must take steps to eliminate the harmful interference, just as an amateur that meets the Part 97 spurious emissions rules may have to install a low-pass filter if he or she interferes with a nearby antenna-connected television or other broadcast receivers.

In ARRL’s experience, however, the majority of broadcast interference reports arise because the fundamental signal overloads sensitive amateur receivers. In that case, the additional filtering or attenuation necessary to protect the receiver must be installed by the amateur operator. Rarely, amateurs can also experience intermodulation from multiple nearby broadcast stations, often with the intermodulation source being on the amateur’s own premise. 

If you are hearing a single broadcast station on a multiple of its operating frequency, you can install a broadcast high-pass filter that will ensure that none of that problem is coming from an overloaded receiver.  For a receive-only test, you can obtain one of the lower-power broadcast high-pass filters to see if it improves receive capabilities. If you have a 10 or 20-dB attenuator, or step attenuator, you can also try that in line on receive. In both cases, and this is important, do not transmit into receive-only filters or typical attenuators or you will damage the filters, attenuators or both.  But if the interference goes away with the attenuator, or drops a lot more than the amount of attenuation, you know that you need to obtain a transmit-rated high pass filter and that, too, should solve your problem.

If the high-pass filter doesn’t solve the problem and/or the noise level drops to match the attenuation used, then the interfering signal is external to your radio and your radio is not being overloaded.  If that is the case, contact ARRL and we may be able to work with the station engineer to find out what is going on. 

 

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