FM 8th Harmonic Interference to a Cellular Service Not Caused by an FM Station: A Case Study

It is well-known?that 7th and 8th harmonic FM broadcast emissions have been a source of contention between cellular carriers and FM stations. Such harmonics are rarely radiated from the broadcast antenna itself. Usually, the cabinet radiation of the FM transmitter is seen as the culprit, even if the cabinet radiation is within the Part 15 specifications. In a special case, the proximity of the FM and cellular antennas can create?an overload situation where the cellular carrier generates?its own interference. A station in Massachusetts was accused of causing 8th harmonic interference and the carrier involved the FCC in the matter. A methodical approach had to be taken to eliminate the assumed causes of the interference and implicate the carrier’s internal receiver overload mechanism as the cause. This paper is a case study of that complaint.?It shows how the possible causes had to?be unpacked to satisfy the complainant and the FCC that the FM station was not the primary cause. The various mechanisms for generating 8th harmonic energy are explored. The?calculations and measurements required to exonerate the FM station are presented.?

David Maxson | Isotrope, LLC | Medfield, Massachusetts, USA