Arthur C. Stewart and John B. Heffelfinger

Abstract: The pressure to provide additional full-time facilities in the AM (standard broadcasting) band in the United States resulted in a rulemaking petition filed with the Federal Communications Commission (FCC) by the National Telecommunications and Information Administration (NTIA). The petition proposed the reduction of the AM channel spacing from 10 to 9 kHz. According to the suggested frequency allocation plan, the maximum frequency change required for any standard AM broadcast station would be ±4 kHz. At the Institute for Telecommunication Sciences, in addition to investigation of several impacts which would result fran the change,a study was conducted both theoretically and experimentally to determine the magnitude of the radiation pattern changes that would be caused by a ±4kHz frequency change for stations using directional antenna arrays. The dollar cost to broadcasters because of the proposed change is directly tied to the engineering costs that would be required in achieving compliance with directional arrays. Additionally, a computer study of the 1430 kHz regional channel, using standard patterns for all occupants of the. channel, was conducted to determine the effects of a ±4 kHz frequency change on the interference levels suffered by co-channel stations. The results of the study demonstrate that the effects of small frequency changes on the radiation patterns of directional antenna arrays are greater in practice than theoretical predictions would indicate, but that required pattern corrections could probably be quickly and easily effected.

Keywords: AM broadcast band; allocation bandwidth; directional antennas; frequency allocation

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