FM Reception Guide: FM Propagation

FM does have limitations, the most significant being that FM signals, like those of TV, are propagated in a line-of-sight path from the transmitter and cannot follow the curvature of the earth.  Because of this, FM signals travel only a fraction of the distance covered by AM and short wave signals which bounce off the ionosphere.
The distance between the receiving antenna and the transmitting antenna also determines FM signal strength. The further apart the two antennas are from each other, the weaker the received signal will be. Generally, an area thirty air miles or more from any FM transmitting antenna can be considered prone to suffer weak or fringe reception. If the two antennas can “see” one another without buildings, trees or hills between them, the signal will be stronger and there will be fewer instances of abnormal reception.

Multipath Interference
Regardless of overall signal strength, a very important consideration is that obstacles, which an FM signal may encounter in the line-of-sight path, tend to reflect and disperse the signal in many directions. FM coverage is limited by obstacles - hills, buildings, trees - all of which reflect and tend to weaken and dissipate the transmitted signal. In many locations, especially in urban areas where many tall buildings interfere with the direct transmitted signal or in suburban areas surrounded by hills or mountains, an FM receiver may pick up a station’s primary signal but, also, several secondary reflections coming from various directions. These reflections arrive at the receiver out-of-phase —slightly delayed in time—with the primary signal and tend to blur or distort the principal signal. The result, known as multipath distortion, is the equivalent of the familiar ghosts in TV reception. The audible effect can range from a low-level fuzziness to a severely distorted sound quality, particularly at the high frequencies or treble.  The degree of distortion depends on the number and relative strength of the reflections.

Indications of multipath interference include distorted sibilants (“sh” sounds), cymbals that tend to shatter rather than ring and the sound of strings that have an overlay of grit or a low-level sputtering hiss. Any or all of these artifacts can diminish or disappear when the receiver is switched to operate in the monaural mode. Multipath distortion is especially troublesome in FM stereo reception. Since stereo broadcasts consist of several signal components transmitted on the same radio wave, any change in the phase or relationship of these components can degrade or destroy the stereo quality. The reason a stereo broadcast may sound more acceptable when received in the mono mode is that multipath effects on the stereo information of the FM signal, as well as the stereo information itself, are effectively removed.

Even the most expensive FM tuners or receivers cannot separate the primary signal from signal reflections. Therefore, the problem of multipath reception must be rectified before the signal arrives at the input of the receiver. Generally, the only satisfactory solution to severe multipath is a directional antenna designed to accept signals from only one direction—from the transmitter—and to reject reflections from other directions. So far, only outdoor antennae have been successfully designed to be sensitive in only one direction.

Fringe Reception
If you live in a fringe area, your reception may be plagued by background hiss audible during quiet passages of music or talk that is drastically reduced when the receiving equipment is placed in the mono mode. You may find it worthwhile to invest in an outdoor FM antenna with more gain than that provided by an indoor antenna.

Using a highly directional outdoor antenna aimed at the antenna of the desired transmitter and designed to reject the unwanted stations and/or reflections (multipath) is the only way to ameliorate interference created by a strong station adjacent to a weak one. In instances where you may want to receive stations, which broadcast from compass points different from that of another favorite station, the antenna should be provided with a rotor that can re-aim the antenna.

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