Frequency-Modulation Distortion Caused by Multipath Transmission

When a frequency-modulated wave is received by more than one path, so that two or more of the voltages which are induced in the antenna have nearly the same amplitude, considerable distortion can result. Large objects, such as hills or high buildings, can reflect and absorb the waves and thus cause interference. Two interfering waves will be in phase part of the time and out of phase part of the time during modulation. This causes amplitude modulation on the resultant carrier, and a sharp irregularity in the instantaneous frequency, corresponding to each hole in the carrier caused by the interference. The distortion can often be greatly reduced by detuning the re ceiver or discriminator until the hole in the carrier coincides with the zero-balance point of the discriminator. Directional antennas are helpful when the signals from the desired station are not coming from the same direction. Formulas are derived for the modulated envelope and for the distortion. A Fourier-series analysis of the distorted audio output makes it possible to calculate the effects of de-emphasis networks and other audio selectivity.