Multiple Echo Distortion in Frequency-Modulation, Frequency-Division-Multiplex Trunk Radio Systems

This paper is concerned with intermodulation distortion in FM, FDM trunk radio systems due to a number of echoes of differing amplitudes and delay times. General formulas are developed, and applied to the case of a 960-channel system subjected to distortion due to echoes from flanges in waveguide runs. Examples considered are 100- and 200-ft runs of RG-50/U waveguide, in 10-ft sections, for use in the 6000-Mc band. The calculations are carried out with the simplifying assumption (for which there is some experimental evidence), that the individual flange reflections are equal and of the same sign. Distortion levels are computed both with equal section lengths and with section lengths made slightly unequal, so as to destroy coherent addition of the individual echoes. While the latter arrangement gives a considerable reduction, as compared with the equal length arrangement, in the peak input standing-wave ratio, the improvement in the distortion is so small as probably not to justify, in practice, any deliberate variation of the lengths.