Transmission Characteristics of a Short-Wave Telephone Circuit

A method of observing and recording the audio-frequency transmission characteristics of a short-wave radiotelephone channel is described. These characteristics undergo rapid changes. They appear to be the result of wave interference between signals arriving at the receiver over paths of different group or electrical length possibly combined with the distortion produced by a progressive change in the angle of rotation of the polarization plane with frequency over the signal band. The persistence of certain pattern shapes during the observation periods and the changes in these shapes from hour-to-hour suggest that they are the result of progressive rather than erratic disturbances in the transmission medium. Times when the audio-frequency characteristics were flat were very rare. However, a considerable departure from flatness may occur without serious effect on the intelligibility of the speech transmission. Synthetic patterns used in the analysis of the characteristics are explained and illustrated. Types of audio-frequency distortion resulting from selective fading are discussed. The effect of frequency or phase modulation in producing distortion on such a circuit is considered. Records are shown of the effect of an automatic gain control, following carrier amplitude variations, upon the audio-frequency transmission characteristic. "Rapid" fading records revealing unlike fading on radio frequencies separated by 170 cycles are included. The seasonal variation in susceptibility of the circuit to this "rapid" fading is illustrated. The records mentioned above are for ordinary modulated carrier transmission and involve the results of interaction between the two side bands in the detection process.