The effect of echo on the operation of high-frequency communication circuits

Echo on high-frequency communication services is defined as the simultaneous reception of signals over both major and minor arcs of the great circle connecting a transmitter and receiver. Two distinct kinds of echo are recognized according to the illumination conditions under which they occur. Echo of the first kind is observed when the great-circle path coincides with the twilight zone surrounding the earth, whereas echo of the second kind, which can occur only on fairly long communication paths, is most severe when the short path is most intensely illuminated. Little can be done to obviate echo of the first kind and it is not, like echo of the second kind, amenable to prediction by available methods of calculating sky-wave field intensities. Radio traffic data are cited which corroborate calculations and show both that echo of the second kind is more severe at times of maximum solar activity, and is less severe on higher frequencies. Conclusions are drawn concerning mode of operation and choice of operating frequency to minimize echo interference.