Short-term and averaged characteristics of nonreciprocal HF ionospheric paths

This paper reports an investigation into the ability of ray theory to explain nonreciprocity on HF ionospheric paths. It first reviews and extends some of the predictions of the ray theory of reciprocity as applied to single and multi-hop propagation paths. In this, ray theory predicts nonreciprocity in phase for single magnetoionic waves transmitted between horizontally polarized antennas. If the antennas are elliptically polarized, there can also be nonreciprocity in amplitudes. It is shown that experimental observations on several circuits using linearly polarized antennas support the validity of predictions based on ray theory. In particular: 1) the correlation between two-way, short-term fading of pulsed signals was positive and high when only one magnetoionic wave was present in the signal, but negative or low when both ordinary and extraordinary waves were present 2) the six-minute averaged path loss in opposite directions was equal within experimental errors for signals that underwent polarization fading. Short-term discrepancies were found and these can be attributed to insufficient averaging of polarization fading effects. Systematic differences in the path loss in opposite directions observed by others could be explained if the antennas were elliptically polarized for transmission paths that deviated from the great-circle path.