COMPUTER PREDICTION OF THE EFFECTS OF HF OBLIQUE-PATH POLARIZATION ROTATION WITH FREQUENCY

Abstract

Abstract : When a CW skywave signal is received on a linearly polarized antenna, polarization (Faraday) rotation produces a variation of received signal strength with radio frequency. The resulting dependence of received signal amplitude on radio frequency imposes a bandwidth limitation on pulsed signals transmitted over ionospheric paths. A measure of this limitation, termed coherent polarization bandwidth, is defined to correspond to the bandwidth in which the plane of polarization rotates 90 degrees. Computer raytracing calculations were performed using a single Champman-layer ionospheric model to determine the one- hop coherent polarization bandwidth as a function of geomagnetic aximuth and frequency. The coherent bandwidth was found to decrease with increasing path length, radio frequency, and geomagnetic azimuth. Assuming a critical frequency of 9 MHz and a path length of 2000 km, the bandwidth increased from a minimum of 140 kHz at 10.5 MHz and from a minimum of 70kHz at 17.5 MHz as the propagation direction varied from geomagnetic north to east. The theoretical effects of polarization rotation with frequency, and also of ionospheric dispersion or phase distortion, on the envelope shape of short-pulse signals (of from 0.5 to 50 microseconds duration) were calculated by computer for the same path. A pronounced waveshape distortion due to the effect of polarization rotation on the pulse envelope was observed when the signal bandwidth appreciably exceeded the coherent polarization bandwidth for the path.