COMMUNICATION BY VHF FORWARD-SCATTER IN AND CLOSE TO THE AUORAL ZONE
Koehler, James Alfred Rudolph
Bailey et al (1955) have show that VHF forward-scatter provides a very reliable method of communication over path lengths of several thousand kilometers. It has been suggested that this mode of propagation may be utilized to provide communications links in auroral latitudes using low power transmitters and very narrow bandwidth receivers. Such systems would only be feasible if the received signal doppler broadening was small. Previous investigation in auroral latitudes by back-scatter techniques has shown that scatter velocities of up to several km/s are not uncommon. A 50 watt transmitter was set up in Fort Smith, N.W.T. using a five-element Yagi antenna and operating on a frequency of 40.49 Mc/s. At Saskatoon, an identical antenna was used in conjunction with a receiver whose bandwidth could be varied from 5 c/s to 100 c/s. By changing the bandwidth an noting its effect on the received signal-to-noise radio, it was possible to estimate that the received signal, under undisturbed and mildly disturbed conditions, had a bandwidth of the order of 10 c/s. This corresponds to a r.m.s. scatterer velocity of about 50m/s. There was some evidence to show that velocities greater than 50 m/s do occur during periods when the ionosphere is greatly disturbed. These periods, however, are always associated with an enhancement int eh received signal amplitude. The experiments indicate that, using a 100 watt transmitter a receiver with a bandwidth of 10 c/s an antenna similar to those used, an information rate of at least 10 words/minute would be possible 99.9% of the time over a path length of 1000 km.