Factors Affecting Modulation Techniques for VHF Scatter Systems

An experimental program at approximately 50 mc has been carried out over a 1295-km east-west ionospheric-scatter path to determine the comminucation capacity of the propagation medium using ordinary modulation techniques. Binary error-rate studies using dual-narrow-band frequency-shift-keyed terminal equipment were made to observe the dependence of error rate in scatter propagation on signal-to-noise ratio, multipath factors, and Doppler shifts. With respect to multipath limitation, using antennas of 6° beamwidth, error rate is found to be independent of transmission speed to 500 binary digits per second. Tests were made using dual-diversity narrow-beam rhombic antennas, and dual- and quadruple-diversity broad-beam Yagi antennas. The use of four Yagi antennas gave very promising results as compared to use of two narrow-beam antennas. Frequency shifts of 2, 4, and 6 kc were used to determine the effect of meteor Doppler components in the received signal, leading to recommendations for minimum frequency shift. Though actual modulation tests have so far been made at a frequency of 49.6 mc only, systematic observations of longdelay signals of the order of 10 to 80 msec were made at 30 and 40 mc, using pulse techniques. These delayed signals arrived at the receiver via the F
2
backscatter mode, and were occasionally of sufficient strength to cause binary errors for circuits operating in these frequency ranges. Studies of intelligibility for voice transmission were made using narrow-band frequency modulation and single-sideband modulation. Using 20-kw power, the frequencymodulation tests with dual-diversity receivers and narrow-beam rhombic antennas gave good results at levels of signal-to-noise ratio exceeded for 90 per cent of the year at 50 mc.