Choice of multi-channel telegraph systems for use on H.F. radio links

Telegraph systems for use over h.f. radio links are generally restricted to low modulation rates (100¿200 bauds) because of the effects of multi-path propagation. The usual practice is to employ frequency-division multiplexing to exploit the bandwidth of the radio channel. Different methods of modulation in the telegraph channels may be used¿frequency-shift, frequency-exchange and phase-shift. The performance of typical systems of each kind has been measured in the laboratory under similar conditions of propagation provided by a fading-circuit simulator with additive uniform spectrum random noise. The relative merits of the systems are discussed in relation to transmission accuracy and bandwidth utilization. These are found to vary according to propagation conditions and to whether automatic error correction is used or not.