From the examination of the channel characteristics experienced in the mobile radio environment, it is apparent that the development of data transmission based upon non-binary systems as an alternative to commonly used binary methods is long overdue. A poly-signal modem has emerged from such a design consideration which is unconventional by virtue of its use of the time/frequency parameters available. Because of the complication of modelling the mobile channel one poly-signal scheme with particular advantage was chosen for analysis and the final design compared with a conventional binary system both in laboratory simulations, and also by field trials. It has been shown that under multipath fading conditions, the poly-signal modem maintains a probability of error curve which is normally associated with white noise interference, while binary error curves are modified to a simple function. The two error curves diverge resulting in an apparent increasing improvement for the poly-signal system as bit error rate reduces. For a bit error rate of 10-5 a 10 dB signal to noise ratio advantage is achieved with the new modem compared with the binary, when both operate at 600 bits per second. The improvement is a result of the immunity the poly-signal scheme has to time dependent interference.