Criteria for the Ultimate Capability of the Optimized Tropospheric Scatter System

Behavior of the tropospheric scatter mechanism is now sufficiently well understood to permit a determination of optimum transmission criteria. The requirements for optimization are established from the channel signal-to-noise ratio equation. It is shown that the optimum is related not only to the basic RF transmission loss, but also to the effective noise figure, antenna gain and medium aperture loss. It is considered that if a minimum product of antenna height, path length, and frequency are maintained, there will be a cubic relation between path loss and frequency. When the frequency dependence of the other factors is also taken into account, it is found that the net result will depend either directly or inversely upon frequency, depending upon the choice of system parameters. On this basis, and by using a graphical solution, an optimum frequency is found, for which the system gain maximizes. It is seen that for a given antenna, the product of frequency and path length is substantially constant. From this, it is found that the minimum product of antenna height, path length and frequency is achieved, so validating the use of the initial cubic relation. It is shown that when the optimum frequency is used, the associated medium aperture loss is rather small, so that improved accuracy in the determination of this factor is unlikely to produce substantial improvement in the optimized system. Also, the plane wave antenna gain is not exceptionally high, so that structures which are more economic than paraboloidal types may be contemplated. Curves for path loss vs distance are derived using optimum frequencies; total loss curves for 99.9 per cent reliability of the optimized system are also shown. This data is used to establish a series of system capability curves which are directly related to transmitter power, noise bandwidth and the signal-to-noise ratio at receiver input. It is considered that with the optimized system, very low power may be used to carry high capacity circuits over quite long spans.