 High System Gain is con- sidered beneficial for the following reasons: S/N J(dB)C(dBm)+ 139 (50.20) 1. It may allow longer hops. - F (dB) 50/16 Line of sight radio systems Improved outage due to multipath fading can be the result of followed by a power attenuator, when the system characteristics increased received signal level. (path length, frequency, antenna gain etc.) permit. It should be apparent that System Gain is an extremely important equipment specification. Yet it is possible to have too much System 50.4.6 Receiver characteristics Gain at some point and actually begin to harm system performance, reliability, or cost. For example, high transmitter power may be The receiver characteristic normally considered is sensitivity, i.e. neither desirable nor beneficial as shall be shown in the following weak signal performance. System Gain has been improved through section. the years by improving receiver sensitivity. Unlike transmitter power output, improved receiver sensitivity is relatively cheap. 50.4.5 Transmitter power output Today's low noise preamplifier transistors are actually cheaper and better performing than the devices of the past (tunnel diode pream- On the subject of transmitter power output, it should first be realised plifiers, parametric amplifiers, low noise mixers, etc.). Unlike trans- that microwave transmitter power output has a direct and strong mitter power output, there is a law of nature limitation on receiver impact on all of the parameters of concern, namely equipment cost, sensitivity: this is the random noise (KTB noise) that, for a given system cost, reliability and performance. In addition, though the bandwidth, is only dependent on temperature. It represents the 'OdB technology of generating transmitter power has changed and gener- Noise Figure' limitation, which in turn directly determines the best ally improved, the amount of transmitter power has not significantly achievable receiver sensitivity. increased. One interpretation of this is that transmitter power is still Today's