A theoretical study of distortion due to multipath propagation is presented. For application in an antenna-diversity system a distortion detector has been developed which indicates not only multipath distortions as a result of great time delays between superimposed waves but also indicates noise interference as well as co-channel interference and nonlinear distortions as a result of large signal interference. Main feature of this detector is its extremely short detection time of 25 µs which is limited to the group delay time of the IF-filters of the receiver. On the basis of this fast indicating distortion detector the efficiency of antenna-diversity is investigated as a function of the number of implemented antennas among which that with the best signal is selected. The results show that the improvement factor increases considerably by the number of antennas applied.