Measurement of Echoes Due to Spurious TEon Modes in a Long-Distance 60-mm Waveguide Communication System

The performance of a long-distance millimeter waveguide communication system can be seriously degraded by slowly decaying echo trains in the impulse response. Slowly decaying echo trains are caused by coupling over long distances of the desired signal mode (TEoi) with forward traveling spurious TEon modes. When many echoes in such an echo train act in phase, the resulting peak distortion on a received signal can be quite large, even with relatively small total echo power. A previous paper 1 described an echo test set (Fig. 1) designed to detect such slowly decaying echo trains. This test set transmits a 20-ns pulse into a 60-mm waveguide line with a shorting plate at the end, samples the signal in 4-ns increments for 1 us near the returning pulse, transforms the data to loss and delay versus frequency, and overlaps data taken at different center frequencies, fc, to extend the frequency range of the data. Measurements were made with this test set as part of a recent 14-km field test of the WT4 system 2 between Netcong and Long Valley, New Jersey. These measurements indicated indirectly that the power in slowly decaying echo trains generated in the 60-mm waveguide itself was too small to degrade system performance. 1 To demonstrate overall system performance, however, we must de3253