Interchannel Interference in FM Systems Produced by Continuous Random Mode Conversion in Circular Electric Waveguide

Long-distance, high-capacity transmission via the T E 0 i mode in the millimeter-wave circular-electric waveguide is an attractive goal be2765 2766 THE B E L L SYSTEM T E C H N I C A L J O U R N A L , N O V E M B E R 1963 cause the theoretical attenuation caused by heat loss decreases monotonically as the frequency of operation increases. However, in order to obtain sufficiently small heat loss attenuation and delay distortion, the operating frequency must be well above the T E 0 i cutoff frequency and far into the multimode region. For example, a two-inch I . D . perfect copper circular guide will have a theoretical TEoi heat loss of 1.54 db per mile at 55 kmc and will propagate 223 additional (spurious) modes. 1 Real waveguides are never perfect right-circular cylinders but have continuous random imperfections arising from the manufacturing or the laying of the waveguide. These random imperfections cause conversion of the TEoi signal mode to spurious modes. 2 We call the attenuation produced by this energy loss "conversion attenuation." Usually more important, the imperfections also cause converted energy in the spurious modes to be reconverted into the TE 0 i signal mode. Since the group velocities of the various modes will be different, the reconverted energy from each mode will be delayed or advanced in time with respect to the energy which has remained in the TEoi signal mode. Thus we have a signal plus a continuum of echoes. The effect causes the amplitude and phase of the TE 0 i wave to fluctuate randomly with frequency.