POWER LOSS FOR MULTIMODE WAVE-GUIDES AND ITS APPLICATION TO BEAM-WAVE-GUIDE SYSTEM

The conventional way of expressing power loss in decibels/meter for a multimode waveguiding system with finite wall conductivity (such as a beam-waveguide (BWG) system with protective shroud) can be incorrect a nd misleading. The power loss tin decibels) for a multimode waveguidin g system is, in general, not linearly proportional to the length of th e waveguide. New power-loss formulas for multimode system are derived in this paper for arbitrarily shaped conducting waveguide tubes. In th ese formulas, there are factors such as [exp(jx)-1]/(jx), where x = (b eta(a) -beta(b))l, with beta(a) and beta(b) being the propagation cons tants of the different propagating modes and l being the distance from the source plane to the plane of interest along the guide. For a larg e BWG supporting many propagating modes, beta(a)'s are suite close to beta(b)'s, thus the mode coupling terms remain important for a very lo ng distance from the source plane. The multinode power-loss formula fo r a large circular conducting tube has been verified by experiments. T his formula was also used to calculate the additional noise temperatur e contribution due to the presence of a protective shroud surrounding a millimeter-wave BWG system.