Propagation characteristics of low-loss tubular waveguides

Tubular metal waveguides of circular cross-section supporting the H01-mode and operated at a frequency well above the cut-off value for the guide are known to be capable, in ideal circumstances, of providing a screened microwave channel having an attenuation of only a few decibels per mile. The paper is concerned with the practicability of this arrangement at a frequency of 35 Gc/s employing straight lengths of copper and aluminium tubes, 0.9¿2.74 in inside diameter, manufactured and installed to commercial tolerances. Using pulses of 0.1 microsec duration at this frequency, it is demonstrated that with solid-drawn tubes of standard production attenuations about 30% above the theoretical value are readily obtainable, and the conditions of propagation are entirely stable. Arrangements for launching the wave both from the usual H01 rectangular-guide source and directly from a magnetron oscillator are discussed. Attention is drawn to the possible application of such a waveguide to trunk communication and to high-power transmission. Although progress has been made on the problem of propagation round bends in the guide, a really satisfactory solution has not yet been found, and further work on that aspect is in progress.