Channel-dropping filter for millimetre waves in circular waveguide

Low-loss waveguide, operated at millimetre wavelengths in the circular H01 mode, is likely to offer a spectrum of usable frequencies for overland trunk telecommunications, extending over more than 10 GHz. To utilise this vast bandwidth effectively requires groups of channel-dropping filters, each separating out a single channel covering 200¿500 MHz. The difficulty is to carry out this operation without introducing large insertion losses, and the paper describes an arrangement that has shown encouraging results. The filter provides for the complete channel-dropping process in circular H01, mode and uses taper sections of semicircular guide to allow only those frequencies above the cutoff to go forward. The dropped channel is extracted by distributed-slot coupling into circular guide, placed between the flat surfaces of the two parallel semicircular guides carrying the transmitted power. Experimental information is given about the performance of a filter of this kind, and special attention is drawn to features which the investigation has shown can be significantly improved