A Coupled "Coaxial" Transmission-Line Band-Pass Filter

Band-pass filters employing tuned coupled circuits are used to advantage at low frequencies and, because they have good electrical characteristics, it may be desirable to adapt them to the very-high frequencies. However, at frequencies bordering on the microwave region, it is no longer possible to treat such circuit elements in the usual low-frequency fashion, the distributed character of the parameters becoming of prime importance. This paper describes a band-pass filter in which the coupled elements consist of a shielded pair of cylindrical conductors. Because the analysis is based upon transmission-line theory, the results are valid and usable at microwave frequencies until transmission other than the TEM mode becomes bothersome.