Concentric Narrow-Band-Elimination Filter

A type of wave filter employing a concentric transmission line is described. Filters of this type are especially applicable to ultra-high-frequency operation where their physical size becomes reasonable and where the conventional lumped-constant filters become practically unrealizable. It is shown that the addition of a simple lumped-reactance element will result in a sharp cutoff characteristic analogous to that exhibited by a conventional m-derived lumped-constant filter but over a limited frequency band. This latter filter has allowed simultaneous transmission and reception using the same antenna, in the 30- to 42-megacycle band, transmitter pouer 150 watts, receiver sensitivity 5 microvolts, with a frequency difference of only 0.9 megacycle. It is shown that a 14 per cent increase in driving-point impedance of the filter can be obtained if the filter line is shortened 0.065 wavelength and tuned to resonance with a shunt capacitance. This result is applicable directly to the use of transmission lines as tank circuits when very high driving point impedance is required.