GB1095274A - Electromagnetic waveguide mode filter - Google Patents

Abstract

1,095,274. Waveguide mode filters. COMPAGNIE GENERALE D'ELECTRICITE. June 2, 1965 [July 30, 1964], No. 23603/65. Heading H1W. A parasitic waveguide mode of order m, (n + p) is eliminated from a waveguide utilizing a transmission mode of order m, n by coupling the transmission guide to an auxiliary dissipative guide dimensioned to transmit a mode of order m, q having a phase velocity equal to that of the parasitic mode in the transmission guide. In the sole example described, a circular transmission waveguide of diameter d utilizing the TE 01 circular-electric mode is coupled, over a certain axial distance, to an auxiliary coaxial guide of diameter D which transmits a coaxial TE 01 mode at a certain phase velocity. This phase velocity may be made equal to that of a parasitic TE 02 mode in the transmission guide by making D/d = r 0,3 /r 0,2 where r 0,2 and r 0,3 are the 2nd and 3rd order zeros of the Bessel junction J 1 (x), x being a normalized radial coordinate. The equation expresses the fact that the parasitic TE 02 mode has the same velocity as the TE 03 mode resulting from the superposition of the TE 02 mode upon the coaxial TE 01 mode. The whole of the parasitic mode energy is coupled to the coaxial guide which is provided with absorptive elements at either end. The inner guide may be made from a dielectric tube 8 provided with a conductive layer in the form of rings 9 separated by coupling slots. Non-uniform coupling, using triangular, exponential or cosine-squared distribution, may be used. Other forms of coupling structure may be used, e.g. circular slots, or a conductive-wire winding. The external conductor may be solid or may comprise a winding 10 of insulated wire supported upon the dielectric internal lining of a conductive tube 12. This lining may be divided into two coaxial parts by a cylinder 13 of fine metallic gauze. The band-width of the arrangement may be improved by providing dielectric or conductive loading at intervals in the coaxial guide (e.g. dielectric load 19) to achieve a non-uniform phase-velocity. The coaxial guide may contain distributed dissipative material or may be coupled by slots to a surrounding dissipative medium.