A new type of directional coupler for coupling coaxial line to TE10waveguide

A directional coupler is described for coupling a waveguide mode to a TEM mode in a coaxial line. In contrast to "loop type" couplers the present device is a "forward" or co-directional coupler making use of a multiplicity of capacitive coupling probes inserted in a longitudinal slot in the broad face of the waveguide. The work refers to an S-band device yielding couplings down to about 15 db with a directivity of over 30 db. An approximate theory for the performance of such a coupler is given in terms of the theory of coupled transmission lines. The modification of the coaxial line progagation constant by the presence of the coupling slot and probes is indicated. The empirical procedure to be followed for designing such a coupler is outlined.