Design and development of compact coaxial to coplanar waveguide directional coupler for HPM measurement

A compact, high-directivity, weak-coupling directional coupler with coaxial-coplanar waveguide structure and diamond-shaped coupling slot was developed at L band for a high-power microwave measurement by both theoretical analysis and experiments. The relation between the dimension of the coupling slot and the characteristic parameters of the coupler was studied by the small-hole coupling theory and the infinitesimal method. According to the analysis results, the directivity could be better than 20 dB while the length of the slot is just less than a single λ because of the use of the diamond coupling slot. For demonstration, a 45-dB L band coaxial-coplanar waveguide directional coupler was designed and verified by test. The experiment results and the theoretical analysis are in good agreement, which indicates that in the range of 1-2 GHz, the coupling was approximately -45 dB and the directivity was better than 17 dB. The power-handling capacity was demonstrated to reach 2 MW with a length (150 mm) of less than λ and a cross section (46 mm × 55 mm) of less than one-twelfth of the BJ-14 waveguide directional coupler.