Wide band strip-line microwave directional coupler for high power operation

Abstract

A wide band compact 20 dB directional coupler has been designed, fabricated and tested for high power operation within frequency band 4–12 GHz. In order to obtain broad band frequency response a three-section asymmetric coupled strip-line directional coupler has been chosen. Since the phase shift between the even and odd modes appears by their propagation through the strip-line discontinuities, a degradation of the coupler isolation and directivity is observed. To compensate these parasitic effects the periodic structures are introduced on the outer edges of coupled strip-lines. The periodic structures delay the even mode more strongly than the odd mode. So the wideband compensation of the difference between phase velocities of even and odd modes are achieved. The coupler shows high electrical performances as well as compact size, low weight and easy-to-manufacture design. The proposed 20 dB directional coupler provides within frequency band 4–12 GHz the coupling characteristic imbalance 0,5 dB, isolation 35 dB, directivity 15 dB and VSWR<1,4. The allowable input power of directional coupler is up to 100 W. The coupler dimensions are 39×35×26 mm.