Ultra-broadband microstrip directional couplers

Abstract

Conventional broadband couplers use multi-sectioning techniques and consist of multiple coupled lines. In this study, in contrast to traditional methods, one pair of coupled lines is used to design multi-octave broadband couplers. In the proposed microstrip couplers, the coupled lines are weakly coupled and are of one-eighth wavelength at the highest operating frequency. The weakly coupled lines exhibit a linear coupling response with a slope of 6 dB/octave. To compensate the coupling response, one-pole low-pass networks with a slope of −6 dB/octave are cascaded with the microstrip coupled lines. To validate the design concept, two broadband microstrip couplers for the frequency range of 30–500 MHz and 1–3 GHz have been designed and fabricated. The couplers exhibit a coupling of 52 and 39 dB, respectively. The broadband couplers provide a ripple of 0.5 dB in the coupling response. The proposed weakly coupled couplers are adequate for high-power measurements. The weak coupling permits lumped-element surface mount devices to be used as the compensating network. The multi-octave couplers are realised in small areas.