Three-line broadband codirectional microwave couplers using planar comb and herringbone microstrip lines

Abstract

Equations relating the scattering parameters to the line parameters of an n-line system of coupled transmission lines are applied to the design and construction of three-line broadband codirectional microwave couplers using planar comb and herringbone microstrip structures. Theoretical curves are produced for the scattering parameters of equal power splitting couplers with a range of output phase differences at the centre operating frequency. A broadband coupler is achieved with a phase difference of ? radians between the output ports at the centre frequency. The bandwidth is ± 65% which is the largest so far reported for a single section uniform device. Three couplers are constructed using various dielectric materials and their performances measured using a time domain automatic network analyser (t.d.a.n.a.), a relatively new broadband microwave measurement technique. One of the couplers is designed to transform the impedance level from 50 ? at the input to 25 ? at the output ports. A possible application of the coupler is demonstrated in microwave GaAs. f.e.t. amplifier design and results are given for a two-device circuit operating around 3 GHz.