A Novel Dual-Band 3-dB Branch-Line Coupler Design With Controllable Bandwidths

This paper presents, for the first time, the design and implementation of a dual-band branch-line coupler with unequal operating bandwidths. The control of bandwidths is accomplished by the use of dual-band quarter-wavelength impedance transformer with controllable phase slopes over the two designated frequency bands. The adjustment of phase slopes is attained by the proper selection of the circuit parameters of the transformer. For design purposes, closed-form expressions are derived for the determination of the electrical lengths and line impedances of the resulting coupler. For experimental verification, both simulated and measured results of dual-band couplers designed to operate at 1 and 2 GHz are shown.