SIW Butler Matrix with Modified Hybrid Coupler for Slot Antenna Array

This paper presents a substrate integrated waveguide Butler matrix with a modified hybrid coupler, operating from 28 to 32 GHz. The modified hybrid coupler is realized by using 90° hybrid coupler followed by a -45° compensating phase shifter. Thus, 45°/135° output phase differences can be obtained using this type of coupler. Compared with the equal-length unequal-width phase shifter, the compensating phase shifter exhibits better phase characteristics, with only 3° phase error over 28-32 GHz. Because the phase of compensating phase shifter is not realized by taking the phase introduced by the crossover as a reference, it gives great flexibility to the design of compensating phase shifter. Adopting the modified hybrid coupler, the designed Butler matrix features the output phases with peak to peak error of 13° and wideband performance from 28 to 32 GHz. The slot array fed by the Butler matrix can radiate four slanted beams with acceptable measured gains, in the range of 9.7~12 dBi for port 1 excitation and 8.4~11.1 dBi for port 2 excitation. The radiated beams can reach a wide azimuthal coverage between ±61°.