A Coupler with Programmable Characteristic for Butler Matrix Applications

This paper presents a coupler with programmable phase-difference characteristic implementable for Butler Matrix applications. To achieve the characteristics, capacitors are placed at each intersection of the series and parallel arms of a branch line-type coupler. To verify the concept, a 3-bit variable phase-difference coupler is designed and simulated to operate at the frequency of 2.4 GHz. By properly selecting capacitance combinations from a 0.3 pF to 2.3 pF capacitance range, the proposed coupler produced eight phase-difference states from -120° to -50° with 10° increments. The designed coupler exhibits good scattering parameter response for all states. The reflection magnitude is better than -10 dB in 290 MHz(12%) bandwidth from the frequency of 2.19 GHz to 2.48 GHz. In such a bandwidth, the transmission magnitude is within -3.1 dB and -4.7 dB, which corresponds to a 1.6 dB imbalance with the maximum isolation of -13.9 dB. The designed coupler is then implemented in a $4 \times 4$ Butler matrix feeding network by substituting conventional hybrid couplers. The Butler matrix constructed with the proposed coupler demonstrates an increase in the number of output phase-differences, small transmission imbalance, good match, and high isolation.