This paper describes the design of a hybrid coupler with a continuously
variable output phase difference. This is achieved by using
reconfigurable transmission lines with electrically tunable lengths
controlled by two biasing voltages through varactor diodes placed across
the coupler branches. The design of the coupler is based on the
quadrature hybrid structure for the case where the output phase
difference is 90° and on the asymmetric structure for the other cases.
The proposed coupler can achieve a tunable output phase difference from
52 to 128 while keeping a coupling coefficient of -3dB (± 0.5dB) over
the entire desired frequency band. To validate the simulated results, a
prototype working at 3.5 GHz was fabricated and tested. The measurement
results show good correspondence with the simulation results, especially
when the output phase difference is 90° while a phase mismatch of less
than 7° was observed for the other cases. The presented coupler would be
a great asset for antenna feeding arrays, especially the Butler matrix.