An Enabling Multi-Operation Branch-Line Coupler

In this paper, an enabling multi-operation branch-line coupler is proposed initially. The proposed design is configured by ±90° and 180° transmission lines, and the main advantage is that the proposed design can be applied in three different system connection schemes as shown in Fig. 1. Corresponding to the three schemes, the coupler can be with three operations (e.g. the operations of balanced input with single-ended outputs, single-ended input with single-ended outputs, and single-ended input with balanced and single-ended outputs). This can increase the flexibility and freedom of the coupler for systems with different connection types. Furthermore, the common-mode suppression can be achieved under the operation. The standard and mixed-mode S-parameters are utilized to deduce the circuit parameters. Two prototypes using 3/4 λ
_g
(λ
_g
is the guided wavelength at the center frequency) transmission line (design I) and coupled line with two shorts (design II) to realize the -90° transmission line were designed, fabricated, and measured to verify the theoretical prediction. The sizes of the two prototypes are 0.98 λ
_g
× 0.2λ
_g
and 0.65λ
_g
× 0.2λ
_g
, respectively. Design I (design II) exhibits the 15-dB operating bandwidths for the three operations of 8% (9%), 7.3% (9%), and 6.7% (7.8%), respectively.