Design of X-band parallel broad-side coupled microstrip couplers on single-layer substrates

In this paper an X-band parallel broad-side coupled microstrip coupler is proposed for the RF module's built-in test (BIT) integrated with the receiver circuit on the same single-layer substrate. The coupler with a coupling coefficient of 10 dB is designed on a single-layer thin substrate with the features of small size, simple structure, and easy integration. The thin substrates, e.g. Rogers 5880 with the thickness of 10 mils, are widely used in RF modules. However, the edge coupled microstrip coupler on such a substrate is hard to provide a coupling coefficient more than 15 dB because of the physical limitation between the conductor lines in the PCB fabrication. The proposed X-band broad-side coupled microstrip coupler is consisted by a microstrip line and a qusi-CPW line. The simulated and measured results show that the coupling goal can be achieved by such a simple method.