Substrate Integrated Coaxial Line based Branch Line Coupler with Broad Out of Band Rejection

This paper reports the design and development of a miniaturized substrate integrated coaxial line (SICL) based branch line coupler operating at 5 GHz with broad out of band rejection. The proposed coupler employs a pair of center tapped unequal length open stubs to reduce the lateral footprint as well as introduce transmission zeros in the out of band region. Equations derived analytically from even and odd mode analysis aid in determining the size reduction factor as well as the position of transmission zeroes in the out of band region. A prototype of the proposed SICL based branch line coupler is fabricated and experimentally verified. The measurement results indicate low amplitude and phase imbalance less than +/- 0.5 dB and +/- 3 degrees in the range of 4.6 GHz to 5.3 GHz, contributing to a fractional bandwidth of 14.14% centered at 5 GHz. Also the proposed fully shielded and self-packaged SICL coupler features out of band rejection up to 4f(o).