A Novel Ultra-Compact Microstrip Coupler With Low Imbalance Magnitudes and Phases for Wireless Networks

Based on stub loaded meandrous coupled lines, a novel microstrip coupler is designed, fabricated and measured for wireless applications. It is very compact with high performance. The size of this coupler is only
$0.0028 \; \lambda $
g2, where it operates at 875 MHz. The values of S11, S21, S31 at the operating frequency are -24.97 dB, -3.058 dB and -3.033 dB respectively. Therefore, the insertion losses are very low and the magnitudes of S21 and S31 are well balanced. The phase imbalance at the operating frequency is 0.5°. From 790 MHz up to 1.18 GHz, S11 is better than -10 dB, the maximum phase imbalance is lower than 4.5° and the maximum magnitude imbalance is lower that ±0.5 dB. Therefore, it has 40% fractional bandwidth (FBW) that covers 0.79 GHz to 1.18 GHz. Its in-band isolation factor is better than -25 dB. The important advantages of this coupler compared to most of the previous designs is having a filtering frequency response and having flat passbands with a maximum group delay of 1 ns.