A Directivity-Compensated Wideband Microstrip Coupler Based on Capacitive-Loading Patches and Reflectionless Amplitude Equalizers

This letter proposes a directivity-compensated wideband coupled-line-based microstrip coupler that incorporates capacitive-loading patches and reflectionless amplitude equalizers. Three tiny rectangular patches, which are loaded symmetrically in-between the coupled lines, compensate for the directivity deterioration caused by substrate inhomogeneity in the microstrip coupler. Two reflectionless amplitude equalizers based on resistor-loaded open-ended microstrip line lossy resonators are cascaded at the coupling port and the isolation port, respectively, are utilized to improve the flatness of the coupling coefficient and the isolation without deteriorating the directivity of the coupler. Apart from the amplitude modification in S-parameters, the phase variations introduced during the design process are also analyzed. A prototype of a 20-dB microstrip directional coupler with its working frequency centered at 4.3 GHz was designed and fabricated. Measurement results demonstrate that the fractional bandwidth of the coupling coefficient with its flatness better than ±0.5 dB is as high as 137%. Measured directivity ranges from 17.5 to 36.0 dB across 1.36–7.25 GHz, demonstrating the effectiveness of the proposed design.