A novel method for enhancing the directivity of coupled-line directional couplers is presented. In a coupled-line directional coupler formed on thin multi-layered substrates, it is difficult to match the coupled-line characteristic impedance with the load impedance. This impedance mismatch causes forward coupling in the directional coupler, thus degrading the directivity. This method utilizes reverse-phase forward coupling with an additional coupled line to cancel the undesired forward coupling due to the primary coupled line. It has been experimentally verified that the directivity of the proposed 20-dB coupler is increased by 13 dB at 840 MHz in comparison with the conventional coupler.
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