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A new type of compact narrow wall directional coupler, whose coupling region is a ridge waveguide having dimensions such that TE10, TE20, and TE30 modes can exist, is proposed and analyzed. Coupling ratio and nonreflecting condition are determined by the degree of interference between TE20 and TE30 modes, and between TE10 and TE30 modes, respectively. The coupler can be used as a variable power divider by varying the coupling slot height. For 3-dB coupling over the 16.0 to 19.0-GHz frequency range, power equality within ±0.5 dB, more than 30-dB isolation and more than 30-dB return loss can be obtained. Experimental coupling results have shown good agreement with the theory.