Abstract: From the transmission line equations single ridge waveguide slot active admittance formula is derived. With three-dimensional electromagnetic simulation software HFSS, the active radiation conductance of the gap in the antenna are obtained. An 8-element single ridge waveguide slot array is designed. Experimental results show that the side lobe is -23.61dB, relative bandwidth for VSWR less than 2 is about 8%, which is about 2 times of that in rectangular waveguide slot antenna.