The paper presents a method of estimation of coupling of a slot coupled H-plane T-junction, in which the primary waveguide is a rectangular one and the T-arm is a circular waveguide. An equivalent network of the junction is determined from a precise formulation for the effect of wall thickness. Appropriate dyadic Green's functions in the two waveguides are used for the determination of equivalent circuit parameters by the variational method. A comparison between the theoretical and experimental results on the variation of coupling with frequency is presented.