 For a signal entering the H-plane port, a well-designed matching structure will prevent any of the power in the signal being reflected back out of the same port.As there can be no communication with the E-plane port, and again considering the symmetry of the structure, then the power in this signal must be divided equally between the two collinear ports.Similarly for the E-plane port, if the matching structure eliminates any reflection from this port, then the power entering it must be divided equally between the two collinear ports.