The design of a patented broadwall waveguide hybrid coupler, derived from the working principle of a septum polarizer, is presented. This $E$ -plane coupler requires a single and large aperture in the common broadwall of the two parallel waveguide transmission lines to function, facilitating its manufacturing and assembly. The working principle of this hybrid coupler is detailed, supported by equations making a link with the well-known 3-dB tandem coupler and enabling the definition of a good starting point for optimization purposes. The concept is further validated through the design, manufacture, and test of a prototype working over the frequency range 17.3–20.2 GHz. Good results are obtained over a fractional bandwidth of 19.4% (17.2–20.9 GHz) with axial ratio and return loss (RL) better than 0.5 and 24 dB, respectively. This operating range may be extended up to about 27% (16.8–22.0 GHz) for less demanding applications (axial ratio below 3 dB and RL better than 20 dB). This is a promising solution for the design of advanced feed systems and beamforming networks part of communication satellite payloads.