A compact and wideband 90° cruciform coupler in SIW technology

A cruciform directional coupler based on substrate integrated waveguide (SIW) technology is presented in this paper. The coupler is composed of crossover of two SIW sections, two inductive metallic posts and air slab in the central region. Design steps are provided. A prototype coupler has been fabricated for X-band operation. The coupler has a wide bandwidth of 40% at the center frequency of 10 GHz while occupies 26 mm × 26 mm in a 1.58 mm thick RT/duroid 5880 substrate with dielectric constant = 2.2, even compact than a super-compact cruciform coupler reported recently. The main advantage of the coupler is its smooth phase response, maximum measured deviation being less than ±0.2° over the whole X-band.