A class of wideband directional couplers suitable for achieving different coupling levels are proposed in this paper. The proposed couplers have been implemented using the double-layer groove gap waveguide at the millimeter-wave frequency range. They are composed of two parallel groove gap waveguide parts placed on top of each other in such a way that the coupling between the two waveguide sections can be achieved via the coupling structures placed in the common wall. Usually, the coupling layer consists of two rows of holes and depending on the number of holes the coupling levels can be controlled. The manifest and applicable property of the proposed directional coupler is that the coupling level can be simply changed using the different coupling layers without changing the top and bottom groove waveguides and transitions. As another significant advantage of the proposed coupler, there is no requirement of good electrical contact among different metallic parts of the structure, which considerably simplifies the manufacturing processes and mechanical assembly at millimeter-waves applications. For verification purpose, three sample couplers with coupling levels of 10, 20, and 30 dB have been designed, fabricated, and measured. To make the device accessible and measurable using the standard rectangular waveguides, two types of transitions from the groove gap waveguide to standard WR-15 are designed. The simulation and experimental results are in good agreement and show that the proposed couplers have large bandwidth for return loss level of 20 dB, coupling level of 10±1, 20±1, and 30±1 dB, and isolation level of 30, 35, and 40 dB over the desired frequency band of 50-70 GHz. The proposed directional couplers can be used as good candidates at millimeter-wave frequencies for probing and the design of compact integrated microwave circuits and systems, such as antenna array feeding networks.INDEX TERMS Directional coupler, gap waveguide technology, millimeter wave, V-band applications.
