Comments on “Super-Compact Substrate Integrated Waveguide Cruciform Directional Coupler”

For the original article by T.Djerafi and K. Wu see ibid. vol.17, no.11, p757-59, Nov. 2007. The authors and their coworkers have proposed new cruciform H-plane waveguide directional couplers with some coupling factors. The design principles for both directional couplers are the same. However, this fact is not mentioned in the paper, it is merely listed in the references. In addition, it would be difficult for most readers to sufficiently understand the contents of [Y.Yumita et al. 2005] because of a Japanese-edited one-page summary, which is listed as six pages by mistake. Shortly after the paper [Y.Yumita et al. 2005], the investigation results and their expansions have been published at two international conference papers. In these papers, the circuit structure and its working principle are clearly explained. The proposed cruciform directional coupler in H-plane waveguide consists of two rectangular waveguides crossing each other at right angles, two cylindrical metallic posts arranged two-fold-symmetrically on a diagonal line of the cross region for producing directional properties, and inductive windows or inductive metallic posts placed at each input/output port for making a matched state. This circuit structure is the same as that in [1]. Since the SIW structure is considered as a quasi H-plane waveguide structure, the above-mentioned design concept can be directly applied to the design of an SIW directional coupler. Actually, we also successfully designed an SIW cruciform 3-dB hybrid [5], [6] based on the same idea as the H-plane waveguide cruciform directional coupler. In order that many readers obtain more information concerning cruciform directional couplers and fairly evaluate the investigation results in the papers and should be added in the references. Of course, however, we can recognize the good experimental results of the paper regardless of this comment.