Compact full Ka‐band waveguide directional coupler based on large aperture array

Abstract

A compact full Ka-band waveguide directional coupler based on novel aperture array is presented. The broadband, good in-band coupling flatness and excellent return loss are achieved by using the proposed novel aperture array in the common broad wall of two paralleled waveguides. For verification purpose, a prototype of the proposed directional coupler has been designed, fabricated and measured. The experimental results show that the return loss and isolation of the proposed coupler are better than 28 and 30 dB, respectively, and coupling strength varies from −20.35 to −19.23 dB over the full waveguide bandwidth. The length of the coupling region is only 8.5 mm.Introduction: Directional couplers are widely used in many microwave and millimetre systems, which require certain power splitting specification. Waveguide structures are often used because of the low insertion loss and high power handling capabilities, especially in high frequency. As an example of many early proposed waveguide directional couplers, Bethe-hole coupler [1] has a single round hole at the centre of the common broad wall between two rectangular waveguides. However, it is difficult to achieve broadband and good coupling flatness. To meet the specifications of systems, many structures have been proposed to enlarge their operating bandwidth and achieve good coupling flatness, including multi-hole coupler [2], multi-aperture configurations [3], multi-port technology [4] and multiple branch structure [5]. These couplers widen the operating bandwidth effectively. However, they normally need several guided-wave lengths to realise broadband. It is difficult to obtain broadband and good coupling flatness in compact size.In this Letter, we will show how bandwidth of up to 41.3% and coupling strength varying in −19.79 ± 0.56 dB over full waveguide bandwidth can be achieved simultaneously in compact size by using the novel aperture array which is presented in Fig. 1. The characteristics and design processes of the proposed coupler are presented as follows. The coupling region (shown in Fig. 1a) is designed with length of only 8.5 mm, that is, 0.7 λ g (guided-wave length of centre frequency). The experimental results show that the return loss of the sample coupler is better than 28 dB in the operating bandwidth varying from 26.3 to 40 GHz, in which the S 21 -magnitude is around −0.35 dB and the isolation is higher than 30 dB. To the authors' knowledge, no such compact-size of 20 dB waveguide directional coupler with full Ka-band and good coupling flatness has ever been reported before. Table 1 compares the proposed coupler with some recently reported couplers [6][7][8][9].