# Compact Full Ka-Band Waveguide Directional Coupler Based on Rectangular Aperture Array with Stairs
## 1 Introduction  
Directional couplers are widely used in modern microwave and millimeter-wave applications such as radar systems, communication systems, and microwave imaging systems, which require power combining, power dividing, or power sampling. Waveguide structures are often used in high-frequency systems or high-power systems, because of their low insert losses and high-power handling characteristics. As early as 1944, an example of waveguide directional coupler named the Bethe-hole coupler [1] was proposed. It has a single round hole on the common broad wall between a pair of rectangular waveguides. However, it is difficult to achieve broad bandwidth, tight coupling strength, and good coupling flatness. Some scholars proposed many new structures to improve the Bethe-hole coupler. The authors in [2] exhibit a cavity-type directional coupler using a simple structure that is easy to manufacture. In Reference [3], the Riblet coupler is presented, which consists of a short-slot hybrid junction. A simple compact narrow-wall directional coupler is developed in [4]. Multi-apertures are used in directional couplers in [5]. Moreover, multiport waveguide directional couplers are designed in [6]. In recent years, to satisfy the requirements of modern systems, many approaches have been presented to enhance coupling strength and enlarge bandwidth, including three coupling branch couplers [7], multilayer couplers [8], branch-line couplers [9], high-order directional couplers [10], and three-dimensional couplers [11]. These structures obviously expand the operation bandwidth. In Reference [12], a directional coupler using a printed ridge gap waveguide is proposed. Some new structures using substrate-integrated waveguide (SIW) are designed in [13, 14, 15, 16, 17]. Large apertures with transition structures [18] are presented to reduce the coupler size. They usually take several operating wavelengthsto widen the bandwidth. It is difficult to achieve broad bandwidth or tight coupling strength with compact size. The Ka band is a commonly used band for satellite communications, radar detection, and microwave imaging. In this article, we explain how coupling strength within \(-3.27\)\(\pm\) 0.53 dB over a full Ka band can be achieved simultaneously in compact size by using the novel rectangular aperture array with stairs, which are shown in Figure 1. The large rectangular aperture array (shown in Figure 1a) with floors (shown in Figure 1b,c) is first used in E-plane coupler. The 3D-structure diagram of the proposed coupler is shown in Figure 1d. This new structure can not only enhance the coupling strength effectively, but also reduce the size of the coupler. The floors (shown Figure 1b) can enhance the coupling strength and match the standard rectangular waveguide. Thus, this design can further decrease the size of the coupler. This structure can make the coupler size smaller than the coupler presented in [18]. The length of the coupling region (shown in Figure 1c) is only 15.82 mm, which is only 1.35 \(\times\)\(\lambda\)g (\(\lambda\)g is operation wavelength of center frequency). A sample coupler was manufactured and measured and is described herein. The coupling coefficient is \(-3.27\)\(\pm\) 0.53 dB in the operation bandwidth varying from 26.5 GHz to 40 GHz, in which the return loss is better than 17.46 dB, and the isolation is higher than 20.82 dB. To the authors' knowledge, no such compact 3 dB waveguide directional coupler with full waveguide bandwidth and good coupling coefficient has ever been presented before. The proposed coupler and some recently reported 3 dB directional couplers [11; 18; 19; 20; 21] are compared in Table 1.