# A Gap Waveguide-Based Compact Rectangular Waveguide to a Packaged Microstrip Inline Transition
## 1 Introduction  
Gap waveguide technology-based components have been getting a lot of attention in the last few years [1; 2]. These components are based on the so-called bed of nails [3], a periodic structure composed by metallic pins covered by a metal lid. This bed of nails creates an artificial magnetic conductor (AMC) or high impedance surface (HIS) that prevents electromagnetic wave propagation in a certain frequency region [4]. Such a property has been successfully used for microwave and millimeter wave circuit packaging, avoiding resonances which could degrade their performance [5; 6; 7]. Taking this structure as the baseline, waveguides can be created if an air channel is formed in between two bed of nail sections, which behave as high impedance surfaces and forbid electromagnetic wave propagation in any direction other than the air channel formed in between them. This channel can have different forms, depending on its height, its section and the presence of a dielectric substrate, leading to different types of waveguides [8; 9; 10]. Based on these waveguides a variety of components [11; 12; 13; 14; 15] have been created. In addition, the reduction of cross-talk effects allows for improving the performances of different components such as grid amplifiers [16]. Finally, gap waveguide technology has been proposed as an alternative solution for antenna array feeding networks [17; 18]. The properties of these waveguides have been explored, and found to be similar to those of an equivalent standard rectangular or ridge metal waveguide. However, there is a fundamental advantage in terms of fabrication that makes these waveguides of great interest, since they do not require electrical contact between the pins and the lid that covers them. In addition to the rectangular or ridge waveguide behavior, the gap waveguide can perform similarly to a microstrip line. That is the case for the ridgegap waveguide (RGW), which overcomes the significant transmission losses that dielectric substrates introduce in planar transmission lines such as microstrips, in the millimeter wave range and above. Therefore, for these waveguides, manufacturing and assembly of sub-millimeter wave components can be largely simplified, since, in particular, gap waveguide components allow H-plane cutting. This property has been explored in the aforementioned components and in other packaging applications. In this field, they have deserved attention for the design of transitions from planar to waveguide technology for frequencies higher than 60 GHz, which can be of interest for integration of MMIC and waveguide-based components [19; 20; 21]. The use of gap waveguides has been shown to simplify assembly and allows working wide dielectric substrates. In this paper, two simplified inline transitions from microstrip to standard rectangular waveguides employing gap waveguide technology are presented. This type of transition has been long studied and some relevant examples can be found in the literature [22; 23; 24; 25; 26]. In this case the design method and configurations have some similarities with that presented in [27; 28], since all of them are based on Chebyshev transformers. However, in this case the transition design benefits from the use of of gap waveguide sections. First, the need for good electrical contact between upper and bottom parts in [27] is alleviated. Moreover, the intricate shapes required in [28] are avoided in this case. Therefore, manufacturing and assembly are simplified, leading to cost reduction with respect to previous designs. Note also that, as a difference to [28], in this work the gap waveguide sections are just used to implement the Chebyshev sections and include the dielectric substrate as a part of the bed of nails, which allows working with wide substrates, without requiring any shaping. However, the design procedure complicates things, since there are limitations in the design caused by the higher-order modes than can propagate in some of the dielectric-filled Chebyshev transformer steps.