# Through-Wire Microstrip-to-Empty-Substrate-Integrated-Waveguide Transition at Ka-Band
## 1 Introduction  
The Empty Substrate-Integrated Waveguide (ESIW), first proposed in 2014 [1], maintains the main features of the Substrate-Integrated Waveguide (SIW) [2], such as small size and integration with other circuits in the same substrate, easy manufacturing, and low cost. Moreover, since the dielectric is removed in ESIW circuits, other advantages are also added: higher quality factors in resonators and better performance (mainly in terms of insertion losses).  
To test the good performance of the ESIW, many different devices (filters, antennas, directional couplers, phase-shifters, etc.) have been properly designed and manufactured, and several transitions to connect the ESIW to microstrip lines have also been proposed [3].  
The first transition proposed [1] had a sharp dielectric taper. In this transition, the transformation of the microstrip mode into the fundamental mode of the waveguide is carried out through a metallic iris followed by an exponential taper. This transition was improved in [4] for thin substrates. To make it possible, an additional taper in the microstrip feedingline was added. The last version of the transition [5] increased its mechanical accuracy adding two air holes and prevented radiation losses with via holes. Nevertheless, all of them include metallized and non-metallized parts, which increases the manufacturing complexity.  
Apart from transitions based on sharp dielectric tapers, some more solutions have been proposed in the literature. In [6], a section of ESIW with increased width is used to avoid the sharp dielectric taper, but the size of the transition and the radiation losses were increased. This transition was then modified in [7] using a tapered artificial dielectric slab matrix, which increased considerably its complexity, and then in [8], where an elliptical dielectric taper was added. Another solution consists of using a through-wire from the microstrip line to excite the ESIW resulting in is a very versatile transition [9]. The last transition proposed use a tapered microstrip transition between the 50 \(\Omega\) microstrip line and the ESIW for impedance and mode matching [10].  
Furthermore, it is possible to find other transitions specifically designed for multilayer devices. The first one was presented in [11] to connect ESIWs in contiguous layers, being necessary the continuous concatenation of several versions of this transition to connect non-contiguous ESIWs. Another one was proposed in [12], allowing the connection of ESIWs separated by more than one layer. The last one is based in a sharp dielectric taper and integrated ESIW devices with a higher number of layers through a transformer at a height adapted to the ESIW [13].  
Among all of the proposed transitions, the most versatile one is the through-wire transition [9] because it is possible to choose any angle between the feeding line and the ESIW, and the height of the microstrip and the ESIW can be independently chosen as both are implemented in different layers. Thus, it is possible to increase the quality factor in resonators and reduce losses in the ESIW. Apart from that, the transition is extremely compact and less sensitive to manufacturing errors. This transition was first presented in the \(X\)-band, with the aim of this paper being the not-always-direct validation of the design method explained in [9] at higher frequencies, more specifically at Ku- and Ka-bands. After the validation process in simulation, a back-to-back transition was manufactured in the Ka-band, as it is the most difficult one as manufacturing tolerances become more important, obtaining measured insertion losses lower than 3.7 dB and return losses higher that 11.7 dB.