# Highly Compact Through-Wire Microstrip to Empty Substrate Integrated Coaxial Line Transition
## 1 Introduction  
Novel empty devices based on coaxial transmission lines have been studied in recent years due to their interesting properties: pure-TEM propagation, non-dispersion, low radiation, low loss and high Q (unloaded quality factor), among others. This good performance can be achieved, essentially, due to the absence of dielectric, which reduces the degradation of electromagnetic waves and allows operation at a very high frequency without severe deterioration. Nevertheless, difficult manufacturing processes, high cost or difficult integration with other planar systems are inherent problems to topologies based on coaxial lines [1]. One possibility to solve the aforementioned drawbacks is the use of empty cavities in multilayer printed circuit boards (PCBs) to provide enhanced, integrated, and self-packaged substrate integrated devices, which is a research area that is attracting much interest in recent years. One of the most popular alternatives to implement this kind of integrated circuit is substrate integrated suspended line (SISL) [2]. In this technology, a layer of dielectric is used to support a metallic strip. This main layer is enclosed by other layers, below and above it, that are emptied around the strip. Finally, the structure is closed with the top and bottom layers that are not emptied, and therefore they isolate the main strip from the outside. This structure forms a completely integrated enclosing for the main line, which, being almost empty, provides better propagation performance than traditionaldielectric-filled planar lines. It is also easy to fabricate and is low cost. Because of its good characteristics, many devices have been implemented using SISL. For example, only in the last year, the literature has offered designs of filters [3, 4], antennas [5, 6], or couplers and power dividers [7, 8], among others. Another popular alternative to provide an empty substrate integrated transmission line is empty substrate integrated coaxial line (ESICL), first presented in [9] and later developed in [10]. ESICL is a structure that can be entirely fabricated with the standard dielectric substrate layers by using exclusively printed circuit board (PCB) standard manufacturing procedures. ESICL is built up by piling five conventional substrate layers, conveniently emptied where necessary, which create a metallic cavity in which a rectangular coaxial conductor is suspended. In comparison with SISL, which is only partially emptied, i.e., a dielectric substrate is used to support the main strip, ESICL provides a completely empty and integrated coaxial-like line, which performs even better than SISL, since guided waves do not interact with a lossy dielectric inside ESICL. In addition, ESICL is a pure TEM line (not quasi-TEM like microstrip or SISL), which is also an important advantage. Devices implemented in ESICL, like filters [10, 11], inverters [12], power dividers or directional couplers [13], can be manufactured quite easily and would achieve very high-quality responses and low losses, with performance much better than devices integrated in conventional planar lines or substrate integrated circuits (SICs), such as substrate integrated waveguide (SIW) [1], and only comparable to other completely emptied integrated waveguides, which is the case of the empty substrate integrated waveguide (ESIW) [1]. Two types of transitions are now currently available to inject power from conventional feeding lines to the ESICL structure: a grounded co-planar waveguide to ESICL (GCPW-ESICL) [10] and a microstrip-ESICL [14]. Both transitions feed the ESICL by the central layer and require a covered pseudo-transformer with a combination of metallized and non-metallized parts; this transformer is the most complicated element, requiring an accurate design and is relatively large. In this work, a new transition is presented that feeds the ESICL structure from the top cover with a through-wire connecting the microstrip line with the inner coaxial conductor.