Darío Herraiz

In this work, corona discharge breakdown thresholds in empty substrate integrated waveguide (ESIW) bandpass filters are investigated and experimentally validated for the first time. The objectives are the analysis and subsequent enhancement of the peak power handling capability (PPHC) of inductive ESIW filters operating in low-pressure environments, where gas breakdown may limit the maximum admissible RF power level. First, the critical regions where the electric field (EF) reaches its maximum amplitude are identified by full-wave electromagnetic (EM) simulations. Then, an enhancement strategy based on the introduction of drilled apertures in the ESIW covers is applied, in order to reshape the field distribution in the most critical zones without compromising the in-band response. The results show a consistent increase in the corona discharge threshold at low pressure, between 2 and 3.1dB, with respect to a reference ESIW bandpass filter of the same electrical specifications.

This letter presents an in-line grounded coplanar waveguide (GCPW) to empty the substrate integrated waveguide (ESIW) transition that enables the low-loss, high-performance microwave systems based on the ESIW technology. The proposed structure uses several matching elements including an ESIW widening, triangular and transverse slots, and an iris—without complex manufacturing dielectric tapers or bulky multilayer structures. A fabricated prototype validates the structure, achieving a return loss level better than 18dB and an insertion loss of ${\mathrm {0.45~}}~\pm ~{\mathrm {0.1~\text {dB}}}$ over 10–17.3GHz, demonstrating a near-octave bandwidth and low-loss performance.

Substrate Integrated Circuits (SICs) represent a significant advancement in microwave communication systems due to their high efficiency, performance, and integration capabilities. Empty Substrate-Integrated Waveguides (ESIWs) are a type of SIC that offers benefits such as cost-effectiveness while maintaining high performance. This paper presents the design and implementation of the first floating patch antenna integrated into an ESIW, fed by a metallic rod. The proposed antenna is designed to operate in the X-band (8–12 GHz), with a resonance peak at 10 GHz. The patch antenna is square, which provides interesting radiation characteristics. It is excited by a metallic rod that connects the patch to the ESIW line, resulting in excellent performance in terms of measured radiation efficiency (over 90%) and −10 dB impedance bandwidth (approximately 20%). The prototype demonstrates minimal differences between the simulated and manufactured versions. These results highlight the potential of ESIW-fed floating patch antennas for advanced satellite communication systems. This will enable the integration of complete communication systems within ESIWs and facilitate the straightforward development of 2D element arrays.

It is well-known that Empty Substrate Integrated Waveguides allow not only the integration of classic waveguides, such as rectangular waveguides, into a printed circuit board maintaining the quality of traditional waveguide devices, but also keeping in these circuits the advantages of planar devices: low cost, high integration and mass production capabilities. Nevertheless, a critical point to consider in the success of manufacturing these devices is the soldering of metal covers that close the structure and create the waveguide. This paper presents a new soldering process that simplifies the former procedures, making it easier and reducing economic and time costs. To validate this new assembling procedure, two prototypes have been manufactured and measured: a back-to-back from microstrip to ESIW, and a Ku-band filter; repeteability of the fabrication process has been proved and high performance results have been obtained.

The electrical response of low-frequency band-pass filters based on periodic substrate integrated waveguide (SIW) technology typically shows permitted and forbidden frequency bands. Therefore, this type of filters can be designed using a conceptually very simple and efficient procedure based exclusively on the study of the dispersion properties of the periodic structure. In this paper, we go a step further with the design of a periodically air-filled SIW band-pass filter in which part of the dielectric substrate is removed to reduce insertion losses, and whose unit cell parameters, which are directly related to the center frequency ( $f_{c}$ ) and bandwidth (BW) of the first passband, and also to the first stopband or bandgap (BG) of the structure, have been appropriately selected for filtering purposes, thus providing some useful design rules. Furthermore, we apply the concept of glide symmetry for achieving a much larger fractional bandwidth (FBW) than that obtained in conventional air-filled SIW filters found in the technical literature. Finite implementations of both periodic structures with and without glide symmetry have been analyzed, showing their filtering response for validation purposes. Additionally, to overcome the matching level restrictions in the resulting air-filled periodic SIWs, a microstrip-to-SIW transition including a novel coupling iris is proposed. A prototype of the proposed air-filled glide-symmetric periodic SIW filter has been manufactured and experimentally validated, illustrating the potential of this technique to obtain large FBWs that can not be achieved in conventional air-filled SIW filters. The proposed filter proves to be a good candidate for millimeter wave applications.

3D printing is one of the most promising manufacturing methods in the most developed technological fields, including microwave hardware fabrication. On the other hand, the well-known manufacturing methods of planar substrate integrated circuits allow high-quality prototypes to be made at low cost and with mass production capabilities. The combination of both manufacturing methods, 2D or 2.5D (substrate integrated circuits) and 3D (3D printed structures), will allow us to take advantage of the main strengths of each technology and minimise disadvantages. In this article, for the first time, a transition structure between the Empty Substrate-Integrated Waveguide (ESIW) technology—a planar waveguide integrated on a printed circuit board—and a standard rectangular waveguide manufactured by 3D printing is proposed. This transition will make it possible to combine planar circuits with 3D structures, thus taking advantage of the benefits of both types of technologies. The fabricated prototype presents low losses (0.6 dB for the transmission coefficient and 15 dB for reflection coefficient), good electrical response (very flat), and simultaneously good mechanical stability and robustness to manufacturing and assembly errors. The proposed design for this transition piece is easily realisable for a wide range of affordable 3D printers. Repeatability is guaranteed and the proposed transition allows us to combine different SIC structures to 3D printed circuits. Hence, this transition will enable advancements in the fabrication of microwave devices, particularly with regard to satellite communications.

Manufacture of hybrid 3D-planar circuits, especially those incorporating empty waveguides on substrates, can benefit from most standardized planar fabrication processes, although they are not exactly the same. For this reason, planar circuit manufacturing methods must be adapted to the requirements of these new circuits. Through numerous fabrications and successful designs, several enhancing strategies have been established to improve all the manufacturing phases to achieve better results. They all have been proved in the following substrate-integrated technologies for the manufacturing of microwave devices: ESIW, ESICL, continuous profile, and microstrip. Thanks to these improvements, good-quality prototypes such as transitions, filters, circulators, couplers, antennas, among others, have been fabricated. Throughout the next chapter, these strategies applied along the manufacturing process will be explained: from the first manufacturing phase to the final welding of the whole circuit and taking into account external elements such as wires that may be present in these structures. For this purpose, some devices that have been published will be used as examples.

This paper presents a feasibility study for designing a floating patch antenna structure fed with a probe from a microstrip. The main premise is to eliminate the dielectric in the patch design, which is equivalent to having an air dielectric and leads to the necessity of proper support to fasten the patch in the air. The novelty of this paper is that this new device, apart from being fed with the though-wire technique directly from the microstrip line, has to be, by design, robust and easy to manufacture, and, at the same time, it has to present, simultaneously, good values in all of the performance indexes. A prototype has been designed, manufactured, and measured with good performance results: a bandwidth higher than 10% around 2.4 GHz, a radiation efficiency higher than 96%, a 9.63 dBi gain, and a wide beamwidth. The main advantages of this prototype, together with its good performance indexes, include its low fabrication cost, low losses, light weight, robustness, high integration capability, the complete removal of the dielectric material, and the use of a single post for feeding the patch while simultaneously fixing its floating position.

Substrate integrated waveguides (SIWs) are a compromise in terms of performance, cost, and volume between classical planar technologies and high quality three-dimensional waveguides. Empty SIWs (ESIW) reduce the losses by removing the dielectric substrate. In order to increase the usable bandwidth of the ESIW, a ridge layer can be added to the ESIW, thus obtaining the so-called ridge ESIW (RESIW). A transition from microstrip to RESIW has already been presented, but no other passive component has yet been implemented in this new technology. In this letter, we present a novel bandpass filter in RESIW, based on a new U-shaped geometry acting as impedance inverter, that can be easily fabricated with planar circuit manufacturing techniques. A design procedure for this filter is presented. A four-pole Chebyshev filter centered at 15 GHz, with a relative bandwidth of 3%, has been manufactured. The measured values for insertion and return losses are, respectively, lower than 1.05 dB and greater than 18 dB in the whole passband.

The Empty Substrate Integrated Waveguides (ESIW) maintain the advantages of the Substrate Integrated waveguide (SIW) (i.e. low-volume, low profile, lightweight, easy manufacturing, and integration in a planar circuit board), and present lower losses and higher quality factors in resonators due to the propagation of the fields through air, instead of through lossy dielectric as in SIW. The operational (monomode) bandwidth of the ESIW can be increased with the Single Ridge ESIW (SRESIW). However, the bandwidth can be further increased with the Double Ridge ESIW (DRESIW). In this paper, a brief study of possible DRESIW geometries has been performed, and two transitions from microstrip line (MS) to DRESIW with a dielectric taper geometry based on different equations are proposed. The new wideband transitions present simulated return losses in back-to-back configurations greater than 20 dB in more than a 95% fractional bandwidth. The transition that presents a better compromise between return losses, bandwidth and ease of fabrication is manufactured. The measured return and insertion losses are better than 19.7 dB and 1.5 dB, respectively, in a 96.4% fractional bandwidth.