Reconfigurable Devices using Liquid Crystal at Microwave Frequencies in Substrate Integrated Waveguide

[ES] La cantidad de servicios de telecomunicación se ha incrementado signiticativamente en las últimas décadas. El uso de teléfonos inteligentes, así como el Internet de las Cosas, está generando una saturación del espectro electromágnetico. Por tanto, los requisitos de los sistemas de microondas han cambiado para adaptarse a estos nuevos avances. Para satisfacer estas necesidades, se busca el desarrollo de dispositivos de bajo coste, volumen, peso y consumo. Además, interesa que sean espectralmente eficientes y fácilmente integrables con otros dispositivos. Entre todos los dispostivos de microondas, los filtros son elementos clave dentro de los sistemas de comunicaciones móviles e inalámbricas. Es por ello que el diseño de filtros que cumplan con los requisitos mencionados se ha convertido en un tema de gran interés. Para dar respuesta a este problema ha surgido la tecnología de Guía de Onda Integrada en Sustrato (Substrate Integrated Waveguide (SIW)), que permite la implementación de filtros con un reducido tamaño y fácilmente integrables con otros dispositivos en tecnología planar. Dicha tecnología presenta unas prestaciones en cuanto a manejo de potencia y pérdidas mejores que la tecnología de circuito impreso (Printed Circuit Board (PCB)), aunque no llegan a ser iguales que las de la guía de onda clásica. Por otro lado, la saturación espectral también lleva al estudio de filtros con respuestas variables en frecuencia, es decir, que puedan cambiar su frecuencia central y ancho de banda con el fin de adaptarse a las necesidades del sistema. Por ello, el objetivo general de esta Tesis es el análisis y diseño de nuevos filtros reconfigurables en tecnología integrada. El trabajo empieza con el estudio de los fundamentos de los filtros de microondas hasta llegar al diseño de resonadores reconfigurables en tecnología SIW usando el cristal líquido como material de reconfiguración. En primer lugar, se ha estudiado la influencia que los cambios en el valor de la permitividad dieléctrica en el interior de las estructuras filtrantes pueden tener en la respuesta de las mismos. En particular, se desarrollan filtros alternando secciones de línea con y sin dieléctrico dentro de una SIW vacía, Empty Substrate Integrated Waveguide (ESIW). Una vez hecho esto, se procede al estudio de materiales que tengan un valor de permitividad dieléctrica variable de alguna forma. En concreto, se ha realizado la caracterización de diferentes mezclas de cristal líquido a la frecuencia de microondas. Dicho material cambia su valor de permitividad cuando se le aplica un campo eléctrico o magnético. Dado que para la reconfiguración de la respuesta de los filtros se requiere de una estructura desacoplada en baja frecuencia, es decir, con más de un conductor, se ha desarrollado una estrategia para el desacoplo de la estructuras ESIW, la tecnología Decoupled Empty Substrate Integrated Waveguide (DESIW). Por último, se han diseñado resonadores en dicha tecnología DESIW, que se han llenado de cristal líquido y aplicado unos campos de polarización, consiguiendo variar su respuesta en frecuencia. Dichos resonadores constituyen el elemento básico para el desarrollo de filtros de microondas. Es por ello que el conocimiento obtenido en la Tesis es una buena base para futuros trabajos esta tecnología que permitan conseguir filtros de altas prestaciones.[CA] La quantitat de serveis de telecomunicació s'ha incrementat significativament en les últimes dècades. L'ús de telèfons intel$\cdot$ligents, així com la internet de les coses, està generant una saturació de l'espectre electromagnètic. Per tant, els requisits dels sistemes de microones han canviat per a adaptar-se a aquests nous avanços. Per a satisfer aquestes necessitats, se cerca el desenvolupament de dispositius de baix cost, volum, pes i consum. A més, interessa que siguen espectralment eficients i fàcilment integrables amb altres dispositius. Entre tots els dispositius de microones, els filtres són elements clau dins dels sistemes de comunicacions mòbils i sense fil. És per això que el disseny de filtres que complisquen els requisits esmentats s'ha convertit en un tema de gran interès. Per a donar resposta a aquest problema ha sorgit la tecnologia de Guia d'Ona Integrada en Substrat (Substrate Integrated Waveguide (SIW)), que permet la implementació de filtres amb una reduïda grandària i fàcilment integrables amb altres dispositius en tecnologia planar. Aquesta tecnologia presenta unes prestacions quant a maneig de potència i pèrdues millors que la tecnologia de circuit imprès (Printed Circuit Board (PCB)), encara que no arriben a ser iguals que les de la guia d'ona clàssica. D'altra banda, la saturació espectral també porta a l'estudi de filtres amb respostes variables en freqüència, és a dir, que puguen canviar la seua freqüència central i l'amplada de banda amb la finalitat d'adaptar-se a les necessitats del sistema. Per això, l'objectiu general d'aquesta tesi és l'anàlisi i el disseny de nous filtres reconfigurables en tecnologia integrada. El treball comença amb l'estudi dels fonaments dels filtres de microones, fins a arribar al disseny de ressonadors reconfigurables en tecnologia SIW usant el cristall líquid com a material de reconfiguració. En primer lloc, s'ha estudiat la influència que els canvis en el valor de la permitivitat dielèctrica a l'interior de les estructures filtrants poden tenir en la resposta d'aquestes. En particular, es desenvolupen filtres que alternen seccions de línia amb dielèctric i sense dins d'una SIW buida, Empty Substrate Integrated Waveguide (ESIW). Una vegada fet això, es procedeix a l'estudi de materials que tinguen un valor de permitivitat dielèctrica variable d'alguna forma. En concret, s'ha realitzat la caracterització de diferents mescles de cristall líquid a la freqüència de microones. Aquest material canvia el seu valor de permitivitat quan se li aplica un camp elèctric o magnètic. Atès que per a la reconfiguració de la resposta dels filtres es requereix una estructura desacoblada en baixa freqüència, és a dir, amb més d'un conductor, s'ha desenvolupat una estratègia per al desacoblament d'estructures ESIW, la tecnologia Decoupled Empty Substrate Integrated Waveguide (DESIW). Finalment, s'han dissenyat ressonadors en aquesta tecnologia DESIW, que s'han omplit de cristall líquid i aplicat uns camps de polarització, i s'ha aconseguit variar la seua resposta en freqüència. Aquests ressonadors constitueixen l'element bàsic per al desenvolupament de filtres de microones. És per això que el coneixement obtingut en la tesi és una bona base per a futurs treballs d'aquesta tecnologia que permeten aconseguir filtres d'altes prestacions.[EN] The number of telecommunication services has increased significantly in recent decades. The use of smartphones, as well as the Internet of Things, is generating a saturation of the electromagnetic spectrum. Therefore, the requirements of microwave systems have changed to adapt to these new developments and related challenges. For achieving these needs, the development of devices with low cost, volume, weight and power consumption is sought. In addition, it interests to be spectrally efficient, to offer high performance, and to be easily integrated with other devices. Among all microwave devices, filters are key elements within mobile and wireless communication systems. In this context, the design of filters that meet the aforementioned requirements has become a topic of great interest. For solving this problem, Substrate Integrated Waveguide (SIW) technology has emerged, which allows the implementation of filters with a small size and to be easily integrated with other devices in planar technology. This technology has better power handling and loss performance than Printed Circuit Board (PCB) technology, although they do not have the performance of the classic waveguide counterpart. On the other hand, the spectral saturation also leads to the study of filters with tunable frequency response, that is, they can change their central frequency and bandwidth, in order to fulfil the changing system requirements. Therefore, the general objective of this PhD Thesis work is the analysis and design of new reconfigurable filters in integrated technology. The work begins with the study of the basics of microwave filters until the design of reconfigurable resonators in SIW technology, using Liquid Crystal (LC) as reconfiguration material. Firstly, the influence that the change of the dielectric permittivity value inside the filtering structures have on the frequency response has been studied. Particularly, filters have been obtained by alternating line sections with and without dielectric material inside an empty SIW (Empty Substrate Integrated Waveguide (ESIW)). Once this is done, it is proceed to the study of materials that have a variable dielectric permittivity value. Specifically, the characterization of different LC mixtures at microwave frequencies has been carried out. This material changes its permittivity value when an electric or magnetic bias field is applied. A low-frequency decoupled structure is required for the reconfiguration of filters, that is, structures with more than one conductor. For that, a strategy for decoupling ESIW structures has been developed, i.e, the Decoupled Empty Substrate Integrated Waveguide (DESIW) technology. Finally, some resonators have been designed in DESIW technology, which have been filled with LC. The use of LC allows to tune their frequency response. These resonators are basic elements for the development of microwave filters. So that, the knowledge obtained in this Thesis work is a good basis for future works in this technology that allow for achieving high performance filters.Sánchez Marín, JR. (2019). Reconfigurable Devices using Liquid Crystal at Microwave Frequencies in Substrate Integrated Waveguide [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/132183TESI

Controlled Out-of-Band Rejection of Filters based on SIW with Alternating Dielectric Line Sections

© 2019 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[EN] A study for managing the out-of-band rejection in a new topology of filters based on substrate integrated waveguide with alternating dielectric line sections (ADLSs) is presented in this letter. ADLS is a filtering structure consisting on line sections of the same width but with alternating air and dielectric filling. The design of the lengths of each section provides the central frequency and the bandwidth of bandpass filter response. The proper selection of the structure substrate dielectric permittivity can increase the rejection band up to 2 f(o). Moreover, the selection of the filter order (i.e., the number of sections with and without dielectric) can affect the depth of the rejection band. A study of the width and the depth of the rejection band is performed with different permittivities and orders for two different filters. Then, for validation purposes, the prototypes of both filters have been manufactured and measured.This work was supported in part by the Generalitat Valenciana Research Project under Grant PROMETEOII/2015/005, in part by the Ministerio de Educacion, Cultura y Deporte, Spain, through the Fellowship Program for Training University Professors under Grant FPU14/00150, and in part by the Ministerio de Economia y Competitividad, Spain, through Research and Development Project under Grant TEC2016-75934-C4-1-R.Sánchez-Marín, JR.; Bachiller Martin, MC.; Nova-Giménez, V.; Boria Esbert, VE. (2019). Controlled Out-of-Band Rejection of Filters based on SIW with Alternating Dielectric Line Sections. IEEE Microwave and Wireless Components Letters. 29(4):258-260. https://doi.org/10.1109/LMWC.2019.2902034S25826029

Reconfigurable Resonator in Decoupled Empty SIW Technology Using Liquid Crystal Material

"This paper is a postprint of a paper submitted to and accepted for publication in Electronics Letters and is subject to Institution of Engineering and Technology Copyright. The copy of record is available at IET Digital Library"[EN] This paper presents a novel continuously tunable microwave resonator based on Liquid Crystal (LC) material. The reconfigurable device is implemented in Decoupled Empty Substrate Integrated Waveguide (DESIW) technology. DESIW technology allows both magnetic and electric DC (or low frequency) biasing of the LC. The resonator is continuously tunable in a range of 600 MHz, resulting in a measured tunability range of 8.5% around a centre frequency of 7 GHz. Moreover, the measured insertion loss of the reconfigurable device is between 4.3 dB and 6.8 dB, thus obtaining a Q-factor of 190-172.This work was partially funded by the Generalitat Valenciana research project PROMETEOII/2015/005, by the Ministerio de Educación, Cultura y Deporte (Spain) under the Fellowship Program for Training University Professors FPU14/00150, and by Ministerio de Economía y Competitividad (Spain) under R&D project TEC2016-75934-C4-1-R.Sánchez-Marín, JR.; Bachiller Martin, MC.; Nova-Giménez, V.; Boria Esbert, VE. (2019). Reconfigurable Resonator in Decoupled Empty SIW Technology Using Liquid Crystal Material. Electronics Letters. 55(16):907-910. https://doi.org/10.1049/el.2019.1088S9079105516Sekar, V., Armendariz, M., & Entesari, K. (2011). A 1.2–1.6-GHz Substrate-Integrated-Waveguide RF MEMS Tunable Filter. IEEE Transactions on Microwave Theory and Techniques, 59(4), 866-876. doi:10.1109/tmtt.2011.2109006Adhikari, S., Ban, Y.-J., & Wu, K. (2011). Magnetically Tunable Ferrite Loaded Substrate Integrated Waveguide Cavity Resonator. IEEE Microwave and Wireless Components Letters, 21(3), 139-141. doi:10.1109/lmwc.2010.2102746Prasetiadi, A. E., Karabey, O. H., Weickhmann, C., Franke, T., Hu, W., Jost, M., … Jakoby, R. (2015). Continuously tunable substrate integrated waveguide bandpass filter in liquid crystal technology with magnetic biasing. Electronics Letters, 51(20), 1584-1585. doi:10.1049/el.2015.2494Yaghmaee, P., Fumeaux, C., Bates, B., Manabe, A., Karabey, O. H., & Jakoby, R. (2012). Frequency tunable S-band resonator using nematic liquid crystal. Electronics Letters, 48(13), 798. doi:10.1049/el.2012.1366Sánchez, J. R., Bachiller, C., Esteban, H., Belenguer, A., Nova, V., & Boria, V. (2017). New decoupled empty substrate integrated waveguide realisation. Electronics Letters, 53(17), 1203-1205. doi:10.1049/el.2017.1240Yang, D.-K., & Wu, S.-T. (2006). Fundamentals of Liquid Crystal Devices. doi:10.1002/0470032030Belenguer, A., Esteban, H., & Boria, V. E. (2014). Novel Empty Substrate Integrated Waveguide for High-Performance Microwave Integrated Circuits. IEEE Transactions on Microwave Theory and Techniques, 62(4), 832-839. doi:10.1109/tmtt.2014.2309637Munk, B. A. (2000). Frequency Selective Surfaces. doi:10.1002/0471723770Esteban, H., Belenguer, A., Sanchez, J. R., Bachiller, C., & Boria, V. E. (2017). Improved Low Reflection Transition From Microstrip Line to Empty Substrate-Integrated Waveguide. IEEE Microwave and Wireless Components Letters, 27(8), 685-687. doi:10.1109/lmwc.2017.272401

Dynamic Permittivity Measurement of Ground-Tire Rubber (GTR) during Microwave-Assisted Devulcanization

[EN] Many efforts are being made to find innovative ways of recycling rubber from end-of-life tires (ELTs), also called ground tire Rubber (GTR). Recycling through devulcanization allows the reintroduction of rubber back into the manufacturing industry. Such a process requires providing enough energy to break the sulfur links, while preventing damage to the polymeric chain. Microwave heating is controllable, efficient, and it does not rely on conventional heating mechanisms (conduction, convection) which may involve high heating losses, but rather on direct dielectric heating. However, to adequately control the microwave-assisted devulcanization performance, a thorough knowledge of the GTR permittivity versus temperature is required. In this work, GTR permittivity was monitored during its devulcanization. A resonant technique based on a dual-mode cylindrical cavity was used to simultaneously heat rubber and measure its permittivity at around 2 GHz. The results show sharp changes in the GTR permittivity at 160 and 190 degrees C. After the GTR cooled down, a shifted permittivity evidences a change in the GTR structure caused by the devulcanization process. Microwave-assisted devulcanization effectiveness is proven through time-domain nuclear magnetic resonance (NMR) measurements, by verifying the decrease in the cross-link density of processed GTR samples compared to the original sample.This research project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement number 870,000. More information at https://cordis.europa.eu/project/id/870000 (accessed on 27 August 2022) and https://valuerubber.eu/(accessed on 27 August 2022). JLV and FMS also thanks the funding from Ministerio de Ciencia e Innovacion (PID2020-119047RB-I00 y PLEC2021-007793), Gobierno de Aragon (EC-22-2021) and CSIC (201860E045).Pérez-Campos, R.; Fayos-Fernández, J.; Monzó-Cabrera, J.; Martín Salamanca, F.; López Valentín, J.; Catalá Civera, JM.; Plaza González, PJ.... (2022). Dynamic Permittivity Measurement of Ground-Tire Rubber (GTR) during Microwave-Assisted Devulcanization. Polymers. 14(17):1-21. https://doi.org/10.3390/polym14173543121141

Improved low reflection transition from microstrip line to empty substrate integrated waveguide

"© 2017 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works."[EN] Substrate-integrated waveguides (SIWs) maintain the advantages of planar circuits (low loss, low profile, easy manufacturing, and integration in a planar circuit board) and improve the quality factor of filter resonators. Empty substrateinte-grated waveguides (ESIWs) substantially reduce the insertion losses, because waves propagate through air instead of a lossy dielectric. The first ESIW used a simple tapering transition that cannot be used for thin substrates. A new transition has recently been proposed, which includes a taper also in the microstrip line, not only inside the ESIW, and so it can be used for all substrates, although measured return losses are only 13 dB. In this letter, the cited transition is improved by placing via holes that prevent undesired radiation, as well as two holes that help to ensure good accuracy in the mechanization of the input iris, thus allowing very good return losses (over 20 dB) in the measured results. A design procedure that allows the successful design of the proposed new transition is also provided. A back-to-back configuration of the improved new transition has been successfully manufactured and measured.This work was supported in part by the Ministerio de Economia y Competitividad, Spanish Goverment, under Grant TEC2013-47037-C05-3-R and Grant TEC2013-47037-C05-1-R, and in part by the Ministerio de Educacion, Cultura y Deporte under the Fellowship Program for Training University Professors under Grant FPU14/00150.Esteban González, H.; Belenguer, Á.; Sánchez-Marín, JR.; Bachiller Martin, MC.; Boria Esbert, VE. (2017). Improved low reflection transition from microstrip line to empty substrate integrated waveguide. IEEE Microwave and Wireless Components Letters. 27(8):685-687. https://doi.org/10.1109/LMWC.2017.2724011S68568727

Targeted Metabolomics Shows That the Level of Glutamine, Kynurenine, Acyl-Carnitines and Lysophosphatidylcholines Is Significantly Increased in the Aqueous Humor of Glaucoma Patients.

The composition of the aqueous humor of patients with glaucoma is relevant to understand the underlying causes of the pathology. Information on the concentration of metabolites and small molecules in the aqueous humor of healthy subjects is limited. Among the causes of the limitations is the lack of healthy controls since, until recently, they were not surgically intervened; therefore, the aqueous humor of patients operated for cataract was used as a reference. Sixteen aqueous humor samples from healthy subjects undergoing refractive surgery and eight samples from glaucoma patients were used to assess the concentration of 188 compounds using chromatography and mass spectrometry. The concentration of 80 of the 188 was found to be reliable, allowing comparison of data from the two groups (glaucoma and control). The pattern found in the controls is similar to, but not the same as, that reported using samples from 'controls' undergoing cataract surgery. Comparing data from glaucoma patients and healthy subjects, 57 of the 80 compounds were significantly (p < 0.05) altered in the aqueous humor. Kynurenine and glutamine, but not glutamate, were significantly increased in the glaucoma samples. Furthermore, 10 compounds were selected considering a statistical score of p < 0.0001 and the degree of change of more than double or less than half. The level of C10 (decanoyl)-carnitine decreased, while the concentration of spermidine and various acyl-carnitines and lysophosphatidylcholines increased in glaucoma. Principal component analysis showed complete segregation of controls and cases using the data for the 10 selected compounds. The receiver operating characteristic curve these 10 compounds and for glutamine allowed finding cut-off values and significant sensitivity and specificity scores. The concentration of small metabolites in the aqueous humor of glaucoma patients is altered even when they take medication and are well controlled. The imbalance affects membrane components, especially those of the mitochondria, suggesting that mitochondrial abnormalities are a cause or consequence of glaucoma. The increase in glutamine in glaucoma is also relevant because it could be a means of keeping the concentration of glutamate under control, thus avoiding its potential to induce the death of neurons and retinal cells. Equally notable was the increase in kynurenine, which is essential in the metabolism of nicotine adenine dinucleotides

Microwave Filter based on Substrate Integrated Waveguide with Alternating Dielectric Line Sections

(c) 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this[EN] A new topology of microwave filter based on sub- strate integrated waveguide (SIW) with alternating dielectric line sections is presented in this letter. The two alternating sections are waveguides without and with a dielectric material acting, respectively, as the inverters and the resonant sections of the pro- posed filter. This topology allows to easily design and manufacture compact filters with lower losses and higher rejection band than other available SIW realizations. For validation purposes, a prototype of a four-pole bandpass filter, based on alternating dielectric line sections, has been designed and manufactured, including successful experimental results.This work was supported in part by the Generalitat Valenciana Research Project under Grant PROMETEOII/2015/005, in part by the Ministerio de Educacion, Cultura y Deporte (Spain) through the Fellowship Program for Training University Professors under Grant FPU14/00150, and in part by the Ministerio de Economia y Competitividad (Spain) through the Research and Development Project under Grant TEC2016-75934-C4-1-R.Sánchez-Marín, JR.; Bachiller Martin, MC.; Juliá Morte, M.; Nova-Giménez, V.; Esteban González, H.; Boria Esbert, VE. (2018). Microwave Filter based on Substrate Integrated Waveguide with Alternating Dielectric Line Sections. IEEE Microwave and Wireless Components Letters. 28(11):990-992. https://doi.org/10.1109/LMWC.2018.2871644S990992281

New Decoupled Empty Substrate Integrated Waveguide Realization

[EN] This work presents a novel Decoupled Empty Substrate Integrated Waveguide (DESIW), which enables AC/DC decoupling in any device that is integrated in it. The decoupling strategy is performed throughout a micro-milling square pattern of easy implementation that increases the insertion losses compared to the standard ESIW. However, the DESIW related losses are still tolerable, allowing the employment of the new periodic structure in many practical applications. In particular, it can be used for the design and manufacturing of reconfigurable devices which need a bias voltage on the whole device, or just on some of its particular areas. A broadband transition from DESIW to microstrip planar lines has been also successfully designed. The new line has been manufactured and measured.This work was partially founded by the Generalitat Valenciana research project PROMETEOII/2015/005, by the Ministerio de Educacion, Cultura y Deporte (Spain) under the Fellowship Program for Training University Professors FPU14/00150 and by Ministerio de Economia y Competitividad (Spain) under R&D project TEC2016-75934-C4-R.Sánchez-Marín, JR.; Bachiller Martin, MC.; Esteban González, H.; Belenguer Martínez, Á.; Nova-Giménez, V.; Boria Esbert, VE. (2017). New Decoupled Empty Substrate Integrated Waveguide Realization. Electronics Letters. 53(17):1203-1205. https://doi.org/10.1049/el.2017.1240S12031205531

Miniaturization of Power Divider and 90º Hybrid Directional Coupler for C-Band Applications Using Empty Substrate-Integrated Coaxial Lines

(c) 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this[EN] This paper presents the practical realization of a power divider and a 90° hybrid directional coupler in an empty substrate-integrated coaxial line (ESICL) for C-band frequency applications. This new type of transmission line is very promising in terms of electric performance, bandwidth, integration with other planar circuits, and manufacturing simplicity. The ESICL has been designed for obtaining a wide monomode bandwidth with a characteristic impedance of 50 . Furthermore, an improvement of the efficient transition between the ESICL and the grounded coplanar waveguide, used as feeding line, has been also proposed. The passive devices built using this technology are reduced in mass and volume, keeping robustness, and providing a well-balanced power division, as well as reduced losses and high isolation in the whole operational bandwidth. Two prototypes have been manufactured and the experimental results are in good agreement with the simulated designs.This work was supported in part by the Generalitat Valenciana Research under Project PROMETEOII/2015/005, in part by the Ministerio de Educacion, Cultura y Deporte, Spain, through the Fellowship Program for Training University Professors under Grant FPU14/00150, and in part by the Ministerio de Economia y Competitividad, Spain, through the Research and Development Projects under Grant TEC2016-75934-C4-1-R and Grant TEC2016-75934-C4-3-R.Merello-Gimenez, JM.; Nova-Giménez, V.; Bachiller Martin, MC.; Sánchez-Marín, JR.; Belenguer Martínez, Á.; Boria Esbert, VE. (2018). Miniaturization of Power Divider and 90º Hybrid Directional Coupler for C-Band Applications Using Empty Substrate-Integrated Coaxial Lines. IEEE Transactions on Microwave Theory and Techniques. 66(6):3055-3062. https://doi.org/10.1109/TMTT.2018.2828089S3055306266