Liquid crystal (LC) is an anisotropic liquid material, which flows like a liquid, but at the same time, its molecules have an orientational order like in the solid state. Thus, LC is a promising dielectric material for designing reconfigurable devices at microwave frequencies. In order to optimize the design of reconfigurable microwave devices, accurate values of the dielectric permittivity and the loss tangent of LCs are needed. However, new LCs are not well characterized at these frequencies because of its recent use for microwave applications. Therefore, the characterization in this frequency range is required for its practical use within microwave components and devices. In this paper, a split-cylinder resonator method is used for the characterization of four different nematic LCs at two frequency points, i.e., 5 and 11 GHz. This characterization includes the extraction of their complex dielectric permittivity values at these frequencies. The employed method allows to obtain the two extreme permittivity values without applying any external electric or magnetic field to polarize the LC molecules. Two different approaches, a modal analysis method and a full-wave numerical technique, have been used for determining the LC parameters obtaining similar results in both cases.

This article presents a new approach to manufacturing and assembling inexpensive, lightweight devices using 3-D-printed waveguides, and surface-mount technology (SMT). Specifically, we have employed stereolithography (SLA) and a combination of electroless and electroplating processes to develop cavity filters in the $X$ -and $C$ -bands. The filters can be efficiently integrated into planar circuits via a reliable and simple transition that enables surface-mount assembly. Two resonator topologies, circular and rectangular, were examined and compared to develop $X$ -band filters, and a triplet topology has been developed for the $C$ -band. The measured frequency responses were consistent with simulations, with insertion losses below 2 and 1.43 dB for $X$ -and $C$ -bands, respectively. We have conducted several Monte Carlo analyses and a dimensional study to assess the impact of manufacturing tolerances on filter performance. Furthermore, a rectangular cavity resonator has been used to approximate the effective conductivity of the deposited copper coating, considering the detrimental effects of oxidation and surface roughness. The resulting effective conductivity of 17.7 MS/m was found to be higher than the current state-of-the-art values. Finally, SMT assembly offers several advantages over traditional methods, including reduced size, simplified assembly, and integration with planar systems. The results presented in this work indicate that combining 3-D printing and SMT provides a simple and efficient solution for developing lightweight and highly integrated components with superior performance.

The use of nematic liquid crystal (LC) mixtures for microwave frequency applications presents a fundamental drawback: many of these mixtures have not been properly characterized at these frequencies, and researchers do not have an a priori clear idea of which behavior they can expect. This work is focused on developing a new procedure for the extraction of the main parameters of a nematic liquid crystal: dielectric permittivity and loss tangent at 11 GHz under different polarization voltages; splay elastic constant $K_{11}$, which allows calculation of the threshold voltage ($V_{th}$); and rotational viscosity ${\gamma }_{11}$, which allows calculating the response time of any arbitrary device. These properties will be calculated by using a resonator-based method, which is implemented with a new topology of substrate integrated transmission line. The LC molecules should be rotated (polarized) by applying an electric field in order to extract the characteristic parameters; thus, the transmission line needs to have two conductors and low electric losses in order to preserve the integrity of the measurements. This method was applied to a well-known liquid crystal mixture (GT3-23002 from MERCK) obtaining the permittivity and loss tangent versus bias voltage curves, the splay elastic constant, and the rotational viscosity of the mixture. The results validate the viability of the proposed method.

Substrate Integrated (SI) technologies, either completely or partially filled with dielectric, as well as completely empty (i.e. without any dielectric material), have been object of intense research in the last years. Their performance in terms of losses, cost and size are mid-way between those of classical planar lines and rectangular waveguides. Many communication devices have already been successfully designed, manufactured and measured in all these SI technologies. But these measurements are mostly done at room temperature under atmospheric pressure conditions. However, the good results (in terms of low loss, profile and cost) of these novel technologies, especially the completely empty versions, close to the performance of the rectangular waveguide, make them a very good alternative for being used in small satellite payloads. Up to now, few attention has been given to the thermal analysis of these SI technologies, still missing a complete comparative study of these effects under both, atmospheric pressure and vacuum conditions. In this work the same filter is implemented on four different SI technologies (including completely and partially filled with dielectric, as well as empty -no dielectric- versions). The four filters are designed, manufactured, and measured at different temperatures according to the thermal testing standards for space applications. The thermal study is performed under atmospheric pressure conditions and, for the first time, under high vacuum conditions. Finally, a comparison with all previous available thermal studies (all of them performed at atmospheric pressure levels) is included.

The space industry is seeking new solutions for the development of lightweight, inexpensive and easy-to-manufacture on-board communication payload devices. One of the proposed technologies is additive manufacturing (AM) of polymeric materials that are subsequently metallized. The devices developed using this technique must be operational in the particular space environment conditions. In this work we study the integrity of filters made by additive manufacturing against the extreme conditions of mechanical stress, temperature and power management as described in the standard of the European Space Agency (ESA) for satellite payload. To carry out this qualification for space, two types of X-band filters with identical frequency responses have been designed and manufactured: a classic H-plane waveguide cavities filter, and three filters on surface-mount technology (SMT). Two of the SMT filters are low height rectangular cavities filters, while the third one is a high circular cavities filter. This selection was intended to verify that the AM technique offered good results regardless of the devices topology. Two mechanical vibration tests were performed: sinusoidal sweep and random vibration. These tests emulate the transport and launching conditions of the satellite payload. Furthermore, it was measured the mechanical natural frequency of the filters, and its variation after being exposed to the tests in order to evaluate their physical integrity. The thermal analysis was performed emulating the temperature profile experienced by the satellite payload during working conditions. Then, the power handling tests were carried out. The secondary electron emission yield of the material (metallized polymer) was firstly measured. The power levels, location and frequency at which a multipactor discharge would occur were analyzed for both, low-height and standard-height, filters. Finally, both filters were subjected to those power levels, under high vacuum conditions, to check if the said multipactor discharges occurred. The mechanical stress and temperature tests prove that the developed filters can survive the hard launching and operation conditions of commercial satellite payloads. The multipactor tests report the high power levels that the devices can handle without risk of radiofrequency (RF) breakage. All these tests allow to fully qualify the proposed manufacturing technique for spatial applications.

Additive manufacturing (AM) can be applied to new scenarios where traditional subtractive techniques limit geometry and assembling freedom. One of these scenarios is the development of high-frequency devices for communication systems, where AM offers the capability to develop high-complexity geometries, reducing the devices’ weight and cost. This is extraordinarily convenient for recent small satellites where size, weight, and integration are vital. This work presents an AM process specially optimized to develop communication devices. Industry needs have been studied, and materials and manufacturing techniques have been chosen accordingly. Two communication devices, a band-pass filter and a horn antenna, have been developed using two VAT photopolymerization (VPP) processes and different resins. Printed devices were metallized using a two-step process based on a first electroless metallisation and a final galvanic plating. The manufactured pieces were sandblasted prior to metallization to increase plating adhesion and reduce surface roughness. Finally, several dimensional analyses were performed to evaluate the effect of finishing and plating on the manufacturing process using scanning electron microscopy, profilometry, and contact metrology. The electromagnetic response of developed devices is excellent and comparable with commercial devices manufactured with subtractive techniques. A preliminary tolerance analysis was carried out to approximate the systematic deviation of the overall manufacturing process, considering the erosion of sandblasting and the thickness of copper plating. These deviations were compensated in the design step to improve the dimensional accuracy of the band-pass filter.