Gerardo Perez-Palomino

This paper describes and validates an advanced synthesis design process of Frequency-Selective Surfaces (FSSs) with elliptic band-stop responses. A systematic procedure based on the Generalized Chebyshev Function and the extracted pole technique enables control of the position of the transmission zeros and the attenuation level to obtain an equiripple rejection response. A systematic process is followed to obtain the lumped LC values of the resonator circuits extracted as poles and the impedance inverters. Then, equivalent dipoles and transmission lines are obtained to carry out the electromagnetic design at normal incidence for a linearly polarized field. The impact of the higher-order modes of the periodic structure on the electrical response of the FSS, which can be relevant due to the stringent selected specifications, has been also analyzed. A fourth-order band-stop filter with a 3 GHz bandwidth centered at 30 GHz and its attenuation at 50 dB has been designed considering three different implementations: two filters using a vacuum as a transmission line with different connection lengths and a third one using a dielectric substrate to enable its manufacturing. In order to verify the design procedure using experimental results, the third filter with printed dipoles in the dielectric substrate has been manufactured and measured, thus validating the developed process.

This paper introduces a low-cost, easy-to-manufacture, dual polarization reconfigurable reflectarray antenna based on Liquid Crystal (LC) that operates at W-band. The antenna is electrically large and is capable of independently steering the beam of two orthogonal polarizations. Two different implementations of single-layer unit cells (single resonant and multi-resonant) capable of providing suitable phase range to independently control the two RF polarizations with enough isolation have been investigated. The single resonant cell was finally used to design, manufacture and test a complete reflectarray antenna made of 55×55 elements, for which an accurate and efficient modeling of the cells was implemented. The effect of the LC bias lines is minimized by following a thorough impact study. At a cell level, the measurements validate both the modeling and the capability of the LC to be locally biased at the same cell provided that the biasing network and the resonators of each polarization are properly distributed. At an antenna level, the measurements are predicted by simulations with excellent accuracy, which validates the design and modeling process. The measured antenna shows 35° of 1D scanning range with 25dBi gain and a maximum SLL of -9dB in the entire range for both polarizations at 98 GHz.

Liquid Crystal-based mm-wave spatially fed antennas with electronic reconfiguration are a promising solution at the higher frequencies required in next generation networks. However, one of the main drawbacks of the technology in these bands stems from the high reconfigurability times they present. Through this work, a relevant step towards overcoming the temporal problem by using Dual Frequency Liquid Crystals (DFLC) is presented. This paper details, for the first time, both the electromagnetic and temporal characterization of four commercially available DFLC mixtures in W-band, enabling their use in designing faster devices. To evaluate the experimental characterization, a reflectarray surface (made of 50x50 cells) specifically designed to achieve fast switching times with a sufficient phase range has been manufactured and measured. For this cell, a preliminary addressing technique based on overdriving has been used, exhibiting reconfigurability (rise and decay) times of 20 ms, one order of magnitude faster than the current state of the art of LC-based mm-wave planar devices. The measured results match the simulations, and reveal that a precisely designed biasing technique using overdrive must be used for DFLC-cells to achieve time reduction. Additionally, the benefits of this technology compared with other LC acceleration strategies in mm-wave are discussed.

The slow response time of planar Liquid Crystal (LC)-based phase-shift metasurface and Reconfigurable Intelligent Surfaces (RIS) cells is addressed in this paper by introducing a polymer network LC (PNLC) mixture suitable at mm-wave bands. Since the conventional effective isotropic model used in optical cells for describing the PNLC is not suitable in RF, an effective anisotropic and uniaxial model for such mixture is provided and experimentally validated at 100 GHz for the first time. In order to compare the temporal performance and tunability of the PNLC, transmissive and reflective cells, containing conventional LC and PNLC, have been manufactured and measured at optical and mm-wave frequencies. The temporal responses of PNLC are also compared for both RF and optical cells, obtaining relevant differences between their improvement factors, which are also discussed. Specifically, a 50 fold response time improvement is attained in cells designed to work at 100 GHz, although at the expense of a 3X tunability reduction. The model, which is robust to varying angle of incidence and cell dimensions, has been experimentally validated by designing and manufacturing a PNLC reflectarray cell of a different geometry. The cell shows reconfigurability times of 210ms, representing a significant improvement with respect to state-of-the-art response time in mm-wave cells, which are in the order of several seconds.

This paper describes and validates for the first time the dynamic modelling of Liquid Crystal (LC)-based planar multi-resonant cells, as well as its use as bias signals synthesis tool to improve their reconfigurability time. The dynamic LC director equation is solved in the longitudinal direction through the finite elements method, which provides the z - and time-dependent inhomogeneous permittivity tensor used in an electromagnetic simulator to evaluate the cells behaviour. The proposed model has been experimentally validated using reflective cells for phase control (reflectarray) and measuring the transient phase, both in excitation and relaxation regimes. It is shown how a very reduced number of stratified layers are needed to model the material inhomogeneity, and that even an homogeneous effective tensor can be used in most of the cases, which allows a model simplification suitable for design procedures without losing accuracy. Consequently, a novel bias signal design tool is proposed to significantly reduce the transition times of LC cells, and hence, of electrically large antennas composed of them. These tools, similar to those used in optical displays, are experimentally validated for the first time at mm- and sub-mm wave frequencies in this work, obtaining an improvement of orders of magnitude.

This work presents, for the first time, a generalized bimode Foster’s equivalent circuit for characterization of 2-D Planar Periodic Structures (PPSs) with arbitrary geometry at oblique incidence. It considers the interactions between the fundamental TE and TM modes without any restriction within the bimode bandwidth of the geometry. The proposed circuit is only composed of frequency-independent LC elements, which can be extracted systematically from electromagnetic (EM) simulations. The reactive immittances obtained in the process fulfill the Foster’s theorem, enabling the design process of PPS-based devices using standardized synthesis techniques from circuit theory. To demonstrate its viability and general nature, equivalent circuits are extracted for different single- and multilayer PPS composed of rotated dipoles under oblique incidence θ=20°, φ=30°, and including dielectrics. Excellent agreement is found between the response of the circuit model and the EM simulation in all cases. Finally, to validate experimentally the proposed equivalent circuit and highlight its applicability, a 90° reflective Linear-Polarization (LP) Rotator centered at 25 GHz and under oblique incidence, θ=30°, φ=0° (TE), is designed, manufactured, and tested. The agreement between the circuit response, the EM simulation and the measurement underlines the potential of the new equivalent circuit for PPS design under oblique incidence.

The use of additive manufacturing and different metallization techniques for prototyping radio frequency components such as antennas and waveguides are rising owing to their high precision and low costs. Over time, additive manufacturing has improved so that its utilization is accepted in satellite payloads and military applications. However, there is no record of the frequency response in the millimeter-wave band for inductive 3D frequency selective structures implemented by different metallization techniques. For this reason, three different prototypes of dielectric 3D frequency selective structures working in the millimeter-wave band are designed, simulated, and manufactured using VAT photopolymerization. These prototypes are subsequently metallized using metallic paint atomization and electroplating. The manufactured prototypes have been carefully selected, considering their design complexity, starting with the simplest, the square aperture, the medium complexity, the woodpile structure, and the most complex, the torus structure. Then, each structure is measured before and after the metallization process using a measurement bench. The metallization used for the measurement is nickel spray flowed by the copper electroplating. For the electroplating, a detailed table showing the total area to be metallized and the current applied is also provided. Finally, the effectiveness of both metallization techniques is compared with the simulations performed using CST Microwave Studio. Results indicate that a shifted and reduced band-pass is obtained in some structures. On the other hand, for very complex structures, as in the torus case, band-pass with lower loss is obtained using copper electroplating, thus allowing the manufacturing of inductive 3D frequency selective structures in the millimeter-wave band at a low cost.

A standardized design method of Planar Periodic Structure (PPS)-based 90° Linear Polarization converters is presented. This procedure enables standardized designs centered at an arbitrary frequency in either reflection or transmission at normal incidence, which may be achieved with no computational cost and can be used to obtain suitable designs at oblique incidence. It uses both Foster’s equivalent circuits and the classic coupler synthesis, which allow the number of layers, the values of the lumped elements, and the lengths and impedances of the spacers needed to fulfill any prescribed requirements in terms of XP, insertion losses and bandwidth to be known. The selection of the PPS scatterer that provides the appropriate values of the circuit elements is also studied. Several designs are presented to validate the proposed method, one of which converts a vertical polarization into the horizontal in transmission, exhibiting simulated results of XP < -30 dB, insertion losses better than 0.1 dB and bandwidth that improves other rotators in the state of the art for the same number of layers. The designed device has been manufactured and experimentally tested, and the results validate the methodology.

A Foster’s equivalent circuit for 2-D Planar Periodic Structures (PPSs) that exhibit an arbitrary geometry is presented for first time in this paper. The proposed 4-port network shows an invariant circuit topology to the PPS geometry and is completely comprised of invariant-frequency lumped elements. The circuit is the simplest in terms of number of elements within the bi-mode bandwidth for a certain geometry, and its topology makes it possible both a standardized process to obtain the equivalent circuit of an arbitrary geometry and the use of the circuit theory to design a multitude of devices. To perform a validation, the equivalent circuit of two different PPSs (a rotated dipole and a defected slotted ring) have been obtained and analyzed in single and multi-layer configurations. The circuit is also used as a tool to develop Elliptical to Linear (or Circular) Polarization converters. One of the designs presented at 20 GHz (in transmission) converts an incident elliptical polarization of Axial Ratio 5 and tilted φ=20° into a purely linear vertical polarization of XP<-30 dB, with near-zero reflections and insertion loss better than 0.1 dB. The designed device is also manufactured and tested, and the measurements are in good agreement with simulations.

During the last decades, the Liquid Crystal (LC) technology has been widely used in the near infrared and visible regions of spectrum to develop tunable devices such as displays or spatial light modulators (SLM), which is the term used in photonic to describe an aperture antenna fed by an external source in reflection or transmission mode, known as reflectarray (RA) and transmitarray (TA) in microwaves. However, the use of the LC to implement tunable devices for millimeter and sub-millimeter waves is very recent. In the particular case of RAs/TAs, some efforts have been made in the last recent years to develop LC-pixels [1]–[3] or full antennas [2]–[3] up to 100 GHz. The main difference between both an LC-pixel in these frequencies and that one developed to operate in photonic applications lies in the use of resonators, which must be included in the pixels (cells) to achieve the appropriate phase-shift (Fig 1). The use of resonators makes the performance of the cells completely different for the critical electrical parameters, which also depends on the particular characteristics of the LC mixture used (dielectric anisotropy and loss tangent) at the operating frequency.