Remarkable progress in terahertz (THz) sources and detectors is followed by the necessity of manipulating of terahertz radiation. Since natural materials can not perform efficient interaction with THz radiation, artificial structures called metamaterials are designed to overcome “THz gap” in this area. A variety of tunable metamaterials using different methods of control are presented and discussed in this review paper.

A model for tunable thin film bulk acoustic resonators (TFBARs) based on ferroelectric films is proposed. The model is based on electromechanical equations taking into account piezoelectric effect and electrostriction effect induced by the dc electric field. The dc field induced shift of the resonant frequency is explained by the high-order nonlinear effects in the ferroelectric material. The main contribution to the tunability of the resonant frequency under dc electric field can be attributed to electrostriction, which is nonlinear with respect to the mechanical deformation. It is shown that the upward or downward shift in the resonant frequency is given by the sign of the nonlinear component of the electrostriction tensor $M$ . The model is verified by comparing the results with the measured microwave input impedance of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ and ${\mathrm{Ba}}_{0.25}{\mathrm{Sr}}_{0.75}\mathrm{Ti}{\mathrm{O}}_3$ based TFBARs. For a positive sign of the nonlinear coefficient of electrostriction $M$ , the model predicts an upward shift of the resonant frequencies (resonance and antiresonance) under dc biasing in case of the TFBAR based on $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ , whereas a negative sign of the nonlinear coefficient of electrostriction $M$ predicts downward shift of the resonant frequencies for TFBAR based on ${\mathrm{Ba}}_{0.25}{\mathrm{Sr}}_{0.75}\mathrm{Ti}{\mathrm{O}}_3$ films.

The electromagnetic properties of electrically small loop antennas for two different applications are discussed: near-field and far-field RFID systems. In the first case miniature loop antennas are designed for the near field wireless biological and medical sensing application. The magnetic field in the near-field zone is used for coupling between the loops. Using regularly placed capacitors between segments of the loop provides a homogeneous current distribution along the electrically small loop resulted in the improved magnetic coupling. In another case the loop antenna with the improved radiation characteristics is designed for far distance RFID reader antenna. The antenna consists of a combination of the noninteracting loop and dipole. Such a combination shows a cardioid-type radiation pattern and doubled antenna gain. Results of simulations and measurements of test structures of the loop antennas are presented.

An original design of 3D antenna for passive UHF RFID tags is presented. Tags using the proposed near omnidirectional antenna are characterized by almost eliminated read-orientation sensitivity as compared to conventional tags based on planar dipole antennas. As a result the novel tags placed on the cardboard box corner can be read independently on their orientation with respect to reader antennas with two orthogonal (linear or circular) polarizations

A new implementation of a beam-steering transmitarray is proposed based on the tiled array architecture. Each pixel of the transmitarray is manufactured as a standalone unit which can be hard-wired for specific transmission characteristics. A set of complementary units, providing reciprocal phase-shifts, can be assembled in a prescribed spatial phase-modulation pattern to perform beam steering and beam forming in a broad spatial range. A compact circuit model of the tiled unit cell is proposed and characterized with full-wave electromagnetic simulations. Waveguide measurements of a prototype unit cell have been carried out. A design example of a tiled 10 × 10-element 1-bit beam-steering transmitarray is presented and its performance benchmarked against the conventional single-panel, i.e., unibody, counterpart. Prototypes of the tiled and single-panel C-band transmitarrays have been fabricated and tested, demonstrating their close performance, good agreement with simulations and a weak effect of fabrication tolerances. The proposed transmitarray antenna configuration has great potential for fifth-generation (5G) communication systems.

An enhancement technique of quasi-lumped inductance Q-factor is proposed in order to improve loss performance of LTCC filters. Effectiveness of the technique developed is demonstrated by experimental characteristics of a 3-pole bandpass LTCC filter with reduced in-band insertion loss. Figure of merit of a filter is introduced as a universal characteristic that allows comparison of different filters by means of design effectiveness.