A miniaturized electrical beam-scanning reflectarray antenna providing a directional beam and wide steering ranges with fixed $\theta $ -polarization is proposed. To accomplish these properties, we devise a new active reflectarray cell that can control both the phase and polarization of incident waves. Accordingly, we can enforce all reflected waves to constructively interfere with each other in all target directions. To feed the proposed reflectarray cells, an omnidirectional monopole antenna is installed at the center of the reflectarray, which illuminates the reflectarray evenly. In addition, because the monopole feeder is directly connected to the reflectarray, we can significantly reduce the overall volume of the antenna down to about $0.5~\lambda ^{3}$ with a small F/D ratio of 0.147. Consequently, the proposed antenna attains a wide scan coverage and directional beam patterns. Moreover, the antenna can also maintain polarization direction stably, which is strongly required for dynamically moving platforms such as drones and unmanned aerial vehicles (UAVs). The experimental results agree well with the predictions, proving the validity of our approach.

A passive radio frequency identification (RFID) tag antenna providing a long recognition distance is proposed, which is mountable on various types of platforms including metallic and highly dielectric objects. To increase the reading distance, the antenna is installed in a recessed metallic cavity. To reduce further the height of the antenna, an artificial magnetic conductor is placed on the bottom face of the cavity. Consequently, the tag antenna not only can be miniaturised, but also can provide long reading distances on any type of platform objects. The measured maximum reading distance and the minimum sensitivity are 12.2 m and −17 dBm, under a reader transmitting 30 dBm with a 6 dBi antenna. Good agreement between the prediction and experimental results verifies that the approach is fairly effective and useful to substantially increase the reading distance.

A Fabry-Pérot cavity (FPC) antenna producing both high-gain and circularly-polarized (CP) behavior is proposed. To increase antenna gain and obtain CP characteristics, a superstrate composed of square patches with a pair of truncated corners is placed above the linearly polarized patch antenna with an approximately half-wavelength distance from the ground plane at the operating frequency. The proposed antenna has the advantages of high gain, a simple design, and an excellent boresight axial ratio over the operating frequency bandwidth. Moreover, used in an FPC antenna, the proposed superstrate converts a linear polarization produced by a patch antenna into a circular polarization. In addition, the cavity antenna produces left-hand circular-polarization and right-hand circular-polarization when a patch antenna inside the cavity generates x-direction and y-direction polarization, respectively. The measured and simulated results verify the performance of the antenna.

We propose an innovative polarization reconfigurable high-gain Fabry–Perot cavity (FPC) antenna. The antenna can electrically select its polarization among three linear polarization (LP) modes of $x$ -, $y$ -, and 45°-direction with stable high gain. To achieve these properties, we devise a novel polarization conversion partially reflective surface (PCPRS) as a superstrate. The PCPRS plays two crucial roles simultaneously; it reconfigures antenna polarization and maintains high gain using an FP resonance between the PCPRS and a ground plane. The FP resonance, which is the key point in our antenna design, is essential to maximize both antenna gain and polarization reconfigurability. Therefore, in order to stably achieve the FP resonance for all three polarization modes, we have designed the PCPRS to keep its reflection behavior as constant as possible for all LP modes. To the best of our knowledge, this is the first FPC antenna introducing a partially reflective surface (PRS) which also functions as an active polarizer. Consequently, the proposed antenna attains remarkable polarization conversion performance with higher than 20 dB cross-polarization suppression level for all polarization modes. Moreover, in all modes, our antenna well maintains reasonably high gain of about 15 dBi with good impedance matching properties. Good agreement between measurements and simulations proves the validity of our design approach.

We propose a new and straightforward dielectric lens design method to increase the gain of integrated lens antennas (ILAs). Our lens is divided into several cone-shaped radial sections. Each section consists of specific permittivity to provide desired phase delays for the waves passing through the section. Our method does not depend on the operating frequencies and the physical appearance of the lens, because all the radial sections share one common phase center as a starting point. Accordingly, to change the main beam direction, we only need to rearrange the sections. However, the required permittivity is all different in every section, which is extremely difficult to be realized using conventional dielectric materials. To fabricate the required permittivity based on an effective medium theory, we use a cutting-edge three-dimensional printing technology. Consequently, we can obtain all the required permittivity using only one base printing material. Through experiments, we can show that our lens not only can form a desired beam but also can increase gain in the target direction, which proves the validity of our lens design method.

A frequency reconfigurable dipole antenna based on a silicon radiator is presented. The silicon radiator is activated with the aid of highly dense solid-state plasma by injecting carriers into the intrinsic region of p-i-n diodes. The fabrication and design guideline of the reconfigurable dipole antenna with this plasma radiator are described. When the plasma radiator is activated or deactivated, the length of the dipole arm changes, which means that the operating frequency of the dipole antenna is reconfigurable. When all the channels in the plasma radiator are activated, the operating frequency is tuned from 6.3 GHz to 4.9 GHz. The measured tunable bandwidth of our fabricated dipole antenna is approximately 31%, which is a practical value in comparison to conventional frequency reconfigurable antennas whose tunable bandwidth is in a range from 20% to 50%. To further support the validity of our results, we provide the well-matched simulation results from an antenna simulation. These results demonstrate that silicon with its commercial technology, which has not attracted attention in comparison to a metal antennas, is a promising tunable material for a frequency reconfigurable antenna. This plasma-based reconfigurable antenna has great potential for use in the dynamic communication environment.