A gain-stabilized broadband Fabry-Perot cavity antenna (FPCA) is presented, based on a single-layer partially reflective surface (PRS). The proposed PRS employs compact double-sided structure in regular hexagonal arrangement to achieve a wide range of positive phase gradient from 7.9-10.9 GHz. The PRS unit is composed of a circular patch and a fan-shaped tripod within a circular aperture, which printed on both sides of the thin dielectric substrate. A slot antenna with non-uniform parasitic patches is applied as the source antenna of the FPCA, which provides a smooth gain while offering a wide impedance bandwidth. Simulation and measurement results verify that the PRS can effectively improve the performance of the FPCA in terms of widening the bandwidth, reducing the size, and improving the gain stability. The proposed antenna has a measured 3 dB gain bandwidth is 7.9-11 GHz (32.8%) with a peak gain is 13.67 dBi, and an overall size of $2.1\times2.1\times0.57\,{\lambda_{0}}^3$ ( $\lambda_0$ being wavelength at the center frequency of 9.5 GHz).

An inhomogeneous-polarizer loaded substrate integrated waveguide (SIW) horn antenna is proposed to obtain wideband circularly polarized (CP) radiation. Firstly, by introducing extended substrate and extra dielectric slabs around the horn radiating aperture, broadband impedance matching of the SIW horn can be realized. Then, a pure dielectric near-field CP polarizer is designed and placed in front of the horn to obtain broadband CP radiation. The polarizer is designed to feature inhomogeneous property for different polarization orientation illuminations. With this property, 90° phase difference for a pair of orthogonal linearly polarized waves can be realized. Two methods are utilized to analyze the inhomogeneous property of the polarizer and to provide detailed parameter estimation steps/formulas. The effects of polarizer size and oblique incidence are also studied and a very wide 3-dB AR beamwidth of 104° is achieved. Measured results indicate that the proposed CP SIW horn can obtain a matching bandwidth greater than 40% (22-34 GHz) and a CP radiation bandwidth of 30% (22-29.8 GHz). Simple structure and wideband CP performance make it an attractive candidate for broadband millimeter-wave applications.

A novel circularly polarized (CP) dielectric resonator antenna (DRA) array is introduced. The DRA element operating at the desired frequency is determined by the dielectric waveguide model (DWM), and two orthogonal modes are generated to excite CP waves. A 4$\times$4-element DRA array is then developed. The array is fed by the cavity-backed slotted substrate integrated waveguide (SIW) without using extra feeding network. Experimental results shows that the array obtains a bandwidth of 240 MHz (9.88-10.12 GHz) for reflection coefficients and axial ratios less than -10 dB and 1 dB, respectively. Moreover, the array obtains a 16 dBic peak gain, a -33.6 dB cross-polarization level at the boresight, namely 0.5 dB AR at the center frequency, which demonstrate the desirable array performance. In addition, above 62.5% aperture efficiency and above 90% radiation efficiency are realized over the operating bands, with about 76% peak aperture efficiency and about 94% peak radiation efficiency. The proposed array can be regarded as an element to form large planar arrays, which will be useful in beamforming systems. Our design idea may open a new way for developing high efficiency large millimeter-wave arrays.

For the antenna optimization, computationally expensive full-wave EM simulations are necessary, making efficient design of antennas a challenge. Since there are only a few local minimums, some existing algorithms without considering this feature need a lot of useless EM simulations, leading to poor optimization efficiencies. In this paper, a hybrid self-adaptive differential evolution algorithm with simplified Bayesian local optimizer (SADE-SBLO) is proposed for improving antenna optimization efficiencies, in which the SADE is used to generate the offspring population. The algorithm also consists of following four modification strategies. (1) An individual parallel prediction method for reducing surrogate model training and prediction times; (2) An offspring quality pre-assessment method for improving offspring quality and further reducing the number of EM simulations; (3) A self-adaptive database increment method for adapting the algorithm to different optimization stages and also serving as a startup switch for the local optimizer; (4) A SBLO for improving optimization efficiency in the later stage. These strategies are closely integrated to make the algorithm better balance exploration and exploitation, reduce useless EM simulations, and converge faster. Four representative antenna cases are optimized. Compared with some existing algorithms such as DE and SADEA, the proposed algorithm is efficient.