A 3D-printed comb-mushroom-like dielectric lens for stable gain enhancement of the printed antipodal log-periodic dipole array (PLPDA) antenna is presented. The low-profile dielectric lens (DL) is specially designed to be easily integrated with the antenna source for improving its gain performance. Periodic comb-like structure is also utilized in the DL to obtain desired effective permittivity according to the effective medium theory. Both the design process and lens fabrication are greatly benefited from 3D printing technology. With this novel manufacturing method, the lens structure can be designed with greater design freedom and flexibility, which is hard to realize by conventional manufacture process. Loaded with the proposed 3D printed comb-mushroom-like DL, the PLPDA antenna has 11.2$\times$0.7 dBi flat gains in the operating frequency band (14-20 GHz), which means a 4.7 dB prominent gain increment and a very flat gain feature in the wideband.

In this letter, a low-profile wideband pattern diversity antenna based on metasurface (MTS) is proposed. Initially, based on the theory of characteristic mode (TCM), the two adjacent intrinsic modes, which correspond to the broadside and conical radiation pattern, respectively, are exploited to realize a wideband pattern diversity characteristic under a low profile. Then, by analyzing the modal current distribution on the MTS, the optimal locations, orientations and relative phases of the four L-shaped feeding probes are determined to excite the above two modes. However, the in-band high-order modes (HOMs) introduced by the feeding structure will deteriorate the radiation pattern. Therefore, chamfering corners on the MTS is used to suppress the HOMs for radiation pattern improvement. Finally, by combining TCM and MTS, the modes are manipulated to achieve a pattern diversity antenna with an extremely low profile of 0.06 λ0 (the free-space wavelength at the center frequency) and an isolation of better than 38 dB, which also maintains an operating bandwidth of 20.24%. Based on these excellent performances, the proposed antenna is an attractive solution for pattern diversity applications requiring low profile, especially in unmanned aerial vehicle communications.

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 wide axial-ratio beamwidth (ARBW) and half-power beamwidth (HPBW) circularly polarized (CP) antenna is realized by arranging two curved patch dipole pairs orthogonally in a cruciform contour that conforms with a hemispheric shell. First, two orthogonal curved dipole pairs are designed via step-by-step investigation of the evolution of traditional crossed dipoles. Then, an antenna with a wide CP working bandwidth and wide ARBWs and HPBWs is developed by enlarging the diameter of the curved arms, replacing the narrow arms with the wide patches and making these patches conformal with the shell surface; this improves the impedance bandwidth (IBW) and further reduces the antenna’s size. Finally, a prototype with reduced dimensions of $0.52\times 0.52\times0.15$ ( $\lambda _{0}^{3}$ ) is designed, fabricated, and measured. The resulting antenna exhibits a −10 dB IBW of 55.8% (1.06–1.89 GHz) and a 3 dB AR bandwidth of 70.9% (1–2.1 GHz), within which the ARBW and HPBW reach maximum values of 202°/196° and 113°/111° in the two orthogonal principal planes at 1.602 and 1.268 GHz, respectively. Furthermore, a 3 dB ARBW exceeding 100°, an HPBW exceeding 96°, and efficiency exceeding 60% are acquired within the 1.252–1.89 GHz (40.6%) target service coverage band.