An extremely broadband circularly polarized (CP) antenna is presented. The proposed antenna consists of a crossed bowtie dipole loaded with four unequal parasitic cross-slots, four parasitic bowtie patches, and four parasitic rectangular strips. The cross-slots can generate a CP mode due to the sequential phases of the crossed bowtie dipole. The parasitic bowtie patches are gap-coupled with two arms of the crossed bowtie dipole, and they are sequentially rotated along diagonals to generate an additional CP mode. The parasitic strips also generate one CP mode due to the sequentially rotated configuration parallel to the crossed bowtie dipole. By properly combing with two CP modes of the crossed bowtie dipole, broadband CP operation is achieved. The experimental and simulated results are in agreement, and the measured results show that the proposed antenna exhibits a very wide impedance bandwidth of 93.1% (2.21-6.06 GHz) and an excellent axial ratio (AR) bandwidth of 90.9% (2.2-6.4 GHz). In addition, the proposed antenna produces a right-hand circular polarization (RHCP) with a peak gain of 8.6 dBi.

An ultrawideband (UWB) full-metal planar array antenna based on a combination of ridge gap waveguide (RGW) and E-plane groove gap waveguide (E-GGW) is presented for millimeter-wave (mmWave) applications. First, the radiation element of a double-step double-ridged slot structure is proposed and designed. By removing the vertical walls of the cavity in the H-plane, the impedance matching characteristics presents a UWB bandwidth of 53%. Second, a UWB double-layer feeding network is achieved by combining RGWs and double-side-pin E-GGWs for the radiation elements. Finally, an 8 $\times $ 8 full-metal planar array antenna with the presented elements and network is designed, prototyped, and measured. The measured results agree well with the simulated results. The proposed antenna provides an impedance matching bandwidth ( $\vert \text{S}_{11}\vert < -10$ dB) of 46.8% from 18.8 to 30.3 GHz. The maximum gain is 27.7 dBi at 27 GHz and a stable gain of 26 $\pm ~1.7$ dBi is obtained over the operation band.

A phase gradient metasurface (PGM) array with beam converge characteristic in reflection direction at 15 GHz and transmission direction at 5.6 GHz is proposed in this letter. Compared with the traditional unidirectional PGM, the proposed PGM can achieve bidirectional radiation at two frequencies. A horn is used as the feed placing at a focal length of 200 mm of PGM array. Simulation results show that the horn has a peak gain enhancement of 11.9 dB at 15 GHz with a -3 dB beamwidth reduction of 33° in reflection direction and a peak gain enhancement of 7.9 dB at 5.6 GHz with a -3 dB beamwidth reduction of 30° in transmission direction. The proposed PGM array achieves beam converge in two directions at two different frequency and has a potential use for communication system consisting of receivers with multi-frequency and multi-location characteristic.

In this paper, a novel design of broadband circularly polarized (CP) folded reflectarray (FRA) antenna is proposed. The CP FRA consists of two kinds of metasurfaces and a linearly polarized (LP) open waveguide antenna as the feeding source. The bottom metasurface functions as the main reflector, which is capable of realizing cross-polarization conversion and beam focusing simultaneously. The upper metasurface is utilized as a subreflector, which could reflect LP wave coming from the feeding source and transmit its orthogonal counterpart into CP wave. For demonstration, the matesurface-assisted CP FRA antenna is fabricated and measured. Numerical results are consistent with experimental ones, illustrating a peak gain of 24.1 dBi and a peak aperture efficiency of 51% at 9.9 GHz. The 3-dB gain bandwidth and 3-dB axial ratio bandwidth are achieved as 27.5% (8.8-11.6 GHz) and 43.9% (8-12.5 GHz), respectively. The proposed CP FRA provides a new avenue to realize broadband high efficiency CP antenna, which may find great potentials in modern wireless communication systems.

A simple multilayer unit cell has been discussed for circularly polarized transmitarray applications in this letter. This element consists of three layers of identical substrates. Each substrate is printed by two sets of parallel dipoles of orthogonal directions on the opposite sides to realize a proper phase difference. The transmission coefficients have been studied to balance magnitude against phase difference. Then, the dipole dimensions are analyzed to obtain a satisfactory transmission. A transmitarray prototype of 137 elements has been designed, manufactured, and tested. The measurements present expected radiation patterns compared to the simulations. The measured result suggests a gain of 24.5 dBi with sidelobe level of -18.7 dB and cross polarization of -20.5 dB at 12.7 GHz, which is equivalent to a 38.9% aperture efficiency. The 1-dB gain bandwidth is 7.1%.

In this letter, a wideband millimeter-wave (mmWave) planar array antenna with a printed ridge gap waveguide (PRGW) feeding network is presented. A planar bowtie antenna fed by the PRGW is employed as the array radiation element which combines the bowtie with an upside-down electromagnetic bandgap part by using lamination. Thus, with this new upside-down structure, the proposed array antenna achieves a stable, wideband, and high-antenna-efficiency performance. A prototype of 8 ×8 array is fabricated and measured. The measured results show that the proposed array has an impedance matching bandwidth (|S ₁₁ | <; -10 dB) of 50.3% from 18.6 to 31.1 GHz. The maximum measured gain is 22.1 dBi at 29 GHz, and the total antenna efficiency is above 60% with a bandwidth of 45% from 18.8 to 29.8 GHz.