An ultra-wideband circularly polarized (CP) planar array antenna built by new radiation elements, the bold-C spiral elements, in millimeter-wave band is presented in the paper. Using the tightly coupled array (TCA) technique, the embedded CP bold-C spiral element in the infinite array has achieved both impedance bandwidth and 3-dB axial ratio (AR) bandwidth of 50.7% covering 18.1–30.4 GHz for the fixed broadside beam. Then, a feeding network is designed by using the dielectric-based inverted microstrip gap waveguide (DIMGW) technology in order to have a compact layout. An 8×8 CP planar array based on the proposed radiating element and the feeding network is designed and prototyped. The measured results show that the impedance bandwidth of the whole array for the reflection coefficient below -10 dB is 49% from 18.2 to 30 GHz, and 3-dB AR bandwidth is 44.9% covering 19–30 GHz.

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 multifunctional tri-band antenna combining transmitarray (TA) and reflectarray (RA) is presented in this communication. Five-layer patches etched on three dielectric substrates with two air gaps are used as the unit cell. Cross dipoles inserted in cross slots control transmission characteristics at Ku-band (14 GHz). Double square rings on the top and bottom layers control reflection characteristics at K (21 GHz) and Ka (30 GHz)-bands, respectively. The antenna has dual-polarized characteristics in each band. The phase shift range that can be obtained is 360° for 3 dB transmission magnitude at Ku-band and 0.15 dB reflection magnitude at K and Ka-bands. The tri-band dual-polarized TA/RA antenna is designed, manufactured, and measured to verify the performance. The simulated results keep consistent with the measured ones. The measured results show that the TA has a peak gain of 23.8 dBi with an aperture efficiency of 40% at 14.25 GHz and 1-dB gain bandwidth of 6.5%. The RA has a peak gain of 28 dBi with an aperture efficiency of 44% at 21 GHz and 1-dB gain bandwidth of 7.3%, and has a peak gain of 31.5 dBi with an aperture efficiency of 47% at 30.5 GHz and 1-dB gain bandwidth of 7.1%.

This letter presents a novel superstrate composed of multilayer metasurface (MM), which is characterized by wideband monostatic and bistatic radar cross section reduction (RCSR) for Fabry-Perot (FP) antenna with magnetoelectric (ME) dipole-fed. The proposed MM consists of four layers, the top three layers form a polarization-selective absorber (PSA), and the bottom layer is a polarization conversion metasurface (PCM). The copolarized incident waves are absorbed by the PSA, while the cross-polarized waves pass through with a low insertion loss. Then, it is reflected by the PCM with 90° polarization rotation and absorbed by the upper PSA. Additionally, the bottom layer of the PCM unit is an improved pattern instead of the ground plane. It is capable of polarization conversion and partial reflection. Therefore, the MM and ME dipole are respectively utilized as the partially reflecting surface (PRS) and the primary source to construct an FP antenna. Simulation and measurement results show that the bandwidth of monostatic RCSR is more than 80%. The FP antenna also achieves a high aperture efficiency of 51%, which is due to the benefit from the widebeam characteristics of the ME dipole.

In this letter, we propose a dual-circularly polarized reflect-transmit-array antenna (RTA) with independent beam control. It can transmit left-hand circularly polarized (LHCP) wave as transmitarray antenna and reflect right-hand circularly polarized (RHCP) wave as reflectarray antenna at 8.7–10.2 GHz. In addition, 360° phase shift for each circularly polarized wave can be achieved by rotating the top and bottom patches. A 20 × 20- unit cell RTA is designed, fabricated, and measured. For the LHCP radiation, the measured results show that the 3 dB gain bandwidth and axial ratio (AR) bandwidth are 13.7% (8.8–10.1 GHz) and 15.9% (8.7–10.2 GHz), respectively. For the RHCP radiation, the measured 3 dB gain bandwidth and AR bandwidth are 11.4% (9.1–10.2 GHz) and 12.6% (8.9–10.1 GHz). The peak aperture efficiencies for LHCP and RHCP radiations are 45.5% at 9.2 GHz and 41.8% at 9.6 GHz, respectively.

This communication presents a novel bifunctional metasurface (MS) combining the reflectarray (RA) and transmitarray (TA) with dual-band orthogonally polarized dual-beam characteristics. The proposed reflect-transmit-array (RTA) is capable of reflecting the $x$ -polarized incidence without changing its polarization state around 10 GHz while transmitting the $y$ -polarized incidence and converting it into the circularly polarized (CP) waves around 14 GHz. To completely cover the working frequency bands of both modes, an antipodal Vivaldi antenna (AVA) is introduced as the source feed due to its intrinsic ultrawideband characteristics. As a proof of concept demonstration, a prototype consisting of 20 $\times20$ elements is also designed, fabricated, and measured. We observe a good agreement between the measurement and simulation results that shows a peak gain of 26.2 dBi and 27.3 dBic, 1 dB gain bandwidth (BW) of 9.8–11.3 GHz (14.2%) and 12.8–14.2 GHz (10.4%), and peak aperture efficiency (AE) of 48.7% and 35.1% for the reflection and transmission modes, respectively.