A novel compact differential-fed dual-polarized filtering patch antenna is proposed. Specific filtering response with radiation nulls is firstly realized for the dual-polarized antenna element by symmetrically loading defected ground structures (DGS), a cross slot and eight shorting pins without extra circuits. The DGS with pins can generate a lower-edge radiation null for a sharp roll-off rate, and simultaneously introduce extra in-band resonance for wide passband. Meanwhile, the cross slot with pins can offer an upper-edge null and an in-band resonance. Combing with inherent higher-order-mode null of the patch itself, the skirt selectivity and stopband suppression at upper band can be significantly improved. Meanwhile, the |Sdd11| at the upper and lower stopbands is close to 0 dB to achieve high stopband suppressions. As a result, a compact high-gain dual-polarized filtering patch antenna is readily constructed. Thanks for complete symmetry of above structures, the proposed method is very suitable for dual-polarized antennas, where all the performance for both polarizations are identical. For demonstration, a prototype of the proposed antenna is fabricated and measured. Good agreements between the measured and simulated results are observed, indicating good performance with a large bandwidth about 23and a peak gain up to 8.9 dBi within the passband, as well as out-of-band suppression levels of more than 20 dB. Compared with other reported filtering antennas, the proposed filtering patch antenna not only exhibits good wideband dual-polarization radiation, but also features high gain in a compact structure for very high aperture efficiency, and ultra-low cross-polarization level due to the differential feeding and complete symmetry.

This article presents a novel broadband dual-polarized filtering patch antenna array using low-temperature cofired ceramic (LTCC) process for 5G millimeter-wave (mmW) applications. The filtering antenna element is implemented using a ± 45° polarized patch antenna fed by two pairs of differential L-type probes. An additional square ring and open strips are connected to each probe, resulting in radiation nulls and realizing the lower/upper stopband rejection level over 24 dB with skirt selectivity. The cross coupling among these structures not only introduces filtering responses, but also contributes to broadening the passband of 24.25–29.5 GHz covering 5G n257/n258/n261 bands. The antenna performance is validated by equivalent circuits based on transmission line models, which reveals clearly the principles of filtering antenna step by step and provides guidelines for further designs. The proposed filtering antenna element is expanded to a $4\times 4$ filtering antenna array with shunt-fed feeding networks. The spacing between elements is $0.48~\lambda _{0}$ . The measured results demonstrate that the array performs good filtering characters including high stopband rejection, high selectivity, and wide upper stopband up to 40 GHz, where the antenna efficiency (considering radiations from all the directions) is less than 5%. In addition, the differential feeds provide symmetric radiation patterns with low cross-polarized counterparts. Compared with other reported works, the proposed filtering antenna/array has excellent performances of wideband, high stopband rejection and wide upper stopband for potential 5G mmW applications.

We review dielectric resonator antenna (DRA) designs. This review examines recent advancements across several categories, specifically focusing on their applicability in array configurations for millimeter-wave (mmW) bands, particularly in the context of 5G and beyond 5G applications. Notably, the off-chip DRA designs, including in-substrate and compact DRAs, have gained prominence in recent years. This surge in popularity can be attributed to the rapid development of cost-effective multilayer laminate manufacturing techniques, such as printed circuit boards (PCBs) and low-temperature co-fired ceramic (LTCC). Furthermore, there is a growing demand for DRAs with beam-steering, dual-band functions, and on-chip alignment availability, as they offer versatile alternatives to traditional lossy printed antennas. DRAs exhibit distinct advantages of lower conductive losses and greater flexibility in shapes and materials. We discuss and compare the performances of different DRA designs, considering their material usage, manufacturing feasibility, overall performance, and applications. By exploring the pros and cons of these diverse DRA designs, this review provides valuable insights for researchers in the field.

A new antenna element type based on the open-ended ridge gap waveguide (RGW) is proposed for phased array applications. This element type is of a particular interest at high millimeter-wave frequencies ( $\geq$ 100 GHz) owing to a contactless design alleviating active beam-steering electronic integration. The key challenge addressed here is a realization of a wide fractional bandwidth and a wide scan range with high radiation efficiency. We demonstrate a relatively simple wideband impedance matching network comprised of an aperture stepped ridge segment and a single-pin RGW section. Furthermore, the $E$ - and $H$ -plane grooves are added that effectively suppress antenna elements’ mutual coupling. Results demonstrate a wide-angle beam steering ( $\geq \text{50}^\circ$ ) over $\geq {20\%}$ fractional bandwidth at the $W$ -band with $\geq {89\%}$ radiation efficiency that significantly outperforms the existing solutions at these frequencies. An experimental prototype of a 1 × 19 $W$ -band array validates the proposed design concept through the embedded element pattern measurements.

A concept of a contactless in-line transition between a monolithic microwave integrated circuit (MMIC) and a ridge gap waveguide (RGW) is proposed and investigated at W-band. The transition employs an E-plane waveguide bifurcation obtained by mounting a GaAs MMIC on a supporting PCB in the opening of an RGW top metal lid. Designed this way, multiple contactless transitions can be placed in a row with an electrically small spacing that makes the transition idea suitable for array antenna front-ends. A transition equivalent circuit is constructed employing a single-mode transmission line model, which is verified through a full-wave simulation. An (85–105) GHz transition design is then developed and experimentally investigated in the back-to-back configuration indicating a (0.5–0.75) dB transition insertion loss. Finally, the performance of a 1-bit phase shifter MMIC, integrated into the RGW using two proposed transitions, is demonstrated.