A compact co-linearly polarized microstrip antenna with identical radiation properties is proposed and validated for in-band full-duplex systems. By exploring a fence-strip resonator (FSR) in the central plane of a patch, good isolation is achieved between the transmitting and receiving ports. The proposed FSR consists of a metallic vias fence and a strip with a distance away from the ground, performing as a pair of distributed inductor and capacitor. When the FSR is resonating, the radiating current is concentrated in the half part of the patch, performing at its half-TM ₁₀ mode with high port isolation. To validate the proposed antenna, a prototype with the size of $0.25\lambda _{0} \times 0.25\lambda _{0} \times 0.04\lambda _{0}$ ( $\lambda _{0}$ is the free-space wavelength at center frequency) has been fabricated and characterized, with experiments consistent well with simulations. A measured port isolation higher than 20 dB is achieved over the operating bandwidth of 2.40–2.52 GHz with the maximum of 30 dB. The proposed antenna is with the merits of compact size, low profile, simple feed, and planar-integrated structure for in-band full-duplex systems.

A dual-polarized microstrip antenna loaded with capacitive via fence is proposed for compact size, wide beamwidth, and high isolation. By exploring a capacitive blind-via fence near the radiating apertures, the field is concentrated between the fence and ground, achieving obvious dimension miniaturization and wide beamwidth for regular dual-polarized microstrip antennas. The proposed blind-via fence is constituted by four rows of metallic wires with subwavelength diameter, period and gap to the ground. Thanks to the field concentration brought by the blind-via fence, high isolation is realized by suppressing the radiation from the feeding probes, similar with two disk-loaded monopole antennas, and forcing the fundamental mode of the patch. To validate the proposed concept, a prototype is fabricated and characterized with the size of 0.19 λ0 × 0.19 λ0 × 0.07 λ0 (λ0 is the free-space wavelength at center frequency). The simulated and measured results agree well, with wide half-power beamwidth of 107° and 105° in E and H plane, respectively. Meanwhile, the port isolation is higher than 32 dB within the operating bandwidth of 2.48 ~ 2.56 GHz. The proposed antenna is with the merits of compact size, wide beamwidth, and high isolation, exhibiting potential usage in diversity or MIMO systems with wide-coverage applications.

We present a high-gain, low-profile dual-polarized microstrip antenna operating in the hybridized higher order mode. The proposed antenna consists of a slot-loaded cross-shaped patch, which can be considered as orthogonally polarized radiating elements operated in the TM ₅₀ mode. Moreover, the four loaded slots can effectively excite the in-phase TM ₁₀ mode such that the superposition of both modes can lead to gain enhancement in the broadside direction, with suppressed sidelobes. The proposed antenna requires only a single-dielectric layer and a simple feeding method, while exhibiting good isolation between the two polarizations. We have developed a prototype, which has a total size of only 1.65 λ ₀ × 1.65 λ ₀ × 0.04 λ ₀ (λ ₀ is the free-space wavelength at the center frequency) and a measured maximum gain of 10.9 dBi. This dual-polarized microstrip antenna with the advantages of high gain, low profile, low cost, and good isolation, is envisioned to benefit many multiple-input, multiple-output and the fifth generation (5G) communication systems.

In this communication, a compact-size and high-efficiency air cavity antenna with a cage-like structure is proposed for 2.4-GHz Wireless Local Area Networks (WLAN). The air cavity antenna is constructed with four sides of hybrid metallized vias, including sparsely arranged blind vias and tightly arranged shorting vias. Firstly, the antenna size is significantly reduced by utilizing the shorting vias, which make the antenna operate at the high-order TM _1/2,1/2 mode. Then, by loading the capacitive blind vias at the radiating apertures, the antenna size is further miniaturized. Notably, high radiation efficiency is obtained attributed to the lower dielectric loss of the air-medium structure. Finally, a prototype of the proposed cage antenna has been fabricated and tested with good agreement between simulated and measured results. In such a compact size of 0.128 λ ₀ ×0.128 λ ₀ ×0.10 λ ₀ (λ ₀ is free-space wavelength at the center frequency), the measured total efficiency over 90% is achieved in the WLAN band by the cage structure. Compared with the existing 2.4-GHz antennas, the proposed one is with the ascendency of compact size and high radiating efficiency for WLAN applications.

An effective decoupling structure is presented for compact, co-polarized, and high-isolation two-port multiple-input–multiple-output (MIMO) patch antenna. The proposed decoupling structure is composed of chip capacitors and metallic vias, essentially behaving as an inductor-capacitor ( LC ) series band-stop filter. Specifically, the LC decoupling structure is periodically positioned along the center of the patch, effectively blocking the radiating current when it resonates with half of the patch. Furthermore, the proposed antenna can be treated as a pair of back-to-back TM _1/2,0- mode printed inverted-F antennas (PIFAs) with zero edge spacing but high isolation. As a validation of the concept, the measured results of a fabricated prototype consist well with the simulated ones, demonstrating good isolation better than 24 dB over the whole impedance bandwidth of 2.41-2.53 GHz. With a compact size of 0.26λ ₀ × 0.25λ ₀ × 0.03λ ₀ (λ ₀ is the free-space wavelength at the center frequency), the peak isolation reaches 38 dB at 2.46 GHz. The proposed structure holds significant promise in MIMO antenna applications due to its merits of compact size, high isolation, and identical broadside radiation property.