The single- and dual-layer metasurfaces are proposed to miniaturize a low-profile wideband antenna. The single- and dual-layer metasurfaces consist of one and two square patch arrays, respectively, both supported by grounded dielectric substrate to form the waveguided metamaterials. After retrieving the effective refractive index along the propagation direction in the waveguided metamaterial, the effective propagation constant is subsequently derived to initially estimate the resonant frequencies of the dual-mode antenna. With the increased effective refractive index, both the proposed antennas realize the gain greater than 6.5 dBi over the bandwidth of ~30% with a reduced radiating aperture of $0.46\lambda _{0} \times 0.46\lambda _{0}$ and a thickness of $0.06\lambda _{0}$ ( $\lambda _{0}$ is the free-space wavelength at 5.5 GHz). Moreover, the dual-layer metasurface provides more freedom to increase the effective refractive index with achievable gap widths compared with the single-layer metasurface.

A patch is proposed to stack above a substrate integrated cavity (SIC) backed filtering slot antenna to enhance antenna frequency selectivity. The antenna is composed of a slot radiator backed by an SIC, a stacked patch, and a microstrip feed line. Two parallel slots are cut onto the top broad wall of the SIC, which is excited by a microstrip line through a nonresonant short slot on the center of the bottom broad wall. The stacked patch is positioned right above the SIC supported by a piece of substrate slab for gain enhancement. Compared with the traditional SIC-backed slot antenna, the proposed antenna achieves higher frequency selectivity as well as enhanced bandwidth and gain. The field distribution and parametric study are conducted to exhibit the operating mechanism of the antenna more clearly. Compared with the conventional SIC-backed slot antenna, the antenna prototype operating at 10 GHz, as example, achieves wider 10 dB impedance matching bandwidth of 8.67%, higher gain of 7.2 dBi, better frequency selectivity of 0.9/0.45 at the lower and upper edges of the operation band, and lower maximum cross-polarization levels of -23/-22 dB in E- and H-planes within the 3 dB beam range, respectively.

Recently, there has been growing interest in the use of metamaterial (MTM)-based lenses, also known as metalenses, as innovative antenna technology. Increasingly widespread applications of metalenses in modern microwave communication and sensing systems have been found, following the development of the first microwave artificial lens in the 1940s based on the concept of an artificial dielectric, which was later broadly termed an “MTM.” This article examines the evolution of metalens antennas over the past 80 years and introduces the principles and technologies underlying their design. It then focuses on the latest progress in the research on and applications of MTMs and metasurface (MTS)-based metalens antennas. The principles and basic structures of transmissive focusing metalens antennas in the microwave band are elaborated. Selected metalens antennas are introduced chronologically, starting with metallic waveguide lens antennas, followed by metallic or metal–dielectric Fresnel zone plate lens antennas, transmitarray lens antennas, MTS lens antennas, and flat transformation-optics-based MTM Luneburg lens antennas. The technical merits, challenges, applications, trends, and future research of each type of metalens antenna are also addressed. The information presented in this article will benefit the research, development, and application of metalens antennas in fifth-and beyond-generation communications, Wi-Fi 6 short-range connections, and next-generation microwave sensing and imaging systems.

This paper presents a new approach for designing differential bandpass filter (BPF) with independently tunable/controllable dual passbands using the element-loaded stepped-impedance resonators (SIRs). The frequency-agile ability of the fundamental and third-harmonic resonant frequencies ( f ₁ and f ₃ ) of the element-loaded SIR against the value of the loaded element under different loading positions on the SIR is theoretically analyzed. It can be found that there must be a position that could be utilized to load element to realize the independent control of f ₁ without affecting f ₃ . This feature is successfully applicable to design independently tunable differential dual-band BPF when the loaded elements are tunable capacitors. Furthermore, by making the loaded elements to be shunt stubs, a new coupling path can be constructed for f ₁ , and then the coupling coefficients for the two bands can be separately varied. Both the operational frequencies and bandwidths of the two bands can be separately adjusted. As a result, the traditional design procedure for the differential dual-band BPF can be simplified significantly. For demonstration, two examples are designed, implemented, and measured, and their simulation and measured results are presented, showing good accordance.

This paper presents a varactor- and stub-loaded dumbbell-shaped resonator (VSDSR) for application in differential dual-band bandpass filter, which possesses a tunable lower passband and a fixed upper passband. Under differential-mode (DM) bisection, the VSDSR behaves as a ring resonator with four resonant modes, which can be categorized into two independent groups for constructing two passbands. At the same time, the common-mode (CM) equivalent circuit of the VSDSR generates multiple CM resonant frequencies and some of them inevitably overlap with the DM ones. Accordingly, the traditional CM suppression methods such as adding shunt stub or lumped element are disabled. To suppress the CM signals in both DM passbands, the defected ground structure (DGS) units are embedded underneath the adjacent edges of the two rings directly to form a CM suppression filter without increasing the circuit size. Meanwhile, they have almost no effect on the DM responses. Benefitting from the bandstop and slow-wave effects of the employed DGS units, the CM noises in both DM bands can be rejected effectively. In particular, the level of the CM suppression in the DM lower passband can be improved by adjusting the location of the DGS units. Furthermore, the proposed CM suppression scheme is adaptive in the tunable lower DM passband.