In order to improve the sensitivity of liquid dielectric constant measurements, a liquid dielectric constant sensor based on a cubic container structure is proposed for the first time. The cubic container, which consists of a dielectric substrate with a split resonant ring (SRR) and microstrip lines, can enhance the electric field intensity in the measuring area. High sensitivity can be obtained from measuring the dielectric constant with the characteristics of the structure resonate. The research results show that the resonant frequency of the sensor is shifted from 7.69 GHz to 5.70 GHz, with about a 2 GHz frequency offset, when the dielectric constant of the sample varied from 1 to 10. A resonance frequency offset of 200 MHz for the per unit dielectric constant is achieved, which is excellent regarding performance. The permittivity of oil with a different metal content is measured by using the relation between the fitted permittivity and the resonant frequency. The relative error is less than 1.5% and the sensitivity of measuring is up to 3.45%.

A 2 × 2 dual-band dual-circularly polarized (CP) Fabry-Pérot cavity (FPC) multiple input multiple output (MIMO) antenna is proposed in this paper. It is formed by arranging four identical FPC antenna elements with 90° rotation. The FPC antenna element is composed of a single linearly polarized (LP) feeder and a chiral metamaterial (CMM)-based partially reflective surface (PRS), which acts as a polarization converter. The feeder is with a hybrid structure of a slot radiator operating in the low band and a patch radiator in the high band. As the orthogonal currents on the end and center arms of the H-shaped patch break the degeneracy of two CP waves at different frequencies, the PRS simultaneously realizes left-hand CP (LHCP) and right-hand CP (RHCP) features at two different bands, respectively. To enhance the port isolation between two adjacent antenna elements, a broadband, wide-angle, polarization-independent metamaterial absorber (MMA) is introduced to suppress the space wave coupling. To verify the design concept, the proposed MIMO antenna prototype is fabricated and measured. The measured bandwidth with a voltage standing wave ratio (VSWR) of less than 2 covers 3.32-3.42 GHz and 6.23-6.93 GHz, and the LHCP pattern and RHCP pattern are realized at 3.33-3.42 GHz (2.7%) and 6.59-6.75 GHz (2.4%), respectively. A maximum gain enhancement is up to 5.31 dBi in the low band and up to 4.3 dBi in the high band. In addition, the port isolation is better than 42 dB, and the ECC is less than -40 dB. With these performances, the proposed MIMO antenna is suitable for vehicular satellite communication applications.

A high-gain and beam-switchable multibeam holographic metasurface antenna (HMSA) array coated with phase-correcting metasurface (PCMS) is proposed. It comprises four identical HMSAs arranged in a crossed-shaped structure. Each beam has a different direction to allow for switchable control according to engineering specifications. The HMSA pattern is generated through holographic interference between the desired beam and the reference wave. A top-hat monopole Yagi radiator is employed as the surface-wave (SW) launcher to produce the reference wave. To enhance the gain, a PCMS is strategically suspended over the HMSA array at a distance of 6 mm. Each cell of the PCMS can provide arbitrary phase compensation in the reference plane to realize the in-phase plane wave. The phase lag arises from nonuniform phase distributions on the MS and different distances from each MS unit cell to the reference plane. It also acts as a superstrate to adjust the free-space wave coupling path. When the free-space wave and SW coupling have the same amplitude but out of phase, they subtract each other. A prototype is fabricated and measured, demonstrating that the use of PCMS can enhance the radiation performance of the HMSA array. This achieves a peak gain of 19.53 dBi, an SLL of less than −14.5 dB, and an aperture efficiency (AE) of more than 26.48%.

A compact high-isolation multiple-input multiple-output (MIMO) microbase station antenna consisting of two dual-polarized antenna elements is presented. The symmetrical magnetic coupling feeding network is used to feed the radiation patch of the dual-polarized antenna and to improve the isolation of the antenna element. By introducing parasitic square patches, the radiation characteristics and isolation of the dual-polarized antenna are improved. In addition, a modified closed-loop metasurface unit is designed and used to construct an isolation wall. When dual-polarized antenna elements are used to assemble a MIMO antenna, the proposed metasurface structure is inserted between them to reduce the coupling between MIMO antenna elements, regarded as a spatial band-stop filter to suppress the spatial electromagnetic waveform of the elements. After loading the metasurface isolation wall, the maximum increase of isolation is 9.17 dB at the center frequency point of 2.6 GHz. For antenna element ports and cross-polarized ports, the isolations are greater than 30 dB, and the isolation of the same polarized ports is better than 25 dB within the frequency band from 2.45 to 2.8 GHz. At the same time, the reflection coefficients of the ports of the elements are less than −15 dB. Finally, a prototype of the proposed MIMO microbase station antenna was fabricated and tested, and the experimental results are proximity with the simulation responses.

This paper presents the axial ratio (AR) bandwidth enhancement of a dual-band dual-circularly polarized (DBDCP) antenna. It utilizes a novel polarization-independent partial reflective surface (PRS) unit cell, which consists of two same metallic patterns with a relative 90o rotation on both surfaces of a substrate. Each metallic pattern is composed of a C-shaped arc and a circular patch. The LP field from the feeder is transformed into two different circularly polarized (CP) field at two respective bands by the unit cell. Due to the asymmetric structure and separated topology of two C-shaped arcs, a slight resonant deviation appears in AR for different polarized waves. Moreover, both x- and y-polarized waves are produced simultaneously by using Fabry-Pérot cavity (FPC) structure when a y-polarized feeder is used. Thus, the AR bandwidths for both x-and y-polarized incident waves are close to each other and combined deliberately into wide AR bandwidth. To validate the design concept, a prototype of the proposed MIMO antenna is fabricated and measured. The measured voltage standing wave ratio (VSWR) of less than 2 covers from 11.55 GHz to 14.8 GHz, and the LHCP pattern and RHCP pattern are realized in low band of 12-12.6 GHz (4.88%) and high band of 14.1-14.61 GHz (3.55%), respectively. The maximum gain is 13.5 dBi (13.1 dBi) at the low-band (high-band). Besides, the port isolation is better than 40 dB, and the ECC is less than -80 dB. With these performances, the proposed MIMO antenna can be employed for vehicular MIMO communications and sensing at Ku-band.