This paper presents a metasurface-based single-layer low-profile circularly polarized (CP) antenna with the wideband operation and its multiple-input multiple-output (MIMO) configuration for fifth-generation (5G) communication systems. The antenna consists of a truncated corner patch and a metasurface (MS) of a $2\times2$ periodic square metallic plates. The distinguishing feature of this design is that all the radiating elements (radiator and MS) are printed on the single-layer of the dielectric substrate, which ensures the low-profile and low-cost features of the antenna while maintaining high gain and wideband characteristics. The wideband CP radiations are realized by exploiting surface-waves along the MS and its radiation mechanism is explained in detail. The single-layer antenna geometry has an overall compact size of $1.0\lambda _{0} \times 1.0\lambda _{0} \times 0.04\lambda _{0}$ . Simulated and measured results show that the single-layer metasurface antenna has a wide 10 dB impedance bandwidth of 23.4 % (24.5 – 31 GHz) (23.4 %) and overlapping 3-dB axial ratio bandwidth of 16.8 % (25 – 29.6 GHz). The antenna also offers stable radiation patterns with a high radiation efficiency (>95%) and a flat gain of 11 dBic. Moreover, a 4-port ( $2\times2$ ) MIMO antenna is designed using the proposed design by placing each element perpendicular to each other. Without a dedicated decoupling structure, the MIMO antenna shows an excellent diversity performance in terms of isolation between antenna elements, envelope correlation coefficient, and channel capacity loss. Most importantly, the operational bandwidth of the antenna covers the millimeter-wave (mm-wave) band (25 – 29.5 GHz) assigned for 5G communication. These features of the proposed antenna system make it a suitable candidate for 5G smart devices and sensors.

In this article, a 4-port circularly polarized (CP) multiple-input multiple-output (MIMO) antenna at 5.9 GHz is presented for the vehicle to everything (V2X) communications. A novel hybrid decoupling structure consisting of circular ring parasites and defected ground structure (DGS) is used to reduce the mutual coupling between the MIMO elements. The circular ring parasites are incorporated between the antennas to reduce the mutual coupling between perpendicular antennas. Whereas, the DGS significantly reduces the mutual coupling between adjacent antennas. These circular ring parasites and DGS affect the electromagnetic field distribution and consequently reduce the mutual coupling. The effectiveness of the decoupling structure is explained through transmission coefficient and surface current distribution. The simulated and measured results show that the proposed MIMO antenna provides overlapping $\text{S}_{\mathrm {11 {}}}$ (−10 dB) and axial ratio (3-dB) bandwidth of 2.04% (5.82 – 5.94 GHz) with a peak gain of 7.68 dBic. Moreover, the fabricated MIMO antenna offers excellent diversity performance, isolation between antenna elements is very high (>34dB), envelope correlation coefficient (ECC) is lower than 0.001, and diversity gain is 9.99 dB, very close to the ideal value of 10 dB. Furthermore, the link budget for the proposed MIMO system is calculated to check the suitability for V2X communications which shows that the antenna can communicate between them over a distance of 1.5 kilometers with a 21 dBm of transmit power. Owing to these qualities, the proposed MIMO antenna can be an excellent choice for V2X MIMO communications.

This paper presents the design and realization of a compact ultra-wideband (UWB) antenna with a rectangular notch wireless area network (WLAN) band that has controllable notched bandwidth and center frequency. The UWB characteristics of the antenna are achieved by truncating the lower ends of the rectangular microstrip patch, and the notch characteristics are obtained by using electromagnetic bandgap (EBG) structures. EBGs consist of two rectangular metallic conductors loaded on the back of the radiator, which is connected to the patch by shorting pins. A rectangular notch at the WLAN band with high selectivity is realized by tuning the individual resonant frequencies of the EBGs and merging them. Furthermore, the results show that the bandwidth and frequency of the rectangular notch band could be controlled according to the on-demand rejection band applications. In the demonstration, the rectangular notch band was shifted to X-band satellite communication by tuning the EBG parameters. The simulated and measured results show that the proposed antenna has an operational bandwidth from 3.1–12.5 GHz for |S₁₁| < -10 with a rectangular notch band from 5–6 GHz, thus rejecting WLAN band signals. The antenna also has additional advantages: the overall size of the compact antenna is 16 × 25 × 1.52 mm³ and it has stable gain and radiation patterns.

This paper investigates an ultra-wideband (UWB) antenna with a triple-rectangular notch band at 5G, WLAN, and Satellite downlink bands. The 5G and WLAN bands are notched by using a pair of electromagnetic bandgap (EBG) structures while the satellite downlink band is notched by using two split-ring resonators (SRRs). The EBG structure is a metallic conductor loaded on the backside of the radiator which is connected to the patch via a shorting pin. The SRRs consist of a pair of metallic rings etched on the backside of the feedline. The simulated and measured results show that the proposed antenna has an operational bandwidth from 3.1 – 11.8 GHz for $\vert \text{S}11\vert < -10$ rejecting 5G sub-6 GHz (3.4 – 3.9 GHz), WLAN (5.15 – 5.825 GHz), and satellite downlink band (7.25 – 7.75 GHz) signals with high selectivity. The peak gain of the UWB antenna is 3.86 dBi but it sharply decreases up to -13 dBi at stop-bands. The antenna also has additional advantages of the overall compact size ( $20\times 26\times1.52$ mm3) with stable gain and radiation patterns at the pass-bands.

This paper presents the performance improvement of a co-planar waveguide rectangularly notched UWB-MIMO antenna. The isolation and gain of the antenna are enhanced by using a parasitic isolator. The antenna consists of four microstrip patch antennas and an isolator. The UWB characteristic of the antenna is achieved by truncating the lower ends of the radiating patch by a semicircle. The rectangular notch characteristic is obtained by adding two electromagnetic bandgap structures on the backside of the antenna, which is attached to the radiator via shorting pin. The performance, especially the decoupling of the MIMO antenna is improved by using a novel parasitic decoupler, which is placed between the antennas to receive uncorrelated signals. The decoupling structure consists of a square shape metallic element with a circular slot inside and a half-semicircle slot edged at each corner. Four rectangular metallic stubs are extended from opposite parallel sides to improve further isolation. The simulated and measured results show that the antenna has a rectangular notch band (5.25–5.85 GHz) across the working band of 3–12.8 GHz. In addition, the antenna has a planar structure with an overall size of 60 × 60 × 1.52 mm³ and offers stable gain, reduced mutual coupling (<−21 dB), and lower envelop correlation (<0.001).

This paper presents a compact two-element MIMO antenna with improved isolation for triple-band applications. The antenna consists of two radiating elements with the shared ground plane and a novel decoupling structure. Each antenna element has three stubs with different lengths, which work as quarter-wavelength monopoles to give a triple-band operation. The decoupling system is made by etching various slots in an inverted H-shape stub attached to two quarter-circles at its lower ends. The simulated and measured results show that the antenna operates (|S₁₁| < −10 dB) at the key frequency bands of 2.4 GHz (2.29–2.47 GHz), 3.5 GHz (3.34–3.73 GHz), and 5.5 GHz (4.57–6.75 GHz) with a stable gain and radiation patterns. Moreover, the MIMO antenna shows good isolation characteristics. The isolation is more than 20 dB, the envelope correlation coefficient is <0.003, and diversity gain is 9.98 dB, within the frequency band of interest. Furthermore, the MIMO antenna has a compact size of 48 mm × 31 mm × 1.6 mm. These features of the proposed antenna make it a suitable candidate for I.S.M., 5G sub-6 GHz, and WLAN applications.