In this paper, a radio frequency sensor for measuring microfluidics dielectric properties is designed based on microstrip meander line. The meander sensor replaces the straight transmission lines with meander transmission lines in the part of the half-wavelength path difference to improve the sensitivity of the sensor and reduce its size. According to the experimental results, the meander sensor based on the meander line has higher accuracy and a lower relative error than the straight sensor in measuring methanol–ethanol mixtures with different molar fractions. The relative error measured by the meander sensor after calibration with an adjustable cavity is less than 1%. It is easier to detect the very slight variation in dielectric properties brought about by microfluidics. The detection technique can be further applied for the accurate detection of dielectric properties of valuable biological samples, providing a more concise and convenient way.

Circular antenna array (CAA) is one of the most widely used antenna array designs. This paper addresses the design challenges of the CAA with the non-uniform single ring, which is placed in an X-Y plane with the best sidelobe level (SLL) and improved first null beamwidth (FNBW). It has been solved using differential evolution, craziness-based particle swarm optimization (CRPSO), and novel particle swarm optimization (NPSO) techniques. An optimal combination of feeding current and inter-element spacing provides an array pattern with the best SLL and improved FNBW, as well as some other parameter calculations of the antenna array like maximum directivity, maximum effective aperture, total effective aperture, maximum beam area, total beam area, circumference, and radius of the CAAs using these techniques. There are six designs of CAAs with different antenna elements (i.e., 10-, 12-, 16-, 20-, 36-, and 64-elements) which have been taken into account. Simulations are done in MATLAB. Based on various simulation results, we can analyze the performance of SLL and FNBW with other parameters using NPSO and compare them with different techniques of CAAs, as shown in the numerical analysis and simulation result section.

For future fifth-generation (5G) smartphones, a high-efficiency multiple-input/multiple-output (MIMO) antenna system capable of operating in Long Term Evolution (LTE) 42/43/46 is suggested. A pair of small microstrip-fed slot antenna and inverted F antenna is part of the single antenna design structure. They are positioned on the longer side of the FR-4 printed circuit board. In an effort to lessen the mutual coupling, two antenna elements have a U-shaped rectangular slot inserted between them. The detailed study of the suggested MIMO system includes measurement of the reflection parameter, analyzing the radiation performance, performing envelope correlation coefficient (ECC) and channel capacity calculations, and analyses of the effects of user’s hand in the vicinity of the antenna array. Simulation and measurement results demonstrate the required performance of the proposed design, that is, greater than 80% of total efficiency, isolation >12 dB, and ECC <0.15. The suggested antenna has the benefits of multiband operation, high efficiency, good isolation, and a small footprint.

This paper presents a broadband circularly polarized (CP) antenna array for millimeter-wave applications, and the antenna array has the advantages of wide impedance bandwidth (IBW), novel CP design, and low profile. The antenna unit consists of a two-layer substrate and two pairs of magnetoelectric dipoles. Stepped microstrip lines coupled by rectangular slits form a feeder network for easy integration. The axial ratio bandwidth (ARBW) is extended because a pair of parasitic patches is loaded and an elliptical perturbation is added. The simulation results show that the antenna has an ARBW of 18.6% (26.4–31.9 GHz) and an IBW of 45.5% (20.6–32.7 GHz), with a gain greater than 7.11 dBic in the IBW. To improve the gain of the antenna, a 2 × 2 antenna array is designed, fabricated, and measured. The measured results show that the array has an ARBW of 16.6% (26.42–31.21 GHz), an IBW of 41.6% (22.28–33.97 GHz), a peak gain of 13.89 dBic in the IBW, the cross-polarization levels in the xoz-plane and yoz-plane are above 20 dB, and a radiation efficiency greater than 89%.

In this study, a high-isolation multiple-input multiple-output (MIMO) microstrip patch antenna (MPA), which utilizes an orthogonal mode cancellation method is proposed. This method employs TM10 and TM01 modes, which are simultaneously excited in the rectangular passive MPA. Initially, a rectangular decoupling structure featuring polarization rotation characteristics is designed. Further studies show that by loading the polarization conversion parasitic structure (PCPS), the electric field of the spatial coupling wave can be transformed from the x-polarized TM10 mode to the y-polarized TM01 mode. Therefore, TM10 and TM01 modes from the excited antenna and decoupling structure are concurrently coupled to the passive antenna, forming an evident weak-field region on the passive antenna. Placing the feeding probe of the passive MPA within the weak-field region prevents signal reception at the port. Consequently, this results in an extremely low mutual coupling of −49 dB at a resonant frequency of 5.8 GHz. Finally, a prototype of the proposed antenna is fabricated and tested, and the measured results closely match the simulated results. Additionally, it is observed that PCPS slightly influences the performance of the MIMO antenna.

A compact 12-element multiple-input multiple-output loop antenna array for a fifth-generation (5G) mobile phone is proposed. The operating band for the proposed antenna covers the long-term evolution 42 band, which spans a frequency range of 3.4–3.6 GHz. The size of the single antenna element is 5.85 × 4.9 mm2 (0.068λ0 × 0.057λ0), and 12 of these elements are positioned along two side edges of the mobile phone. This antenna is suitable for high screen-to-body ratio devices. The measured 6-dB impedance bandwidth is from 3.4 to 3.73 GHz. The measured total efficiency of the proposed antenna is 39–56% and the peak gain is 0–3.2 dBi. The transmission coefficient is less than −12 dB, the envelope correlation coefficient is less than 0.42, and the channel capacity is 45–50 bps/Hz.

An intra-band pattern-corrected decoupling vertical conducting wall is realized by dielectric substrate with conductor cladding on both side wall between two tightly spaced H-plane microstrip patches with λ0/20 edge-to-edge spacing. The wall is grounded and two symmetrical slots are etched on the vertical substrate. The measured results agree with the simulations, showing that the slotted vertical wall reduces the mutual coupling within the bandwidth to −30 dB and corrects the radiation beam tilt to be within −4.5° to 3° from the broadside direction. A gain reduction of 0.6 dB is observed compared to the gain without the slotted decoupling wall.

For forthcoming wireless applications, a small and highly decoupled complementary split ring resonators (CSRR)–loaded co-planar waveguide (CPW)–fed antenna for dual-band applications is investigated. The low-profile antenna consists of a CSRR-loaded rectangular radiating element with a truncated bottom, giving a wideband performance over the frequency ranges of 5.28–5.52 GHz and 6–7.2 GHz. The antenna has been printed on a widely used FR4 substrate measuring 7.5 × 10.5 × 1.6 mm3 in volume. This research’s suggested antenna is turned into a 4 × 4 multi input multi output (MIMO) construction using a 25 × 25 mm2 printed circuit board. Individual antennas were isolated by nearly 20 dB without using a decoupling device. The antenna has been built, and the measured and simulated results correspond well. Computing envelope correlation coefficient (ECC), channel capacity (CC), and channel capacity loss (CCL) further validates the antenna’s performance (−). The antenna has an overall gain of around 2.54 dBi and a radiation efficiency of approximately 89% throughout the relevant spectral range, which is much better for wireless applications. The suggested antenna’s omnidirectional emission pattern makes it a potential contender for future wireless and cellular applications.