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A differentially fed hybrid dipole, slot, and patch antenna with unidirectional radiation patterns is designed in this article. The antenna consists of a metal strip, a patch loaded with a slot, and a simple wideband 180° out-of-phase power splitter (OPS). The OPS can generate two signals with the same amplitude but accurate phase difference of 180° to achieve one-port differential feeding. The hybrid modes of different types of antennas, i.e., dipole, patch, and slot, are excited to obtain five bands. Additionally, the TM01, TM21, and TM03 modes of the patch antenna are excited. By loading the slot and the metal strip, the radiation pattern of TM21 mode is converted from conical to broadside, and the ground walls further concentrate the radiation pattern. The ground walls also enhance the antenna gain, with the increments of 1 dB at 3.5 GHz, 3.4 dB at 4.65 GHz, 4.1 dB at 5.5 GHz, and 1.4 dB at 6.3 GHz. Finally, the antenna is fabricated and measured. The measured results show that the antenna covers five impedance bandwidths (IMBWs) of 1.85–1.91 GHz (3.2%), 3.13–3.66 GHz (15.6%), 4.62–4.98 GHz (7.5%), 5.10–5.82 GHz (13.2%), and 6.10–6.57 GHz (7.4%) for $\vert \,\,{S} _{11}\vert dB. The radiation patterns at each band are toward the broadside, with peak gains of 5.1 dBi at 1.85 GHz, 8.8 dBi at 3.45 GHz, 8.8 dBi at 4.7 GHz, 12.2 dBi at 5.5 GHz, and 9.7 dBi at 6.3 GHz.

In this paper, a $\varepsilon $ -negative metasurface superstrate is proposed for mutual coupling reduction of large antenna arrays. Unlike the previous decoupling metasurface works that are mostly confined to two-ports antennas, the proposed decoupling superstrate can be extended to massive multiple-input multiple-output (MIMO) systems. A $4\times 4$ antenna array is used as an example to illustrate the decoupling performance of the proposed metasurface. With the decoupling metasurface, the worst mutual coupling of the antenna array is improved by 8 dB over the operation bandwidth with a maximum mutual coupling reduction of 25 dB. Moreover, the decoupling metasurface also help restore the radiation patterns, bring down the active voltage sanding wave ratio, and broaden the bandwidth of the array.

Herein, the mesoporous composite materials, mTiO2–ZrO2/PTAy-x (PTA = phosphotungstic acid, x represents the molar ratio of Ti to Zr, y represents the weight percent of PTA in mTiO2–ZrO2/PTA samples), have been prepared by the surfactant-assisted sol–gel method combined with a subsequent hydrothermal treatment. The as-prepared catalysts were fully characterized by FT-IR, XRD, SEM, TEM, XPS, DRS, PLs and nitrogen adsorption–desorption techniques. The results indicated that mTiO2–ZrO2 sample with the TiO2 and ZrO2 molar ratio of 3 exhibits good crystallinity, large internal surface areas and narrow-sized pores. The PTA clusters incorporated in mTiO2–ZrO2 frameworks preserve intact Keggin structure according to EDX, FT-IR and diffuse reflectance UV/vis/NIR spectroscopy. The band gap energy of as-prepared composites is reduced compared with single components. The PL experiments revealed that the obtained mTiO2–ZrO2 can strongly retard the recombination of photogenerated electrons and holes compared with a single oxide, and the introduction of PTA species can further improve the light adsorption and the photochemical stability, therefore efficiently promoting photocatalytic activity of the ternary composite materials. Photocatalytic experiments confirmed that mTiO2–ZrO2/PTA10-3 displayed highly efficient and durable activity for the photodegradation of methyl orange under simulated sunlight and photoreduction of Cr(VI) under visible light. The study of the photocatalytic mechanism revealed that superoxide radicals (∙O2−) and holes (h+) are the main active species. These materials are very stable in the photocatalytic reactions, and therefore, they are promising and efficient catalysts for practical applications.

A defected ground structure (DGS) loaded with meandered lines is proposed to reduce mutual coupling of a dual-band planar multiple-input multiple-output (MIMO) antenna. To verify the effect...

This paper provides an effective technique to reduce the mutual coupling between two adjacent probe-fed microstrip antenna elements operating at 3.5 GHz in multiple input multiple output (M...

The fifth generation (5G) millimeter-wave (mmWave) handset demands a cost-effective mmWave array antenna with beam steering capability to overcome the high-pass loss and to ensure seamless connectivity. Unlike sub-6-GHz handsets, emerging mmWave handsets usually employ phased array antennas with a reasonably large number of elements. Unfortunately, due to the legacy of a few antennas in sub-6-GHz handsets, the mutual coupling effect on the mmWave handset has not been thoroughly investigated. In this paper, we study the mutual coupling effect on the mmWave handset performance by comparing array antennas with different inter-element spacing and different configurations. It is found that mutual coupling tends to increase the active reflection (especially at large scanning angles), which in turn reduces the realized gain and maximum scanning angle of the phased array antenna. For a sub-6-GHz multiple-input multiple-output handset with two or four antenna ports and fully digital precoding/decoding, 10-dB isolation is usually regarded as good enough. It is shown in this paper, however, that the outage capacity of the mmWave handset can be clearly improved by reducing the mutual coupling.

A dual-polarized antenna array for 4G base stations is presented in this paper. The antenna array is a 2×4 uniform planar array and is composed of printed dual-polarized dipole antennas, operating in 1.7-2.2 GHz. By loading the metal plate and using antenna decoupling surface, the mutual coupling of the array is below -20dB.

In this paper, a new method to compare the effectiveness of a stirrer by calculating the number of characteristic modes of the stirrer is proposed. We find that, for Z-shaped stirrers, the more characteristic modes a stirrer has, the better it works in the reverberation chamber. The number of characteristic modes refer to the number of modes whose characteristic angels fall between 135 and 225 degree. The effectiveness of a stirrer is measured in terms of the field uniformity in the reverberation chamber. The field uniformity is defined as the standard deviation (STD)of the average power of the total electric fields at the eight vertices of the working volume. Three types of stirrers with the same rotational volume are used for verifications.

A novel miniaturized dual-band dipole array antenna consisting of two parallel $1 \times 4$ subarrays for use in base stations operating in the digital cellular system (DCS) and wideband code division multiple access (WCDMA) bands is presented. Size miniaturization is realized by utilizing band notch dipoles (BNDs) as array elements and artificial magnetic conductors as reflectors for reductions in width and profile height of the array antenna, respectively. Using the BNDs of high radiation suppression at desired notch frequencies, the radiation from DCS subarray is suppressed in the WCDMA band and vice versa. High isolation between the subarrays, therefore, is achieved while reducing the subarray separation. In addition, a dual four-way unequal power divider is designed for beam pattern synthesis. The measured results show that realized gains of 12.1–12.6 and 12.4–13.1 dBi, respectively, within the DCS and WCDMA bands, as well as the first null depth and sidelobe level, respectively, of lower than −20 and −16 dB, are achieved simultaneously, indicating that the proposed array antenna is suitable for potential base station applications.

A low-profile dual-polarized dipole antenna using wideband artificial magnetic conductor (AMC) reflector with stable radiation pattern for use in 2G/3G/4G base stations operating from 1.7 to 2.7 GHz is presented. The antenna consists of a pair of crossed-dipoles and a wideband AMC reflector which consists of an AMC surface, a metallic ground plane, and the air substrate between them. The AMC is designed to operate with 90° reflection-phase bandwidth of 1.64–2.88 GHz (54.8%). Compared with using perfect electric conductor reflector, the profile height of the dual-polarized dipole antenna using AMC reflector is reduced by half from 36 to 18 mm, resulting in the lowest-profile 2G/3G/4G base station antenna. Six metallic ground walls are used to stabilize the radiation pattern, and the resulted half-power beamwidths are around 68±3° and 81±3° in the H- and V-planes, respectively, across the operating band. More measured results show that wide impedance bandwidths of 1.67–2.98 and 1.64–3.05 GHz for the two input ports, isolation of more than 25 dB, and cross-polarization of less than −20 dB are achieved.

It is commonly believed that orthogonal polarisations lead to negligible correlation. In this Letter, it is shown that for the closely spaced patch antennas, orthogonal polarisations alone do not ensure uncorrelation. The surface waves must be suppressed (in combination with orthogonal polarisation) to achieve uncorrelated antennas. This is validated by different realisations of closely spaced patch antennas. The final design with reduced mutual coupling and correlation has been fabricated and measured. Good agreements between measured and simulated results are observed.

A square patch radiator supports both TM 01 and TM 10 modes with equal resonant frequencies. Feeding the patch in its diagonal can excite both modes with equal amplitudes and phases. By utilising this property, an effective technique is proposed to realise dual-band dual-polarised patch antenna. While the lower resonant frequency can be calculated using an existing analytical formula, an expression of the higher resonant frequency is still missing in the literature. To this end, an approximate expression of the higher resonant frequency is devised using the equivalent transmission line model in this work. It is a function of the distance between the shorting post and the feeding location. In order to validate the accuracy of the expression, three different antenna configurations are designed and manufactured. Good agreements between theoretical, simulated, and measured results are observed.

A low-profile dual-polarized patch antenna with stable radiation pattern for use in base stations operating from 1.7 to 2.7 GHz is presented. The antenna consists of a square patch radiator placed at 20 mm ( $0.14\lambda _{0}$ at the center frequency of 2.2 GHz) above a square ground plane. For bandwidth enhancement, the square patch radiator is dual fed using four probes, each having a U-shape. Two wideband 180° baluns are designed using composite right/left handed transmission lines to generate two signals with same amplitude, but phase shift of 180° for the probes. The radiation pattern is stabilized using four slot groups on the ground plane and a metallic ground wall around the radiating patch. The antenna is studied and designed using computer simulation. Results show that the simulated half-power beamwidths have variations of only 4° and 5° in the elevation (EL) and azimuth (AZ) planes, respectively, across the operating band. Measured results show that the antenna has the impedance bandwidths of 1.68–2.83 GHz and 1.7–2.85 GHz for the two input ports, isolation of more than 38 dB, and cross-polarization of less than −20 dB at the boresight in both the EL and AZ planes. The measured boresight gains are 7.8 ± 0.6 dBi for both two ports, and the radiation efficiencies are 74%–84% and 73%–82% for ports 1 and 2, respectively.

The mutual coupling between antenna elements significantly deteriorates the performance of antenna arrays. Many decoupling techniques have been developed over the years. A key step of decoupling processing is to obtain the decoupling matrix (or mutual impedance matrix). However, the accurate decoupling matrix is unavailable in practice because mutual coupling depends not only on the geometry of the array, but also on the excitations of the elements. Unfortunately the decoupling errors can also lead to severe performance degradation of antenna arrays, especially for the adaptive beamformers. In this paper we propose a decoupling error reduction method based on the reduced- rank multistage Wiener filter (MWF). The reduced-rank MWF projects the weight vector of beamformer onto the Krylov subspace, which has a regularization effect that can reduce the performance degradation caused by random errors in the array. Besides having a simple implementation structure, the proposed method is computationally efficient. Moreover, simulation results show that the proposed method considerably improves the signal-to-interference-plus-noise ratio (SINR) of the beamformer when there exist decoupling errors in the array.

This paper presents a radar cross-section (RCS) reduction technique by using the coding diffusion metasurface, which is optimised through a random optimization algorithm. The design consists of two unit cells, which are elements &lsquo, 1&rsquo, and &lsquo, 0&rsquo, The reflection phase between the two-unit cells has a 180&deg, &plusmn, 37&deg, phase difference. It has a working frequency band from 8.6 GHz to 22.5 GHz, with more than 9 dB RCS reduction. The monostatic RCS reduction has a wider bandwidth of coding diffusion metasurface as compared to the traditional chessboard metasurface. In addition, the bistatic performance of the designed metasurfaces is observed at 15.4 GHz, which shows obvious RCS reduction when compared to a metallic plate of the same size. The simulated and measured result shows the proficiency of the designed metasurface.

In the present paper, a new technique to reduce mutual coupling between linearly arranged patch antenna array is presented. Antennas are arrange in close space with edge-to-edge separation of 0.096λ and center-to-center spacing of 0.29λ, where λ is the free space wavelength. Two separate rectangular shape and T-shape parasitic elements are used to cancel the surface current (SC) in order to reduce mutual coupling. The parasitic elements are placed in between the antennas with an extension of the T-shape towards the boundary of patches. The obtained results indicate that mutual coupling for two consecutive antennas are less than -15 dB over the entire bandwidth.

Multiple-input multiple-output (MIMO) systems promise high data rate and have been employed ubiquitously in modern telecommunications. However, due to limited space for MIMO antennas and finite angular spread of the propagation channel, antenna mutual coupling and correlations are inevitable, which can degrade the MIMO performance considerably. Since the antenna correlation is a function of antenna patterns and angular distribution of the spatial channel, it is less intuitive than the mutual coupling for antenna engineers. As a result, MIMO antenna designers usually focus on mutual coupling reduction, hoping that the correlation will be improved as well. But, does mutual coupling reduction guarantee low antenna correlation? Unfortunately, this important question has not been well answered in the literature. We try to address this issue in this work.

An optically transparent metasurface (MS) is proposed to design a resonant cavity fed by a patch antenna operating at 5.6 GHz. In the proposed MS, a transparent micro metal mesh conductive (MMMC) film is used as the transparent conducting film (TCF), and it has a high optical transmittance of more than 75% and a low sheet resistance of 0.7 &Omega, /sq. The MS is composed of a layer of glass substrate and a layer of MMMC film. The unit cell of MS consists of a square patch using MMMC film patterned on a square glass substrate. The transparent MS, patch antenna, ground plane, and air-filled half-wavelength cavity form a resonant cavity antenna, to achieve an improved gain. The MS is designed, optimized and analyzed using the EM simulation software CST. Results show that the MS can improve the simulated boresight gain from 4.7 to 13.2 dBi by 8.5 dB, without affecting the impedance bandwidth (IMBW) much. The losses of MS with different values of sheet resistance are also studied, showing the MS using MMMC with sheet resistance of 0.7 &Omega, /sq has very small losses.

This paper provides a systematic review of the mutual coupling in multiple-input multiple-output (MIMO) systems, including the effects on performances of MIMO systems and various decoupling techniques. The mutual coupling changes the antenna characteristics in an array, and therefore, degrades the system performance of the MIMO system and causes the spectral regrowth. Although the system performance can be partially improved by calibrating out the mutual coupling in the digital domain, it is more effective to use decoupling techniques (from the antenna point) to overcome the mutual coupling effects. Some popular decoupling techniques for MIMO systems (especially for massive MIMO base station antennas) are also presented.

An X-band metasurfaced (MS) antenna is proposed for circular polarization (CP) operation. The antenna consists of a patch antenna and a MS, which are printed on a single substrate, resulting in low profile. Results show that, with using the MS, the antenna can achieve an impedance bandwidth (IMBW) of 9.24–10.16 GHz (0.92 GHz, 9.5%) and an axial-ratio bandwidth (ARBW) of 9.54–9.94 GHz (0.4 GHz, 4.1%), and increase the boresight gain by about 2 dB on average within the ARBW.

A compact monopole antenna with a volume of only 8×32×5 mm3 for smartphones applications is presented. The antenna consists of two meander monopoles, having an S-shape and L-shape, on one side of the substrate and a tuning stub on the other side. Simulated and measured results show that the antenna has a dual-band of 824–960 MHz and 1700–3800 MHz which can cover the frequency bands for the GSM850, GSM900, DCS, PCS and UMTS systems and also Band 40, Band 41, Band 42 and Band 43 in the LTE system. The results in gain, efficiency and radiation pattern are also presented.

In this paper, a wideband circularly polarized (CP) slot-antenna array is presented. A feeding network with four output ports is designed using metamaterial transmission line (MM TL) to feed four slot antenna elements, forming a sequential-rotation array (SRA) and resulting in CP operation. The antenna array is studied and designed using computer simulation and measurement. The measured results show that the antenna array has a wide impedance bandwidth (IMBW) of 0.92–3.65 GHz (2.73 GHz, 119.5%) and axial-ratio bandwidth (ARBW) of 1.43–3.59 GHz (2.16 GHz, 86.1%).

A broadband dual-circular polarization (CP) antenna is presented in this paper. The antenna consists of square radiator fed by a feeding network with four outputs. The feeding network is designed using a 3-dB hybrid-coupler chip to obtain a sequential-phase difference of 90° at the four output ports. Simulation shows that the antenna has an impedance bandwidth (IMBW) of 37% (1.1–1.6 GHz) for |S11

A planar 8-port MIMO antenna consisting of only 4 slot elements is proposed in this paper. The antenna has a symmetrical structure and is designed to operate from 2.40–2.48 GHz. Each of the 4 slot elements is serving as a common radiator for 2 input ports. A T-shaped defected ground structure (DGS) is cut in the middle of the slot element to increase isolation between the two input ports to more than 10 dB and reduce the Envelop-Correlation Coefficient (ECC) to below 0.1. The proposed antenna is studied using computer simulation and fabricated for measurement using the Satimo Starlab system.

A defected ground structure (DGS) with two resonances is proposed to reduce mutual coupling of planar multiple-input-multiple-output (MIMO) antenna. A dual-band MIMO antenna is used for study. The antenna consists of two planar-inverted F-antenna (PIFA) elements printed symmetrically for dual-band operation. The defected ground structure (DGS) is placed between the two PIFA elements to reduce mutual coupling. The antenna is studied and designed using computer simulation, and then fabricated for measurement using the Satimo Starlab system. Results show that the MIMO antenna has the measured bandwidths of 1.56–2.78 GHz (1.22 GHz or 56%) and 4.7–5.82 GHz (1.12 GHz or 21%) in the lower and higher frequency bands, respectively, with mutual coupling of less than −15 dB.

A wideband metasurfaced (MS) slot antenna is proposed for circularly polarized (CP) operation. The antenna consists of a crossed slot antenna (CSA) and a MS placed at a short distance above the CSA. The CSA has two slots placed in orthogonal positions and with distributed elements to generate two E-fields with 90°-phase difference for CP radiation. A microstrip line is used to single feed the CSA at the center. Simulations and measurements are used to study and design the MS slot antenna. The measured results show that, with the use of the MS, the impedance bandwidth can be increased from 4.24–6.78 GHz (2.54 GHz, 46.1%) to 3.36–6.91 GHz (3.55 GHz, 69.13%), and the axial-ratio bandwidth (ARBW) can be increased from 4.38–4.96 GHz (0.58 GHz, 12.4%) to 3.96–6.02 GHz (2.06 GHz, 41.28%). Within the ARBW, the realized co-polarized gain can be increased by about 3 dB on average.