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A frequency-reconfigurable bandwidth-enhanced dielectric patch antenna is investigated in this paper. The modified feeding structure and air tunnel are introduced to incorporate the TM10 mode and antiphase TM20 mode, thereby expanding the antenna bandwidth. Meanwhile, the air tunnel provides space for the placement of frequency tuning components. After that, according to the E-field distribution of the modes, two pairs of varactor-loaded microstrip lines are implanted in the air tunnel and sides of the DP resonator to independently tune the TM10 mode and antiphase TM20 mode, respectively. Finally, the approximate synchronous tuning of two modes is realized through the appropriate parameter combination of the frequency tuning structure. During the tuning process of the operating frequency, the bandwidth enhancement characteristic of the resultant antenna is obtained while maintaining high gain, good efficiency, and low profile (0.06λ0). The fractional bandwidth of each state varies from 7.9% to 13.1%. The -10 dB frequency coverage of the antenna is 22.6% (4.28-5.37 GHz). As the operating frequency shifts down, the peak gain varies from 8 to 6.2 dBi while the peak efficiency changes from 93% to 82%.

A wideband circularly polarized (CP) endfire planar magneto-electric dipole (ME-dipole) antenna based on substrate integrated waveguide (SIW) is proposed for millimeter-wave (mm-wave) applications. The antenna mainly consists of an open-ended rectangular SIW radiating like a magnetic dipole, a parallel electric dipole, and a double-sided parallel-strip line (DSPSL) which connects the two dipoles and provides 90 o phase difference between them to generate CP radiation. To increase the axial ratio (AR) bandwidth, a pair of triangular slots are etched at the end of the upper and lower broad walls of the SIW, and moreover, a specific bowtie-shaped electric dipole is used instead of the traditional linear one. It has been shown that both the slots and the bowtie-shaped electric dipole can regulate or tune the magnitude of the radiation field of the electric dipole, making it tend to balance with radiation field generated by the magnetic dipole and hence leading to a good CP performance. The antenna has a single-layer planar structure, and it shows a -10-dB impedance bandwidth of over 50%, a 3-dB AR bandwidth of 36.6%, and a peak gain of 9.3 dBic.

A polarization-dependent transmissive and reflective phase-gradient metasurface (PGMS) is investigated and experimentally verified in the $Ku$ -band. The PGMS is based on a single-layer substrate-integrated element, which consists of a cross-shaped dielectric unit and a couple of metallic strips/patches that are printed on the dielectric unit. Taking advantage of both the dielectric and the metallic components, the proposed element can transmit or reflect the linearly polarized (LP) incident wave depending on its polarization. Moreover, the phase of the transmitted or reflected wave can be independently controlled by different phase-control parameters. As a result, two independent foci can be obtained on the transmission and reflection sides of the PGMS for two orthogonal LP waves. For demonstration, a prototype of the PGMS is fabricated and tested. Both the transmissive and reflective foci are observed at the target positions. Also, the PGMS is shown to be able to manipulate the wavefronts independently on its two sides by adjusting the phase distributions. These features make the proposed PGMS a great choice for use in polarization identification, near-field energy transfer, or far-field wireless communication.

A novel decoupling method is investigated for the multi-input multi-output (MIMO) microstrip patch antenna (MPA) array. This method makes use of the TM10 and TM02 modes, which are simultaneously excited in the rectangular MPA. It is found that by adjusting the frequencies of the two modes, their $E$ -fields coupled to the adjacent passive MPA can counteract each other to generate a specific null-field region. When the feeding position of the passive MPA is located in the null-field region, the MPA cannot be excited effectively, and hence an extremely low mutual coupling is obtained without requiring any extra decoupling structure. To validate the feasibility of this decoupling method, an E- and an H-plane-coupled $1\times 2$ MIMO MPA arrays are respectively simulated, fabricated, and measured. The isolation of the E-plane-coupled array is enhanced from ~11 to 57 dB, while that of the H-plane-coupled array is improved from ~18 to 51 dB.

A simple decoupling method of using specific higher-order modes to improve the isolation of dielectric resonator antenna (DRA) elements is investigated. The higher-order modes have the feature that they can be excited by the active (source) DRA while unexcitable by means of the feeding network. As a result, the coupled fields in the passive (coupled) DRA are unable to reach the feeding port, leading to a very low mutual coupling level. When the coupled modes in the passive DRA have the same radiation characteristics with the mode that excited in the active DRA, the distortion of radiation patterns can also be avoided. This method is found effective for reducing both the E-plane and H-plane coupling. As no extra decoupling element is needed, the entire antenna system is very simple. For verification, an E-plane-coupled $1\times 2$ multiple-input-multiple-output (MIMO) DRA array has been fabricated and measured. The results show that using a proper mode, the isolation can be enhanced from about 15 to 45 dB at 5.25 GHz.

A method of designing a miniature planar dual-band horizontally polarized (HP) omnidirectional antenna is presented. It combines an electrically small (ES) lower band omnidirectional loop antenna with an upper band Alford loop antenna on a single substrate. Its frequency ratio is higher than two. A technique that effectively increases the bandwidth of the Alford loop is developed. To reduce the gain variation in the azimuthal plane, the ES loop is fed by four symmetrical radial strips extended from a circular patch. For demonstration, the ES loop and Alford loop were designed for the 2.4 and 5 GHz Wi-Fi bands for Wi-Fi applications. The antenna prototype has a diameter of $0.296\lambda _{0}$ and a height of $0.012\lambda _{0}$ only, where $\lambda _{0}$ is the wavelength in air at the lowest operating frequency. It covers the Wi-Fi bands of 2.4–2.5 and 5.1–5.9 GHz, with a high total efficiency of ~90%. Based on the single-element design, a four-element multiple-input–multiple-output (MIMO) antenna is developed for Wi-Fi applications. The MIMO antenna has a size of $0.93\times 0.93\times 0.012\lambda _{0}^{3}$ , with a small gain variation of less than 3.2 dB in the azimuthal plane. The isolations between any two antenna elements are higher than 16 and 24.5 dB for the 2.4 and 5 GHz bands, respectively.

This communication describes a wideband switched-beam antenna fed by a compact single-layer beamforming network. First, a novel hybrid coupler structure that can provide an arbitrary phase difference over a bandwidth larger than 58% is proposed with the theoretical analysis. Based on this compact coupler configuration, a wideband $4 \times 4$ Butler matrix is devised with the need for standalone phase shifters and crossovers eliminated. The proposed Butler matrix achieves the widest bandwidth among the single-layer structures up to present. Consequently, a wideband switched-beam antenna array is realized by utilizing the log-periodic endfire antenna elements with bowtie dipoles on the Butler matrix. For validation, a broadband switched-beam antenna array is designed, fabricated, and measured. A stable beamforming performance is ascertained with a gain variation smaller than ±1.66 dB over a bandwidth from 1.98 to 3.14 GHz.

A circularly polarized (CP) multiple-input–multiple-output (MIMO) antenna is investigated. This MIMO antenna consists of two identical dielectric resonator antennas (DRAs), which use the same feeding cross slots to generate the same CP fields. Four metal strips are printed on the lateral sides of each DRA to change the rotation direction of the $E$ -field coupled in the passive (coupled) DRA, vary its polarization property, and thus make it opposite (orthogonal) to that of the active (driven) DRA. Thanks to the polarization orthogonality, the isolation between the two CP DRAs is improved significantly, without occupying extra space and destroying the antenna performance. In addition, using this decoupling scheme, the two CP DRAs can operate not only separately but also simultaneously. For verification, a prototype operating at the 2.4 GHz WLAN band is fabricated and measured. In the operating band (2.38–2.52 GHz), the MIMO antenna obtains a maximum improvement of 31 dB and an average improvement of 11 dB in isolation, at a center-to-center distance of $0.4\lambda _{0}$ .

The coexistence of microwave (MW) and millimeter wave (MMW) technologies is considered as the development tendency of the future 5G wireless communication system. In this paper, a single-port, single-layer, dual-band antenna with large frequency ratio is proposed to support the MW and MMW applications. This antenna mainly consists of a stub-loaded microstrip line in the center, two slot-loaded rectangular patches on the lateral sides, and two thin microstrip lines in-between which connect the former two. The stub-loaded microstrip line works as a series-fed MMW linear array at 26 GHz, and simultaneously it serves as the feed line for the MW patches operating at 4.85 GHz. The connection lines are elaborately designed to isolate the MW patches from the MMW signal, guaranteeing the radiation performance of the antenna at the two bands without requiring complicated filter. For verification, a prototype is fabricated and measured. The measured results show that the antenna has an impedance bandwidth of 3.1% and a peak gain of 8.93 dBi at the 4.85-GHz MW band, while an impedance bandwidth of 19% and a peak gain of 13.57 dBi at the 26-GHz MMW band.

An advanced characteristic mode tracking method is investigated in this article. The tracking method is based on the eigenvector correlation, and a novel tracking error correction algorithm (TECA) is proposed to solve the challenging issues caused by the mode swapping and the mode degeneracy. As the core of the modal tracking method, the TECA can quickly find out the cross-mode pairs according to the eigenvalue differences among different modes, identify the wrongly mapped modes using the eigenvector correlation, and finally correct all the possible tracking errors. The detailed process of each step is described and discussed. In addition, the characteristic modes of two classical fractal antenna structures are investigated for validation. It has been shown that the modes including the swapped modes and degenerated modes are all tracked correctly, verifying the modal tracking method is reliable and efficient. Compared with the previous approaches, the proposed method is not subject to the number of tracked modes and the magnitude of eigenvalue. Also, it requires less computational time. Moreover, the sensitivity to the sizes of frequency step and mesh is reduced to some extent.

A single-layer low-profile circularly polarized (CP) filtering patch antenna is investigated for wireless local area network (WLAN) applications. The CP antenna mainly consists of a feeding loop, four L-shaped radiating patches, and a square ground plane. To integrate the filtering function, four thin microstrip stubs and four shorting pins are introduced to the patches, and two U-shaped slots are etched in the feeding loop. These parasitic elements can realize the bandpass filtering function without requiring an external filter. In addition, these elements are skillfully added into the CP antenna, without increasing either the antenna footprint or profile. To verify the design, a prototype operating at 2.4 GHz WLAN band is fabricated and tested. The measured results show that the prototype has a low profile of 0.02 λ 0, a passband bandwidth (| S 11| < –10 dB and axial ratio (AR) < 3 dB) of 4.1%, and a peak gain of 8.3 dBic. Moreover, the prototype also shows a sharp roll-off rate near the passband edge and a good out-of-band suppression level of more than 20 dB in the stopbands.

A series-fed millimeter-wave (mm-wave) microstrip linear array with single-layer structure and wide operating bandwidth is proposed and investigated. The antenna array simply consists of a microstrip line and a series of short stubs that are periodically introduced on one edge of the microstrip line, at an interval of about one guide wavelength. Owing to the perturbation of the additional stubs, the original symmetrical E-field distributions on the two opposite edges of the microstrip line become asymmetrical, and the intensities of the equivalent magnetic currents are unequal and unable to cancel each other out as in a pure microstrip line. As a result, the stub-loaded microstrip line can radiate effectively, producing broadside radiation patterns. The introduction of stubs can also generate an extra resonant mode, which combines with the resonant mode of the microstrip line, yielding a wide bandwidth of over 20%. In addition, the microstrip array is simply fed by a coaxial probe, without involving any complex feeding network. Therefore, the array configuration is very simple and compact. For demonstration, a $1 \times 4\,\,1$ -D microstrip linear array operating at 27 GHz is fabricated and tested. The measured results show that the array has an impedance bandwidth of 21.40%, an average realized gain of 12.18 dBi, and a first sidelobe level (FSLL) of about −13 dB. Moreover, the FSLL can be decreased to −28.4 dB by tapering the width of the microstrip line. Finally, the design is extended to realize a $4 \times 4\,\,2$ -D microstrip planar array. This planar array can achieve an impedance bandwidth of 19.15% and an enhanced gain of 18.30 dBi.

A planar endfire antenna with enhanced front-to-back ratio (FTBR) and miniaturized size in its endfire direction is investigated in this communication. The antenna mainly consists of two closely placed dipoles with a small spacing of $0.04~\lambda _{0}$ . By using a novel topology, the desired phase difference between the two dipoles is obtained to improve the peak FTBR up to 30 dB and the peak gain to 6.5 dBi. Furthermore, a parasitic strip and a series of grooves are introduced to the antenna to adjust the phase difference and the current distributions of the two dipoles, effectively expanding the 20 dB FTBR bandwidth from 2.3% to 13.3%. Consequently, the antenna with an overall size of $0.161\,\,\lambda _{0} \times 0.43 \lambda _{0}\times 0.009\lambda _{0}$ obtains an overlapping bandwidth ( $\vert \text {S}_{11}\vert \lt - 10$ dB and FTBR >20 dB) of 8.3%, an average gain of 5.1 dBi, and a high efficiency of about 90%.

A planar 2 × 2 filtering patch antenna array with high selectivity and wide stopband is investigated for marine communications. The 2 × 2 patch array is composed of a driven patch and four identical parasitic patches placed nearby. A forked-microstrip-line-coupled rectangular slot is used to excite the driven patch, while the neighboring parasitic patches are capacitively coupled by the driven patch. Owing to the hybrid feeding effect of the forked feed line, a radiation null can be generated at the lower band-edge. Simultaneously, another radiation null can be generated at the upper band-edge owing to the radiation cancellation effect between the driven patch and the parasitic patches. Consequently, no power dividing feeding network is required for the 2 × 2 array, and the quasi-elliptic bandpass filtering response is achieved without utilizing any complex filtering/non-filtering circuits. In addition, the proposed filtering array exhibits a wide stopband capable of suppressing the second-harmonic radiation effectively. Based on this 2 × 2 array, a larger 4 × 4 filtering array is further designed by just using a traditional four-way power dividing network. For validation, both of the 2 × 2 and 4 × 4 filtering patch arrays operating at 5.25 GHz are fabricated and tested. The measured results show that the two patch arrays have comparable impedance bandwidths of about 4.4%. The in-band peak gains are given by 11.1 and 15.5 dBi respectively, which are suitable for the long-distance shore-to-ship communications.