Presents the table of contents for this issue of the publication.

"Presents a listing of the editorial board, board of governors, current staff, committee members, and/or society editors for this issue of the publication."

A circularly polarized (CP) broad-beam parallel-plate (PP) antenna is investigated. The difference between the phase velocities of two PP orthogonal modes is used to obtain CP fields. The two orthogonal modes are excited by an inclined slot fabricated on the ground plane that connects the two PPs. By tuning the slot length, the impedance can be easily matched. To reduce the cross-polarized fields, a modified CP PP antenna with semicircular plates is introduced. ANSYS HFSS was used to simulate the antennas and prototypes were fabricated to verify the simulations. Both the original and modified CP PP antennas have an overlapping antenna bandwidth of ~13.0% and an antenna gain of ~8 dBic. As compared with other broad-beam antennas, the two PP antennas have simpler and larger structures, easing their fabrications especially at millimeter-wave frequencies. A design guideline is developed.

A novel design concept to reduce high H-plane cross-polarization of a shorted patch antenna (SPA) with broadside radiated patterns is presented by loading a pair of open-ended stubs. Initially, the nonbroadside $\vert E_{\mathrm {\theta }}\vert $ component of the traditional SPA at TM $_{\mathrm {1/2,2}}$ mode is demonstrated to be much larger than its broadside $\vert E_{\varphi }\vert $ component in H-plane or yoz plane radiation patterns. In order to maintain its peak radiation in the normal direction, the far-zone radiated fields of the antenna are theoretically studied and analyzed. The results demonstrate that the dual nodal lines of the TM $_{\mathrm {1/2,2}}$ modal fields could be gradually moved far away from each other by increasing the stub length. As such, its $\vert E_{\varphi }\vert $ and $\vert E_{\theta }\vert $ components in H-plane or yoz plane radiation patterns could be progressively strengthened and weakened, respectively. After that, by reallocating these dual nodal lines around the corners of the radiating patch, the nonbroadside radiated fields of TM 1/2,2 mode are successfully transformed into the broadside radiation, while keeping a low H-plane cross-polarization level. Additionally, the symmetric open-ended stubs are folded so as to reduce the dimensions of the core radiator. In final, the proposed SPA is fabricated and measured. Simulated and measured results are found in good agreement with each other, and both of them illustrate that the antenna has satisfactorily acquired a normal radiation pattern at reshaped TM 1/2,2 mode. Most importantly, its high H-plane cross-polarization is dramatically reduced from −4.3 to −21.4 dB as compared to the traditional SPA at TM 1/2,0 mode.

A single planar inverted F-antenna is proposed employing two input ports to optimally power-combine the output signals of two nonlinear power amplifiers inside the metal-only low-loss antenna structure. High power-added efficiency (PAE) at back-off power levels is reached through a Doherty combiner architecture, where the optimal power combination is seen to generally require the synthesis of a nonsymmetric antenna input impedance matrix. The dual-fed antenna is compact and exhibits close to single-mode radiation properties; it has a relatively stable gain pattern despite the nonequal Doherty power amplifier (PA) output powers. An integrated active prototype has been designed, fabricated, and characterized over-the-air in both an anechoic and a reverberation chamber. Good agreement is observed and an uncertainty analysis is performed. The Doherty transmitter features a max-PAE of 58% at 2.14 GHz and a 6 dB back-off PAE of 52%; a minimum system power gain of 9.5 dB (max 12 dB); and a maximum system output power of 43.5 dBm.

A novel feeding method to dielectric loaded substrate-integrated waveguide (SIW) horn antennas is presented, resulting in equal half power beamwidths (HPBWs) and low sidelobe levels (SLLs) in both the E-plane and H-plane. The concept is to synthesize a unique electromagnetic field distribution using four vertically stacked SIWs. In particular, a uniquely adjusted excitation of magnitudes and phases of the four SIWs is employed as the feeding of the SIW horn, emulating the field distribution of pyramidal horn antennas. This approach realizes the phase correction of the electric field at the horn aperture and concentrates the radiation to the endfire direction to reduce the sidelobes. To prove the concept, a stacked dielectric loaded SIW horn antenna fed by four TE 10 excitations was simulated at 35 GHz, yielding a gain of 15 dBi, an HPBW of 33° for both the E-plane and H-plane, and an SLL of approximately −24 dB for both the E-plane and H-plane. To achieve the desired excitation at each port, a narrowband SIW power splitter was designed to adapt a 3.5 mm coaxial connector to the four waveguide ports with the required phase shift. The entire feeding and antenna structure were fully substrate integrated and fabricated on Rogers 5880 panels. The measured radiation patterns show good agreement with the simulation with the best performance at 35.1 GHz.

Plasmonic nanoantennas are typically designed with RF-inspired rigorous parametric optimization processes that lack proper physical insights. In this article, we demonstrate a systematic optimization approach for nanoantennas based on characteristic mode analysis (CMA). A complex geometry, designated as split-ring two-wire antenna (SRA), is selected and optimized using the CMA technique. CMA identifies the dominant modes of the structure at the frequency of interest as well as explains the dependence of the modes on the structure’s shape, size and material properties. These insights from CMA have been used in this study to efficiently optimize SRA shape, size, and material which yield more than 700% near-field intensity enhancement (NFIE) at the desired operating frequency. This proposed CMA-based optimization method can be adapted easily for many other nanoantenna applications, facilitating the development of improved nanostructures.

In this article, a compact low-profile antenna for octa-band unbroken metal-rimmed smartphone applications is proposed. The antenna consists of a capacitive coupling rectangular vertical plate and two new-type extended ground branches. For lower frequency, the LTE700, GSM850, and GSM900 bands are provided by the bezel mode which is generated through capacitive coupling excitation of a rectangular vertical plate to the bezel. For upper frequency, the DCS, PCS, UMTS, LTE2300 and LTE2500 bands are covered by the multiple modes of two ground branches. With the proposed structure, the operating bands can be expanded and optimized simply. All the mentioned bands are achieved in the size of ( $70 \times 7 + 20 \times 2$ ) $\times $ (1.5 + 0.8) mm 3 on the 130 mm $\times \,\, 70$ mm system board. The antenna is fabricated and measured to verify this proposal. The tested results include reflection coefficient, radiation patterns, efficiency, and gain.

A wideband circularly polarized (CP) waveguide array antenna is proposed in this article. The proposed array antenna consists of four antipodally ridged elements and a compact feeding network with two orthogonal septa inserted to a stepped cavity. Both the components are wideband and they can work independently or together. The proposed topology gives a good solution to feed a $2 \times 2$ waveguide subarray which is the basis of large-scale arrays. It exhibits a larger working bandwidth than the array antennas fed by high-order mode cavities. A prototype is fabricated using 3-D printing and milling technologies. Measured results demonstrate a bandwidth of 40.3% from 9.9 to 14.9 GHz, in terms of the reflection coefficients less than −15 dB and the axial ratios smaller than 3 dB. An expanded $8 \times 8$ CP array antenna is explored based on the proposed $2 \times 2$ subarray to show the potentials of our design, and a total efficiency over 78% within a bandwidth of 38.7% is obtained by simulation.

This article presents randomly grouped subarray techniques to reduce the number of phase shifters in a 2-D phased array while maintaining a certain scan range in the azimuth and elevation planes and keeping sidelobes below a specified level. It is shown that controlling random groups of elements in such a manner suppresses these grating lobes and allows the use of fewer phase shifters. Guidelines for finding the best partition of a phased array into random subgroups are presented. It is shown that 75% reduction in phase shifters can be achieved while maintaining a 40° scan range in the azimuth plane, 15° scan range in the elevation plane, and keeping −10 to −12 dB sidelobes without tapering and with 6 dB taper. Other partitions are also presented with up to 88% reduction in phase shifters while maintaining a 5° scan range in elevation. Measurements on a $16\times16\,\,2$ -D phased array at 14 GHz are used to confirm the performance of randomly partitioned phased arrays. Application areas are in arrays with limited scan angle in the elevation plane, such as automotive radars, aircraft landing systems, and point-to-point communication systems.

Design of compact wideband circularly polarized (CP) antennas is challenging due to the necessity of simultaneous handling of several characteristics [reflection, axial ratio (AR), and gain] while maintaining a small size of the structure. Antenna redesign for various operating bands is clearly more difficult yet practically important because intentional reduction of the bandwidth (e.g., by moving the lower edge of the operating band up in frequency) may lead to a considerable size reduction, which can be beneficial for specific application areas. This article proposes a rigorous approach to rapid redesign of miniaturized CP antennas involving inverse surrogate models and fast electromagnetic (EM)-based parameter tuning. Our methodology allows for a precise control of the lower operating frequency of the CP antenna (both in terms of the impedance and AR bandwidth) and accomplishing the geometry parameter scaling at an extremely low cost of a few EM analyses of the structure at hand. Our methodology is demonstrated by redesigning a compact wide slot CP antenna in the range of 3.2–5.8 GHz. The proposed approach can be used for fast rendering of the bandwidth/size tradeoffs (the footprints obtained for the verification structure range from 783 to 482 mm 2 ), thus determining the most suitable designs for particular applications. The numerical findings are experimentally validated.

A low-profile differentially fed dual-polarized antenna with high gain and isolation is proposed for fifth-generation (5G) microcell communications. By introducing two pairs of symmetrical meandering conductors to connect both ends of the radiating cross slots, a compact-size antenna with wide bandwidth can be achieved. By further adopting an artificial magnetic conductor-backed (AMC) reflector that is arranged below the radiating patches with a distance of 6 mm, the antenna profile can be reduced from $0.25\lambda _{c}$ to $0.13\lambda _{c}$ (where $\lambda _{c}$ is the free-space wavelength at the center frequency), while both the gain and frequency bandwidth are also enhanced. Furthermore, owing to the orthogonal differentially fed structure, the antenna element displays a high port isolation of 37 dB. Finally, by meticulously designing the feeding network, much narrower beamwidth can be yielded, and hence, the unidirectional gain is further improved. The measured results show that a lower-frequency bandwidth of 13.5% (3.26–3.73 GHz) with a gain of 15.7 ± 0.1 dBi and upper-frequency bandwidth of 7.6% (4.68–5.05 GHz) with 15.55 ± 0.05 dBi gain can be obtained by the proposed antenna array. In addition, port isolation better than 28 dB and stable radiation patterns can be achieved. With the aforementioned characteristics, the proposed antenna is a good candidate for future 5G microcell communications.

A composite right/left-handed (CRLH) leaky-wave antenna (LWA) can effectively scan the radiation beam from backward-to-forward direction. However, in most cases, a large range of frequency sweep is required to achieve a wide-angle beam scan, which could limit their applications. An in-depth study is conducted on an equivalent circuit model for a CRLH LWA unit cell to find the controlling parameters on the frequency sweeping range. A systematic design guideline is given for a CRLH LWA for a wide-angle beam scan in a flexibly chosen frequency range. It is shown that beam scanning by sweeping frequency in a target range can be achieved by systematically designing the unit cell parameters. To verify our approach, a novel CRLH unit cell is developed and used to design an LWA for a wide-angle beam scan in a narrow frequency range. Finally, the concept is validated through realization of the antenna and its measurement. The measured results show that the antenna prototype can scan its beam from −56° to +51° when frequency sweeps from 5.1 to 6.11 GHz (i.e., 18.02% of fractional bandwidth).

An antenna capable of directly phase modulating a radio frequency (RF) carrier is discussed, designed, and measured as both an antenna and a modulator. Access point densification for the Internet of Things will be expensive in part due to the cost and inefficiency of amplifying waveforms with large peak-to-average power ratios for downlink transmission. Directly modulating at the antenna means only a carrier wave has to be amplified, reducing the cost of densification. Here, reconfigurable frequency selective surfaces are suggested as phase modulators. The design process for producing a phase modulating antenna is detailed, and a prototype is fabricated that is capable of up to 8-PSK modulation with 5.3 dB variation in constellation points and a peak gain of 2.3 dB. When implemented in an end-to-end communications system, the antenna exhibits only 1.5 dB drop in performance compared with instrument grade modulation in an additive white Gaussian noise (AWGN) channel.

A simple single-layer reconfigurable leaky-wave antenna (LWA) is presented that has polarization agility and beam-scanning functionality. This LWA system realizes a scanned beam that can be switched between all of its linear polarization (LP) and circular polarization (CP) states using only one dc biasing source. A slot-loaded substrate-integrated waveguide (SIW)-based LWA is first explored to attain CP performance with continuous beam scanning through broadside. This CP LWA realizes a measured CP performance with a 3 dB gain variance within 2.75–3.35 GHz for scan angles ranging from −28.6° to +31.5°. A row of shorted stubs is then incorporated into the CP LWA to obtain similar LP performance. Finally, by introducing p-i-n diodes into this LP LWA configuration to facilitate reconfigurable connections between the main patch and the shorted stubs, the radiated fields can be switched between all of its CP and LP states. The measured results of all three antennas confirm their simulated performance. It is demonstrated that the main beam of the polarization-reconfigurable LWA can be scanned from −31.5° to +17.1° with gain variations between 9.5 and 12.8 dBic in its CP state and from −34.3° to +20° with them between 7.8 and 11.7 dBi in its LP state.

Wearable electrically small loop antennas (ESLAs) are introduced to monitor joint flexion and rotation while overcoming limitations in the state-of-the-art. The reported approach is not restricted to laboratory environments, does not suffer from integration drift and line-of-sight, and does not impede natural movement. Our previous work introduced wrap-around coils that addressed the challenges above but were limited to monitoring joint flexion. In contrast, a new class of ESLAs is herewith proposed, placed longitudinally across the joint, and operated at 34 MHz to monitor both flexion and rotation. An added advantage versus our previous design is a remarkable improvement in flexion angle resolution: the transmission coefficient dynamic range for 0° – 100° flexion improves by 18.8 dB in this article. Two ESLAs are shown to accurately detect flexion/rotation for angular resolutions up to 10°. However, if higher resolution is desired, ambiguities arise. To tackle this, a three-ESLA system with integrated post-processing is proposed that achieves resolution as high as 2°. Simulations and in vitro experiments are in excellent agreement. Guidelines for system design suited to diverse applications are discussed, and conformance with safety standards is ensured. In future, ESLAs can be seamlessly integrated in garments, enabling transformative benefits to healthcare, sports, and beyond.

This article presents the electromagnetic (EM) wave routing technique using single-epsilon-high (single-EH) anisotropic medium to generate dual-beam radiation at 28 GHz band for next generation communication systems. This technique is implemented using SIW dipole antenna loaded with the single-EH anisotropic medium realized using modified asymmetric electric- LC (ELC) metamaterial unit cell loaded vertically in front of the radiator. The technique is first verified using simulations and then practically realized, and the results are in good agreement with the predicted theoretical results. The effect of the thickness of the media loading the antenna is investigated and dual-beam radiation in the frequency band 26–31 GHz is obtained by choosing the appropriate number of ELC-slabs. The measured results confirm 26–31 GHz impedance bandwidth and dual-beam radiation directed along 50° and 130° with 8 dBi beam peaks.

A compact folded-patch antenna with a dimension of 45 mm $\times45$ mm $\times3.2$ mm ( $0.137\lambda \times 0.137\lambda \times 0.0097\lambda$ ) is proposed for on-metal applications. A T-shaped L-probe is employed for feeding the top-loaded radiating folded patch. It is found that good impedance matching can be easily achieved between the tag antenna and the microchip with the use of the T-shaped feeder. Also, the maximum power transmission coefficient can be significantly enhanced from 0.45 to 1. By adjusting some of the design parameters, the tag’s footprint area can be effectively reduced by 50% while maintaining considerably good read performance. The proposed tag antenna can not only radiate efficiently with a high radiation efficiency of 73.8% but also be read from a long read distance of 19 m when placed on a 20 cm $\times20$ cm metal plate with reference to a transmitting power of 4 W EIRP.

We propose a hybrid dispersion compensation method to design wideband fixed-beam leaky-wave antennas (LWAs), with the tilting angle customizable in both forward and backward quadrants. In a previous work, a triangular dispersive prism constituted by metallic pins was presented to compensate for the dispersion of traditional LWAs and achieve a squint-free radiation in a relative bandwidth of 20%. However, that design suffered from the drawbacks of large geometry and limited angular range. To overcome these limitations, here a broadband gradient metasurface based on geometric phase theory is loaded in front of the prism to customize the radiation angle and reduce the total prism size. In addition, a novel ridged gap waveguide is employed to improve the linearity of LWA dispersion, which is beneficial to maintain the bandwidth of LWA with reduced size. Two examples are presented to demonstrate the excellent performance of the designed antennas. One antenna radiates at an angle of 39.5° in a squint-free bandwidth of 20%, but the prism size is decreased to a quarter of the original one, while the other realizes a broadside radiation with a squint-free bandwidth of 18%. Good agreement is exhibited between simulated and experimental results. The proposed method can be further extended to compensate for the dispersion of antenna arrays and improve the angular stability of the radiation beams in a large frequency band.

A low pulling effect self-isolated harmonic active radiator operating around 28 GHz is proposed, investigated, and demonstrated in this article. Thanks to the inherent self-isolation property of a feedback loop of the proposed active radiator, a superiority of pulling effect mitigation, harmonic output, and simple structure is achieved. As part of the loop, a partially air-filled dual-mode substrate integrated waveguide cavity with a dielectric-loaded broadside slot is designed to realize the functions of resonators, harmonic diplexers, and antennas concurrently. The radiator oscillates at a fundamental frequency which is set by the first mode of the cavity resonator, and the generated second harmonic signal is extracted and radiated by the diplexer and antenna functions, respectively. Self-isolation is achieved because a pulling signal is attenuated by the synergetic effect of the reverse isolation and harmonic separation of the loop amplifier and diplexer, respectively. Measured results of a fabricated prototype show a maximum equivalent isotropic radiated power of 15 dBm at 28.15 GHz and a phase noise of −108 dBc/Hz at 1 MHz offset. Load pulling and injection pulling are reduced by a factor of 10 and 17, respectively, compared with an active radiator transmitting a fundamental signal around 28 GHz. Such a robust and compactly integrated structure is a good candidate for low-cost millimeter-wave Internet-of-Things applications in connection with identification, sensing, tracking, and communication.

An approach toward designing and building of a compact, low-profile, wideband, unidirectional, and conformal imaging antenna for electromagnetic (EM) head imaging systems is presented. The approach includes the realization of a custom-made flexible high-permittivity dielectric substrate to achieve a compact sensing antenna. The developed composite substrate is built using silicon-based poly-di-methyl-siloxane (PDMS) matrix and microscale of aluminium oxide (Al 2 O 3 ) and graphite (G) powders. Al 2 O 3 and G powders are used as fillers with different weight-ratio to manipulate and control the dielectric properties of the substrate for attaining better matched with the human head and reducing antenna’s physical size while keeping the PDMS flexibility feature. Using the custom-made substrate, a compact, wideband, and unidirectional on-body matched antenna for wearable EM head imaging system is realized. The antenna is configured as a multi-slot planar structure with four shorting pins, working as electric and magnetic dipoles at different frequency bands. The measured reflection coefficient (S11) shows an operating frequency band of 1–4.3 GHz. The time-average power density and the amplitude of the received signal inside the MRI-based realistic head phantom demonstrate a unidirectional propagation and high-fidelity factor (FF) of more than 90%. An array of 13 antennas are fabricated and tested on a realistic 3-D head phantom to verify the imaging capability of the proposed antenna. The reconstructed images of different targets inside the head phantom demonstrate the possibility of utilizing the conformal antenna arrays to detect and locate abnormality inside the brain using multistatic delay-multiply-and-sum beamforming algorithm.

Electrodynamic characteristics of a straight strip antenna located at a plane interface of a uniaxial metamaterial and an isotropic magnetodielectric are studied using the integral equation method. The antenna is perpendicular to the anisotropy axis of the metamaterial filling the half-space on one side of the interface and is excited by a given voltage. The cases of metamaterials with hyperbolic and nonhyperbolic dispersion are considered. A closed-form solution for the current distribution of an infinitely long strip is obtained and the input impedance of such an antenna is found. Based on this solution, the limits of applicability of the transmission line theory for determining the characteristics of the antenna are established and a generalization to the case of a finite-length strip antenna is discussed.

We present the design, manufacture, and measured performance of a dielectrically loaded quad-ridge flared horn (QRFH) feed for decade bandwidth radio astronomy application. The introduction of the dielectric load improves the QRFH beamwidth control in H-plane at the mid and upper frequency range. Consequently on the reflector, illumination efficiency, phase efficiency, and the intrinsic cross-polarization ratio (IXR) have been improved. The dielectric load is made from homogeneous low-loss polytetrafluoroethylene and has a low profile with a cylinder shape for simple installation at the center of the QRFH. The dielectrically loaded QRFH presented here covers 1.5–15.5 GHz with a calculated average aperture efficiency above 50% on a f/D = 0.3 prime-focus reflector. We present a calculation of system noise temperature and sensitivity for the QRFH on a 100 m prime-focus reflector. Measured beam patterns of the QRFH are in good agreement with the simulations over the full frequency band. The input reflection coefficient was predicted to be below −10 dB across the bandwidth. We present a tolerance analysis that explains why the measured one deviates.

In this article, a class of wideband transmit arrays (TAs), composed of cascaded anisotropic impedance surfaces (AISs), for circularly polarized (CP) multibeam generation from a single feed horn are reported. The dispersionless phase compensation is achieved by the Berry phase (BP) via imposing a spatially dependent rotation angle on the TA unit cells. A homogenized model for the BPTA unit cell is proposed and utilized for obtaining a wideband response by tailoring the dispersive properties of the AIS layers. Two modeling methods, an analytical vectorial field analysis and a full-wave strategy incorporating the homogenized model, were employed to efficiently evaluate the performance of the BPTAs. In order to validate the proposed unit cell and the modeling methodologies, a Q-band single-beam BPTA is demonstrated, which achieves a peak gain of 30.2 dBi and a 1 dB bandwidth of 11.1% within which the axial ratio is smaller than 2 dB. Furthermore, by employing the intersection approach for pattern synthesis, several Q-band BPTAs supporting multiple concurrent symmetric/asymmetric CP pencil beams and circular-shaped flat-top beams are designed. A BPTA prototype for producing quad CP pencil beams with unequal gain values was fabricated and characterized, yielding good performance with an overall operational bandwidth of about 11%. The proposed BPTAs are promising candidates for point-to-multipoint communication and point-to-multiregional coverage in wideband millimeter-wave communications for wireless and satellite applications.

An energy-efficient 5G phased array incorporating a novel vertically polarized (V-pol) endfire planar folded slot antenna (PFSA) for user devices (UE) is presented. First, we analytically amend existing millimeter-wave (mmWave) hybrid beamforming architectures that have precluded the uniqueness of 5G antennas and UEs. The total power consumption of the derived switchable 5G UE antenna system is estimated to be reduced by approximately 70% in comparison with a recently reported fully digital mmWave 5G UE antenna system under identical conditions. This is attributed to the ability to deactivate certain RF chains based on the directive nature of antenna elements. The PFSA featuring a height profile of less than 1/ $9~\lambda _{ {0}}$ is derived from a planar folded slot structure. The designed and fabricated $1\times 4$ PFSA array features an impedance bandwidth of approximately 4 GHz with a center frequency of 37–39 GHz and the gain of 7.7 dBi with antenna efficiency of 94.12% at 39 GHz. An mmWave 5G beamforming module is demonstrated using the presented energy-efficient 5G beamforming architecture and V-pol endfire PFSA array. The fabricated module achieves a measured EIRP of 18.2 dBm and a scanning range of ±50° in azimuth at 28 GHz.

In this article, the scan blindness in a T-printed dipole is analyzed, and an elimination strategy is proposed. First, the main cause of scan blindness is analyzed. The scan characteristics are obtained using an active element pattern (AEP) with an infinite rectangular lattice arrangement. Based on the propagation of a guided wave along the antenna row and the electric-field ( $E$ -field) distribution observed during simulations, an equivalent circuit model for a unit cell of the T-printed dipole is obtained. A quasi-transverse electromagnetic (TEM) guided wave is predicted using the dispersion relation curve obtained from the equivalent circuit, and it is proven that the calculated curve is in good agreement with the eigen mode simulations and measured trajectory of the scan blind angle, for different frequencies. Next, slits and stubs are introduced as parasitic structures to eliminate the scan blindness and improve the antenna scan range. To confirm the effects of these parasitic elements, a linear array simulation is performed, which confirms the suppression of a quasi-TEM guided wave. Analysis of the active reflection coefficient and dispersion diagram indicates that the scan characteristics have been improved by the addition of parasitics. Four types of array prototypes are fabricated and their measurements validate the scan blindness prediction and confirm the proposed mechanism of scan blindness and its improvements.

In this article, we investigate the radiation and impedance properties of arrays of tilted dipoles. A spectral periodic method of moments (MoM) is developed for the analysis of infinite arrays with arbitrarily tilted dipole elements, in free space or with a backing reflector. With the aid of this analysis method, the radiation characteristics of arrays of stacked dipoles over a ground plane are studied, explaining the variation of the patterns as a function of the interelement distance and the angle of inclination of the elements. Finite linear arrays of tilted dipoles are also investigated, to assess the dependence of the array characteristics on the number of elements. The developed method can be used to design arrays with nonsymmetric radiation patterns for angular filtering or pattern shaping.

An endfire circularly polarized (CP) complementary antenna array is proposed for 5G applications. The proposed antenna is realized using a single-layered printed circuit board (PCB) with plated-through-hole technology and metal blocks. The antenna element consists of an open-ended substrate-integrated waveguide (SIW), an electric dipole, a double-sided parallel-strip line (DSPSL), and two metal blocks. It is simple in configuration and can achieve wide impedance and axial ratio (AR) bandwidths, stable gain and radiation patterns, and low back radiation. To increase the gain, a $1 \times 8$ antenna array is formed by integrating eight antenna elements with a planar 1–8 SIW feed network. The measurement results show that the proposed array can achieve an overlapping impedance and AR bandwidth of 23.8% from 56.3 to 71.5 GHz with an endfire left-handed CP (LHCP) gain from 14 to 15.3 dBic. The proposed array possesses all the salient features of the complementary source in symmetric and stable radiation patterns, low back radiation, and wide bandwidth.

This article presents an enhanced single-sideband time-modulated phased array (ESTMPA) using modulating pulses with stepped waveforms. Based on the in-phase/quadrature (I/Q) complex modulation technique, this phase-only weighting array generates a scanning beam at the 1st sideband. The proposed modulating pulses realized through a reconfigurable power divider in I/Q time modulator can avoid the power loss from the switches during switch-OFF state and eliminate the maximum undesired sideband—the 5th harmonic in STMPA. As a result, it brings a power spectrum with less undesired sidebands, lower sideband level (−16.9 dB), higher harmonic efficiency (94.96%), and wider allowable signal bandwidth (eight times as wide as that of the conventional time modulated array). To experimentally verify the feasibility of the proposed design, a wideband enhanced I/Q time modulator and its corresponding eight-element ESTMPA are designed and manufactured. A detailed study on the effect of the magnitude and phase deviations in the circuit and the transition period of modulating pulse are presented. The measured results of power spectrum and radiation pattern have a good agreement with the simulated ones.

New electromagnetic (EM) structures are demonstrated to realize low-radar-cross section (RCS) antennas by making effective use of frequency-selective absorber (FSA). According to the well-known reciprocity principle, the two-layered FSA can be considered as an actual receiving antenna and then transformed to a circularly polarized (CP) antenna. At the radiation state, a truncated patch resonator on the bottom layer is fed by a coaxial probe so as to produce CP wave, and it further excites the slots on the upper layer, resulting in its radiation toward free space, and when the antenna at the stealth state, the detective incident wave can be effectively absorbed outside the radiation band, thus achieving RCS reduction. The design strategies are then explained and verified with the aid of the corresponding equivalent circuit models. Two examples of $2 \times 2$ and $4 \times 4$ antenna arrays were designed to validate the flexibilities of the proposed design method. Finally, the $4 \times 4$ antenna array was fabricated and measured, and reasonable agreement is achieved.

In this article, we consider the problem of beampattern synthesis with sidelobe control using constant modulus weights. We specifically focus on two applications, i.e., notching and sidelobe-level (SLL) control (SLC). The considered problem is a challenging NP-hard due to the employed constant-modulus weights. Here, we convert the problem into an intermediate form by incorporating auxiliary variables and changing the form of the constant modulus. Then, we decompose the intermediate form into subproblems of single constraint each. Finally, we propose an iterative solution following the alternate direction method of multipliers (ADMM) and Proximal frameworks. The proposed solution solves the original problem by iteratively solving the subproblems in coordinated logical order. Importantly, it combines the convergence speed of ADMM and convergence property of Proximal methods. In addition, it always satisfies the constant modulus constraint. The solution has a significant contribution to mitigate interference using notching and to realize RadCom. Simulation results show that the proposed solution has a better performance in terms of computational speed, notching, and constant SLL than the previous works.

In this article, we studied the characteristics of multi-mode propagation and electric field distribution for a single layered and multi-layered periodic infinite-length square chain structures. We also studied the characteristics of mode propagation in silver and dielectric circular-rod chain structures. This article examines three modes of propagation in three Bragg propagation periods of a square chain structure. Each mode exhibits different forms in each of the three propagation periods, including the propagation, leaky, and cut-off modes. We also consider multiple complex characteristics. For example, the directions of wave and energy propagation paths have two forms: syntropy and reverse (one-way and two-way). The leaky wave has a variety of distribution states, for instance, attenuation and divergence. By analyzing these new physical properties, we have established a solid foundation for better development and application of leaky antennas and other applications. The method in this article was designed and validated for general metal structure applications.

This article focuses on the design of a transparent circularly polarized (CP) antenna subarray integrating with Cube satellite’s (CubeSat’s) solar panels. The subarray antenna employs two techniques including the Fabry–Perot cavity (FPC) and sequential rotation-feeding network. These techniques are used to generate CP along with a high level of directivity over a broad bandwidth. The main aim here is to propose a multifunctional antenna with the high radiation properties and the capability of power harvesting, simultaneously. To harvest power using a solar panel, the transparency requirement should be satisfied. Hence, the proposed design is sputtered with the thin layer of indium–tin–oxide (ITO) with a thickness of 200 nm. However, it seems to be sacrificed a large amount of conductivity at the frequency band of interest (x-band). Correspondingly, the performance of conductivity and transparency for both the materials of ITO and copper (Cu) as coated on the proposed design is determined. Alternatively, as an optically transparent conductor (OTC), a combination of both coating layers, including ITO and Cu with thicknesses of 200 and 5 nm, respectively, is applied. In the scattering point of view, the proposed design is also capable of suppressing the radar cross-section (RCS) for the space missions with the aim of space-based observations.

A novel iterative convex optimization approach is proposed for the synthesis of low-sidelobe 4-D heterogeneous arrays in the presence of mutual coupling effect. To realize wide bandwidth and wide-angle scanning, a tightly coupled dipole element (TCDE) is selected as the basic element. Due to the strong coupling between antenna elements, the active element pattern (AEP) and active reflection coefficient (ARC) are included in the proposed approach in order to evaluate the overall mutual coupling and port matching. Specifically, a nonconvex programming problem in terms of the given maximum sidelobe level (SLL), the ARC or reflected power at the center frequency, and the given maximum sideband level (SBL) at sidebands is established. After the application of some mathematical transformations, the nonconvex programming problem is decomposed into a convex optimization problem at the center frequency and an iterative convex problem (ICP) at sidebands. Owing to the efficiency of the convex optimization, the two problems can be efficiently solved. The proposed approach is applied to synthesize low-sidelobe patterns while minimizing the SBL in a 32-element cone-shaped 4-D heterogeneous array. The simulated and measured results verify the effectiveness of the proposed approach and show the advantages (improved gain) of the heterogeneous array.

A thin laminated composite is proposed in this article for highly efficient microwave absorption at both high frequency and low frequency. It is composed of two layers of carbon-fiber-reinforced planar structure. In each layer, periodically arranged fiber array with the axes of the fibers parallel to each other is embedded into a ceramic matrix which is used to bind the fibers together and to provide necessary mechanical or chemical properties. Stacking up two layers of the planar structure with the fibers in different layers orientated into different directions produces a lamina which can be further parallely stacked up to provide the whole laminated material. The influence of the structural and electromagnetic parameters on the microwave absorption efficiency is first studied in detail with scattering matrix and boundary-mode matching method. Then the genetic algorithm and the sequential quadratic programming methods are used to form a two-step strategy based on the mode-matching method to optimize the structural and electromagnetic parameters for a highly efficient microwave absorption in both high-frequency and low-frequency bands. Numerical examples are also given to demonstrate the high absorption efficiency of the designed laminated material.

With the development of radar and communication system, frequency selective rasorber (FSR) is in great need. In this article, a flexible rasorber based on graphene is presented and investigated. A passband with small insertion loss and two absorption bands with high absorptivity below and above the passband is realized. Without welding lumped elements, our design can be applied to wide wavelength regions from microwave to low terahertz range, and also conformal situations. Besides, by integrating graphene sandwich structure (GSS), tunable transmission at the passband of the rasorber is realized, which makes our rasorber have the function of energy manipulation. For experimental demonstration, the rasorber prototype is fabricated and measured at microwave region; the fabricated sample shows good flexibility and good tunability, demonstrating that our radome has great application prospect in the field of electromagnetic stealth, compatibility, and protection.

Adopting the direct $Z$ -transform ( $\text{D}Z\text{T}$ ) method due to its higher accuracy instead of the approximate $Z$ -transform ( $\text{A}Z\text{T}$ ) methods and efficient and switchable truncations between the 1st- and 2nd-order uniaxial perfectly matched layers (UPMLs) with the complex-frequency-shifted (CFS) scheme are shown to terminate the relevant finite-difference time-domain (FDTD) regions. The proposed $\text{D}Z\text{T}$ -CFS-UPML formulations can possess the switchable function in terms of relevant FDTD problems so that the optimal performance can be obtained with the tradeoff among memory requirement, CPU time, and absorption accuracy. For the FDTD problem with the strong evanescent and weak low-frequency propagating waves, the proposed $\text{D}Z\text{T}$ -CFS-UPML formulations can be switched to the 1st-order PML truncation, and for the other cases with both low-frequency propagating and strong evanescent waves, the 2nd-order PML is the best choice. Two numerical simulations have been carried out to illustrate the validity and flexibility of the proposed approach.

A finite-difference time-domain (FDTD) method is developed to analyze electromagnetic scattering from 3-D fully anisotropic periodic structures impinged by obliquely incident plane waves. Starting from Maxwell’s curl equations, we employ material transformation matrices to link the update of the electric and magnetic fields in the FDTD method. The problem under consideration is Bloch–Floquet periodic in the horizontal directions but finite in the vertical direction. At the FDTD truncation boundaries in the horizontal directions, the Bloch–Floquet periodic boundary conditions (BPBCs) are applied, while the perfectly matched layer (PML) absorbing boundary condition is implemented for the vertical direction. We design three different anisotropic models in 3-D simulations to validate our method with a commercial software package COMSOL. These examples show the accuracy and efficiency of the FDTD method for analyzing the propagation characteristics, including reflectance, transmittance, absorptance, and complex reflection and transmission coefficients for the fully anisotropic periodic structures.

Modeling wideband plane-wave propagation through a stratified magnetized plasma sheath is important toward predicting the quality of communications during re-entry, scattering caused by electron density perturbations, and many other applications. When using the finite-difference time-domain (FDTD) method to solve the problem, modeling the incident field in the simulation domain is difficult if the incident field travels obliquely to the grid axes. In this article, a total-field/scattered-field (TF/SF) formulation whereby a plane-wave source is introduced into cold layered magnetized plasma has been developed for the FDTD method. Based on phase matching theory, twelve auxiliary 1-D propagators are specified to calculate the correction field values for the TF/SF boundary. A modified convolution perfect match layer (CPML) is implemented to terminate the 1-D propagators. Numerical results are also presented to show the accuracy and effectiveness of the method. Although this method is developed for magnetized plasma, the extension to other anisotropic dispersive layered media is straightforward.

This article presents a new self-dual integral equation (SDIE) for electromagnetic scattering from arbitrarily impedance boundary condition (IBC) objects including partly coated objects. The proposed SDIE is constructed by using the combined field integral equation (CFIE) and IBC, shorted as C-SDIE. To overcome the difficulty of discontinuous surface impedance from nonuniform IBC/partly coated objects, the discontinuous Galerkin (DG) method is applied to discretize the C-SDIE. Numerical experiments confirm that the DG-C-SDIE has promising numerical performance in terms of accuracy and efficiency. Furthermore, the domain decomposition preconditioning based on DG is employed to further enhance the proposed DG-C-SDIE for large-scale, multi-scale objects. The numerical results demonstrate the capability of the proposed DG-C-SDIE.

An iteration-free finite element domain decomposition method (DDM) is proposed for the simulation of 3-D electromagnetic problems. The finite element tearing and interconnecting (FETI) with Robin-type transmission condition is introduced to construct a domain decomposition framework. Based on hierarchical ( $\mathcal {H}$ -) matrix algorithm, the numerical Green’s function of each subdomain can be represented by a data-sparse matrix form. Subsequently, the original 3-D problem is reduced to a 2-D problem on the skeleton. Derived from the finite element discretization of pseudo-differential operator, the skeleton matrix can also be efficiently approximated with an $\mathcal {H}$ -matrix by assembling all the numerical Green’s functions of subdomains. Based on $\mathcal {H}$ -LU factorization algorithm, the computational complexity and memory requirement for the direct solution of the skeleton equation can be reduced to be logarithmic linear. The resulting direct FETI (D-FETI) method completely depends on direct method without any iterative process. Hence, it does not suffer from the issue of convergence rate, and avoids redundant computations for multiple right-hand side problems. Numerical examples demonstrate the effectiveness of the proposed method.

Achieving adequate coverage with high gain antennas is key to realizing the full promise of the wide bandwidth available at centimeter/millimeter bands. We report extensive outdoor measurements at 28 GHz in suburban residential areas in New Jersey and Chile, with over 2000 links measured for the same-street link types [vegetation blocked line of sight (LOS)] from 13 streets and other-street link types [true non-LOS (NLOS)] from 7 streets, using a specialized narrowband channel sounder at ranges reaching 200 m. The measurements, applicable to fixed wireless access, involved a 55° transmit antenna placed on the exterior of a street-facing window and a 10° receive horn antenna spinning on top of a van mast at 3 m height, emulating a lamppost-mounted base station (BS). The measured path gain-distance dependence is well represented by power-law models, and azimuth gains at the base are degraded through scattering by more than 4.3 dB for 10% of links. It was found that, with 51 dBm effective isotropic radiated power (EIRP) at the BS and 11 dBi antenna at an outdoor- mounted terminal, 1 Gb/s downlink rate can be delivered up to 100 m from a BS deployed in the same street with 90% coverage guarantee.

Wave attenuation through rain with different rainfall rates at millimeter wave ( $f = 77$ GHz) and low-terahertz (Low-THz) ( $f = 300$ GHz) frequencies is studied in this article. Rain has pronounced impacts on electromagnetic wave propagation and one of the well-known effects is attenuation of the transmitted wave. Attenuation at both frequencies and hydrometeor properties [rainfall rate and drop size distribution (DSD)] are measured simultaneously. The measured DSD is fit with gamma and Weibull distributions and is also compared to the frequently used distribution Marshall and Palmer (MP) model; Weibull is shown to be a better fit to the measured DSDs. Theoretical prediction of attenuation as a function of rainfall rate (up to about 20 mm/h) is determined using Mie scattering theory, and the fit gamma and Weibull, and MP distribution models; as well as using the International Telecommunications Union Radiocommunication Sector (ITU-R) recommendation. The calculations are evaluated by comparing them to the experiment. The measured results at 77 GHz best agree with the ITU-R recommendation whereas at 300 GHz, the calculation based on Mie scattering and the Weibull distribution exhibits the best fit to the measured data. The measured data that exceed the theoretical prediction are analyzed and interpreted based on their corresponding observed drop size properties, for the first time.

This article aims at studying the angular scattering properties of bianisotropic metasurfaces and clarifying the different roles played by tangential and normal polarization densities. Different types of metasurfaces are considered for this study and are classified according to their symmetrical/asymmetrical and reciprocal/nonreciprocal angular scattering behavior. Finally, the article presents the relationships between the symmetrical angular scattering properties of reciprocal metasurfaces and the structural symmetries of their scattering particles. This may prove to be practically useful for the implementation of metasurfaces with complex angular scattering characteristics.

A number of new results aiming at facilitating the use of the $\alpha $ - $\eta $ - $\kappa $ - $\mu $ fading model are presented. These results include: 1) fast convergent, with no recursions, series representations for the envelope probability density function (PDF) and cumulative distribution function (CDF); 2) higher order moments of the envelope; 3) moment generating function (MGF) of the envelope; 4) asymptotic behavior of the envelope PDF and also of the envelope CDF; 5) a procedure for parameter estimation; 6) new closed-form expressions for particular cases of the envelope distribution; 7) new mathematical identity for the generalized Laguerre polynomial and the generalized hypergeometric function; and 8) approximate, but tight, series representation for the phase PDF. As application examples, the following are given: 1) outage probability; 2) outage capacity; 3) amount of fading; and 4) bit error rate, for which the asymptotic behavior is also presented.

A variant of the direct optimization method for point-to-point ionospheric ray tracing is presented. The method is well suited for applications where the launch direction of the radio wave ray is unknown, but the position of the receiver is specified instead. Iterative transformation of a candidate path to the sought-for ray is guided by a generalized force, where the definition of the force depends on the ray type. For high rays, the negative gradient of the optical path functional is used. For low rays, the transformation of the gradient is applied, converting the neighborhood of a saddle point to that of a local minimum. Knowledge about the character of the rays is used to establish a scheme for systematic identification of all relevant rays between the given points, without the need to provide an accurate initial estimate for each solution. Various applications of the method to isotropic ionosphere demonstrate its ability to resolve complex ray configurations including 3-D propagation and multi-path propagation where rays are close in the launch direction. Results of the application of the method to ray tracing between Khabarovsk and Tory show good quantitative agreement with the measured oblique ionograms.

In this article, a method is presented to estimate the effective electrical parameters of the scatterers for multi-view multi-static two dimensional transverse electric (2D-TE) scattering configurations. The derived method needs the shape and position information of scatterers a-priorly to model the targets as circular cylinders (i.e., an effective radius and center is determined for each target) with constant electrical parameters. Next, the method employs the virtual experiments to focus the incident magnetic field as a $J_{0}$ on a specific target. After focusing the excitation and neglecting the multiple scattering between targets, the scattered field is in $H_{1}^{(1)}$ form. Next, the error between the simulated and the virtual fields is calculated. Then, the effective electrical parameter is estimated as the minimizer of the error. Lastly, for the cases where the shape and/or position information are noisy, the introduced method links the deviation of the electrical parameters to deviations of the effective radius and center. From this relation, the method presents a way to estimate the maximum deviations in the electrical parameters, given the maximum deviations in effective radius and center. The efficiency and accuracy of the proposed formulations are tested with both numerical and experimental examples.

This article presents an electromagnetically powered stent designed for hyperthermia treatment of in-stent restenosis. The stent device based on medical-grade stainless steel serves as a radio frequency (RF) inductive receiver to produce mild heating wirelessly through resonant-coupling power transfer, while acting as a mechanical scaffold inside an artery similar to commercial stents. The device and its custom transmitter are prototyped and optimized to show efficient wireless power transfer and stent heating through in vitro tests. The inductive stent with its helical pattern is gold coated to achieve a $3.5\times $ higher quality ( $Q$ ) factor, improving heating performance of the device. The combinational use of independent resonant antennas with the power antenna is found to significantly boost stent temperature by up to 96% with an intermediate tissue layer. Upon matching the frequencies at which the $Q$ factors of the inductive stent, power antenna, and booster antenna are peaked, the stent excited through 10 mm-thick tissue exhibits a temperature increase of 18 °C, well over a necessary level for targeted hyperthermia treatment. The prototype achieves heating efficiencies (HEs) of 15.5–3.2 °C/W with a tissue thickness of 5–15 mm. These results indicate that the proposed resonant-heating stent system with the prototyped transmitter is promising for further development toward its clinical application.

Dielectric loss occurring in tissues in close proximity to UHF-implanted antennas is an important factor in the performance of medical implant communication systems. Common practice in numerical analysis and testing is to utilize radiation efficiency (RE) measures external to the tissue phantom employed. This approach means that RE is also dependent on the phantom used and antenna positioning, making it difficult to understand antenna performance and minimize near-field tissue losses. Therefore, an alternative methodology for determining the intrinsic radiation performance of implanted antennas that focuses on assessing structural and near-field tissue losses is presented. The new method is independent of the tissue phantom employed and can be used for quantitative comparison of designs across different studies. The intrinsic RE of an implant antenna is determined by assessing the power flow within the tissue phantom at a distance of at least $\lambda _{g}/2$ from the radiating structure. The simulated results are presented for canonical antennas at 403 and 2400 MHz in homogeneous muscle and fat phantoms. These illustrate the dominance of propagating path losses in high-water content tissues such as muscle, whereas near-field dielectric losses may be more important in low-water tissues such as fat due to the extended reactive near-field.

This article presents a general technique for the far-field transformation of near fields measured over arbitrary surfaces and multimode probe correction in an efficient and accurate way. The use of not only one but several spherical wave expansions to model the antenna under test fields allows to incorporate full probe correction for arbitrary orientations with low computation cost. Combining this approach with a hierarchical subdomain decomposition strategy, the transformation problem is solved with a low computational complexity. In this method, the fields produced by neighbor subdomains are gradually aggregated and interpolated following a multilevel scheme, leading to a good scalability with frequency compared with other matrix-based transformation methods. The numerical and experimental results are provided to show the efficiency and capabilities of the proposed algorithm.

A novel direction-finding method with time-modulated array (TMA) is proposed for modulating the echo linear frequency modulation (LFM) signal and analyzing its harmonic characteristics. Unlike conventional analysis method where each coefficient of harmonic components is analyzed by discrete Fourier transform, in the proposed method, the harmonic coefficients of modulated signals are calculated by using the pulse compression technology. After the modulated LFM signal is compressed by the matched filter, a closed-form expression of the corresponding output signal is derived and the harmonic coefficients of the input modulated signals can also be obtained. Meanwhile, in order to avoid being affected by other harmonic components in the process of analyzing each harmonic coefficient, the constraints to acquire independent harmonic coefficients are highlighted. Numerical simulations are provided to verify the performance of the proposed method, and a simple S-band eight-element TMA is constructed to experimentally verify its feasibility.

Misalignment measurement is important in radio frequency orbital angular momentum (RF-OAM)-related communication and radar applications. Misalignment causes distorted vortex wavefront and OAM spectra dispersion, therefore a conventional gradient method cannot be used directly. We propose a misaligned measurement method for RF-vortex beams based on image processing. Theoretical analysis and numerical simulations of the misaligned OAM wavefront demonstrate that the misalignment information can be measured by extracting the features of the OAM mean value distribution and the OAM variance value distribution. It is found in this paper that the extrema of mean OAM distribution and OAM variance distribution correspond to the tilted singularity point and original pivot point, separately. These extrema points can be found by image processing algorithms that recognize round speckles. A proof-of-concept experiment is presented, which employs a 2-D scanning probe to measure a 10°-tilted OAM beam with mode +1. The tilt measurement error is only 0.39°.

A low-profile vertical polarized antenna is presented for radio frequency identification (RFID)-enabled intelligent parking system applications. In order to reduce the crosstalk between antennas and false detection of vehicles in the parking zone, a bidirectional antenna with low vertical obstacle is required by industry. To that end, a low-profile cavity-excited magnetic dipole-based antenna with vertical polarization is designed. Two vertical sides of the antenna are kept open for the easy access of the reader and powering components. In order to increase the polarization purity along the intended sides, a rectangular truncated ground plane is proved to be beneficial compared to a typical square shaped one. The overall structure of the on-road antenna over the ground plane is around $0.2\times 0.55\times 0.06\,\,\lambda _{0}^{3}$ ( $\lambda _{0}$ is the free space wavelength of the center frequency). The design process and operating principle of the antenna are described in detail along with the effects of different parameters. The antenna is prototyped and measured. The antenna attains high reflection coefficient of −20 dB over 912–930 GHz covering the intended Australian ultrahigh-frequency (UHF) RFID band. The antenna demonstrates bidirectional radiation patterns having a maximum gain of 5 dBi over the required band with −20 dBi cross-polarization level. On-road measurements of the prototype show that the 3 dB beamwidth extends to ±90° elevation angles, which provides high read range while the tags embedded in the vehicle’s license plate are in low elevation. The antenna has a rugged structure to sustain more than 10 years in regular road and traffic conditions. By meeting all the requirements, it has been proved to be an effective part of the on-road RFID-enabled intelligent parking system.

Monolithic integration of 3-D printed gradient index (GRIN) dielectric loading with a double-ridged rectangular horn standard gain horn (SGH) and its impact on antenna performance are demonstrated. Results show that the GRIN loaded horns improve sidelobe levels (SLLs) and beam symmetry, while maintaining good impedance match and relatively high radiation efficiency over more than an octave bandwidth from 7.5 to 18 GHz. Moreover, they have low weight and show good robustness to fabrication imperfections. This work paves the way for seamless integration of complex wideband antennas in 3-D printed systems such as unmanned aerial vehicles (UAVs).

An electrically small Huygens circularly polarized (HCP) rectenna whose antenna is directly matched to its rectifying circuit is developed and experimentally validated in the ISM band at 915 MHz. The HCP antenna is a near-field resonant parasitic (NFRP) design consisting of a driven element and a crossed pair of balanced electric and magnetic NFRP dipole elements. The rectifier circuit is highly capacitive. It is a full-wave design based on two HSMS286C Schottky diodes. Two innovative driven element designs are explored that facilitate the effective conjoin of the antenna to different classes of external circuits. An HCP antenna with a driven spiral line element is demonstrated. Its input impedance is capacitive and, hence, the system is ideal for matching directly to an inductive external circuit. An HCP antenna with a driven loop element is investigated in a similar manner. The system has an inductive input impedance that is subsequently optimized to be conjugately match to the capacitive rectifier directly. The measured HCP rectenna prototype has broad-angle capture capacity, avoids any polarization mismatch issues, and has exceptional ac to dc conversion efficiency, the maximum reaching 90.6%. It is an ideal candidate for wireless power transfer (WPT)-enabled Internet-of-Things (IoT) applications.

This communication presents a beam-deflection short backfire antenna (BD-SBA). The proposed BD-SBA differs from the conventional SBA in that the main reflector of the BD-SBA is a phase-modulated metasurface instead of a perfect electric conductor (PEC). The phase-modulated metasurface consists of 15 columns of unit cells which are designed to change the phase of the reflected waves. Therefore, the beam of the BD-SBA can be deflected to a certain angle. First, several BD-SBAs with linear phase-modulated metasurfaces are studied, which indicates that the beam of antennas point at different directions. Second, the BD-SBAs with nonlinear phase-modulated metasurfaces are studied to compare the difference between metasurfaces with linear and nonlinear width variation of patch. The simulated results show that the metasurface with nonlinear variation can obtain better performance than the former. To verify this design method, the BD-SBA with $\Delta \theta = 10^{\circ }$ was fabricated and measured at ${X}$ -band. The overall height of the proposed antenna is 0.35 wavelength at 10 GHz. The measured and simulated results are in good agreement; the measured beam-deflection angle, realized gain, and aperture efficiency are 10°, 14.8 dBi, and 68.3% at the center frequency, respectively. Due to the beam-deflection function, medium-to-high gain characteristics, and low profile, the proposed BD-SBA can be expected to find potential applications in wireless backhauling.

In this communication, a metasurface-based decoupling method (MDM) is proposed to reduce the mutual couplings at two independent bands of two coupled multiple-input-multiple-output (MIMO) antennas. The metasurface superstrate is composed of pairs of non-uniform cut wires with two different lengths. It is compact in size and effective in decoupling two nearby dual-band patch antennas that are strongly coupled in the H-plane with the edge-to-edge spacing of only 0.008 wavelength at low-frequency band (LB). The antenna is fabricated and measured and the results show that the isolation between two dual-band antennas can be improved to more than 25 dB at both 2.5–2.7 GHz and 3.4–3.6 GHz bands, while their reflection coefficients remain to be below −10 dB after the metasurface superstrate is introduced. Moreover, the total efficiency is improved by about 15% in the low band and the envelope correlation coefficient (ECC) between the two antennas is reduced from 0.46 to 0.08 at 2.6 GHz and 0.08 to 0.01 at 3.5 GHz. The proposed method can find plenty of applications in dual-band MIMO and 5G communication systems.

In this communication, a compact saber-like antenna with low wind drag is proposed for on-board communication application, under the requirements of polarization diversity and omnidirectional coverage. The proposed antenna consists of a thin resonant cavity for omnidirectional horizontal polarization (HP), and a folded slot is colocated with the cavity to provide omnidirectional vertical polarization (VP). Isolation lower than −20 dB between two feeding ports is achieved in a quite compact dimension of $30 \times 29 \times 9$ mm 3 ( $0.24\lambda _{0} \times 0.24\lambda _{0} \times 0.07\lambda _{0}$ , $\lambda _{0}$ is the free-space wavelength at 2.44 GHz), also with the omnidirectional patterns for both polarizations. A prototype of the proposed antenna is fabricated and tested to verify the design strategy. Compared with previous designs, the cross section area for wind drag is reduced for more than 28.5%, exhibiting potential usage for high-speed moving antenna carriers.

This communication introduces a new structure of a partially reflective surface (PRS) to realize gain enhancement for a millimeter-wave Fabry–Pérot Cavity (FPC) antenna. It is the first attempt at applying a Fresnel zone plate (FZP) to a single-layer PRS, which contributes to a significant gain improvement. The proposed antenna consists of a feeding source, a substrate-integrated quasi-curve reflector, and the proposed PRS. With the quasi-curve reflector, multiple resonate modes are excited, providing a wide 3 dB gain bandwidth. For validation, a prototype of millimeter-wave FPC antenna is designed and measured. The antenna can be implemented by low-cost and mature printed-circuit-board (PCB) technology. The antenna yields a measured impedance bandwidth of 17.8% from 54 to 64.5 GHz and a 3 dB gain bandwidth of 13.3% from 56 to 64 GHz. The measured peak gain is 21 dBi at broadside direction. This antenna finds potential applications in high-speed communications at the millimeter-wave spectrum.

A rigorous mathematical solution to the problem of synthesizing wire antennas using the multipole expansion method (MEM) is presented. The input current and required size of the antenna are estimated using the knowledge of the required field pattern. This solution is based on three steps. First, obtaining the expansion’s weighting coefficients that carry both the signature of the current and size of the antenna. In essence, the analysis of the direct modeling needs to be addressed first in order to define these coefficients. Second, extracting the coefficients from the required field pattern using the natural orthogonality property of the spherical harmonic functions. Finally, performing a numerical search for the argument of the spherical Bessel functions to satisfy the ratio between two successive moments. Once the size of the antenna is found, a recursive expression is used to obtain the input current. For the current distributions assumed herein, the odd order coefficients of the MEM are shown to be the only necessary coefficients to construct the radiation far-field. Also, it is found that the estimated model parameters are in excellent agreement with the analytical ones.