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For the first time, the ionic polymer metal composite (IPMC) actuator is integrated with a RFID (Radio-Frequency Identification) tag antenna for achieving frequency reconfiguration in the UHF (Ultra-High-Frequency) band. Here, the IPMC movable flap serves as an actuator that can effectively tune the tag resonant frequency. By applying a 2-layer crenellated structure and employing a surface patterned electrode for restraining back relaxation, the deflection of the IPMC actuator can be significantly improved. The IPMC actuator can move in two directions, enabling two-degree frequency tuning. Numerical and experimental data confirm that the tip displacement of the IPMC actuator can be enhanced up to 266% with the use of the 2-layer crenellated structure. The IPMC actuator allows the resonant frequency of the tag antenna to be tuned back by as much as 35 MHz, after deviating due to placing on an unintended object. UHF RFID application is also performed using a portable commercial RFID reader. Good frequency reconfiguration and broad tuning range, along with far read distances, have been achieved with our tunable UHF RFID tag antenna.

We propose a low-profile and high-gain pattern-reconfigurable planar array antenna (PRPAA) based on digital coding characterization. The radiating elements in the PRPAA are designed ingeniously to realize digital “0” and “1” elements. By dynamically encoding the digital radiating elements with different coding patterns, various radiation beams can be achieved and switched in real time based on one antenna aperture. More importantly, for different coding patterns, the number and direction of the radiation beams of the digital PRPAA can be well predicted. We fabricate such a PRPAA and validate its digital-controllable radiation patterns experimentally. This communication provides many opportunities to realize low-cost, lightweight, high-gain, and multi-unit PRPAA, which offers promising applications in massive multi-in multi-out (MIMO) antenna array system, smart antenna, radar system, and dynamic microwave imaging.

The present-day ocean remote sensing instruments that operate at low microwave frequencies are limited in spatial resolution and do not allow for monitoring of the coastal waters. This is due to the difficulties of employing a large reflector antenna on a satellite platform, and generating high-quality pencil beams at multiple frequencies. Recent advances in digital beamforming focal-plane arrays (FPAs) have been exploited in this paper to overcome the above problems. A holistic design procedure for such novel multibeam radiometers has been developed, where: 1) the antenna system specifications are derived directly from the requirements to oceanographic surveys for future satellite missions and 2) the numbers of FPA elements/receivers are determined through a dedicated optimum beamforming procedure minimizing the distance to coast. This approach has been applied to synthesize FPAs for two alternative radiometer systems: a conical scanner with an offset parabolic reflector and a stationary wide-scan torus reflector system, each operating at $C$ -, $X$ -, and Ku-bands. Numerical results predict excellent beam performance for both systems with as low as 0.14% total received power over the land.

With the demanding system requirements for the fifth-generation (5G) wireless communications and the severe spectrum shortage at conventional cellular frequencies, multibeam antenna systems operating in the millimeter-wave frequency bands have attracted a lot of research interest and have been actively investigated. They represent the key antenna technology for supporting a high data transmission rate, an improved signal-to-interference-plus-noise ratio, an increased spectral and energy efficiency, and versatile beam shaping, thereby holding a great promise in serving as the critical infrastructure for enabling beamforming and massive multiple-input multiple-output (MIMO) that boost the 5G. This paper provides an overview of the existing multibeam antenna technologies which include the passive multibeam antennas (MBAs) based on quasi-optical components and beamforming circuits, multibeam phased-array antennas enabled by various phase-shifting methods, and digital MBAs with different system architectures. Specifically, their principles of operation, design, and implementation, as well as a number of illustrative application examples are reviewed. Finally, the suitability of these MBAs for the future 5G massive MIMO wireless systems as well as the associated challenges is discussed.

The technology to send a magnetic field in a particular direction, known as energy beamforming, has been recently introduced as a magnetic field shaping technology in nonradiative wireless power transmission. In general, one of the most efficient conditions for energy beamforming is that the magnetic fields induced by each antenna should be synthesized to head the same direction. To synthesize the magnetic fields induced at each antenna, interference by mutual inductance that can occur between transmitting (TX) antennas should be minimized. In addition, energy should not be exchanged between the TXs, otherwise it lowers the transmission efficiencies of TX and receiving antenna. In this paper, we present an optimal antenna structure that minimizes the mutual inductance between two TX antennas. First, we have analyzed the mutual inductance between TX antennas that have asymmetric sizes with different antenna lengths and arrangement angles. The directivity of the magnetic field vector is also investigated through an experimental analysis of an antenna structure. Finally, it has been verified that the optimal TX antennas for energy beamforming should be symmetric, which means that all the length of antennas are same and disposed perpendicular to each other. The experimental results show that the deviations of magnetic field directivity for symmetric and asymmetric antennas are 0.045 and 0.355, respectively, which shows that the symmetric structure shows 8.2 times larger consistency over the asymmetric structure.

The data model for direction of arrival (DOA) estimation is revisited and studied with more attention to the practical implementation of the antenna arrays and verified through measurement results. A complete data model for a single-channel direction finding system that includes, for the first time, the effects of the switching mechanism among antennas is presented. In order to combat coupling among antenna elements and the radio frequency switch nonidealities, a comprehensive model is developed, which includes more accurate noise modeling. The effect of the phase center (PC) in DOA estimation is then discussed and an improved data model is presented, which includes the antenna’s PC movement. The Cramer–Rao bound for circular arrays including the newly introduced parameter for the PC is derived. The new model shows significant improvement in the estimation accuracy.

A concept of scalable antenna arrays with an even number of radiating elements and nonuniform element spacing has been proposed. It has been shown that the proposed arrays can be fed with a frequency-selective network based on a well-known directional filter and with such a network the beamwidth can be kept constant across one frequency octave. The appropriate adjustment of coupled-line sections in such a directional filter allows for flexible selection of transmission coefficients between two outputs, and therefore, can be used for gradual switching of the signals between radiating elements. The proposed frequency-selective feeding network has been applied in a six-element antenna array operating in 1.75–3.5 GHz frequency range in which constant beamwidth around 16° has been achieved.

Brain-machine interface (BMI) is a multidisciplinary field that has been recently developed in an attempt to help restore functionalities for paralyzed individuals. One of the key components for the implementation of a wireless BMI necessitates unique designs for both the internal brain and external head antennas. In this paper, we initially revisited the design of an optimized 1-mm3 implantable antenna transferring power and data with a reduced size low profile external reader antenna by utilizing radio-frequency identification (RFID)-inspired backscattering. Detailed computational assessments and specific absorption rate evaluations are performed. Prototypes were characterized in terms of link efficiency through a realized RFID link with up to −25 dB link efficiency. The noise analysis for antennas in biological systems was performed using two novel absorption-noise models. And finally a channel capacity estimation was performed, proving that the BMI antenna link could support up to 100 recording channels. An end-to-end BMI antenna system characterization is detailed in this paper for multichannel implanted neural recording applications.

In this paper, a novel reconfigurable open slot antenna has been proposed for LTE smartphone applications to cover a wide bandwidth of 698–960 and 1710–2690 MHz. The antenna is located at the bottom portion of the mobile phone and is integrated with metal rim, thereby occupying a small space and providing mechanical stability to the mobile phone. Varactor diode is used to cover the lower band frequencies, so as to achieve a good frequency coverage and antenna miniaturization. The operational principles of the antenna are studied and the final design is optimized, fabricated, and tested. It has achieved the desired impedance bandwidth and the total efficiency of minimum 50% in free space throughout the required bands. The antenna performance with mobile phone components and human hand is also been studied. Furthermore, the SAR in a human head is investigated and is found to be within allowable SAR limits. Finally a multiple-input multiple-output antenna configuration with high isolation is proposed; it has an identical reconfigurable open slot antenna integrated at the top edge of the mobile phone acting as the secondary antenna for 698–960 and 1710–2690 MHz. Thus the proposed antenna is an excellent candidate for LTE smartphones and mobile devices.

Computational imaging at microwave frequencies has gained traction due to its potential for obtaining high-quality images with fast acquisition rates. Complex and diverse radiation patterns form the cornerstone of this approach. Electrically large antennas, such as mode-mixing cavities and metamaterial apertures, have proven to be effective platforms for generating such waveforms. Due to the complex nature of these antennas, near field scanning is often required to characterize their radiation patterns. However, accurate knowledge of the produced waveforms’ spatial distribution, with respect to the physical position of the antenna, is imperative. This relies on precise alignment between the antenna and the near field scan stage during the characterization process—a requirement that is especially cumbersome to achieve when operating at high frequencies. We present an effective method to address this problem; by introducing RF markers into the antenna the position of the antenna under test within the near field scanning setup can be obtained directly from the measurements. The proposed method is experimentally verified through comparison with measurements made using optical photogrammetry. The proposed process will find application in the alignment of computational and multistatic imaging systems, commonly used in security screening and threat detection, as well as in tiled electrically large antenna structures.

This paper presents a tunable decoupling and matching concept for compact multielement mobile terminal antennas. Instead of optimizing the antenna structure itself to achieve this purpose, the antenna elements are designed to be generic, small and placed closely to each other. The potential of reconfigurable decoupling and matching of such an antenna system, purely controlled by tunable decoupling and matching networks, is studied. The concept is evaluated by different classes of examples. First, fundamental aspects of decoupling and matching are discussed on the basis of several generic antenna matrices. Thereafter, a three-element antenna system of a mobile terminal is investigated by numerical simulations to calculate the achievable bandwidth of each tuning. This study is concluded by presenting efficiency measurements of a lab mock-up of a two-element mobile terminal antenna.

We address the synthesis of isophoric sparse ring arrays for full-Earth coverage from Geostationary Earth Orbit (GEO) satellites by means of steerable beams, switchable between two different widths. In particular, we pursue the desired beam zooming/shrinking through two interleaved sparse arrays deployed over the available circular aperture. To this aim, we propose two different antenna architectures. In one case, the two beams are radiated by two separate interleaved arrays; in the other case, the two beams are instead radiated by two interleaved arrays that share a common part of their layouts. Both the proposed architectures allow us to exploit a synthesis procedure that calculates the two interleaved arrays separately, through the cascade of two steps, both implemented with a (computationally) very efficient deterministic density-tapering approach. The proposed strategy allows to obtain isophoric sparse layouts, satisfying the required design constraints with a significant reduction of the control points with respect to the solution achievable exploiting noninterleaved isophoric sparse arrays.

For accurate beam analysis of a radiating antenna array on a complex platform, the mutual coupling and platform scattering effects are important. We proposed a beam pattern analysis method for an antenna array on a complex platform, which combines the active element pattern method of the receiving mode and hybrid geometrical theory of diffraction antenna current Green’s function technique. This proposed method clearly and mathematically divides the beam pattern of the antenna array on a complex platform into mutual coupling and platform scattering components. For a 12 linear dipole array embedded in a complex aircraft, analyses of the beam pattern and boresight error using the proposed method were verified with the method of moment results, which were simulated with FEKO. With reasonably good accuracy, the proposed method can efficiently analyze a beam pattern reflecting the mutual coupling effect and the platform scattering effect. Additionally, convex optimization with the proposed method is simply applied to synthesize the compensated beam pattern.

A wide-angle scanning planar phased array with magnetic current elements is proposed. A pattern reconfigurable technique is used to design the element that enhances scanning gain and decreases the sidelobe level throughout the entire scanning range. The array is comprised of eight elements in a $2 \times 4$ arrangement with uniform spacing. The proposed phased array operates at 5.8 GHz and can scan with 3 dB beamwidth the entire upper ground elevation plane from −90° to +90° enabled by a two-step pattern reconfigurability mechanism consisting of: 1) coarse-angle scanning and 2) fine-angle scanning. Significant outcomes also include the reduced sidelobe level (less than 7.8 dB) and the particularly small fluctuation (±0.75 dB) of the gain during scanning over a scanning range of 150° (from −75° to +75° in the elevation plane). With the absence of any structure above the ground level, the high efficiency, and the coverage of the entire upper half-space, this proposed antenna array is very attractive for a variety of phased array applications, particularly those that require a flush-mounted structure.

A metamaterial (MTM)-based thin planar lens antenna is proposed for spatial beamforming and multibeam massive multiple-input multiple-output systems. The antenna consists of a planar lens and a linear array of receive/transmit elements. To lower the insertion and reflection loss, the lens is formed by the two-layered ultrathin MTM-based surface separated with air and fed by substrate integrated waveguide-fed stacked-patch antennas. The effects of the focal-to-diameter ( f/D) on the power distribution of the lens are investigated to work out a design method. A planar lens antenna fed with seven elements is, for example, designed to operate at 28-GHz bands. The measured results show that the proposed antenna can achieve a scanning coverage of ±27° with a gain tolerance of 3.7 dB and a maximum gain of 24.2 dBi with an aperture efficiency of 24.5% over the operating bandwidth of 26.6-29 GHz. The lens antenna also features the advantages of compact size, low cost, lightweight, simple feeding network, and easy integration with other circuits for the next generation mobile communication and radar systems.

A novel approach to full-polarimetric short-range imaging with rotating arrays is proposed. Instead of taking advantage of all four possible combinations of the polarizations of transmit and receive antennas as in the typical full-polarimetric imaging systems, the suggested antenna array acquires three different co-polarized measurements at each spatial sampling point with its rotation. A simple algebraic operation is derived to accurately retrieve the full-polarimetric signals in the H/V polarization basis from the co-polarized measurements. After the retrieval of the full-polarimetric signals, the traditional imaging algorithms can be applied to reconstruct the polarimetric images. The effectiveness and accuracy of the suggested approach to fullpolarimetric imaging are validated on the signal and image levels through both numerical simulations and experiments. The results show that the proposed approach can accurately retrieve the fullpolarimetric signals and provides an alternative method to signal acquisition for full-polarimetric imaging.

Bolometer antennas are required to have not only a large gain–bandwidth product but also a fast heating rate suitable for video rate operation. However, while the gain–bandwidth product of an antenna increases with the fraction of the antenna bulk volume occupied by its enclosing sphere, its heating rate decreases. In this paper, we show that for a typical heating rate requirement, wire antennas such as dipole and bow-tie antennas are preferable over planar antennas such as triangular and pentagonal antennas. More complex bent-wire antenna geometries achieved by a genetic algorithm are shown to yield better performance.

A new low-profile, full-space, electronic beam-steering antenna architecture which combines the full-space beam-steering properties of reconfigurable transmitarrays with the low-profile feeding characteristics of leaky-wave antennas is proposed. The design uses an integrated leaky-wave feed to spatially distribute power across a reconfigurable transmitarray aperture in a low-profile manner while individual element phase control using varactor diodes enables full-space pencil-beam-steering. A $6\times 6$ element array was fabricated and experimentally verified, and full-space (both azimuth and elevation) beam-steering was demonstrated at angles up to 45° off broadside with a total efficiency for all scan angles on the order of 25%–35%. The design demonstrates a tenfold reduction in the overall thickness over the original transmitarray, improved aperture amplitude distribution control, as well as improved spillover control. The design also acts as a lower cost, scalable alternative to phased arrays as it does not require a complicated beamforming network and the feed and aperture are easily scaled.

We introduce a systematic approach to the shape optimization of compact, single-aperture MIMO antennas. Because the characteristic modes of a radiator represent its complete set of possible responses to an excitation, any port on the antenna must display the properties of a combination of one or more of these characteristic modes. By restricting our consideration to a class of symmetric antennas, the lowest order characteristic modes of a structure can be separated with practical decoupling networks, studied, and excited independently. We show that the quality factor of each characteristic mode effectively bounds the performance of any individual port excitation, and can be used to evaluate the fitness of the antenna for multiport excitation. Under this framework, we apply a genetic algorithm (GA) to synthesize low $Q$ MIMO antennas while minimizing conductor area. Feed locations are specified on the optimized shape based on the weighted excitation strength of the desired modes, and a two-port MIMO antenna is implemented and measured, verifying the proposed theory.

A direction of arrival (DoA) estimation technique for broadband microwave signals is proposed using a single ultrawideband antenna. It is inspired by the sound source localization ability of a human auditory system using just one ear (monaural localization). By exploiting the incident angle-dependent frequency response of a wideband antenna, the DoA of a broadband microwave signal can be estimated. The DoA estimation accuracies are evaluated for two antenna configurations and microwave signals with different signal-to-noise ratios. Encouraging the DoA estimation performance of the proposed technique is demonstrated in both simulation and experiment.

A novel design of a low-profile wideband monopolar antenna is proposed with integration examples for vehicles and helmets. Initially, a monopolar antenna size is reduced by adding four symmetrical tapered slots on an edge-shorted patch. The antenna is first designed to be integrated onto a large flat ground plane that is applicable for vehicular installation. This design is later adapted for integration onto a helmet with a finite-curved ground plane, where the helmet material is included as part of the design. This study shows that a curved ground plane improves the radiation patterns at broadside and further reduces the antenna height. Both vehicle and helmet antennas have been optimized to operate in a wide frequency range from about 800–2300 MHz. These antennas radiate in a monopole-like pattern with vertical polarization and high radiation efficiency that makes them suitable for transmission applications. Good agreement between simulation and measurement is obtained, which ultimately validates the proposed design technique.

The robustness of wearable UHF-band planar inverted-F antennas (PIFAs), with respect to body–antenna separation and human tissue dispersion, is addressed through numerical investigations. The main goal is gaining physical insights into the relationship between the grounded antenna performance and the distribution of the electric and magnetic energy densities in the antenna near-field region close to the ground plane border. A criterion for choosing a proper shape of the antenna ground plane is suggested, which can improve the antenna robustness with respect to the random variations of the body–antenna coupling scenario, but with a minimal impact on the antenna size.

In this paper, a novel planar reconfigurable monopulse antenna with compact size is proposed and studied for indoor smart wireless access points (WAPs) application. Circular polarization is achieved by using a dual-mode substrate integrated waveguide (SIW) slot antenna. Two diagonal ports are individually used to excite the SIW element antenna to achieve a 180° phase-shifting excitation. A reconfigurable $2 \times 2$ prototype is developed with the proposed element antenna, and it uses eight PIN diodes as four switches to implement the reconfiguration. Sum beams, difference beams in azimuth and elevation planes, and bidifference beams are generated. Measurement results verify the designed prototype, showing that the antenna has a null depth of over 30 dB and a peak sum beam gain of 12.29 dBi. The measured 10-dB return loss bandwidths are 5.72–5.96, 5.75–5.95, 5.73–5.97, and 5.76–5.94 GHz for the sum, elevation difference, azimuth difference and bidifference beams, respectively. And their measured 3-dB axial ratio (AR) bandwidths are 5.84–5.88, 5.86–5.90, 5.85–5.9, 5.85–5.9, and 5.84–5.91 GHz. The capability of reconfigurable monopulse beams ensures that the proposed antenna is a good candidate for smart WAP applications.

A triple-band multibeam multiple-input multiple-output (MIMO) antenna is investigated for access points (APs). The triple-band antenna covers 2.4–2.5, 3.4–3.7, and 5.1–5.9 GHz frequency bands with high gains and directional radiation patterns for WLAN/WiMAX applications. The MIMO antenna consists of four triple-band antenna elements rotated around a circle for space and polarization diversities. The multibeam antenna is composed of four MIMO antenna units for the coverage of a conical area. The triple-band multibeam MIMO antenna is designed and the MIMO performance is evaluated. It is shown that low capacity loss and high diversity gain can be obtained with the low correlation and low branch power ratio between the MIMO antenna units. It is found that the four-beam MIMO antenna is suitable for large public places with high density clients where enhanced frequency reusability and reduced maintenance efforts are preferred.

A concept of dual-band multibeam antenna arrays that allows for efficient utilization of a common aperture has been proposed. The presented antenna arrays allow for achieving $N$ -independent beams pointing at $N$ different directions at higher frequency and $N$ -independent beams at lower frequency pointing at $N/2$ different directions and having two orthogonal polarizations. As feeding networks, novel modified Butler matrices (BMs) have been proposed in which directional couplers having different properties in two frequency ranges have been for the first time utilized. To confirm the theoretical analysis, a dual-band multibeam antenna array fed by a modified $4\times4$ BM has been developed operating in 2.4 and 5.2 GHz frequency ranges.

A novel compact four-port reconfigurable multiple-input multiple-output (MIMO) antenna design for IEEE 802.11 applications is proposed. In one configuration, the antenna provides four ports operating from 4.9 to 5.725 GHz with isolation between antennas greater than 14 dB. In the second configuration, it provides a two-port antenna operating at 2.4–2.5 GHz together with another two-port antenna operating at 4.9–5.725 GHz all with isolations greater than 18 dB. The structure consists of four-slot antennas, and two of the slots are made reconfigurable by including MEMS switches in the slots. The overall antenna size is compact and occupies $46\times20\times1.6\;{\rm mm}^{3}$ and is printed on FR-4 printed-circuit-board. The proposed antenna is investigated by simulation and measurement, and results include radiation patterns, S-parameters, and signal correlations and branch power ratio (BPR) between ports. These show that in typical wireless environments’ envelope, cross correlations of less than 0.2 between the ports are obtained. The effect of the RF MEMS switches on the antenna performance is also addressed.

A novel all-textile UWB antenna with full-ground plane and high on-body fidelity is presented in this work. Due to the combined requirements of backward radiation reduction in WBANs and high-fidelity prerequisite for UWB pulse transmission/reception, a multilayered structure is implemented. The structure consists of two patches and a ground plane implemented on two layers of 2-mm-thick substrate. The full ground plane on the lower layer shields users against on-body radiation. Numerical and experimental evaluations both in free space and on human body prove the antenna operation within the FCC UWB bandwidth of 3.1–10.6 GHz. Its high-measured on-body system fidelity values of between 95% and 97% at a distance of 1 m show that the textile UWB antenna is suitable for body-worn UWB impulse radio communication.

A quasi-analytical solution is derived for the transmittance between two OAM antennas. It makes use of the properties of the modified circle polynomials, related to Zernike functions, and of integration in the complex plane of the radial spectral coordinate. The resulting formulation can be estimated extremely fast and is shown to exhibit several expected properties. An asymptotic expression is proposed and validated.

This work proposes an efficient optimization methodology of antenna array excitations accounting for the effects of mutual couplings and real radiating patterns of array elements with complex geometry. The method is formulated as a linear programming problem providing multibeam patterns with specified beamwidths, constraints on sidelobe and cross-polar levels, and null pointing directions. A matrix formulation is used to integrate the optimization procedure and a finite-array analysis based on the infinite-array approach, in which the periodic radiating element is characterized by a full-wave and hybrid procedure, combining the domain decomposition technique, the finite-element method, and modal analysis. Numerical examples of arrays of open-ended waveguides, microstrip antennas, and dielectric resonator antennas are presented.

The pattern synthesis problem of maximizing the gain at a scanning direction subject to a maximum sidelobe level for a reactively loaded antenna array is formulated as a quadratically constrained quadratic program (QCQP). This formulation is derived from the scattering matrix and the active element patterns of the array considering both fed and loaded antennas as ports. Since the resulting QCQP is nonconvex, the loads are computed from the solution of the semidefinite relaxation of the problem by forcing the feasibility of the constraints in a meaningful physical sense, i.e., by explicitly imposing passive or purely reactive loads. Numerical results of dipole arrays with various configurations are presented and compared to previous works.

A foam-embedded wideband antenna array is used to build a portable system for congestive heart failure (CHF) detection. In addition to the array, which has a pair of four-element antenna subarrays, the system includes a portable vector network analyzer (VNA), a switching system, and a laptop, which contains control, signal processing, and image formation algorithms. The main aim of the built system is to provide a noninvasive, convenient, low cost, fast, and reliable platform for detection and monitoring of CHF. In that regard, the antenna elements are built using a three-dimensional (3-D) structure, which utilizes a combination of loop, monopole, and parasitic patches to cover the needed band (0.7–1 GHz) while keeping the size compact. A differential technique is utilized to distinguish between healthy and unhealthy cases. The performance of the system is tested on an artificial phantom and a phantom with a pair of lamb lungs, which are confirmed to have electrical properties close to those of humans. Several possible cases are investigated to validate the reliability of the system in the early detection of CHF.

This paper presents experimental investigations and analysis of ultra-wideband localization of body-worn antennas in an indoor environment. Three-dimensional (3-D) tracking of wearable antennas placed on the upper human body is achieved using a compact and cost-effective antenna and time of arrival (TOA) localization techniques. Various features such as rms delay spread, kurtosis, and signal amplitude have been used to classify the channel type between each antenna location and base station. Non-line-of-sight identification and mitigation techniques have been used to enhance the accuracy of the localization technique applied. Geometric dilution of precision of the base-station configuration and numerical analysis on the radiation patterns for different locations of the body-worn antenna has been studied in terms of variation in localization accuracy. High-accuracy 3-D localization results in the range of 1–2 cm are obtained with the antennas placed on the limbs and 1–3 cm for the torso region. The accuracy achieved is 3 times better than commercially available UWB localization and tracking systems. The significantly high-accuracy results obtained for all antenna locations demonstrate the reliability and suitability of UWB technology for various indoor tracking and localization applications in the fields of healthcare, sports, and well-being.

When a receiving antenna array operates on a complex environment, the ideal phase differences of the receiving voltages of the antenna array are significantly distorted by both mutual coupling and platform scattering. This distortion causes performance degradation of the direction of arrival (DOA) algorithms. For estimating and compensating for the phase distortion, an analysis of the receiving voltages of the receiving antenna array on the platform is required. In this paper, we propose a hybrid UTD-ACGF technique for modeling port voltages of the receiving antenna array on a complex environment. The complex environment is modeled by UTD technique, and the antenna array is modeled by ACGF. In order to explain the coupling effect between the complex environment and the antenna array, we use the first perturbation series approximation based on the exact mutual coupling perturbation series. The hybrid UTD-ACGF results are verified through comparison to MoM results. Finally, we statistically define a distortion matrix that has a relation between the distorted port voltages and the ideal port voltages. The distorted port voltages are evaluated using the hybrid UTD-ACGF technique as a backbone. In order to enhance the performance of the DOA algorithm, we efficiently estimate and compensate for the distortion matrix. A DOA simulation using the MUSIC algorithm is performed to confirm the compensatory effect of the distortion matrix.

An approach to controlling the two-way pattern (i.e., the product of the transmitting and receiving antenna patterns) for a distributed digital subarray antenna (DDSA) is presented. Collectively combining periodic widely separated subarrays results in grating lobes (GLs), and they are unwanted because of the ambiguities that accompany them. The approach to GL control presented here involves a combination of conventional suppression methods on the transmitting side and in the digital beamforming on the receiving side, filling the gaps between the subarrays with “virtual” elements, thus forming a contiguous array. Individually, the transmitting and receiving antenna patterns may not have adequate performance, but it is demonstrated that if designed to complement each other, the two-way pattern can achieve ultralow sidelobe performance. The effects of receiving element signal-to-noise ratio (SNR) and errors in calibration are also addressed.

While transmit and receive beamforming for the multiple-input multiple-output (MIMO) point-to-point channel is a well-studied topic, existing algorithms cannot specify effective beamformers for certain situations. To address this deficiency, we present an iterative algorithm that can establish beamformers based on different channel information available to the transmitter and receiver, different receiver architectures, and different constraints on the power radiated by the transmit array. Simulation results based on modeled and measured channels demonstrate that the algorithm effectively accommodates a wide range of conditions and that the performance achieved with the approach matches that obtained using existing algorithms that are applicable only to specific conditions.

This paper concerns the study of the mm-wave reconfigurable antennas based on S-PIN diodes suitable for using in mm-wave communication systems. The investigated antennas were designed, fabricated, and subjected to the measurements in both steady and transient states. One selected antenna was incorporated into a wireless system to determine its suitability for operating in such a system as a time-modulated linear array. By means of appropriate switching of the antenna, multiple radio frequency (RF) chains were simulated in a receiver having only one RF chain. This allowed applying a combining technique for improving the system performance in a presence of a strong interfering signal.

In this article, a dual-band phased-array antenna element is presented. The proposed element is particularly designed to possess frequency tuning capabilities by variable-impedance matching. It is demonstrated that the operational frequency band of the antenna shifts from L- to S-band (about 2.5:1 frequency ratio) with in-band frequency tuning of about 25%. The compact size and the wide-angle scanning properties, make the antenna element suitable for phased-array application. Other advantages of variable-impedance matching for phased-array systems, such as sustain wider scan angles and minimize unwanted signal interferences, coming from spurious directions, are also discussed.

A novel compact-range wireless access system is proposed for multi-Gb/s data transfer in the 60-GHz band. Large-array antennas in the access point generate a quasi-plane wave and form a stable communication zone proportional to the antenna size. The link budget based upon the Friis transmission equation is not applicable as ever in this compact-range communication system. The signal coverage up to 10 m still falls within the Fresnel region of the large-array antenna. Mobile users everywhere within the coverage receive almost constant field strength free of multipath interference. To demonstrate the viability, circularly polarized waveguide slot arrays of various sizes are designed and fabricated. After the evaluation of conventional antenna radio frequency (RF) performance, bit-error rate (BER) and signal-to-noise ratio (SNR) measurements are conducted to evaluate the prototype of a compact-range Gigabit wireless access system.

Communication with unlicenced 60-GHz band is attractive for wireless networks since a free-of-charge bandwidth of 7 GHz is available. The antenna array, conducting transmitter/receiver beamforming, is often used to enhance the transmission range. However, to conduct beamforming, the transceivers have to know the angles of signal arrival. Many angle-of-arrival (AoA) estimation algorithms are well known. Unfortunately, most of them are developed for digital arrays, requiring an analog-to-digital-converter (ADC) and a digital-to-analog converter (DAC) for each antenna element. For an array with a large number of antennas, the implementation cost will be very high. Recently, hybrid antenna array, in which multiple antennas share an ADC/DAC, was proposed as a low-cost solution. However, the existing AoA estimation algorithms for hybrid arrays can only support one AoA; their applications are then limited in multipath indoor environments. In this paper, we propose new AoA estimation algorithms to solve the problem. Using an augmented beam-space approach, we extend the use of the well-known AoA estimation algorithms, MUSIC and ESPRIT developed for digital arrays, to the scenario of hybrid antenna arrays. Simulations show that the proposed algorithms, although developed for hybrid arrays, can have similar performance as MUSIC and ESPRIT in digital arrays.

The design of a novel partially reflective surface (PRS) antenna with the capability of beam steering is presented in this paper. The beam steering is realized by employing a reconfigurable PRS structure to achieve a changeable reflection phase as well as using a phased array as the source to excite the PRS antenna. A prototype antenna including the biasing network is fabricated and measured. It achieves a consistent beam steering from – 15° to 15° with respect to the broadside direction across an overlapped frequency range from 5.5 to 5.7 GHz with measured realized gains over 12 dBi. Good agreement between the simulated and measured results for the input reflection coefficients and radiation patterns is achieved, which validates the feasibility of the design principle. Compared with other beam steering PRS antennas, the proposed one enables a larger beam steering angle with comparable gains, requires a simpler biasing network, and is more compact.

Beamforming in series-fed antenna arrays can inherently suffer from beam-squinting. To overcome the beam-squinting problem, low-dispersion, fast-wave transmission lines can be employed. Such transmission lines can be designed by loading a regular transmission line with non-Foster reactive elements (e.g., negative capacitors and inductors). As a result of a recent development, these non-Foster reactive elements can be implemented using loss-compensated negative-group-delay (NGD) networks, providing a solution to the stability issues associated with conventional non-Foster networks. In this work, transmission lines augmented by loss-compensated NGD networks, representing the non-Foster reactive-element loading, are employed for designing wideband fast-wave, low-dispersion transmission lines. This work consolidates this non-Foster reactive element loading method with earlier efforts where NGD networks were used to implement zero-degree phase shifters for beamforming at the broadside direction, and generalizes these methods for arbitrary-angle beamforming from backfire to endfire including the broadside direction. Experimental results are presented for a wideband linear four-element transmitting array feed network for beamforming at 30° with respect to the broadside direction in the frequency range 1–1.5 GHz. By connecting this feed network to four wideband tapered-slot antennas, the beamforming performance is experimentally verified inside an anechoic chamber. Moreover, the antenna array is experimentally tested for transmission of a narrow pulse, where low distortion is observed at the beamforming angle over the entire operating bandwidth. The physical length of the feed network is realistic and is 0.96 wavelengths long at the center of this frequency range. In addition, switched-line phase shifters are employed for squint-free beamforming in three other angles: 60°, 0°, and $-30^{\circ}$ .

Nowadays, broadband satellites operating at Ka-band and providing high capacity (above 100 Mbps) are mostly based on single-feed-per-beam (SFB) antenna configurations. This antenna farm, using typically three or four reflectors to produce the full dual-band (Tx/Rx) multiple beam coverage, provides high performance but leaves no or little room on the spacecraft for other missions. Accommodation constraints and the desire of operators to maximize and diversify their revenues per satellite have led to several studies on innovative antenna solutions enabling to reduce the number of reflector apertures. Multiple-feed-per-beam (MFB) antenna configurations, using only two apertures, are currently under development for broadband missions at Ka-band. C-band is a more mature business and operators are contemplating the possibility to apply the benefits of multiple beam antennas (MBA) to this frequency band. Due to the lower operating frequency, accommodation constraints are even more stringent and although still scarcely discussed, MFB antenna configurations at C-band are clearly of interest. This paper presents investigations on this topic. In particular, it is found that the MFB architecture applied in C-band is a more realistic approach when compared to the SFB option. Beam Forming Networks (BFNs) with periodic structure is the key to get compact feed clusters that can be allocated in large and medium-size platforms. A new four color scheme compatible with a very compact MFB feed array architecture is introduced.

In recent years, spacecraft requirements have trended toward smaller, lighter, and less expensive systems with more capabilities. At the same time, demands on the communication systems have increased, including faster data rates, lower power budgets, reduced system volume, and smaller link margins. The combination of these factors has posed numerous engineering challenges for antenna system designers, often forming complex tradeoffs among design parameters such as bandwidth, pattern control, beamwidth, and antenna size. In this paper, we show how a challenging set of antenna requirements were met for NASA’s recent LADEE mission. We focus on the difficulties inherent in the requirements for both omnidirectional and medium gain antennas, both in S-band. We present techniques used to develop a requirements-compliant system based on our research in antenna synthesis methods. Compared to the conventional antennas considered by NASA for the mission, the antennas we developed yielded 65% increased downlink coverage and 44% cost savings for the mission. The deployed flight antennas were the only antennas on the mission and performed above expectations during the 8-month mission, which concluded in April 2014.

We develop a general approach to cross correlation in antenna systems suitable for applications to spatial diversity and MIMO systems. The far-field correlation is expressed in terms of the currents on the antennas. The basic strategy proposed here is to perform the overall evaluation of cross correlation by means of superposition integrals involving contributions emerging from all possible mutual correlations between the point sources on the antenna currents where interactions are mediated by a new cross-correlation Green’s function. The method is verified and demonstrated in several numerical examples and a design methodology aiming at maximizing the diversity gain is outlined and illustrated. It is also shown that arbitrary antenna arrays can be reduced to suitable models involving only infinitesimal dipoles, in effect enabling us to compute the total diversity gain using the cross-correlation Green’s function. The formulation provided here gives the electromagnetic aspect of spatial diversity an articulated form proper for design and development of practical communication links using multiple antenna systems.

This study proposes a simple self-calibration system that involves applying the phase-match method (PMM) in the far-field calibration of a deployed active array. When the proposed self-calibration approach is employed, an antenna array system can be maintained at a ground-based station, and no expensive and complex test equipment is required. An EM-based simulation method was applied to validate the proposed method numerically. Finally, an eight-element microstrip array operating at 3.6 GHz was designed to study the proposed method experimentally. The measured results validated the effectiveness of the proposed method in calibrating deployed antenna array systems.

A novel design of dual-band reconfigurable slot antenna is proposed in the paper. The configuration of the proposed antenna is based on a folded slot with a branch edge formed by multiple strips loaded with circuit components. In the design, the antenna is decomposed into networks, and the corresponding equivalent circuits are deduced. By applying resonant condition on the equivalent circuit at the desired center frequencies, the circuit components can be determined. The proposed design is later transformed into a reconfigurable antenna by using varactors. The antenna configuration is also modified for applying biasing voltages to varactors. Dual-band reconfigurable operation of antenna is studied and demonstrated for S-band applications at 2.45 GHz and GPS at 1.227/1.381/1.575 GHz. The design approach is described in details. Also, antenna measurements are conducted for design validation.

The effect of directional antenna elements in uniform circular arrays (UCAs) for direction of arrival (DOA) estimation is studied in this paper. While the vast majority of previous work assumes isotropic antenna elements or omnidirectional dipoles, this work demonstrates that improved DOA estimation accuracy and increased bandwidth is achievable with appropriately-designed directional antennas. The Cramer–Rao Lower Bound (CRLB) is derived for UCAs with directional antennas and is compared to isotropic antennas for 4- and 8-element arrays using a theoretical radiation pattern. The directivity that minimizes the CRLB is identified and microstrip patch antennas approximating the optimal theoretical gain pattern are designed to compare the resulting DOA estimation accuracy with a UCA using dipole antenna elements. Simulation results show improved DOA estimation accuracy and robustness using microstrip patch antennas as opposed to conventional dipoles. Additionally, it is shown that the bandwidth of a UCA for DOA estimation is limited only by the broadband characteristics of the directional antenna elements and not by the electrical size of the array as is the case with omnidirectional antennas.

For coherent MIMO radar the optimal target signal processing can be achieved for any transmitted waveforms or radiation beam pattern, making transmit beamforming through waveform design possible without degrading target detection performance. In this work, an innovative waveform optimization approach termed phase-only variable metric method (POVMM) is proposed for coherent MIMO radar waveform design to form a desired transmit beam pattern such as one with radiation nulls in certain directions. The waveform design is carried out by minimizing the radiation powers of the MIMO radar antenna in the selected directions with optimization variables constrained to the waveform phases only. The gradient function of the cost function with regard to waveform phases is analytically derived for the optimization and the POVMM is developed based on the variable metric methods with a flexible search step sizing strategy for improving optimization efficiency. The proposed approach is validated with various designs and simulations.

In this paper, we evaluate adaptive impedance tuning in combination with intrinsically unmatched antennas in order to reduce the form factor of handset antennas. Thereby, we are able to shrink the size of an antenna element by a factor of four compared to an intrinsically broadband matched antenna while maintaining a similar total efficiency in usage scenarios. The antenna concept is based on an unmatched coupling element in order to obtain a flat impedance behavior, which is relatively insensitive to the user interaction. It enables a broadband adaptive matching by a tunable $\Pi$ -section network using varactors. The network takes into account the parasitic effects of the impedance detector and the ESD protection circuit.

A systematic framework is presented for designing an antenna with arbitrary shape and expanding its input impedance and pattern bandwidths using reactive loading. This method, unlike other design procedures, allows us to control the current distribution over the antenna structure with potentially fine granularity. This technique is applied to wire and microstrip antennas and it is shown to achieve a very good impedance and radiation pattern performance over the desired band. The loads are combination of Foster and non-Foster elements, which are generally required to obtain such dramatically improved performance.