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The impedance expansion method (IEM), which has been previously proposed by the authors of this paper, is a circuit modeling technique for electrically very small devices. This paper describes a circuit modeling procedure that utilizes the IEM and eigenmode analysis for a wireless power transfer system. First, a multimode circuit model that can represent the higher order resonances in a broadband frequency range is presented. Subsequently, a single-mode circuit model—consisting only of passive elements—that can represent both the radiated and conduction loss powers at approximately its operating frequency band is presented. [ABSTRACT FROM AUTHOR]

In this paper, the performance of Fresnel lens antennas is investigated in detail. The performance is evaluated in terms of aperture efficiency and stable radiation pattern over a wide frequency band. This paper proposes an efficient technique for enhancing aperture efficiency by smoothly compensating for the spherical phase front arriving upon the lens surface. The feeder has been optimized to fit an axially symmetric cosn-like radiation pattern. For this, an accurate technological process has been used to manufacture a lens that allows for smooth compensation of the phase shift. The lens is compared with a classic Fresnel lens having the same physical dimensions and feeding system. The improvement is validated by measurement, which has revealed a maximum measured gain of 38.9 dBi corresponding to a maximum measured aperture efficiency of 59% with a −2 dB radiation frequency bandwidth of 29.25 GHz around 90 GHz. This amounts to an aperture efficiency enhancement of 60% and −2 dB radiation frequency bandwidth enhancement of 72% as compared with those of the classic Fresnel lens. [ABSTRACT FROM AUTHOR]

Numerous medical operations employ blood transfusions, requiring X-ray irradiated blood for safety concerns. Current irradiation techniques can be significantly improved by replacing standard visual indicators with wireless dosimeter tags that automate the process, reducing inefficiencies, and eliminating blood wastage. A key requirement of the proposed dosimeter tag is flexible and efficient antennas that can be mounted on blood bags. This paper presents the design of a low-cost inkjet-printed dipole antenna on flexible Kapton substrate for a 2.45 GHz radio frequency identification (RFID) dosimeter tag. The tag is to be used in a lossy blood environment, which can severely affect antenna radiation performance. To mitigate this, the concept of artificial magnetic conductor (AMC) unit cells is investigated for best impedance and gain performance. When integrated with a dipole radiator, the fabricated AMC-backed antenna maintains broadside radiation with gains of 4.1–4.8 dBi under planar and bending conditions, and on a lossy blood bag. In a rectenna configuration, the antenna can power sensors for ranges up to 1 m. Measured output dc voltages up to 1.7 V are achieved across a 25 $\text{k}\Omega $ resistor. This antenna design is flexible, compact, efficient on lossy structures, and suitable for direct integration with biomedical sensing chips. [ABSTRACT FROM AUTHOR]

This paper presents electric and magnetic probes to measure radio frequency (RF) electric and magnetic fields for measuring the time-varying RF fields used in magnetic resonance imaging (MRI). A small single loop (2 cm) and short dipole antenna (2 cm) were developed to monitor the near field magnetic and electric exposure during the medical device testing. Theoretical analysis for each designed probe was performed to convert the real time receive signals to the exposed electric and magnetic field. Probes in this paper were tuned and matched at center frequency of 127.6 MHz for 3T MRI scanners to improve the accuracy and sensitivity. Different cables were designed to reduce the E-field pick up and achieving the most accurate measurement setup. Probes were fabricated on a single sided printed circuit board, FR4 of thickness 1.57 mm and a copper thickness of 35~\mu \textm . The measured S-parameters of the magnetic and electric field probes show less than −44 and −45 dB return loss at 127.6-MHz center frequency of the RF Birdcage. [ABSTRACT FROM AUTHOR]

In this paper, a theoretical analysis of orbital angular momentum (OAM) wave propagation properties in terms of reflection and refraction is introduced by decomposing OAM waves into infinite plane waves in the spectral domain with different elevation and azimuth angles. Transformations of phases and amplitudes at the interface of two media for different incident angles and elevation angles are numerically analyzed. Similarly, based on the idea of decomposing OAM waves, reflection and refraction properties of OAMwave propagating through slabs are also analyzed and simulated. Finally, experiments on OAM waves with both mode numbers 1 and 2 are conducted to demonstrate the reflection and refraction properties of OAM waves. [ABSTRACT FROM AUTHOR]

Fifth generation (5G) mobile networks will introduce several new frequencies for short-range high-capacity communications. Future handsets must also support current frequency bands for backward compatibility and long-range communications. This paper presents a proof-of-concept solution for co-designed millimeter-wave (mm-wave) and Long Term Evolution (LTE) antennas in a metal-rimmed handset. The design shows that both antenna types can be accommodated in a shared volume and be integrated into the same structure. Presented antennas operate at 700–960 MHz, 1710–2690 MHz, and 25–30 GHz. Simulations and measurements suggest that the system can be designed in such a way that the mm-wave antenna does not hinder the low-band performance. LTE antennas generally reach over 60% total efficiency while the mm-wave module has a peak gain of 7 dBi with measurement-verified beam-steering capability. The proposed design proves that 5G mm-wave antennas can be embedded to 4G systems without greatly sacrificing display size or sub-6 GHz antenna performance. [ABSTRACT FROM AUTHOR]

This paper proposes a method to represent and predict received signal instantaneous polarization states as a function of arbitrary transmit polarization states for time-dispersive channels. The model incorporates frequency-dependent characterizations to represent the polarization dispersion behavior of a received signal across its bandwidth in a multipath channel. The representation takes advantage of signal dispersion features typical of depolarized signals, and employs subbanded characterizations to achieve polarization representations exhibiting high degrees of polarization in each subband. Experiments are conducted that demonstrate the utility of the modeling approach to predict polarization dispersion responses for arbitrary transmit polarizations based on measurements from two orthogonal transmit polarizations. [ABSTRACT FROM AUTHOR]

This paper presents an eight-band antenna for full-screen metal frame LTE mobile phones with a size of 142 mm $\times79$ mm $\times7.5$ mm. The proposed antenna mainly consists of two monopole branches, a match circuit, and two ground branches. The two monopole branches and the match circuit are used to cover the lower band (698–960 MHz) and part of the higher band (1710–2690 MHz). One of the two ground branches is used to cover the other part of the higher band, and the other is mainly used to make the lower band matched. The merit of the proposed antenna is that eight bands can be realized under the conditions of an only 2 mm ground clearance and a metal frame; it is very suitable for full-screen metal frame mobile phones. A prototype is manufactured and measured. The measured −6 dB impedance bandwidths are 300 MHz (665–965 MHz) at the lower band and 1190 MHz (1610–2800 MHz) at the higher band. The LTE700, GSM850, GSM900, DCS, PCS, UMTS, LTE2300, and LTE2500 bands are covered. The measured efficiency is bigger than 42%, so it is practical for mobile phone applications. [ABSTRACT FROM AUTHOR]

Frequency-selective surfaces (FSSs) are widely employed in antenna’s radar cross-sectional reduction, which plays a key role in improving survivability and penetration ability of future military systems. In practical, dual-band bandpass FSSs with large band ratio (BR) are urgently needed for some military equipment, such as homing head, that can detect both middle and far range targets. In this paper, a novel method for designing dual-band bandpass FSS with large BR is proposed. The method can provide a large BR response, by utilizing a spurious-free structure and combing resonant and nonresonant elements. The detailed analysis is presented with the aid of equivalent circuit model as well as closed-form equations to reveal the operating mechanism. A design example is simulated, fabricated, and then measured. Two excellent passbands are obtained with a large BR value of 15.3 in experiment. Moreover, a −10 dB fractional bandwidth of 164.3% is realized from 1.31 to 13.35 GHz, without observing any spurious transmission windows. In conclusion, our proposed method fills the gap of the existing dual-band FSSs to obtain a large BR value of more than 6 and could be a good guidance for designing the future multiband FSSs. [ABSTRACT FROM AUTHOR]

The synthesis of time-modulated arrays (TMAs) for physical layer secure communications is carried out in this paper. The problem is cast as the transmission from the TMA of a signal undistorted within a desired angular region, while the signal distortion is maximized elsewhere. Toward this aim, a customized optimization strategy based on a binary genetic algorithm is proposed to define the optimal ON–OFF sequence controlling the radio-frequency switches modulating the signal transmitted by the elements of the TMA. Anad hocfidelity factor metric is used to quantify the signal distortion as a function of the angular direction. A set of numerical examples is reported and discussed to illustrate the behavior of the proposed synthesis approach as well as to give some insights on its potentialities and achievable performance. Moreover, this paper shows, for the first time, the results of an experimental validation when considering a real TMA. [ABSTRACT FROM AUTHOR]

This paper presents a simple, compact, and low-power method for generating extremely low-frequency radio signals below ~500 Hz. This paper is motivated by the prohibitively large size and low efficiency of conventional antennas operating in this frequency range. The successful generation of time-varying magnetic fields produced by the physical rotation of a 3 cm3 permanent magnet is demonstrated. Ground plane and receiver orientation effects are analyzed both theoretically and experimentally. Observations as a function of distance from the source indicate that the fields are dominated by the static component, rather than by the radiation component, of the generated field. Based on these observations, the electromagnetic radiation emitted by a spinning magnet source is predicted to be weak. Nevertheless, static magnetic signaling using this technique is proven to be both possible and practical at frequencies below ~500 Hz and at distances in excess of 100 m. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to formulate an aggressively thinned sparse antenna array suitable for orthogonal multibeam receiver applications. The power patterns of $M \times N$ element planar rectangular array are first reduced to orthogonally placed cross multiplicative subarrays. These arrays are then redistributed using a compressive sensing (CS) approach in order to achieve array thinning along two 1-D subarrays for a fixed steered beam projection. A multibeam synthesis approach is then implemented which permits efficient beam maxima as well as a null placement of multiple interlaced far-field patterns. Numerical examples are presented to show the implementation of the proposed approach. [ABSTRACT FROM AUTHOR]

The theory, modeling, measurement, and analysis of the radiation characteristics of bond wire antennas (BWAs) is presented in this paper. We commence with rigorous formulations and derivations of analytical models for calculating the vector potential, radiated electromagnetic fields, and directivity of full-loop BWAs (FL-BWAs), considering nonconstant current distribution along the BWAs. Based on the image theory, we derive an analytical model for calculating the directivity of half-loop BWAs (HL-BWAs) from that of FL-BWAs. Very good correlation is obtained between the directivities calculated analytically using the derived models, and numerically using commercial full-wave solvers, thus validating our analytical approach. We apply the verified models to thoroughly analyze the radiation characteristics of FL-BWAs and HL-BWAs. The radiation characteristics of an HL-BWA are based on the assumption that its reference plane is infinitely large and perfectly conducting. However, for the development of wireless systems, reference planes of finite sizes are required. Therefore, we investigate the impact of realistic planes sizes. Our results reveal that the radiation characteristics of BWAs referred to as “HL-BWAs” in published literature differ significantly from those of true HL-BWAs. Therefore, we introduce the term “quasi-half-loop BWA (QHL-BWA)” to describe BWAs with small reference planes. For experimental verification, QHL-BWAs which operate in the 60 GHz band were fabricated using the wedge–wedge bonding process. Both S-parameter and radiation pattern measurements were performed. Very good correlation was obtained between measurement and simulation results. [ABSTRACT FROM AUTHOR]

This paper presents a short-term analysis of the radio propagation channels within vegetation media at the most commonly used wireless network frequency bands: 2.4, 3.5, and 5.8 GHz. Fading effects underlying this short-term analysis would determine whether the radio channel could support a stable link or not. The configuration used in this study is a peer-to-peer system, where the transmitter and the receiver are located at low heights inside several vegetation media, including forests and meadows. The distribution function that best fits the received power was determined to be Weibull, and the evolution of its parameters was studied as the distance between transmitter and receiver increases. [ABSTRACT FROM PUBLISHER]

Frequency-agile pathloss models for urban street canyons are discussed in this paper. The models are floating intercept (FI), fixed reference (FR), and ITU-R M.2135 urban microcellular (UMi) line-of-sight (LOS) and Manhattan-grid non-LOS (NLOS) models. These models are parameterized based on channel sounding campaigns in three cities covering radio frequencies ranging from 0.8 to 60 GHz. Fitting the models with measured pathloss reveals that the models are usable to cover the considered frequency range. The FI and FR models are equally simple and robust, with a slight advantage of the FI model in accuracy because of the larger number of model parameters. The original M.2135 LOS model is based on a two-ray model that includes a break point (BP). The model is extended for a better fit with measurements by including new model parameters such as a pathloss offset and a BP scaling factor that represent local scattering conditions of surrounding environments. The new model parameters are found frequency dependent in many cases. The original M.2135 model is furthermore simplified in NLOS scenarios while maintaining the model accuracy. The model parameters are derived using maximum likelihood estimation, which also showed that the modified M.2135 model offers up to 50% better accuracy compared to the FI and FR models in terms of the employed log-likelihood function (LLF). The improvement in accuracy is particularly remarkable in NLOS scenarios. A full set of parameters is provided for the models, allowing a choice for any given requirements on accuracy and complexity. Finally, applicability of the proposed models to other street canyons is discussed using independent pathloss measurements. [ABSTRACT FROM PUBLISHER]

The conventional time-modulated arrays (TMAs) have a low radiation efficiency, since each antenna turns off at specific time slots. In this paper, first, the efficiency of TMA is investigated analytically. Next, the optimized switching sequences are proposed to enhance the radiation efficiency of a specific sideband, while preserving the level of the other sidebands. The practical hardware limitations are considered in the switching sequence design. Moreover, a low-cost flexible eight-element printed dipole array operating at 1.2–1.4 GHz is implemented to verify the proposed algorithms. The measured radiation patterns indicate that by applying the proposed sequences, the TMA efficiency improves by 8.3 dB for the fundamental frequency. The efficiency enhancement for the first and second sidebands is 2.9 and 2.95 dB, respectively. In addition, a fast and low-cost setup for TMA measurement using the commercial software defined radio dongle is implemented in this paper. [ABSTRACT FROM AUTHOR]

Target probing based on wideband pulse radar is a challenging task for radio frequency simulation (RFS) in anechoic chamber. Due to the long time duration of pulse radar signal and the limited space of anechoic chamber, the reflected signal returns before the pulse radar signal is fully transmitted, which makes it difficult for the signal reception. As a consequence, the target high-resolution range profile (HRRP) cannot be obtained for target imaging and recognition. In this paper, we propose a novel strategy that contains two steps to solve this problem and obtain the target HRRP. First, the randomly interrupted transmitting and receiving (R-ITR) method is proposed to receive the pulse radar signal in RFS. Second, the target HRRP is reconstructed by compressive sensing (CS) since the R-ITR echo is piecewise sparse. The protocol to choose R-ITR parameters and performance of HRRP reconstruction are provided. Simulations and experiments with different R-ITR parameters are also conducted to verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

A multilevel subarray RF modularization technique is presented to construct a hierarchical beamforming network (BFN) for phased array of antennas. In contrast to the conventional BFN architecture, which feeds each antenna element by using individual set of RF devices, the restructured RF modules may simplify the BFN architecture. They may dramatically reduce the cost without significantly sacrificing the radiation performance. In this technique, the RF devices to feed antenna elements are subdivided into groups for modularization. Each module is formed by very few elements of RF devices to form a multilevel BFN, where the number of RF elements in each module are the integer factors of array size. Each module at the same layer consists of identical RF structures to minimize the mass manufacture cost. In this paper, the theoretical foundation is developed to provide design guidelines. Examination of radiation discrepancy is also investigated to demonstrate the feasibility and validate the proposed technique. [ABSTRACT FROM AUTHOR]

A differential microstrip antenna with improved gain for RF energy harvesting is presented in this paper. The developed antenna can be used in either center grounded or differential configuration. The antenna is designed and fabricated for GSM900 band (890–960 MHz). The antenna has a gain of 8.5 dBi at the center frequency and exhibits VSWR \boldsymbol\leq 2 for frequencies between 870 MHz to 1.05 GHz. The efficiency of the antenna is 80%. The developed antenna finds its application in energy harvesting, RFID tags and in wireless communication circuits, where differential inputs/outputs are needed. A complete differential RF energy harvesting system with a peak efficiency of 65.3% for a load of 3 \mathbfk\boldsymbol\Omega is also developed. [ABSTRACT FROM PUBLISHER]

An implementation of circularly polarized crossed dipoles in millimeter-wave band is presented in this paper. Novel detached crossed dipoles have been designed to tackle the problems confronted by single-fed crossed dipoles in mm-wave band. An evolution procedure from conventional crossed dipoles to the proposed ones is given. Meanwhile, a metallic ridge is added in the cavity to provide a robust feeding structure. Measurements of an optimized antenna prototype show that it can achieve a bandwidth from 27 to 36.8 GHz for both \ AR\leq \ 3\ \ dB and \vert S11\vert\leq-{\ {10}}\ {\ {dB}} and an average broadside gain of 12.4 dBi. The measured results agree reasonably well with simulations. For reference to practical designs, studies on five critical parameters have also been performed in this paper. [ABSTRACT FROM PUBLISHER]

In this paper, a novel configurable loop array antenna based on resonator-loaded microstrip lines (RLML) is proposed to enlarge the interrogation zone of the reader antenna for near-field ultrahigh-frequency (UHF) radio frequency identification (RFID) system. The proposed loop array antenna is composed of a number of RLML loop units. By configuration of the RLML loop units sharing a common side with its adjacent RLML loop units which carry reverse-direction flowing current in the ${x}$ -axis and ${y}$ -axis directions, loop array antennas are created to generate a strong and uniform magnetic-field distribution over a large configurable interrogation area. As an example, loop array antennas with $1\,\times \,8$ and $2\,\times \,3$ RLML loop units implemented onto a piece of an FR4 printed circuit board (PCB) are designed, fabricated, and measured. The measured bandwidth ranges from 894 to 964 MHz for a $1\times 8$ array antenna with a reading area of $570 \,\,{\mathrm{ mm}}\times 150\,\,{\mathrm{ mm}}$ and 917–932 MHz for a $2\times 3$ array antenna with a reading area of $270 \,\,{\mathrm{ mm}}\times 240\,\,{\mathrm{ mm}}$. The reading distances of an $1\times 8$ array antenna for loop-like tag, dipole-like tag, and antimetal tag are achieved up to 22, 42, and 45 mm, respectively. For a $2\times 3$ array antenna, the reading distances of three tags are achieved up to 23, 43, and 40 mm. [ABSTRACT FROM AUTHOR]

The future terahertz (THz) communication is a promising solution to the high data rate and short-range communication due to its broad available bandwidth. However, the high propagation path loss and the limited output power of the amplifier require highly directive antennas with high antenna gain in order to realize a reasonable signal-to-noise ratio (SNR). An angle-of-arrival (AoA) estimation is necessary to adjust the main lobe direction of the antenna. A major challenge of the AoA estimation is the compromise between accuracy and time consumption due to the big number of main lobe directions. This paper proposes a novel efficient AoA algorithm, which utilizes the fact that the power-angular spectrums (PASs) of different frequencies are highly correlated and the THz communication devices are equipped with radio frequency (RF) frontends of lower frequency. By means of a rough and quick estimation at the lower frequency, the range of the true AoA can be significantly confined and the precise estimation can be efficiently carried out. To begin with, an antenna model with discrete main lobe directions is proposed and the relationship between antenna gain, half power beamwidth (HPBW), and number of main lobe directions is analyzed. The PAS correlation is then validated with channel sounding measurement. The two-step AoA estimation algorithm is derived, formulated, and finally validated with hardware demonstration. [ABSTRACT FROM AUTHOR]

Phased arrays are a form of steerable antenna whose architecture makes the system complex and generally expensive. Their versatility and performance motivates the development of alternative structures which allow us to obtain simplified designs as powerful as the original approach but significantly cheaper. In this paper, a novel technique which collects information only from one antenna during each radio frequency cycle but introduces a restriction in the working bandwidth has been proposed. By studying the effect of the sampling order and optimizing it using backtracking or a genetic algorithm, the bandwidth is increased up to almost four times the original value for small arrays, and up to 25% of the carrier frequency for large arrays using maximum delay sorting. Finally, effectiveness of nonperiodic sampling order, which converts interfering harmonics into distributed background noise, is also shown. [ABSTRACT FROM PUBLISHER]

When multiple radio frequency sources are connected to multiple loads through a passive multiport matching network, perfect power transfer to the loads across all frequencies is generally impossible. In this two-part paper, we provide analyses of bandwidth over which power transfer is possible. Our principal tools include broadband multiport matching upper bounds, presented herein, on the integral over all frequency of the logarithm of a suitably defined power loss ratio. In general, the larger the integral, the larger the bandwidth over which power transfer can be accomplished. We apply these bounds in several ways. We show how the number of sources and loads, and the coupling between loads, affect achievable bandwidth. We analyze the bandwidth of networks constrained to have certain architectures. We characterize systems whose bandwidths scale as the ratio between the numbers of loads and sources. The first part of this paper presents the bounds and uses them to analyze loads whose frequency responses can be represented by analytical circuit models. The second part analyzes the bandwidth of realistic loads whose frequency responses are available numerically. We provide applications to wireless transmitters where the loads are antennas being driven by amplifiers. The derivations of the bounds are also included. [ABSTRACT FROM AUTHOR]

Skin-mounted electronics is the new frontier for unobtrusive body-centric monitoring systems. In designing the wireless devices to be placed in direct contact with the human skin, the presence of the lossy body cannot be ignored because of strong electromagnetic interactions. In this paper, the performance of epidermal antennas, for application to radio frequency identification (RFID) links in the UHF band, was investigated by means of numerical simulations and laboratory tests on fabricated prototypes. The analysis demonstrates the existence of an optimal size of the antennas (from 3 to 6 cm for loops and from 6 to 15 cm for dipoles) and of upper bounds in the achievable radiation gain (less than −10 dB in the case of 0.5 mm thick application substrates) as a consequence of the balance between the two opposing mechanisms of radiation and loss. This behavior, which is controlled by the hosting medium, does not depend on the antenna shape, even if the loop layout permits considerably minimizing the device size. Even the conductivity of the antenna trace plays only a second-order role; low-cost inkjet printable paints with conductivity higher than 10^4 S/m are suitable to provide radiation performance comparable with the performance of copper-made antennas. Starting from the investigation of the above cited physical phenomena, including the effect of common classes of suitable substrate membranes, guidelines are finally derived for the optimal design of real RFID epidermal antennas. [ABSTRACT FROM PUBLISHER]

The approach, proposed in “Range-Angle-Dependent Beamforming of Pulsed-Frequency Diverse Array,” IEEE Trans. Antennas Propag. vol. 63, no. 7, pp. 3262–3267, Jul., 2015, is intended to achieve a quasi-static transmit beampattern with pulsed-frequency diverse array (FDA). The derived constraint is too rigorous; thus, FDA approaches the conventional phased array to some extent. This paper proposes an effective approach to resolve the time-varying issue of FDA at the receiver based on the model established in the above paper. The proposed approach exhibits a more flexible capability in receive beamforming as the constraint imposed on the pulse duration is released. [ABSTRACT FROM AUTHOR]

In this paper, a novel idea to reduce the number of active elements in a large periodic array is presented and demonstrated firstly by an E-plane microstrip array. High aperture efficiency can be achieved by making use of mutual coupling between array elements, and then the total production cost of the array is greatly decreased by reducing the number of active array elements and associated components. A simple way to control the mutual coupling of the microstrip array is given as an example in this paper. Simulations and measurements show that when the active element spacing is as large as 1.6\lambda0 (\lambda0 is the free-space wavelength at the operating frequency), a - 18.3 dB side lobe level (SLL) and an approximately equal aperture efficiency can be achieved. No obvious performance degradations are found compared to a conventional array with half the element spacing (0.8\lambda0) and an identical total size. Then, simulations and measurements of an H-plane sparse array are given based on the same idea, followed by the simulated results of a planar two-dimensional (2-D) one. [ABSTRACT FROM AUTHOR]

This paper presents the feasibility of deploying a large-scale tunable absorber based on discrete plasma-shells. The proposed conductor-backed absorber is realized by integrating ceramic gas-encapsulating chambers (plasma-shells) and a closely coupled lossy resonant layer that also serves as a biasing electrode to sustain the plasma. Two topologies comprising lossy inductive or capacitive layers are investigated to realize tunable microwave absorbers. The plasma is sustained by a sinusoidal radio frequency (RF) voltage source coupled directly through the walls of the plasma-shells. These active frequency-selective absorbers are analyzed using a transmission line approach to provide the working principle and its frequency tuning capability. By varying the voltage of the sustainer, the plasma can be modeled as a lossy, variable, frequency-dependent inductor, providing a dynamic tuning response of the absorption spectral band. A prototype plasma-tuned absorber is fabricated and measured in a free space environment to validate the concept. A good agreement between the equivalent circuit model, full-wave electromagnetic simulation, and the measurement results is obtained. [ABSTRACT FROM AUTHOR]

In this paper, a novel module named time-modulated amplitude-phase weighting with multiple branches (TMAPW-MB) is proposed to realize harmonic beamforming. The harmonic beamforming is then explored to eliminate the conventional digital phase shifters and digital attenuators in antenna arrays. The input and output of general linear frequency modulation (LFM) are investigated in a four-branch TMAPW-MB module. The analysis and numerical results exhibit the attractive capability in the suppression of unwanted harmonic component in radio-frequency (RF) channels. Then, the proposed TMAPW-MB modules are employed in an $X$ -band $1 \times 16$ Vivaldi antenna array for amplitude-phase weighting and, therefore, beamforming. The numerical results show that radiation patterns with −25.0 dB sideband levels (SBLs), −20.0 dB sidelobe levels (SLLs), and ±50° beam scanning range could be realized through the proposed TMAPW-MB module. Meanwhile, due to the removed quantization errors in traditional digital phase shifters and digital attenuators, the proposed module ensures real-time, continuous beam scanning, and precisely controlled SLL and SBL in antenna arrays. [ABSTRACT FROM AUTHOR]

Magnetic resonance imaging (MRI) is one of the most powerful imaging modality in clinics and is essential for the diagnosis of strokes through carotid artery imaging. The limiting factor for high-quality MRI is the signal-to-noise ratio (SNR) performance of the radio frequency (RF) coils. The current RF surface coils, however, are made of rigid or semiflexible materials with very limited bending properties. As a result, their SNR is limited because they cannot be placed very close to the imaging area, thus receiving noises from parts of the human body, which are not intended to be imaged. Taking advantage of the computerized embroidery and laser cutting technology, in this paper, we utilize electrotextile to design, fabricate, and measure multilayer RF coil array system for 3 Tesla (3T) MRI to improve the SNR performance. The proposed RF coil array system provides an ergonomic and high-performance solution to the 3T MRI systems. A roadmap to systematically design electrotextile RF coil arrays is proposed. RF coil array is characterized to have the accurate resonant frequency, good impedance matching, and low mutual coupling. In addition, magnetic field distribution, bending effects, and human body effects are also discussed. A systematic method to characterize the performance of the electrotextile pattern is studied and used to assist the development and performance characterization. Finally, the high resolution and high SNR images of various kinds of phantoms are obtained using the University of California at Los Angeles (UCLA) Antenna Lab electrotextile coil array after its integration with the 3T MRI scanners at UCLA David Geffen School of Medicine Translational Research Imaging Center. Compared with the conventional surface coil, more than 10 dB SNR increase is observed at the depth of 0.5 cm and 3 dB increase at the depth of 3 cm. [ABSTRACT FROM AUTHOR]

This paper describes 3-D numerical simulations and radio frequency (RF) measurements of wave propagation in microwave-heated magnetized plasmas of ion sources. Full-wave solution of Maxwell’s equations has been addressed through the finite-element method commercial software COMSOL. Our numerical model takes into account the strongly inhomogeneous and anisotropic magnetized “cold” plasma medium. The simulations reproduce the main features of the wave–plasma interaction of the flexible plasma trap (FPT) that recently came into operations at INFN-LNS. A two-pin RF probe has been ad hoc developed and used as a plasma-immersed antenna for measuring local wave electric fields in the FPT device. The measurements of plasma electron density and RF electric field, performed for different external magnetic field configurations, allowed direct comparison with the assumed simulation model. [ABSTRACT FROM AUTHOR]

This paper presents the design and validation of a tuned time-domain electric field probe for mapping of radio frequency (RF) exposures used during testing of magnetic resonance imaging (MRI) conditional medical devices. The probes were 5 and 10 mm short dipole antenna, developed as a tradeoff between spatial resolution, linearity, and sensitivity. The probes were tuned and matched at a center frequency of 127.6 MHz, which corresponds to the RF frequency for 3T MRI scanners. To improve the accuracy and sensitivity, an RF low noise amplifier with high gain and very low noise figure was developed, followed by distributed $\boldsymbol {\lambda /4}$ baluns along a triaxial cable to reduce the electric field pickup in the MRI environment. The probe was fabricated on a double-sided printed circuit board, FR4 thickness of 1.57 mm and a copper thickness of $35~\mu \text{m}$. Theoretical analysis was performed to calculate the exposed electric field from the real-time receive signals. To verify the probe performance finite-difference time-domain method simulations were compared to the actual measured electric fields. Developed probe was tested in a commercially available 3T RF exposure system to determine the probe dynamic range and linearity. [ABSTRACT FROM AUTHOR]

Ambient electromagnetic and RF energy is an ubiquitous energy resource that is found everywhere but difficult to harvest because of the time-varying orientation of incoming radiations and the low efficiency of RF rectifiers resulting from a low input power density operation. In response to these two challenges that come with the design of rectennas for ambient energy harvesting, this paper discusses a figure of merit to evaluate the rectenna performance that combines the rectification efficiency, the radiation efficiency, and the spherical coverage of the antenna. To illustrate this proposed figure of merit, a rectenna with a strengthen RF-to-dc efficiency and also a full spatial coverage is designed using beam-forming networks. The resulting system is demonstrated for a wireless sensor mounted on a pole. To maintain a reasonable size of the prototype, a miniaturization technique of a $4 \times 4$ Butler matrix (BM) is investigated, which allows for the reduction of the footprint of the BM by 2 compared to its classical microstrip design counterpart at 2.4 GHz. The overall rectification efficiency and dc power patterns are reported for the elevation plane of one multidirectional rectenna at 2.4 GHz for an incident power density of $0.45\,\,\mu \text {W}\cdot \text {cm}^{-2}$. It is shown that the system can triple the dc output power compared to a three-dipole counterpart occupying the same area. Furthermore, while the maximum harvesting capability at $1 \,\,\mu \text {W}\cdot \text {cm}-^{2}$ does not exceed $28\pi $ %.steradian in the literature, it reaches $43.6\pi $ %.steradian with the proposed demonstrator. [ABSTRACT FROM AUTHOR]

Impedance loading is a common technique traditionally used in the RF to enhance the performance of an antenna, but its application in the optical regime is not as rigorously studied. This is mainly due to a lack of exact analytical expressions that can be used to rapidly predict the radiation properties of loaded nanoantennas. This paper will derive a set of useful analytical expressions for the far-field radiation properties of loop antennas loaded with an arbitrary number of lumped impedances that are valid from the RF-to-optical regimes. The analytical expressions will be validated with full-wave solvers and can be evaluated more than $100\times $ faster. The ability to perform such rapid evaluations enables, for the first time, large-scale single-objective and multi-objective optimizations. A series of optimizations reveal that electrically small superdirective antennas can be achieved at a variety of far-field angles through capacitive loading, paving the way for a pattern reconfigurable antenna. In addition, gains of greater than 3 dB can be achieved for electrically small antennas over a fractional bandwidth of 28%. Finally, it is shown that impedance loading can be used to achieve circularly polarized radiation from a single loop. [ABSTRACT FROM AUTHOR]

Reverberation chambers (RVCs) are used to provide an efficient and reliable testbed for emulating the wireless channel effects in the laboratory environment. In the literature, various techniques have been studied to enrich the channel emulation capabilities of RVCs. However, insufficient control of some of the emulated channel characteristics (such as time and frequency dispersions) still leaves an open problem for RVC-based channel emulators. In this paper, novel channel emulation techniques by using RVC are introduced to enhance the flexibility in controlling the emulated time and frequency dispersions of the channels. Surface acoustic wave delay lines and power controllers (i.e., amplifiers and attenuators) are used to add degrees of freedom for manipulating the power delay profile during delay spread emulation. This technique also overcomes the challenge of achieving higher delay spread values with benchtop-sized RVC. The second technique is aimed to control the Doppler spectrum shape emulated in the chamber by judiciously placing radio frequency absorbers around the mechanical stirrer. Experimental and mathematical analyses are conducted to evaluate the proposed techniques, and the results show that the proposed methods can improve the channel emulation in the RVCs. [ABSTRACT FROM AUTHOR]

An $N$ -element array with receivers subject to an $N$ -point spatial fast Fourier transform (FFT) leads to $N$ directionally orthogonal radio frequency (RF) beams. FFTs are fast algorithms for computing the $N$ -point discrete Fourier transform (DFT) at reduced complexity. The brute-force computation of a DFT requires $\mathcal {O}(N^{2})$ multiplications while an FFT provides the same computation at $\mathcal {O}(N \log N)$ multiplications. The digital chip area and power consumption of the DFT computation are still dominated by the multipliers required by the FFT used. In this paper, an approximation to the 16-point DFT is proposed which maintains mathematical properties close to the ideal 16-point DFT to obtain 16 RF beams by computing an approximate spatial DFT in every clock cycle at significantly lower area and power in the digital realization. The proposed approximation can be implemented using FFT-like fast algorithms that are multiplierless, thereby further reducing the digital chip area and power consumption associated with multiplication in a conventional FFT approach to zero. A 16-beam beamformer employing a 16-element linear array of patch antennas, direct-conversion receivers, and a Xilinx Virtex-6 field-programmable gate array-based real-time digital back-end clocked at 240 MHz are described as an example realization of 16 complex-valued (IQ) receive-mode RF beams, centered at 2.4 GHz with 120 MHz of bandwidth per beam. [ABSTRACT FROM AUTHOR]

In this paper, Bragg and non-Bragg scattering mechanisms and frequency shifts from time-varying periodic water wave are studied from three aspects: wave tank measurements with an ultrahigh frequency radar, numerical simulation using the method of moments, and theoretical derivation applying the small perturbation method. The scattering field, radar cross section (RCS), and frequency shifts are discussed in both horizontal and vertical polarizations. The wave tank observations show that backscattering enhancement occurs when water wavelength is an integer multiple of Bragg wavelength, and there are several Doppler harmonics with frequency shifts of water-wave frequency and its integer multiples. Numerical simulations indicate that these Doppler harmonics except the one associated with the water-wave phase velocity are caused by the water surface edge effect. Moreover, theoretical analyses, numerical simulations, and wave tank experiments all show a clear exponential relationship between backscattering RCS and wave height. In addition, we further analyze the bistatic scattering and find that the scattering field is composed of plane waves propagating in the directions determined by water wavelength and radio wavelength, the bistatic frequency shift is the harmonic frequency of water wave, and the bistatic RCS also has an exponential relation with water-wave height. [ABSTRACT FROM AUTHOR]

Electrically small antennas (ESAs) can be passively matched only over very narrow bandwidths and the resulting antennas have low gains. These are the major limiting factors for ESAs used in transmit applications, especially at high-frequency (HF) and lower Very HF frequency bands. This paper discusses the challenges of transmit ESA matching circuit design and the design process of a new non-Foster transmit matching architecture for electrically small monopole antennas that achieves wide bandwidth, high transmission efficiency (transducer power gain), and stability at the same time. The proposed circuit is composed of a current buffer (for high isolation), a transformer (for real-part matching), and a negative impedance converter (for imaginary-part matching). The measured −6 dB (−10 dB) $|S_{11}|$ fractional bandwidth of the proposed non-Foster transmitting system is 110% (39%), while the maximum bandwidth that can be achieved is 0.076% (0.047%) when matched with the conventional passive matching. The transmission efficiency of the system is improved by as much as 34.4 dB compared to the same antenna without the proposed non-Foster matching circuit, and it retains an enhanced efficiency over the entire frequency band of operation (26–89 MHz). The system remains stable within this frequency band. The measurement results of the compact and broadband transmitting antenna prototype verify the design concept. [ABSTRACT FROM AUTHOR]

This paper presents the design of dual-port subarray modularization integrating a three-port circulator, realized by using substrate integrated waveguide (SIW) technology, and a subarray of antenna elements for dual uplink and downlink beamforming. In particular, a $2\times2$ element module is implemented and fabricated on a standard printed circuit board substrate to demonstrate its radiation characteristics and operational mechanism. In this design, the SIW structure is used to reduce the radiation from microstrip lines at millimeter-wave frequencies and avoid contaminating the radiation patterns of subarrays. Greater than 5 dBi suppression has been achieved for good compatibility with the integration of radio frequency devices. The measured results show that the subarray modularization improves the insertion loss and results in an excellent isolation for independent transmitting and receiving beamforming network implementation. Numerical and experimental results are shown to verify the performance of these designs. [ABSTRACT FROM AUTHOR]

This paper presents a dual-polarized conformal loop antenna for capsule endoscopy systems. The antenna is designed on the capsule shell, with two orthogonal counterparts to generate dual-polarized radiation. This design exhibits broad impedance bandwidths of 271 and 247 MHz for each port, covering the entire Medical Device Radiocommunications Service band as well as the 433.05–434.79 MHz Industrial Scientific Medical band. Due to the highly orthogonal current distributions of the proposed structure, an isolation of higher than 33 dB is achieved among the operating frequency bands in measurement. The dual-polarized antenna also shows an omnidirectional radiation property and a low cross-polarization level of around −40 dB, which helps to enhance communication quality, regardless of position and orientation. Whether other electronics and biocompatible coatings are included or an electromagnetic interference shielding is considered, the antenna still presents a robust performance. The cubic tissue phantom model is utilized for the initial design and optimization of the antenna structure. Moreover, the CST voxel human model is applied to study the conformal antenna in a realistic environment. Measurements are implemented using a solid tissue-mimicking phantom, and performances consistent with theoretical predictions are achieved. Finally, the wireless biotelemetry property is evaluated via the calculation of the communication link budget with an external antenna. [ABSTRACT FROM AUTHOR]

This paper proposes a new single-radio-frequency (RF) compact beamforming scheme that forms the beam by adjusting variable loads. In a conventional phased-array beamformer, half-wavelength antenna spacing is required in order to achieve a reasonable performance without taking into account the mutual coupling. On the other hand, a conventional electronically steerable parasitic array radiator (ESPAR) system requires smaller antenna spacing, but it provides a lower beamforming performance compared to the phased-array beamformer. The proposed method delivers a beamforming performance close to that of a phased array, without having any constraints on the antenna spacing. In the proposed structure, the RF front-end is connected to each antenna via a tunable load, whereas all antenna elements are also terminated by other tunable reactive loads. Simulation results confirm that the proposed scheme provides better interference suppression and achieves more average capacity compared to ESPAR technique. To further validate the beamforming ability of the proposed method, a testbed with 1/20 of the wavelength antenna spacing was fabricated. A good agreement is observed between the measurement and simulation results. [ABSTRACT FROM PUBLISHER]

A new in-phase/quadrature (I/Q) complex modulation technique to realize single-sideband time-modulated phase-only weighting on an array antenna is proposed. Based on modulating rectangular pulses and trapezoidal pulses, the theories of generating a scanning beam at the single positive sideband are presented. It was found that using trapezoidal pulse modulation can obtain excellent performance in power efficiency and undesired sideband level than using rectangular pulse modulation. Unlike conventional time-modulated antenna array where each array element is connected to a RF switch, in the proposed scheme, each element is connected to an I/Q channel modulator which is composed of two Wilkinson power dividers, two RF switches, two 0/\pi phase shifters in RF channels, and one \pi/2 fixed-phase shifter in the control circuit. By properly controlling both the phase shifters and the switches in time domain, the single-sideband modulation can be realized with the uniform amplitude and variable phase in frequency domain. The I/Q modulator acts as a phase shifter and can be used to form a phased array. In this paper, the main efforts have been paid to find the pulse sequences to suppress the spurious bands. To validate the proposed technique, simulated results of an eight-element single-sideband time-modulated linear array obtained from the array factor and full-wave analysis are reported. In the case of using rectangular pulses, the power efficiency of the scanning beam is 91.47% and the peak level of the highest undesired harmonic is -13.98\;\mathrmdB . In the case of using trapezoidal pulses, the best values can reach 99.79% and -27.98\;\mathrmdB . [ABSTRACT FROM PUBLISHER]

The metasurface (MS) antenna is an important microwave component in the communication system due to its unique beam radiation capability. However, current studies mainly pay attention to improve the performance of the main lobe of the MS antenna, leaving the sidelobe unexplored although it is also essential in some practical applications. In this paper, several phase- and amplitude-control reflected MSs have been proposed to simultaneously manipulate the antenna’s main lobe and sidelobes. The MSs consist of modified I-shaped particles which can independently manipulate the phases and amplitudes of the cross-polarization waves by changing the split size and orientation, respectively. A focusing phase distribution and different Taylor amplitude distributions have been fixed on the MSs. By illuminating the MSs with a self-made antenna, we have successfully designed four MS antennas. For the first antenna, we solely pay attention to improve the main lobe and achieve a high gain of 20.7 dB at 10 GHz. For the other three antennas, we also aim to manipulate their sidelobes. The resultant sidelobe levels (SLLs) are about −25 dB in the $xoz$ plane for the second antenna, −29 dB in the $yoz$ plane for the third antenna, and both of the former two characteristics for the fourth antenna. Compared with the first MS antenna, the last three antennas suffer from gain reductions of 2, 1.7, and 3 dB, respectively. These proposed MS antennas provide a new way to manipulate both main-lobe levels and SLLs and also greatly promote the integration of MSs and antennas. [ABSTRACT FROM AUTHOR]

Conducted cable setups have been dominantly utilized in the industry for performance testing of multiple-input-multiple-output (MIMO) terminals. The wireless cable method, which can achieve cable connection functionality without actual radio frequency cable connections, is a promising alternative. To date, the wireless method has been only discussed for $2\times 2$ MIMO terminals in the literature. However, the algorithm is not directly applicable for high-order MIMO terminals, due to the high computation complexity to determine the calibration matrix and high system cost to implement the calibration matrix. In this paper, an efficient particle swarm optimization (PSO) algorithm is proposed to determine the calibration matrix for high-order MIMO systems. Furthermore, a novel implementation of the calibration matrix in the radio channel emulator is proposed, which can significantly reduce the system cost. To validate the proposed algorithm, two MIMO mockups, each equipped with four antennas, were measured in an anechoic chamber. The measured results demonstrated the effectiveness of the PSO algorithm to establish wireless cable connections for $4\times 4$ MIMO terminals. [ABSTRACT FROM AUTHOR]

A low profile and metal-mountable ultrahigh frequency (UHF) tag antenna, which has a geometrical dimension of 47 mm $\times47$ mm $\times0.8128$ mm ($0.143\lambda \times 0.143\lambda \times 0.0025\lambda$), is proposed. It has four identical open stubs attached to the circumference of a circular patch for performing coarse- and fine-tunings on the tag’s resonant frequency without affecting the read performances. At the center of the patch, a rectangular slot of $0.121\lambda \times 0.009\lambda \times 0.0025\lambda $ is etched for accommodating a microchip. Good impedance matching between the tag antenna and the microchip can be easily achieved by changing the dimensions of the center slot. It is also found that the introduction of the slot is crucial for bringing down the tag’s resonant frequency to the UHF band. In this paper, an equivalent circuit is also derived for studying the impedance characteristics of the proposed tag antenna. Measurements have been carried out with the use of an effective isotropic radiated power of 4 W, and maximum read distances of 3.45 m and ~1.7 m are attainable, respectively, when the tag is placed on metal plate and dielectrics. It is found that the resonant frequency of the proposed tag antenna is stable and not affected much by its backing objects. [ABSTRACT FROM AUTHOR]

In this paper, a compact patch array antenna with reconfigurable polarizations and 2-D switchable beams is developed based on the spatial phase shift technique. A compact element antenna is proposed with switchable ports for implementing the spatial phase shift technique, which is composed of a square driven patch, two sets of perturbation segments connected to the driven patch via p-i-n diodes for polarization diversity, a parasitic patch, and three switchable feeding probes integrated with a designed single-pole triple-throw switch. For circular polarized waves, individual excitation at the three feeding probes can obtain a 90° or 180° phase difference because of the spatial relative position between the feeding probes. The radiation beam and polarization of the $2{\times} 2$ array antenna can be dynamically reconfigured among nine beams and dual-circular polarization modes by properly switching these feeding probes and perturbation segments. In addition, the 3 dB axial-ratio bandwidths are effectively enhanced by using the spatial phase shift technique. To validate the proposed concept, a prototype operating at 4.8 GHz is designed, fabricated, and measured. The measured 10 dB return loss bandwidths for all operating states can cover a frequency band of 4.63–5.07 GHz with measured gains ranging from 7.5 to 10.5 dBic. [ABSTRACT FROM AUTHOR]

This paper presents the details and results of double-directional propagation measurements carried out in two indoor office environments: a semi-open, cubicle-type office space and a more traditional work environment with closed offices. The measurements cover seven frequency bands from 2.4 to 61 GHz, permitting the propagation characteristics to be compared over a wide range of candidate radio frequencies for next-generation mobile wireless systems, including ultra high frequency and millimeter-wave bands. A novel processing algorithm is introduced for the expansion of multiband measurement data into sets of discrete multipath components. Based on the resulting multipath parameter estimates, models are presented for frequency-dependent path loss, shadow fading, copolarization ratio, delay spread, and angular spreads, along with their interfrequency correlations. Our results indicate a remarkably strong consistency in multipath structure over the entire frequency range considered. [ABSTRACT FROM AUTHOR]

This paper investigates on an effective approach to enhance frequency sensitivity of the classic wideband EH1-mode microstrip leaky-wave antenna (MLWA). Nonuniform loadings of shorting pins are installed into the traditional EH1-mode microstrip line, which provide a unique discontinuity affecting the mutual intersection of the passband and stopband, thus generating a newly created stopband within the continuous EH1-mode fast-wave region. After broadening the stopband, the slope of normalized phase constant is remarkably raised up and the original wide bandwidth of the operating region is effectively reduced, resulting in a dramatically enhanced frequency sensitivity. An equivalent-circuit model of the proposed EH1-mode MLWA with nonuniform loading is built up at first, then its nonuniform loading discontinuity effect on dispersion transformation between passband and stopband is further explicated. Moreover, full-wave numerical extraction is also conducted to accurately demonstrate the modified phase constant under varied lateral pins’ loading positions and strip widths. After thorough discussion, one set of optimized parameters are selected into antenna design. Compared with the conventional counterpart, the proposed EH1-mode MLWA with nonuniform loading of shorting pins notably increases the frequency sensitivity from 0.038°/MHz to 0.135°/MHz. [ABSTRACT FROM AUTHOR]

This paper presents an extremely low-profile frequency-tunable composite antenna with enhanced gain and an omnidirectional radiation pattern. The antenna is based on a recently developed very short two-legged monopole antenna and consists of one driven element and eight identical parasitic coupled elements. By utilizing the mutual coupling between elements, the radiation resistance of the antenna is significantly increased, leading to considerable gain enhancement and better matching performance. The composite antenna has the same height as the single monopole ( $\lambda _{0}/150$ ) but a larger lateral dimension ( $\lambda _{0}/16$ ). The performance of the proposed antenna is compared with that of a single monopole antenna. It is shown that the peak gain of the composite antenna is about 10 dB higher than that of the monopole antenna. The design is then modified for frequency tunability to achieve frequency agility and higher bandwidth for certain modulations like frequency hopping spread spectrum. The fractional bandwidth of the tunable antenna is 3.32%, and the gain is further improved at higher frequencies. A new method of effectively measuring a monopole antenna with an electrically small ground plane using cascaded transformers is also presented and discussed. [ABSTRACT FROM AUTHOR]

A triple band differential rectenna for RF energy harvesting applications is proposed in this paper. The rectenna is designed to operate in frequency bands of universal mobile telecommunication service (2.1 GHz), lower WLAN/Wi-Fi (2.4–2.48 GHz), and WiMAX (3.3–3.8 GHz). For designing the proposed rectenna, first a differentially fed multiband slot antenna that works as the front-end receiving unit is designed, fabricated, and tested to check its performance. It is observed that a peak antenna gain of 7, 5.5, and 9.2 dBi is achieved at 2, 2.5, and 3.5 GHz, respectively. In the next step, a triple band differential rectifier is designed using the Villard voltage doubler where interdigital capacitors (IDCs) in lieu of lumped components are used. The full rectifier circuit comprising of the rectifying unit and impedance matching circuit is fabricated and tested to check its performance in the desired bands. The peak RF-dc conversion efficiency of 68% is obtained using the three-tone measurement. In the final stage, both antenna and the rectifier circuit are integrated through SMA connecter in order to implement the proposed rectenna. Measurement of the proposed rectenna shows an approximate maximum efficiency of 53% at 2 GHz, 31% at 2.5 GHz, and 15.56% at 3.5 GHz. [ABSTRACT FROM AUTHOR]