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This paper presents an innovative method for fabricating reflectarray antennas using inkjet printing technology on flexible substrates, markedly enhancing integration and manufacturability compared to traditional PCB methods. The technique employs inkjet printing to deposit conductive inks directly onto a flexible polyethylene naphthalate (PEN) substrate, seamlessly integrating feed and reflectarray components without complex assembly processes. This streamlined approach not only reduces manufacturing complexity and costs but also improves mechanical flexibility, making it ideal for applications requiring deployable antennas. The design process includes an origami-inspired folding of the substrate to achieve the desired three-dimensional antenna structures, optimizing the focal length to dimension ratio (F/D) to ensure maximum efficiency and performance. The feed and the reflectarray geometry are optimized for an F/D of 0.6, which achieves high gain and aperture efficiency, demonstrated through detailed simulations and measurements. For normal incidence, the configuration achieves a peak gain of 9.3 dBi and 48% radiation efficiency at 10 GHz; for oblique incidence, it achieves 7.3 dBi and 40% efficiency. The study underscores the significant potential of inkjet-printed antennas in terms of cost-efficiency, precision, and versatility, paving the way for new advancements in antenna technology with a substantial impact on future communication systems. [ABSTRACT FROM AUTHOR]

In this paper we describe a class of printed patch antennas that involve 3 metal layers completely integrated within a single dielectric, which are of very high efficiency and significant gain. The antennas presented have dual linear polarization and sufficient impedance matching bandwidth to be suited for sub-6 5G applications, producing advantageous antenna arrays monolithically integrated into a single planar PCB that are thin and lightweight. The feeding mechanism of the antennas is through an intermediate dedicated metal layer designed appropriately as a coupler, coupling the input power at the feeding ports of the structure to the antenna without a direct metal contact to the radiating patch. The multilayer design allows monolithic integration of multi-element combiners and phase shifters within the PCB of antenna arrays and results in compact monolithic scanning arrays. Theoretical full-wave simulations and measurements of fabricated prototypes are in very good agreement and validate the proposed designs. [ABSTRACT FROM AUTHOR]

Handshake mechanism plays a significant role in neighbour discovery (ND) algorithms in mobile ad hoc networks (MANETs) with directional antennas. In this letter, aiming at 1) the MANET scenario of uniformly‐distributed nodes and 2) the random beam‐scanning manner, the upper and lower bounds of ND time of the network are theoretically derived for both 1‐way and 2‐way. This analysis indicates that the ND time is approximately linear growth with NDT‐Mutual, where the NDT‐Mutual is defined as the expected time for a node to complete mutual discovery with its neighbours. Then the theoretical expression for NDT‐mutual is derived, which is a function of the number of nodes. The asymptotic analysis reveals that when the number of nodes is very small, the NDT‐mutual of 2‐way is lower than 1‐way, while 1‐way performs better when the number of nodes approaches infinity. The number of nodes when 1‐way and 2‐way perform as well is further presented, which can be utilized to guide the selection of handshake mechanism in MANETs with directional antennas. Simulation results demonstrate the accuracy of our modelling for ND and the effectiveness of using NDT‐mutual to approximate who performs better with 1‐way and 2‐way. [ABSTRACT FROM AUTHOR]

Abstract Handshake mechanism plays a significant role in neighbour discovery (ND) algorithms in mobile ad hoc networks (MANETs) with directional antennas. In this letter, aiming at 1) the MANET scenario of uniformly‐distributed nodes and 2) the random beam‐scanning manner, the upper and lower bounds of ND time of the network are theoretically derived for both 1‐way and 2‐way. This analysis indicates that the ND time is approximately linear growth with NDT‐Mutual, where the NDT‐Mutual is defined as the expected time for a node to complete mutual discovery with its neighbours. Then the theoretical expression for NDT‐mutual is derived, which is a function of the number of nodes. The asymptotic analysis reveals that when the number of nodes is very small, the NDT‐mutual of 2‐way is lower than 1‐way, while 1‐way performs better when the number of nodes approaches infinity. The number of nodes when 1‐way and 2‐way perform as well is further presented, which can be utilized to guide the selection of handshake mechanism in MANETs with directional antennas. Simulation results demonstrate the accuracy of our modelling for ND and the effectiveness of using NDT‐mutual to approximate who performs better with 1‐way and 2‐way.

This study investigated the effects of the difference of beam-pointing directions on the Doppler measurements from a spaceborne Ku-band precipitation radar. The Doppler measurement system was a displaced phase center antenna (DPCA) using two antennas that are displaced in the along-track direction. When the beam-pointing directions of the two antennas deviate, Doppler measurements can degrade. The well-known formula for the uncertainty in Doppler velocity measurement is extended to include the effect of the difference of the beam-pointing directions, and a simulation confirmed the formula. The results show that the uncertainty increases from 0.21 to 0.38 m/s for a case of $S/N = 20$ dB when one antenna footprint center is displaced from the other by half of the half-power beamwidth. [ABSTRACT FROM AUTHOR]

Traditional high-gain antennas, like parabolic reflectors and phased arrays, are challenging to implement on small satellites (SmallSats) due to their reduced volume when stowed for launch. Therefore, a deployable volume-efficient Miura-ori (VEMO) reflectarray aperture is presented that achieves high gain (35.3-dBi) and high volume efficiency (96%). The design is based on a modified Miura-Ori origami folding pattern that allows for ultra-high volume efficiency and simple deployment. In its stowed state, the aperture fits in a cuboid volume (8.0-cm $\times8.0$ -cm $\times2.5$ -cm) that occupies just 16% of the volume of a 1U CubeSat. The aperture is deployed by releasing stored energy from strategically placed neodymium magnets in the magnetic longitudinal offset (MaLO) hinge system. The aperture has a large reflecting surface area (24.8-cm $\times24.8$ -cm) that is stabilized by the MaLO hinges. The antenna operates in the Ka-band (38-GHz), and provides low side lobe level (−17.5-dB), high cross-polarization discrimination (37.1-dB), and acceptable efficiency (27.4%). The main advantage of the proposed VEMO RA antenna is that it achieves high gain with a design that is straightforward, low-cost, and easily deployable while occupying only a small portion of the payload volume. These characteristics are ideal for many SmallSat applications, such as satellite communications, remote sensing, and deep space research.

A rectangular metallic leaky waveguide loaded with liquid crystals (LC) and operating in its fundamental TE mode at 1 THz is proposed to mitigate the well-known trade-off between directivity and tunable angular range of dynamic beamscanning antennas. The radiating aperture consists of a partially reflecting sheet (PRS) realized through a one-dimensional array of longitudinal slots etched on one of the metallic walls of the waveguide to efficiently couple with the propagating TE leaky mode. The antenna performance is evaluated by defining suitable figures of merit that take into account the beamscanning feature and the gain peak. These figures of merit are evaluated for different combinations of the antenna design parameters. After optimization, a tunable angular range of about 28° is reached, while maintaining a gain of around 7 dBi. A leaky-wave analysis of a simplified model of the structure allows to design the antenna without resorting to computationally expensive optimization processes. More rigorous models are then considered to accurately analyze the LC dynamics and the radiating properties of the PRS. The three-dimensional structure is finally validated through full-wave simulations, showing a remarkable agreement with the theoretical predictions obtained with the simplified model.

This paper introduces a procedure for multi‐objective optimisation, for compact wideband superdirective antenna array design, using network characteristic modes. The two main objectives considered are the maximisation of the directivity in the end‐fire direction, simultaneously with the minimisation of the input reflection coefficient in a large bandwidth. The proposed method is used thereafter for designing a three‐element superdirective small Yagi antenna, with an overall dimensions of 0.4λ × 0.37λ × 0.18λ. The length of this mini yagi antenna is 60% shorter than a classical one, with the same performances of gain and bandwidth. The measured results show that the antenna bandwidth, maximum gain, and efficiency are of 11.2%, 8 dBi, and 95% respectively. [ABSTRACT FROM AUTHOR]

A broadband, unidirectional, phased array is proposed. Single antenna element of the prototyped array is a broadband windmill-like shaped antenna. Four elements of this antenna are fed by a power divider and are configured in a novel semicircular-shaped, angular-phased array. A phase difference of $\pi $ /4 was introduced by optimizing the angular path difference among four antenna elements in the array. Total 21.5% of size reduction is achieved by introducing angular-phased array in comparison with the conventional linear-phased array. The array of radius 65 mm is prototyped on the Rogers RO4232 substrate for the operating frequency range 9.35–42.89 GHz. The proposed array configuration can maintain an average gain of 8.54 dB and average radiation efficiency of 69.71% in the entire operating frequency range while exhibiting a peak gain of 11.86 dB. The presented array exhibits highly directional behavior in the endfire direction, with the 3-dB angular beamwidth in the range from 15.1° to 76.7°. To the best of the authors’ knowledge, any comparable state-of-art has not been reported yet in the literature that could provide such a large impedance bandwidth in sub-mmWave range. The proposed array may find its applications for future indoor and outdoor short-range communication networks. [ABSTRACT FROM AUTHOR]

In this letter, a fast multinull steering technique is demonstrated by a linearly polarized linear antenna array. The new technique is based on the electric fields generated by each antenna element on a far-field arc of the antenna array, which are extracted in advance by simulation or measurement. The power transmission efficiency is used as the performance index to control the pattern nulls. By solving a matrix eigenvalue equation, in which the matrix is made up by selecting the corresponding electric fields on different positions on the arc standing for the null directions, the eigenvector corresponding to the zero eigenvalue is identified and used as the optimized distribution of excitations (ODEs) for the antenna array. Single-null and multinull patterns are tested from the ODEs and the results indicate that the number and directions of the multiple nulls can be controlled in the elevation space. The simulated and measured 2-D null patterns are shown in good agreement. [ABSTRACT FROM AUTHOR]

This letter proposes a new ultrahigh frequency (UHF) flexible planar biconical antenna design method for partial discharge (PD) detection in gas-insulated switchgear. Continuous transverse branching increases the electrical length to expand the bandwidth and miniaturize the antenna. For comparison of the effects of the lateral branch structure model, the optimized dimensions of both designed antennas are 150×150 mm, and both use polydimethylsiloxane, flexible material with a thickness of 0.5 mm, as the substrate. The results show that the S11 of the antennas is less than −10 dB in the bands of 0.3–0.5, 0.64–1.25, and 1.4–3.0 GHz, and the planar biconical antennas still maintain good bandwidth and gain under different bending radii of 0, 150, 350, and 500 mm. The PD test platform was built to test the antenna. The experimental data show that the two flexible antennas designed in this letter can detect the PD-radiated electromagnetic wave signal with a discharge of 15 pc before and after the bending deformation and meet the UHF partial discharge detection requirements. [ABSTRACT FROM AUTHOR]

This letter proposes a wideband high-gain left-/right-handed circularly polarized (L/RHCP) patch antenna using a mirror flip approach. The antenna consists of a ring-shaped metal square patch with truncated corners and a two-port feed network in a simple construction. The dual-port feed and truncated corners realize a wide axial ratio bandwidth (ARBW). Moreover, the L/RHCP is easily changed by mirroring the flipped feed network without redesigning the overall topology. The impedance bandwidth (IBW) (S11 < −10 dB) of 58.6% (2.97–5.43 GHz)/47.6% (2.8–4.55 GHz) and 3 dB ARBW of 44.3% (3.09–4.85 GHz)/34.6% (2.96–4.2 GHz) are achieved for L/RHCP antenna. Finally, two prototypes were fabricated and measured for validation. The proposed L/RHCP patch antenna was found to be insensitive to feed position. The IBWs of the proposed LHCP and RHCP antennas showed a wide degree of overlap. The low profile, high gain, small size, and higher robustness of our antenna than traditional patch antennas demonstrate the advantageous of our design for communication, power transmission, and medical fields. [ABSTRACT FROM AUTHOR]

The papers in this special section focus on the state-of-the-art of applied and analytical research, including the latest technological advancements and the use of innovative materials and methods to disrupt the way beam-steering antennas are designed and to reveal novel approaches for the design and analysis of antennas for the next generation of satellite communication systems. [ABSTRACT FROM AUTHOR]

A novel low-cost high-gain beam-steering antenna terminal topology for global data connectivity with low-earth-orbit satellite constellations in the UHF 900 MHz ISM band is reported. It is shown how a simple printed-circuit-board half-width microstrip leaky-wave antenna (LWA) with passive frequency-beam scanning capacity provides an energy-efficient scanning method avoiding more power-demanding and expensive beam-steering techniques (such as electronic scanning, mechanical rotation, or beam switching). For that, iridescent channel-agile operation and cross-layer optimization are needed. As an example, two single-element LWAs and an LWA array are designed to scan the celestial vault with an angular elevation scanning range of 130° and using the available LoRaWAN protocol channels from 902 to 928 MHz. It is reported a scanned-beam peak gain above 9 dBi for the compact printed-circuit LWA array design with 55 cm × 27 cm aperture dimensions. [ABSTRACT FROM AUTHOR]

A variety of novel orbital angular momentum (OAM)-based communication or sensing systems have attracted much attention over the past decade, the superiority is brought about by nothing other than its orthogonality or vorticity. Plane spiral OAM (PSOAM) mode-group (MG) technique as a reconfigurable beamforming method can be used for building a multiple-in-multiple-out (MIMO) system to achieve the reduction in subchannel correlation, which benefits from the spiral phase distribution within the mainlobe. However, the demultiplexing process still depends on the MIMO algorithm, in which the receiver complexity is same as the conventional MIMO systems. In this article, a PSOAM MG orthogonal multiplexing communication link at the X-band using partial arc sampling receiving (PASR) scheme has been demonstrated experimentally. The MG channels have a good isolation of about 15 dB, and the existing performance can ensure the reliable 16-QAM wireless transmission. Besides, a real-time dual-channel video transmission experiment has been carried out to verify the channel isolation caused by MG’s orthogonality intuitively. More importantly, the demultiplexing procedure using the PASR scheme can be implemented by simple analog phase shifting operation with a lower receiver complexity compared with the conventional MIMO systems. [ABSTRACT FROM AUTHOR]

We propose a wideband multibeam circular array antenna for directional networking applications at VHF/UHF frequencies. We introduce a directional antenna element by combining the radiation patterns of a monopole and a loop element. Combining the figure-eight-shaped and omnidirectional radiation patterns of the loop and the monopole antenna results in a directional antenna element. We demonstrate that a significantly larger bandwidth can be achieved by moderately relaxing the size constraints placed on the antenna element. A circular array consisting of eight of these radiating elements was developed. This array has a low profile, fitting in a 4 $\times $ 4 $\times $ 1 ft3 volume. The antenna shows a well-matched frequency response with S11 below −10 dB over an octave frequency bandwidth of 225–450 MHz. The good impedance matching of the array is maintained if the array elements are individually excited (e.g., to provide eight beams) or if multiple adjacent elements are excited together to provide fewer, more directional beams. We validated our simulation results by fabricating and experimentally characterizing a 3:1 scaled model of the proposed antenna. The proposed array can achieve gain values of 6–8 dBi within its operational bandwidth if each radiating element is excited individually. This gain increases to 10–12 dBi by exciting three adjacent elements. [ABSTRACT FROM AUTHOR]

This article proposes a single-layered electronically tunable annular surface plasmon polariton (SPP)-based leaky-wave antenna at 60 GHz for continuous azimuthal beam scanning. Initially, the groundless annular SPP transmission line is designed having specialized feeding mechanism which is operational at 60 GHz under SPP mode. Periodic radiating semicircular patches are incorporated into the vicinity of the TL that form an additional momentum caused to convert the slow-wave SP mode to radiating leaky-mode and placement of extra metallic ground makes the radiated beam unidirectional. Furthermore, implementation of the varactor diodes with imposed suitable switching conditions make the geometry electronically tunable to achieve a continuous complete azimuth coverage. The theoretical predictions, numerical simulations and the experimental validations of the proposed structure show good agreement. The proposed antenna show an overall scanning range of 275°, peak gain of 19.9 dBi with average sidelobe level of −10 dB and minimum cross polar level of −20 dB. Availing the benefits from the low-profile compactness and improved performance, the proposed groundless LWA appears as a promising candidate for integration in 60-GHz mm-wave applications. [ABSTRACT FROM AUTHOR]

A spoof surface plasmon polariton (SSPP) antenna with dual-band endfire gain and small frequency ratio (FR) is presented. It uses an asymmetric structure to support two independent surface waves with different nonlinear phase constants. Due to the different phases of the superposed $E$ -field at the terminal of the proposed structure, dual-band endfire gains and endfire radiation null are achieved. The odd- and even-mode feedings, providing additional phase differences, are also investigated. Three prototypes, with FRs of 1.06, 1.11, and 1.5, are designed, simulated, and measured, respectively. All results validate the mechanism and demonstrate that the FR of the proposed antenna can be achieved easily in the range of 1.06–1.5 by choosing different unit cells and feeding modes. Moreover, the measured endfire out-of-band gain suppression, over 17 dB, presents a good directional interference inhibition. This small-FR endfire antenna shows a broad prospect for band coverage and directional noise inhibition with very close operating frequencies. [ABSTRACT FROM AUTHOR]

The high directionality of mmWave communication makes its line-of-sight (LoS) path susceptible to blockage when the user is moving. Most existing solutions have very stringent requirements on the antennas of the transmitter and the receiver, which are hardly met by today’s consumer-level commercial off-the-shelf (COTS) mmWave products. In reality, a COTS device uses low-resolution wide-beam antennas, and hence cannot support the aforementioned methods for NLoS beam switching in response to the LoS blockage. In this paper, we develop a new method to support high-resolution mmWave multi-path channel resolving based on coarse-grained wide-beam phased array antennas. We design a novel real-time beam-switching algorithm that allows COTS devices to estimate the location and reflection coefficient of the dominant reflectors. Whenever the current LoS is blocked, our algorithm can compute in real-time the best alternative beam direction based on estimated reflectors to establish a strong NLoS link. We implemented the proposed algorithm on a COTS mmWave device and evaluated the system’s performance on the physical and transport layer. Our experiments demonstrate the effectiveness of our algorithm on estimating dominant reflectors and calculating strong alternative beam directions, and its efficacy in providing robust connections for COTS mmWave devices. [ABSTRACT FROM AUTHOR]

Existing cellular network is designed mainly for terrestrial users (TUs) with down-tilted antennas, which couldn’t provide efficient 3D coverage for UAV users. This letter proposes up-tilted antenna model to provide 3D seamless coverage for cellular-connected UAVs, and derives coverage probability for both aerial users (AUs) and TUs via stochastic geometry, where the mutual interference between up-tilted and down-tilted antennas is characterized. Specifically, practical antenna model with tunable half power beamwidth $\theta _{k}$ is adopted and power constraint between up-tilted and down-tilted antenna is considered by modelling power allocation parameter $\alpha $ , where the interference Laplace transform is rederived considering the mutual interference. In addition, the theoretical analysis is based on semi-closed expression of coverage probability, which can be used to explore the impact of various system parameters. Numerical results show that when 10% of the power is allocated to up-tilted antenna ($\alpha =0.9$), coverage probability of AUs can be significantly increased by 192%, while coverage probability of TUs reduced by less than 1%, compared with only down-tilted antenna. [ABSTRACT FROM AUTHOR]

As the importance of spaceborne antennas keeps increasing in contemporary very low frequency (VLF: 3–30 kHz) transmitting systems, figuring out the radiation patterns of VLF spaceborne antennas of different lengths has become a critical issue that needs urgent solution. In this article, we propose an analytical method for calculating the radiation pattern of a VLF linear antenna immersed in an anisotropic magnetoplasma. The core idea of the approach is: 1) to consider the influence of current inhomogeneity to the radiation pattern so that the method is suitable for antennas of arbitrary length and 2) to derive the correction factor of the antenna under the far-zone condition. The results show that the radiation patterns for the extraordinary wave (E-wave) and ordinary wave (O-wave) are quite different, but in the near zone, both modes will contribute to the total field collectively. The radiation in the far zone mainly depends on the E-wave, but unlike isotropic cases, the radiation pattern in the anisotropic environment is always oriented along the geomagnetic field, with the shape of the pattern not influenced by the antenna length anymore. This is because the radiation field of a linear antenna can be regarded as the field excited by an electric dipole multiplying the correction factor, and variations on the correction factor only change the magnitude of the field, instead of the pattern. [ABSTRACT FROM AUTHOR]

This article presents a Tx/Rx dual-band combined-aperture variable inclination continuous transverse stub (VICTS) antenna for low-earth-orbit (LEO) satellite-on-the-move (SOTM) applications. The beam directions for the Tx and Rx bands are kept consistent during beam scanning, so that the antenna allows a duplex link. The topology of the combined-aperture VICTS antenna is derived from the requirement for consistent beam directions. Nonuniform slow wave structures (SWSs) are used to achieve the required phase difference and phase-changing rate (PCR) between the radiation slots in the two bands, while maximizing the antenna efficiency. Predeflection technique is applied to the radiation slots to reduce the gain drop at a wide beam scanning angle. A prototype operating at K- and Ka-bands is designed, fabricated, and measured. Experimental results show that the antenna has a consistent beam direction during scanning in the frequency ranges of 19–21 GHz (Rx band) and 29–31 GHz (Tx band). The maximum beam directions in Tx and Rx bands exceeds ±47° at a relative rotation angle of 40°, and the gain drop during the beam scanning is reduced within 2.3 and 3.5 dB. [ABSTRACT FROM AUTHOR]

In this letter, a hybrid wideband decoupling method based on far-field radiation regulation by employing the specially engineered periodic near-field resonators, which consist of split-ring resonator (SRR) arrays, is presented for the two collinear transceiving horn antennas with extremely large element spacing. Two E-plane-coupled antennas operating at X-band are arranged with center-to-center distance of 8 m (over 200λ). By allocating the SRR arrays in close proximity to each horn antenna, the radiation power in the specified horizontal directions pointing to each other could be significantly reduced without disturbing their radiation characteristics in the broadside thus leading to the conspicuous port isolation enhancement. It is validated that more than 25 dB mutual coupling level reduction is achieved over the frequency range of 8–11.5 GHz (∼35.9% fractional bandwidth) when the SRR arrays are integrated. Finally, a prototype was fabricated and measured. Corresponding measurement results are in good agreement with the simulated ones. [ABSTRACT FROM AUTHOR]

This paper investigates the best antenna characteristics to harvest ambient RF waves maximally. The proposed approach analyzes and compares key design aspects, including multi-polarization, multi-port, multi-band, omnidirectional, quasi-isotropic, and non-quasi-isotropic pattern options, over the same antenna size defined by the enclosing sphere radius. First, the ideal isotropic and the Dirac-directive antennas are theoretically analyzed and compared within a general Gaussian channel to derive the most suitable design options. Then, several test antennas, covering the design options, are designed, fabricated, characterized, and compared within a full-wave antenna simulator under spherical incidences and polarizations. Further experiments in real ambient environments complete the tests. Two optimum antenna solutions emerge from the results: the quasi-isotropic antenna and the collocated LP-port diversity system with a power combiner. The crucial feature is optimizing antenna solutions for lower frequency bands with significant power densities that compensate for the available power ${\lambda }^{2}$ -factor.

Multipole expansions are an essential analysis tool in the foundations of the descriptions of the electromagnetic fields radiated by electric and magnetic sources. Nevertheless, practical antenna systems generally rely on them as an academic explanation, not as a fundamental building block. An overview of the recent surge in interest in multipole sources and their fields to achieve useful radiated and scattered fields with, for example, high directivities in preferred directions is given. Topics include Huygens sources, dielectric-based Mie-tronics, edge-singularity multipoles, and exotic metamaterial-inspired superdirective lenses and radiators. While there has been a never-ending stream of physics publications, little has happened in the engineering electromagnetics community. I will try to answer the title with examples that may stimulate interest in the field.

An antenna-in-module (AiM) architecture at millimeter-wave frequencies for a base transceiver system (BTS) is developed for electromagnetic (EM) radiation with an agile polarization tunable capability. This may reduce the polarization mismatch to a user equipment (UE) antenna, where the mismatch becomes severe in wide-angle scans. The AiM first implements a dual-polarization feature on its antenna elements and then integrates active radio frequency (RF) phase shifters and power amplifiers/attenuators to provide dynamically adjustable weightings. Considering the received fields of a UE antenna, the weightings of the AiM antennas are appropriately tuned to obtain the maximum signal strength from the UE antenna. In this article, the theoretical concept and the antenna architecture are presented. Full-wave simulations using HFSS and the measurement results of the AiM prototype are presented to validate its feasibility. [ABSTRACT FROM AUTHOR]

A novel low-profile vertically polarized planar endfire smart antenna that can achieve 360° beam-steering and high beam-shaping capability is proposed. The microstrip antenna is printed on a single thin substrate with a vertically polarized omnidirectional driven source and 36 tunable slot-integrated microstrip vertically polarized resonators (SMVR) for shaping the endfire beams. To verify the design approach, a prototype operating at the 5 GHz WLAN band is fabricated and tested. The results show that the antenna operates from 5.55 to 5.75 GHz and is capable of scanning a single endfire beam from 0° to 360° in the full azimuth plane. The scanning resolution and the maximum realized gain are 15° and 8.2 dBi, respectively. Moreover, multiple radiation pattern shapes including omnidirectional beam, dual-beams, and tri-beams can also be generated by the antenna. The overall antenna height is only $0.06~\lambda $ at 5.6 GHz. This design provides a new antenna configuration for low-profile vertically polarized endfire beam-steering and beam-shaping antenna and is promising for future wireless communication systems with lost-cost beamforming property. [ABSTRACT FROM AUTHOR]

Compact and lightweight antennas are required for future ground communication systems to provide wide-angle $4\pi $ steradians coverage for man-pack radio communication units carried by soldiers on the ground. Two innovative “Bunker” antennas at K-band are described in this article. The novelty includes antenna deployment from folded to unfolded configurations, compact size, low mass, $4\pi $ steradians coverage, low cost, wide bandwidth performance, and built-in radome for protection from severe environmental conditions. Design of these antennas, trades, RF simulations, mechanical design, antenna deployment, and material selection leading to product development are described for the first time in this article. A prototype antenna has been fabricated and measured results are presented. An excellent correlation between measured and simulated patterns has been achieved. The Bunker antenna has very wide frequency bandwidth of 86% covering Ku and Ka secondary frequency bands in addition to its primary K-band and thus could in the future replace three antennas on a dedicated man-pack with a single tri-band antenna solution. [ABSTRACT FROM AUTHOR]

A planar bidirectional circularly polarized (CP) antenna using orthogonal magnetic dipoles (OMDs) without an extra phase shift line is proposed in this work. It consists of a rectangular microstrip magnetic dipole antenna (MMDA) and a loop radiator with two arms directly connected to the open edge of MMDA. The working principle is revealed by the equivalent sources model to provide a clear physical insight at first. The theoretical investigations imply that by controlling the width and the feed locations of the loop antenna on the aperture, the phase and magnitude for CP radiation can be realized in both endfire and backfire directions without introducing an additional phase shift line structure. For demonstration, one antenna prototype is designed and fabricated. The experimental results indicate that an overlapping bandwidth of 5.91% from 2.30 to 2.44 GHz is attained. Compared with the counterpart using OMDs, the proposed antenna exhibits a low design complexity and improved gains of 3.56/1.83 dBic in the endfire/backfire directions. [ABSTRACT FROM AUTHOR]

Despite the rapidly increasing interest in analog multibeam antennas, there has been a lack of systematic theoretical approaches to synthesizing circuit-type multiple beamforming networks, such as the Blass matrix and the Nolen matrix. To address the issue, this article presents a new concept, the generalized joined coupler (GJC) matrix, which encapsulates both the Blass matrix and the Nolen matrix, as well as their variants, and presents a novel theoretical framework for generating individually and independently controllable multiple beams using the GJC matrix. A GJC matrix has $N $ columns to feed $N $ antenna elements and $M $ rows to feed $M $ beams, and the direction of each individual beam can be controlled by tuning the phase shifters in the associated row of the GJC matrix. In this article, a matrix theory is developed, and an optimization algorithm is proposed to provide a mathematical tool for synthesizing such matrices and, consequently, the multiple beams. Using a particle swarm optimization algorithm, numerical results demonstrate that multibeams with independent control of individual beam directions and sidelobes can, indeed, be synthesized in a systematic manner. Specifically, two GJC matrix variants, the Blass-like matrix and the Nolen-like matrix, are investigated. [ABSTRACT FROM AUTHOR]

This article proposes a one-tier ultra-dense cellular network framework based on the characteristics of directional beamforming to explore the downlink signal-to-interference ratio (SIR) performance. The features of the proposed cellular network include: small base stations (SBSs) are equipped with directional antennas; the beam boresight directions of antennas are adaptively adjusted with the locations of users on the two-dimensional horizontal plane while the boresight directions are perfectly aligned with their associated users. In addition, an angle fading model, which follows from the exponential distribution, is developed to simulate the signals attenuation of interference links. The angle fading of the interference signals depends on the angle offset between the locations of users and the beam boresight directions of their interfering SBSs. With the assistance of stochastic geometry, the analytical expressions of coverage probability and average ergodic rate are derived by combining with the probability density function of angle offset variable. Then, the meta distribution of SIR is obtained to capture the performance changes of individual links for users. The numerical results indicate that the performance advantages of the proposed directional network obviously exceed that of the conventional isotropic antennas case. Monte-Carlo simulations validate the correctness of numerical results. [ABSTRACT FROM AUTHOR]

The adoption of abundant millimeter-wave (mmWave) spectrum offers higher capacity for short-range connectivity in various Unmanned Aerial Vehicle (UAV)-centric communications scenarios. In contrast to the conventional cellular paradigm, where the coordination of connected nodes is highly centralized, the distributed deployments, such as those operating over unlicensed frequency bands, maintain robust interactions in the absence of central control. These agile decentralized systems are being naturally created by dynamic UAV swarms that form a temporary 3D structure without reliance on remote management or pre-established network infrastructures. While much effort has been invested in the performance assessment of distributed, directional, and 3D systems individually, a combination of these three angles allows capturing more realistic UAV swarm scenarios and produces a novel research perspective. This work addresses one of the fundamental challenges in mmWave-based 3D networks– directional deafness– which is known to adversely affect the overall system performance. Particularly, we develop a mathematical framework by taking into account the peculiarities of 3D directional and distributed deployments. We provide a holistic analytical assessment of directional deafness and propose several powerful approximations that capture realistic antenna patterns. [ABSTRACT FROM AUTHOR]

Antenna-free channel models can reflect real multipath propagation and can be applied widely for performance analysis, simulation, and physical emulation, combined with specific antennas used in the communication systems. However, the millimeter-wave (mmWave) channel measurement is usually performed by steering horn antennas, and the measured channel responses are actually spatial convolution of the channel propagation models and antenna pattern, which is commonly referred to as the antenna embedding effect. In this work, we propose a novel antenna de-embedding algorithm based on the deconvolution with Tikhonov regularization. By suppressing parts of the observed responses which are disguised by noise, the Tikhonov regularization facilitates the deconvolution of antenna pattern and enables the extraction of propagation models. In particular, in order to minimize the impact of deconvolved noise, we design an optimization algorithm to obtain the appropriate regularization factor with low computational complexity. To validate the proposed approach, we have performed an indoor mmWave channel measurement campaign using two different steering horn antennas. The principal peaks in the synthesized channel responses are accurately reconstructed, and the signal-to-noise ratio (SNR) is improved. The experiments verify that the proposed scheme de-embeds effectively the antenna effect and leads to the antenna-free channel models. [ABSTRACT FROM AUTHOR]

A novel single-fed horizontally polarized pattern-reconfigurable antenna is proposed with the key feature that is highly pattern-reconfigurable. The antenna is able to generate a large variety of radiation patterns, including omnidirectional, single-directive, and multi-beam patterns without using phase shifters. The antenna is based on a planar geometry, which consists of an Alford loop and four compact parasitic pixel rings with 60 integrated controlling units utilizing p-i-n diodes. Experimental and simulation results are provided for the proposed antenna design operating at 5 GHz. The results show that the single-beam steering with 7.3 dBi gain through 360° in the azimuth plane is achieved. Moreover, flexible dual-beam steering with one beam fixed and the other beam scanning independently among 360° in the azimuth plane is also demonstrated where each beam has 5 dBi gain. Other radiation modes, such as the omnidirectional mode, the tri-beam-steering mode, and the elevation plane steering mode, are also verified. The features of high pattern reconfigurability, 360° single-beam- and multi-beam-steering ability, full 3-D space scanning, planar geometry, and compatibility with a digital controller make it useful in various sub-6 GHz wireless applications, such as wireless power transfer, RF sensing, and analog precoding. [ABSTRACT FROM AUTHOR]

In this article, an antenna with quasi-isotropic radiation pattern is proposed in a 915 MHz industrial, scientific, and medical (ISM) band for robust communication with external devices. The overall structure of the proposed antenna is a folded inverted-F antenna (FIFA). In the simulation, the proposed antenna is integrated with dummy electronics to form a fully encapsulated capsule endoscope architecture. These dummy electronics are encapsulated in a copper cylindrical bucket used as the antenna ground to avoid electromagnetic (EM) interference between these dummy electronics and the antenna. The spherical phantom model is used in the initial design of the proposed antenna. Subsequently, an anatomical realistic phantom was used to evaluate the performance of the proposed antenna in a more realistic human environment. The simulated gain variation (GV) of the proposed antenna in the regular spherical phantom is about 8 dB, and the reflection and other factors caused by the irregular shape of the realistic human tissue environment lead to the GV of the antenna in the anatomically realistic phantom that is about 4 dB. The measurement of reflection coefficient and radiation pattern is carried out in a semisolid muscle-mimicking phantom using a fabricated prototype of the proposed antenna. The measured GV value is about 10.5 dB. Finally, the specific absorption ratio (SAR) was calculated to meet the human safety regulations and the communication link budget was calculated to evaluate the wireless biological telemetry performance of the proposed antenna. [ABSTRACT FROM AUTHOR]

This communication presents a shorted trapezoidal substrate integrated waveguide (SIW) antenna with a quasi-hemispherical pattern composed of a 64-element (8 $\times $ 8) 2-D planar phased array. The proposed low-profile SIW antenna is analyzed in terms of resonating mode and magnetic current and is simulated. The proposed antenna is composed of a trapezoidal patch and shorting vias. The upper side of the trapezoid is the shorting wall, and the middle of the bottom side provides shorting with a via. The resonance produced by the structure causes the magnetic currents to have three different directions. Hence, the single element has a quasi-hemispherical beam pattern on a large ground. The 64-element (8 $\times $ 8) 2-D planar phased array was implemented and measured. Because the constituent elements have a quasi-hemispherical pattern, the phased array antenna has a wide scanning range in all azimuth directions. The measured scanning range with 5 dB gain fluctuation in the $\phi = 0^\circ $ plane, $\phi = 45^\circ $ plane, and $\phi = 90^\circ $ plane are 170°, 143°, and 168°, respectively. [ABSTRACT FROM AUTHOR]

Traditional directional modulation techniques always sacrifice the effective radiated power in order to produce the effect of directional modulation, thus resulting in low radiation efficiency and leading to low channel capacity or short transmission distance. Generally, a signal is first modulated at the baseband and then modulated again at the antenna. The two modulations are always regarded as two separate steps. Therefore, these directional modulation techniques fail to maximize their modulation efficiency, resulting in possible performance degradation. Aimed at this problem, this article proposed a novel secure transmission approach named joint space–time modulation (JSTM) for improving the transmission efficiency of secure communication by using 4-D antenna arrays. By selecting the appropriate modulation strategy, the space–time degrees of freedom are integrated and the signal modulation can be completed with high efficiency, thus being able to reduce the attenuation in the signal modulation process. The proposed approach is able to improve the overall gain of the 4-D arrays close to that of the phased arrays while ensuring good security performance. Numerical and experimental results demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

A novel low-profile phased antenna with dual-port and its two types of 1-D linear array to 2-D space wide-angle beam scanning are proposed. The antenna unit is composed of fishbone structure and rhombus parasitic patch, which is folded into n-shaped to low profile $0.08\lambda $. By adjusting the phase difference ($\Delta \varphi$) from 0° to 180°, the antenna working mode is changed from even mode to hybrid mode and to odd mode eventually. Therefore, the proposed antenna unit realizes wide-angle scanning characteristics and continuous switching state from broadside lobe to endfire lobe. Furthermore, the proposed phased antenna element is extended to two kinds of 1-D linear phased array (Array#1 and Array#2) based on the generalized principle of the pattern multiplication. The fabricated Array#1 can realize beam scanning range $- 116^{\circ }< \theta _{\mathrm {m}} < 0^{\circ }$ (xo z plane), $0^{\circ }< \theta _{\mathrm {m}} < 63^{\circ }$ (yo z plane), and $90^{\circ }< \Phi _{\mathrm {m}} < 169^{\circ }$ (xoy plane) and the fabricated Array#2 can achieve $0^{\circ }< \theta _{\mathrm {m}} < 60^{\circ }$ (xo z plane), $0^{\circ }< \theta _{\mathrm {m}} < 65^{\circ }$ ($\text{y}{o}\text{z}$ plane), and $90^{\circ }< \Phi _{\mathrm {m}} < 152^{\circ }$ (xoy plane). The proposed phased antenna unit and its 1-D array can realize 2-D space wide-angle scanning, improving scanning performance and expanding scanning dimensional space in phased array. [ABSTRACT FROM AUTHOR]

Millimeter-wave (mm-Wave) band channel modeling and characterization are essential for system design and analysis in the fifth generation (5G) and future communication systems. Reliable channel sounding in the deployment scenarios is required for accurate and realistic channel modeling and characterization. In the state of the art, directional scanning sounding (DSS) and virtual antenna array (VAA) sounding are two popular methods due to their simplicity and cost-effectiveness. The DSS and the VAA methods are typically based on mechanically rotatable directional antennas and mechanically movable omnidirectional antennas, respectively. However, the spatial resolution is limited by the directivity and high sidelobes of the directional antennas used in the DSS method. The conventional VAA method also has limitations in terms of the low signal-to-noise ratio (SNR) and the unavailability of suitable omnidirectional antennas that support mmWave (above 60 GHz) band measurements and wideband horizontal polarization measurements. In this article, a novel directional antenna-based VAA framework in combination with the associated beamforming algorithm is proposed. Compared to the state-of-the-art methods, the proposed framework can achieve high angular resolution and high SNR for mm-Wave channel measurements without introducing additional cost and measurement time. Furthermore, it is a generic solution that can be applicable for arbitrary frequency bands and polarizations, unlike the conventional VAA method. To validate the effectiveness and robustness of the proposed method, experiments in two scenarios (a clean anechoic chamber and a realistic indoor meeting room) were conducted over 28–30 GHz with two types of directional antennas (i.e., a horn antenna and a corrugated antenna). Besides, the advantages of the proposed method are highlighted with a comparison to the conventional VAA and DSS methods. [ABSTRACT FROM AUTHOR]

This communication presents a unique design of a dual-polarized wideband passive beam-switching antenna at millimeter-wave (mm-wave) bands for multiple-input–multiple-output (MIMO) pattern diversity applications, which exhibits low correlation between the ports and a broad tilt bandwidth (BW). A magnetoelectric (ME) dipole fed by a printed ridge gap waveguide (PRGW) as a quasi-TE source is considered as the elementary antenna in the Ka-band. To achieve beam deflection in each quadrant, a four-port MIMO prototype consisting of four ME-dipoles, positioned perpendicular to each other, is integrated with a meta-lens that consists of three layers of $12\times12$ dual-polarized split-ring resonators (SRRs) offset from the center of each antenna. The beamforming structure is able to generate four fixed beams, one in each quadrant at an elevation angle of $\sim 40^{\circ }$ with respect to the broadside direction by exciting each port sequentially, while the other ports are terminated. Moreover, the proposed orthogonal structure contributes to low correlation between the four ports, while achieving dual-polarization, which highlights the suitability of the antenna for 5G mm-wave applications. The measured results from a fabricated prototype demonstrate a −10 dB impedance BW of 40% over 24–36 GHz, tilting BW of 32.2%, and a peak gain of 13.4 dBi at 29 GHz, while an isolation better than 30 dB is achieved over the whole frequency band. Moreover, a measured radiation efficiency in excess of 85% is obtained, due to the use of the low-loss PRGW feedlines. [ABSTRACT FROM AUTHOR]

There is a rapid trend in various industries to replace the metallic pipes by nonmetallic ones. This is due to the certain properties, such as high strength, lightweight, resilience to corrosion, and low cost of maintenance for nonmetallic pipes. Despite the abovementioned advantages, nonmetallic pipes are still affected by issues, such as erosion, defects, damages, cracks, holes, delamination, and changes in the thickness. These issues are typically caused due to the manufacturing process, type of carried fluid composition, and flow rate. If not examined well, these issues could lead to disastrous failures caused by leakages and bursting of the pipes. To prevent such major failures, it is extremely important to test the pipes periodically for an accurate estimation of their thickness profile. In this article, we propose a nondestructive testing (NDT) technique, based on near-field microwave holography, for identifying the fluid carried by a nonmetallic pipe and estimating the pipe’s thickness profile. Identifying the carried fluid helps improve the thickness profile estimation. The performance of the proposed techniques will be demonstrated via simulations and experiments. [ABSTRACT FROM AUTHOR]

A double integral operation is required to obtain gain in an array antenna in general, and only an omnidirectional antenna is represented in a closed form. In this letter, as a result of radio-wave propagation research, a closed-form expression that can be applied to a small dipole antenna or a half-wavelength dipole antenna is shown by using an equation applicable to spatial correlation in a three-dimensional multipath environment. In addition, the maximum gain characteristics of the half-wavelength dipole element linear array antenna are analyzed using the equation. [ABSTRACT FROM AUTHOR]

A design approach to a planar dual-sense bidirectional circularly polarized (CP) antenna is advanced. The resultant antenna can exhibit orthogonal CP radiations in arbitrary directions between 90° and 180°, i.e., the flared angle formed by pure left-handed CP (LHCP) and right-handed CP (RHCP) directions could be set in a wide range from 90° to 180°. The operational principle is revealed by an equivalent source model at first. Then, prototype antennas are advanced by incorporating a high-order mode resonant, V-shape dipole to a planar magnetic dipole, and a parasitic electric dipole. As numerically simulated and experimentally validated, the flared angle formed by LHCP and RHCP directions can be adjusted as 90° and 135°. In the 90° case, the presented bidirectional CP antennas can exhibit impedance bandwidth of 6.6%, 3 dB axial ratio (AR) bandwidth of 13.0% in the $+ x$ -axis, and 8.9% in the +z-axis. The advanced low-profile (height < 0.05 wavelength) antenna can simultaneously provide spatial and polarization diversity abilities in one principal-cut elevation plane. Therefore, it would be a promising colocated, spatial/polarization diversity antenna in future wireless systems. [ABSTRACT FROM AUTHOR]

A multiport circular array of endfire Yagi–Uda monopoles with a full beam coverage of the azimuth plane is developed in this communication. The known issue that a vertical monopole antenna on a finite-sized horizontal ground plane radiates an elevated beam is investigated. It is demonstrated that this effect can be mitigated by inserting resonant structures, i.e., slots, into the ground plane to redirect the radiated beam back to the horizontal direction. An endfire monopole-based Yagi–Uda antenna is then developed that radiates a directed endfire beam. A multiport circular array of these antennas is optimized to provide beam coverage over the full 360° azimuthal plane. A sector of this circular array of monopole-based Yagi–Uda antennas was simulated, fabricated, and measured to verify the concepts. Good agreement between the simulated and measured results is confirmed. [ABSTRACT FROM AUTHOR]

This article presents an evaluation of the performance of two different approaches for beamforming (BF) in a fifth generation (5G) handset, based on measured channels from a dual-polarized transmitter (Tx) to a mockup handset with a seven-element receiver (Rx) array. The measurements at 21.5 GHz include both small-scale and large-scale channel changes and were conducted for different users holding the mockup at different locations in an indoor corridor environment. The unique measurements enable novel results based on the statistics of the power achieved with BF, using channel state information (CSI) measured at a 90 Hz rate, and both when using beam scanning (BS) and the equal gain combining (EGC) reference case. For the case of ideal CSI knowledge, BF gains of up to about 18 dB were found. However, when the density of the angle scanning grid is reduced and the CSI is delayed to realistic values, the average performance decreases by 8 dB. More results are given in the article. [ABSTRACT FROM AUTHOR]

Electrically small, high-directivity antennas are in demand for a variety of current and future wireless applications. An electrically small directive antenna (ESDA) that requires only one specially engineered port to excite a set of multipoles is demonstrated in this article. Four 90° copper sectors are combined with additional structures and fed with a coaxial cable. Two resonant quadrupoles (equivalent to two pairs of resonant electric dipoles) and one magnetic dipole are excited. Both high radiation efficiency and good impedance matching are achieved. Theoretical calculations, numerical simulations, and experimental measurements are shown to be in good agreement. An optimized prototype is designed, fabricated, and tested. The measured results confirm that it is a supergain system. The unidirectional ESDA has a peak directivity of 6.71 dBi, a peak realized gain of 6.31 dBi, radiation efficiency of 94.5%, and a front-to-back ratio of 14.89 dB at its resonance frequency, 814 MHz. Its height is $0.06 \lambda _{\mathrm{ res}}$ , and $ka = 0.98$. These measured realized gain and directivity values exceed both the Harrington and Kildal–Best $ka$ -based upper limits. [ABSTRACT FROM AUTHOR]

This article investigates the pattern synthesis of the conformal phased array antenna (PAA) by using the deep deterministic policy gradient (DDPG) algorithm, and a nearly full solid angle for beam steering is realized. The beam steering capability of the planar and conformal PAAs is theoretically compared at first, and a conclusion is obtained that conformed to the conical-and-cylindrical structure can help to achieve ultrawide-angle beam steering. Next, a typical deep reinforcement learning algorithm, which is the DDPG algorithm, is utilized to deal with the fast beam steering problem of the conformal heterogeneous PAA. By virtue of the strong fitting ability of the DDPG algorithm for high-dimensional continuous nonlinear problems, the performance of fast beam steering is achieved within a wide-angle range within (−150°, 150°). Finally, a prototype of $1\times17$ conformal PAA is fabricated for measurement and verification, and the measured results are in good agreement with the simulation results. [ABSTRACT FROM AUTHOR]

An endfire frequency scanning tapered slot antenna (FSTSA) is proposed and investigated. The proposed FSTSA is a two-conductor structure with a gradient slots array etched only on one of two inner edges in the FSTAS. The asymmetrical inner edges enable the antenna to excite two modes with different phase constants simultaneously along the two inner edges. Due to dispersion characteristics of spoof surface plasmon polaritons (SSPPs), the phase difference between the EM waves along the two conductors changes versus frequencies, which enables the main lobe of the FSTSA to scan through the endfire direction. An analysis model is conceived to provide insights into the design, which accurately predicts main lobe directions and initial values of the proposed antenna. The proposed antenna is fabricated, and the measured results show that the main lobe of the antenna scans from −15° to 14° (endfire direction at 0°) from 6.60 to 7.70 GHz, with the scanning rate of 26.4°/GHz. This antenna could be suitable for radar, plane integrated communication system, and certain other beam-scanning applications. [ABSTRACT FROM AUTHOR]

For the near-field synthetic aperture interferometric radiometer (SAIR), the Fourier transform relationship between the visibility function and the near-field brightness temperature (BT) distribution is not valid. It is a challenging task for near-field SAIR imaging to realize very close-range accurate imaging with a large field of view (FOV). This article presents a new SAIR near-field imaging algorithm based on the angular spectrum theory to realize the passive millimeter-wave (PMMW) imaging, called synthetic-angular-spectrum imaging (SASI) algorithm. This SASI algorithm mainly addresses data samples of a 4-D visibility function acquired from planar arrays. First, we invert 4-D visibility samples into the angular spectrum domain via the Fourier transformation. The obtained angular spectrum data cannot be directly used to estimate the near-field BT distribution. Second, a dedicated phase factor compensation is adopted for handling this problem. The dimension-reducing accumulation is employed to generate the synthetic angular spectrum (SAS) of near-field BT distribution. Finally, we reconstruct the BT image by using the generated SAS data. Simulation and experiment results show that the presented SASI algorithm has the superiority on image reconstruction quality, especially for the off-axis regions, compared with the existing near-field SAIR imaging methods. [ABSTRACT FROM AUTHOR]