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Phased arrays are extensively used in many modern beam-scanning applications such as radar and satellite communications. Electronic beam scanning makes phased arrays an important aspect of modern antenna array systems. This Tutorial aims to promote the basic understanding of the principle and operation of a phased array to general undergraduate students. This paper starts with a discussion on the theory of operation and some basic definitions of antenna parameters followed by derivations of two-element and N-element array patterns and, finally, a five-element array design. The essential hardware based on Phase-Locked Loops (PLLs) as phase controllable RF sources required to build an array and the basic tools required for software and measurement set-up to demonstrate the beam-scanning phased array operation are presented. This enables students to quickly understand and set-up an experiment to verify the phased array operation with commercial off-the-shelf components. In addition, the hardware and software necessary for autonomous control are discussed. By combining basic concepts of phase arrays with a series of simple coding and intuitive laboratory experiments, students can easily understand the Uniform Linear Array (ULA) scanning operation. [ABSTRACT FROM AUTHOR]

This paper presents a framework for achieving machine learning (ML)‐assisted direction‐of‐arrival (DoA) accuracy enhancement using a millimetre‐wave (mmWave) dynamic aperture. The technique used for the enhanced DoA estimation accuracy leverages an over‐sized lens‐loaded cavity antenna connected to a single RF chain in the physical layer and a computational method in the computational layer of the framework. It is shown for the first time that by introducing a reconfigurable mode‐mixing mechanism inside the over‐sized lens‐loaded cavity hardware, a greater number of spatially orthogonal radiation modes can be achieved giving rise to many cavity states. If the best cavity state is determined and selected by means of design exploration using a contemporary ML‐assisted antenna optimisation method, the computational DoA estimation accuracy can be improved. The mode‐mixing mechanism in this work is a randomly oriented metallic scatterer located inside an over‐sized constant−ϵr lens‐loaded cavity, connected to a stepper motor that is electronically controlled by inputs from the computational layer of the presented framework. Measurement results in terms of near‐field radiation mode scans are included in this study to verify and validate that the proposed ML‐assisted framework enhances the DoA estimation accuracy. Moreover, this investigation simultaneously provides a simplification in the physical layer implementation of mmWave radio hardware, and DoA accuracy enhancement, which in turn lends itself favourably to the adoption of the proposed framework for channel sounding in mmWave communication systems. [ABSTRACT FROM AUTHOR]

We demonstrate a silicon (Si)–gallium arsenide (GaAs) semiconductor-based holographic metasurface antenna operating at 94 GHz. The metasurface antenna molds the radiated beam by resorting to a holographic approach involving the modulation of a guided-mode generated by a pillbox beamformer. The pillbox beamformer is fed by three integrated horns in substrate-integrated waveguide (SIW) technology located in the focal plane of the parabolic reflector of the pillbox beamformer. The three horns correspond to three independent beams and allow beam steering by switching from one feed to another. The proposed metasurface antenna is planar and extremely low-profile, thus suitable for small platforms such as CubeSats/SmallSats. A prototype validates the concept demonstrating a directivity as high as 31.9 dBi and a reflection coefficient lower than −15 dB at 94 GHz. [ABSTRACT FROM AUTHOR]

In this paper, we propose a new architecture and provide performance analysis for frequency diverse array (FDA) Radar combined with the index modulation (IM) techniques. Here, the IM concept is applied upon frequency offsets in an FDA. Information is transmitted by a dynamically selected subset of antenna element indices applying predefined carrier frequency offsets, which generate different radiation patterns. It is shown that this enables the capability for simultaneous wireless communication and FDA Radar that has the range-angle coupling issue resolved. [ABSTRACT FROM AUTHOR]

A novel physical effect of angular super-resolution based on the direction-of-arrival information sampling using highly oscillatory evanescent harmonics in dense antenna array environment is demonstrated theoretically and experimentally. This phenomenon enables realisation of novel, ultra-compact antenna terminals (ATs) with superdirective radiation patterns and spatial auto-focusing characteristics for any direction of arrival of the excitation waveform. The use of these ATs should enhance the signal-to-noise ratio at the receiver, increase overall energy efficiency of the wireless communication or power transmission systems and lead to substantially lower interference and EM pollution across the wireless network area. Special attention is given to the antenna array operating in the presence of a flat metal ground plane or a ground plane with EM band-gap structure. Near-field scattering phenomena arising as a result of the antenna array coupling to the ground planes of both types and their effects on the far-field radiation patterns are analysed in detail. The results of the study should be important for the development of the electrically compact ATs to be employed in the energy-efficient wireless communication systems including the Internet-of-Things, wireless sensor networks and wireless powering systems. [ABSTRACT FROM AUTHOR]

A new strategy for remote reconfiguration of an antenna array far field radiation pattern is described. The scheme uses a pilot tone co-transmitted with a carrier signal from a location distant from that of a receive antenna array whose far field pattern is to be reconfigured. By mixing the co-transmitted signals locally at each antenna element in the array an IF signal is formed which defines an equivalent array spacing that can be made variable by tuning the frequency of the pilot tone with respect to the RF carrier. This makes the antenna array factor hence far field spatial characteristic reconfigurable on receive. For a 10\,\times\,1 microstrip patch element array we show that the receive pattern can be made to vary from 35 to 10 degrees half power beam width as the difference frequency between the pilot and the carrier at 2.45 GHz varies between 10 MHz and 500 MHz carrier. [ABSTRACT FROM PUBLISHER]

A design methodology is presented which applies band-pass filter principles to form wideband small-aperture evanescent-mode waveguide antenna designs. This approach permits matching of the complex antenna aperture admittance of an evanescent-mode open-ended waveguide to a real impedance generator, and thereby to an SMA coaxial feeding probe. A reflection coefficient of less than -10 dB was obtained over a measured bandwidth of 15%, from 2.30–2.68 GHz, in 2.72 GHz cutoff waveguide. Capacitive posts and evanescent-mode waveguide sections are used to form the resonators/coupling elements of the antenna's filtering sections. Measured realised gain variation from 3.5–4.7 dBi is observed across the bandwidth. The antenna's maximum aperture dimension is <0.5\lambda0 at the upper frequency in the bandwidth and so it is suitable for use in a wide angle scanning phased array. [ABSTRACT FROM PUBLISHER]

We investigate the situation where there are obstructing elements present in the near field of a retrodirective array. We describe three scattering cases, (1) by an array of straight wires, (2) by low loss medium density fibre board partially obscuring the array, and (3) by concrete blocks, totally and then partially obscuring the array. For all scenarios retrodirective action was shown to be able to provide various degrees of automatic compensation for loss in gain relative to that which would have occurred for a conventional (non-retrodirective) array in the presence of the same scattering screens. Gain improvements of up to 10 dB were observed when the retrodirective array was used. In addition we show how the induced variation of received and re-transmited amplitudes across the array, caused by the scattering screens, is the principle mechanism causing deterioration of the retrodirective arrays monostatic response. [ABSTRACT FROM AUTHOR]

Experimental results are presented for the focusing capability of an active phase conjugating lens for a single and a dipole source pair and these are compared with predictions. In addition for a single source we illustrate the ability of the lens to project a null at the lens focus instead of a peak. A scheme is also presented such that when a source or pair of sources is imaged through an identical pair of passive scatterers located symmetrically about the lens that imaging with sub-wavelength resolution is possible. The rationale for the operation of the lens and aberrations observed due to its finite array size is discussed and is supported throughout by means of numerical simulation. [ABSTRACT FROM AUTHOR]

We show that by introducing a gap at the center of the helical sections (where the current is minimum) of a λ/2 quadrifilar helix antenna (QHA) and varying the axial length and radial gap between the overlapping volutes, the antenna gives a 28% impedance bandwidth which is nine times the bandwidth of a conventional QHA. A 16% bandwidth with a front to back ratio of ≥ 14 dB is achievable with 5-14% reduction in the size of the QHA. The structure can yield a monopole radiation pattern suitable for terrestrial applications or a hemispherical pattern suitable for satellite use. The simulation results are validated by measurements at L-band. [ABSTRACT FROM AUTHOR]

We present the results of a parametric study devised to allow us to optimally design a patch fed planar dielectric slab waveguide extended hemi-elliptical lens antenna. The lens antenna, 11λ × 13λ in the lens plane and 0.6λ thick, constructed from polystyrene and weighing only 90 g is fabricated and characterized at 28.5 GHz for both single and multiple operating configurations. The lens when optimized for single beam operation achieves 18.5 dB measured gain (85% aperture efficiency), 40° and 4.1° half power beamwidth for E plane and H plane respectively and 10% impedance bandwidth for -10 dB return loss. While for optimized for multiple beam operation it is shown that the lens can accommodate up to 9 feeds and that beam symmetry can be maintained over a scan angle of 27° with a gain of 14.9 to 17.7 dB, and first side lobe levels of -11 to -7 dB respectively. Over the frequency range 26 to 30 GHz the lens maintains a worst case return loss of -10 dB and port to port feed isolation of better than -25 dB. Further it is shown that residual leaked energy from the structure is less than -48 dBm at 1 cm, thus making a low profile enclosure possible. We also show that by simultaneous excitation of two adjacent ports we can obtain difference patterns with null depths of up to -36 dB. [ABSTRACT FROM AUTHOR]

It Is shown that a sido-fed bifilar helix antenna with a single feed, can generate a slant 45° linearly polarized omnidirectional toroidal pattern. The antenna has a low profile and does not require a gronrd plane. The biller helm antenna provides slant 45° polarization over a solid angle of almost 4π sleradians as compared to a crossed dipole which generates a tilted 45° linearly polarized pattern only over a solid angle of 1.1 4π sterndians. The computed results are validated by experimental data. [ABSTRACT FROM AUTHOR]

Analysis of a microwave lens formed using a pair of frequency selective surfaces comprised of back-to-back double-periodic wire arrays interconnected through phase-conjugating microwave circuitry is given in this paper. It is shown that such a structure when illuminated with a plane wave generates a phase-conjugated forward-transmitted plane wave which experiences negative refraction. Negative refraction makes it possible to produce focusing of wave packets incident on such structures both in the far- and near-field regions. We provide analytical formulation of the problem and develop a rigorous calculation of the EM field transmitted through the structure: i) when excited by a plane wave; and ii) due to an electric line source suspended above it. Numerical simulations illustrate the focusing properties of the proposed lens. These properties are shown to include half wavelength compression of the focused line source image in the transverse direction in the far-field zone. A physical explanation Of why this effect occurs is also provided. [ABSTRACT FROM AUTHOR]

This paper discusses the design and operational properties of a circular array of slot coupled active GaAs metal-semiconductor field-effect transistor (MESFET) rectangular microstrip patch antennas arranged for quasioptical power combining at X band. The active antenna radiating elements are injection locked by slot coupling to a radial cavity waveguide. Spatial power combining efficiency of 75% and dc to radio frequency (RF) conversion efficiency of 10% at 28 dBm EIRP was achieved at 10.38 GHz. Retrodirective action was also demonstrated by using the active elements as self-oscillating mixers pumped at 20.76 GHz. [ABSTRACT FROM AUTHOR]

In this paper, we describe the operation of a low profile, 3 mm deep, circular disk antenna which requires a small ground plane, and incorporates a shorting bar which can be rotated. The position of the shorting bar determines the return loss and radiation characteristics of the antenna element. In one mode at 2.34 GHz the antenna exhibits a return loss of better than -25 dB, 5 dBi gain, and a conical/monopole beam pattern suitable for radar tracking/mobile communications applications. When the short circuit is positioned at 90 degrees the return loss at 3.52 GHz becomes -18 dB, 7 dBi gain, and the antenna exhibits normal linear polarized broadside cosine radiation characteristics. In addition when the short circuit is positioned at zero degrees at 3.52 GHz the antenna gives -1 dB return loss and by reciprocity will act as a reflector to incoming energy. Theoretical results are provided in support of the experimental results observed and are used to explain the operating mechanisms of the antenna. [ABSTRACT FROM AUTHOR]

Antennas with dual linear slant polarization over a wide band are necessary for modern personal communication base-station applications. A traveling-wave subarray is used to generate wide-band dual slant 45° polarization. The antenna exhibits well-defined radiation patterns over a bandwidth of 1.7 to 2.7 GHz and has an impedance bandwidth of 3:1 for < 1.5 voltage standing wave ratio (VSWR), the worst case isolation between polarizations is 28 dB. In addition, the antenna exhibits, 37° half power beamwidth, 35 dB on-axis cross polarization, and 23 dB front-to-back ratio with 20-dB worst case sidelobes, at 2 GHz. A theoretical basis for the operation of the array is presented which shows that its operation can be readily extended to permit the generation of dual slant polarization with any slant angle. [ABSTRACT FROM AUTHOR]

A new quadrifilar antenna has been developed for generating circularly polarized backfire radiation. The antenna consists of two orthogonal rectangular conducting loops, each incorporating capacitive coupling and fed using either a single or two coaxial cables. Though the geometry is much simpler than a conventional quadrifilar helix antenna, the radiation pattern performance is very similar. Measured and simulated patterns are compared for two antennas with different feed arrangements. It is shown that the resonant structure can produce a cardioid pattern with a directivity of 4.5 dB (120° 3-dB beamwidth) and a front-to-back ratio of more than 20 dB at the center operating frequency. A 10% impedance bandwidth (VSWR < 2) over which the axial ratio is less than 3 dB is demonstrated. [ABSTRACT FROM AUTHOR]

A new printed wire antenna with circular polarization properties is presented. The geometry of the printed wire which takes the form of a figure-of-eight has a total length of 1.3λ[sub 0] and serves as a nonresonant traveling-wave antenna. It is shown that a 3-dB axial ratio bandwidth of 15% can be achieved. The half-power and 3-dB axial ratio beamwidth is approximately ±45°, while its gain is of the order of 6.5 dB. [ABSTRACT FROM AUTHOR]

We examine the behavioral characteristics of PON type heterodyne retrodirective arrays (RDAs) and provide a simple, yet comprehensive modeling equation that allows the retransmission response of the RDA to be determined. Using the modeling philosophy developed, the optimum array size required for a prespecified beam pointing error is determined. Then, deleterious effects such as mixer-leakage, phase-conjugate circuit gain imbalance, and cable phase error that occur due to the presence of real components are quantified experimentally for a four-element RDA, these are then correlated with theory. Specific recommendations on the permissible tolerances allowable for these parameters are given in order to assist design specification adherence. In addition, it is shown in theory and confirmed by measurement, how radiation element aberration due to diffraction and mutual coupling can be used to enhance the azimuthal tracking response of the array. This paper allows the first comprehensive, practical, and theoretical appraisal of the design limitations that exist for this type of array. The insights provided by this paper should be of value to practicing antenna engineers. [ABSTRACT FROM AUTHOR]

A novel wire-loop antenna is introduced for circular polarization. By introducing a small gap at the middle of two nonradiating sides of a one-wavelength square wire loop, a 90° time-phase difference between the currents on the other two radiating sides can be achieved. Then by twisting the square loop to make the radiating sides orthogonal the necessary conditions are satisfied for circularly polarized radiation. The antenna yields nearly ideally circular polarization at broadside as well as excellent input impedance properties near the design frequency. The radiation pattern has a broad beamwidth and good rotational symmetry about the axis. For a demonstrator operating at 1 GHz, a 3-dB axial ratio was maintained over ±30°. The mechanism of circular polarization is analyzed based on numerical simulation. Experimental results are presented to verify the theoretical predictions made. [ABSTRACT FROM AUTHOR]

Presents a study which solved the inherent disadvantage of narrow bandwidth of microstrip antennas by a wide-band feeding mechanism. Discussion of patch antenna fed by single stepped T-feed; Information on wide-band patch antenna with two proximity coupled printed stepped T-shaped feeds; Generation of a broad-band circularly polarized signal.

Proposes a novel conjugate mixer topology to overcome the drawback of conventional Pon retrodirective antenna systems. Use of a harmonic mixer instead of the conventional fundamental type; Double balanced structure with a novel phasing strategy.

ABSTRACT A dual/triband evanescent waveguide antenna element is presented. The antenna operates in the 740-790, 1.9-2.2, and 2.5-2.7 GHz frequency bands. It measures 55 × 27.5 × 53 mm3 and occupies a small volume making it attractive for miniaturized applications. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:540-542, 2015 [ABSTRACT FROM AUTHOR]

This article discussed the application of a synthesis method for, and the subsequent experimental characterization, of an active microstrip patch antenna. The active antenna incorporates a GaAs MESFET on the patch periphery and the circuit acts as a self-oscillating radiating element. The MESFET and the antenna are mounted on the same side of the circuit, thereby preserving the conformal nature of the structure. The synthesis approach described is based on the method of harmonic balance and allows optimal FET terminations to be selected for a predefined, added power at a specified frequency of operation. Application of a time domain simulator is illustrated in order to demonstrate start-up performance and limit cycle formation in the presence and absence of a frequency entrainment signal. The issue of stability of the active element is also addressed using a modified device/load line method. Theoretical and experimental results are presented for the external quality factor, locking power/phase variation, and power and frequency pushing characteristics of the active antenna. Finally, the effect on oscillator behavior of the mutual coupling between the microstrip patch and the active antenna distributed embedding elements is discussed. [ABSTRACT FROM AUTHOR]

We present a simple method of forming a switchable radar cross-section (RCS) in evanescent waveguide.Here, the antenna can be selected to be matched to free space, or to act as an almost perfect reflector of incident energy via a single SPST switch located at the antenna aperture. With the aperture switch open, the antenna is matched over a measured bandwidth of 17.5%, from 2.35 to 2.8 GHz, for reflection coefficient <−10 dB, in 2.725 GHz cutoff waveguide. With the aperture switch closed, a minimum reflection coefficient of −2.5 dB across the bandwidth is observed, proving that the antenna has the capacity to be made RCS reconfigurable. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1849-1851, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26981 [ABSTRACT FROM AUTHOR]

In this paper, we describe a canonical proof based on the use of multiport scattering parameters which shows that in the presence of multiple coupling paths between elements that is not possible to individually control the power returned to each transmitter in a retrodirective array which is responding to stimulation by multiple transmitters. It is proven that it is not possible to maximize retransmission to only one of the transmission elements at the expense of the others if more than one transmitter is broadcasting, nor is it possible to simultaneously equalize the signal sent to all k broadcasting antennas when they each transmit unequal power levels. [ABSTRACT FROM AUTHOR]

A printed circular open-loop antenna is introduced as a simple structure for producing circular polarization, the antenna is fed with a coaxial probe. By introducing a gap within the circular loop a traveling-wave current is excited and thus circularly polarized radiation can be achieved. An optimized circularly polarized antenna is designed through numerical analysis using a so-called parametric method of moment technique. Experimental verification of the new antenna is presented. The antenna has a 3-dB axial ratio bandwidth of 12% with an input VSWR of less than 2. [ABSTRACT FROM AUTHOR]