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Antenna array

An antenna array, also known as an array antenna or phased array, is a system consisting of multiple individual antennas arranged in a specific geometric configuration to function collectively as a single antenna, thereby enhancing the transmission or reception of radio waves through controlled interference patterns.^[1] These arrays improve key performance metrics such as gain, directivity, and beamwidth compared to a single antenna element, with the number of elements ranging from two to thousands depending on the application.^[1] By adjusting the amplitude and phase of signals fed to each element, antenna arrays enable electronic beam steering without physical movement, allowing real-time redirection of the radiation pattern.^[2] The fundamental principle underlying antenna arrays is the array factor, which describes how the fields from individual elements combine in the far field to form the overall radiation pattern; for a uniform linear array of N elements spaced by distance d, the array factor is given by \left| \frac{\sin\left(\frac{N}{2} (\beta d \cos\phi + \psi)\right)}{\sin\left(\frac{1}{2} (\beta d \cos\phi + \psi)\right)} \right|, where \beta = 2\pi/\lambda is the propagation constant, \phi is the observation angle, and \psi is the progressive phase shift for steering.^[2] Constructive interference maximizes radiation in desired directions, while destructive interference suppresses it elsewhere, achieving higher directivity and reduced sidelobes.^[3] The far-field approximation holds for distances r much greater than the Rayleigh distance \pi W^2 / \lambda, where W is the array aperture size.^[2] Antenna arrays are classified by geometry and feeding method, including linear arrays (elements along a straight line for one-dimensional beam control), planar arrays (two-dimensional grids for shaped beams in applications like radar), and conformal arrays (curved surfaces for aerodynamics).^[1] Phased arrays, a common type, use phase shifters for dynamic control, while fixed arrays rely on static feeding networks.^[3] Benefits include interference cancellation, diversity for improved signal-to-interference-plus-noise ratio (SINR), and adaptability in modern systems.^[1] In practice, antenna arrays are essential in wireless communications (e.g., 5G base stations with massive MIMO), radar systems for target tracking, satellite communications for beam forming, and radio astronomy for high-resolution imaging.^[3] Challenges involve mutual coupling between elements, which can distort patterns, and the increased complexity in calibration and power distribution.^[2] Advances in digital beamforming and active arrays with integrated amplifiers further expand their capabilities for multifunctional operations.^[1]

Fundamentals

Principle of Operation

An antenna array consists of multiple individual antennas, known as elements, arranged in a specific geometric configuration and excited by electrical signals to collectively produce a desired radiation pattern. The fundamental principle of operation relies on the superposition of electromagnetic fields radiated by each element, where constructive interference amplifies the field strength in targeted directions, while destructive interference suppresses it elsewhere. This interference pattern is shaped by the relative amplitudes, phases, and positions of the elements, enabling control over the direction, width, and sidelobe levels of the beam without mechanical movement.^[2]^[4] In the far field, where the distance from the array is much greater than the array dimensions (typically r \gg \frac{2D^2}{\lambda}, with D as the maximum array dimension and \lambda the wavelength), the total electric field is the vector sum of contributions from each element. Each element's field can be approximated as \mathbf{E}_n \approx E_0 e^{-j\beta r_n}, where \beta = 2\pi / \lambda is the propagation constant and r_n is the distance from the n-th element to the observation point. The phase differences arise from both the geometric path lengths and any intentional excitation phases, leading to the array's directional properties.^[2]^[5] The radiation pattern of the array is described by the product of the single-element pattern (element factor) and the array factor (AF), which captures the geometric and excitation effects assuming isotropic elements. For a uniform linear array of N elements spaced by distance d along the z-axis, with progressive phase shift \alpha between elements, the array factor in the azimuthal plane is given by:

\text{AF}(\theta) = \sum_{n=0}^{N-1} e^{j n \psi}, \quad \psi = \beta d \cos\theta + \alpha

This simplifies to the closed-form expression:

\text{AF}(\theta) = \frac{\sin(N \psi / 2)}{\sin(\psi / 2)} e^{j (N-1) \psi / 2}

The maximum occurs at \psi = 0, yielding \text{AF} = N, and the pattern exhibits grating lobes if d > \lambda / 2. By adjusting \alpha = -\beta d \cos\theta_0, the main beam steers to angle \theta_0 electronically.^[5]^[4] This superposition principle extends to two- or three-dimensional arrays, where the AF becomes a product of factors along each axis, allowing for more complex beam shapes. In receiving mode, the principle is reciprocal: the array collects signals with enhanced sensitivity in the desired direction due to the same interference effects. Applications leverage this for applications like radar and communications, where beam steering provides rapid adaptability.^[2]^[5]

Advantages and Limitations

Antenna arrays provide high directivity and gain, enabling the compensation for propagation losses in high-frequency applications such as millimeter-wave communications, where individual elements would otherwise suffer from severe signal attenuation.^[6] This gain enhancement is achieved through constructive interference of signals from multiple elements, allowing for narrow beamwidths and improved signal-to-noise ratios in point-to-point and point-to-multipoint links.^[6] Additionally, arrays facilitate electronic beamforming, which permits rapid steering of the radiation pattern without mechanical movement, offering flexibility in tracking targets or serving multiple users simultaneously.^[6] In spacecraft telemetry, arrays of small antennas demonstrate superior reliability compared to large single-dish systems, as they avoid mechanical vulnerabilities and enable modular expansion for increased sensitivity.^[7] Sparse antenna array configurations further enhance these benefits by reducing the number of elements required, thereby lowering manufacturing costs, system weight, and mutual coupling effects while maintaining low sidelobe levels through optimized positioning.^[8] For instance, nonuniformly spaced arrays can achieve desired radiation patterns with up to 40% fewer elements than uniform dense arrays, improving efficiency in resource-constrained environments.^[8] Overall, the scalability of arrays supports wideband operation and miniaturization, particularly at shorter wavelengths, making them essential for modern wireless systems.^[6] Despite these strengths, antenna arrays face notable limitations, including scan loss, where beam gain degrades as the steering angle increases due to increased path differences among elements, often modeled as a cosine function with exponents greater than unity.^[6] Mutual coupling between closely spaced elements in dense arrays can distort the radiation pattern and reduce efficiency, necessitating advanced compensation techniques.^[6] At millimeter-wave frequencies, fabrication tolerances become critical, as small dimensional errors amplify performance degradation, and environmental factors like precipitation exacerbate propagation losses.^[6] In sparse designs, while element reduction mitigates some issues, challenges persist, such as slight broadening of the main beam and the computational complexity of nonlinear optimization for element placement to avoid grating lobes.^[8] For space applications, arrays introduce phasing difficulties, especially under adverse conditions like weather interference at Ka-band, and require more extensive electronics for control, increasing overall system complexity.^[7] These factors can limit the practical deployment of arrays in scenarios demanding high precision or simplicity.

Mathematical Modeling

Array Factor

The array factor (AF) is a mathematical function that characterizes the directional properties of an antenna array arising from the geometric arrangement, spacing, and relative excitations of its elements, under the assumption of identical isotropic radiators with negligible mutual coupling. It multiplies the individual element pattern to yield the total array radiation pattern in the far field, enabling analysis of beam formation, directivity, and interference effects.^[9]^[10] For a uniform linear array of N elements aligned along the z-axis and spaced a distance d apart, the far-field array factor is derived from the superposition of spherical waves emanating from each element. In the far-field approximation, where the observation distance r satisfies r \gg \frac{2 (N d)^2}{\lambda} and phase variations across the array are linear, the electric field contribution from the n-th element (indexed from 0 to N-1) is proportional to e^{j k r_n} / r_n, with r_n \approx r - n d \cos\theta and k = 2\pi / \lambda the wavenumber. Including a progressive phase shift \psi for beam steering, the total AF becomes the complex sum

AF(\theta) = \sum_{n=0}^{N-1} e^{j n (k d \cos\theta + \psi)},

where \theta is the polar angle from the array axis. This geometric series sums to the closed-form expression

AF(\theta) = \frac{\sin\left[ \frac{N}{2} (k d \cos\theta + \psi) \right]}{\sin\left[ \frac{1}{2} (k d \cos\theta + \psi) \right]},

often normalized by dividing by N so the maximum value is 1. The derivation relies on the far-field phase approximation and assumes equal amplitude excitations; the sine terms arise from the formula for the sum of a finite geometric progression with common ratio e^{j (k d \cos\theta + \psi)}.^[9] The array factor's magnitude determines key performance metrics: the main lobe direction is steered to \theta_0 where k d \cos\theta_0 + \psi = 0, or \cos\theta_0 = -\psi / (k d); the half-power beamwidth narrows with increasing N or d / \lambda, approximating $0.886 \lambda / (N d) radians for broadside arrays (\psi = 0); and sidelobe levels depend on uniformity, with the first sidelobe at approximately -13.2 dB for large N. For endfire arrays (\psi = -k d), the beam aligns along the array axis, but grating lobes may appear if d > \lambda / 2. These properties highlight the AF's role in optimizing directivity, which scales as N times the element directivity for large arrays.^[9]^[10] In two-dimensional planar arrays, the AF generalizes to a double sum over row and column indices, such as for an M \times N rectangular grid with spacings d_x and d_y,

AF(\theta, \phi) = \sum_{m=0}^{M-1} \sum_{n=0}^{N-1} e^{j [m (k d_x \sin\theta \cos\phi + \psi_x) + n (k d_y \sin\theta \sin\phi + \psi_y)]},

which separates into products of one-dimensional AFs for uniform rectangular lattices. This separability simplifies synthesis for shaped beams or scanning, though assumptions of isotropic elements and no coupling limit accuracy for dense arrays (d < \lambda / 2). The AF thus provides a foundational tool for predicting array behavior prior to full electromagnetic simulation.^[9]

Mutual Coupling and Element Interactions

Mutual coupling refers to the electromagnetic interaction between closely spaced antenna elements in an array, arising from the near-field exchange of energy that alters the current distribution on each element.^[11] This phenomenon is particularly pronounced when element spacing is less than half a wavelength, leading to deviations from ideal isolated-element behavior.^[12] In transmitting arrays, mutual coupling modifies the radiated fields by influencing the excitation currents, while in receiving arrays, it affects the induced voltages and the array manifold, impacting signal reception.^[11] The coupling strength depends on array geometry, element type, and operating frequency, with surface waves and space waves contributing to the interaction in planar structures.^[13] The primary effects of mutual coupling include distortion of the array radiation pattern, variation in input impedance with scan angle, and degradation of overall performance metrics such as gain, directivity, and efficiency.^[12] For instance, in phased arrays, coupling causes impedance mismatches that increase voltage standing wave ratio (VSWR), limiting the usable scan angle—for short dipole elements, this may restrict scanning to about 47° in the H-plane before VSWR exceeds 3:1.^[12] In receiving scenarios, it corrupts direction-of-arrival (DOA) estimation and beamforming algorithms like MUSIC, as the received signal manifold no longer matches the ideal array factor.^[11] Additionally, coupling can reduce signal-to-noise ratio (SNR) in compact arrays and introduce polarization changes, particularly in circularly polarized designs, where unmatched loads exacerbate depolarization.^[12] These interactions are more severe in finite arrays than in infinite approximations, with central elements showing up to 10% deviation in performance.^[12] Mathematically, mutual coupling is modeled using the impedance matrix \mathbf{Z}, where the diagonal elements Z_{ii} represent self-impedances and off-diagonal Z_{ij} (for i \neq j) capture mutual impedances between elements i and j.^[11] For an N-element array, the relationship between voltages \mathbf{V} and currents \mathbf{I} is given by \mathbf{V} = \mathbf{Z} \mathbf{I}, and when connected to loads \mathbf{Z}_L, the currents satisfy (\mathbf{Z} + \mathbf{Z}_L) \mathbf{I} = \mathbf{V}_s, where \mathbf{V}_s is the source voltage vector.^[11] In receiving arrays, the receiving mutual impedance Z_{t_{ij}} is more appropriate, defined as Z_{t_{12}} = V_1 / I_2 for a plane wave incident on element 2 inducing voltage V_1 on element 1, which accounts for directional dependence.^[13] The array factor incorporating coupling adjusts the element excitations accordingly, as in the two-element case: the total field E = I_1 f(\theta) + I_2 f(\theta) e^{j k d \cos \phi}, where I_1 and I_2 are solved from the impedance equations.^[11] Embedded element patterns, derived from the active impedance Z_A = Z_{ii} + \sum_{j \neq i} Z_{ij}, provide a practical way to characterize these interactions.^[12] Element interactions extend beyond impedance to scattering parameters, where the mutual coupling matrix (MCM) models the array manifold in signal processing, enabling compensation through calibration or decoupling networks.^[11] Techniques to mitigate effects include physical decoupling via metamaterials or defected ground structures, achieving isolation levels of -20 to -40 dB, and digital methods like pre-distortion of excitations based on measured Z_t.^[13] For example, in simulations of a seven-element receiving array at around 39 dB SNR, accurate modeling using receiving mutual impedances has been shown to improve direction-of-arrival (DOA) estimation.^[11] Overall, while mutual coupling poses challenges, its proper analysis ensures robust array designs, as emphasized in foundational works on array theory.^[12]

Classification

Periodic Arrays

Periodic antenna arrays consist of multiple antenna elements arranged in a regular, repeating geometric pattern with uniform spacing between elements along one or more dimensions, enabling predictable radiation patterns through constructive and destructive interference. This periodicity distinguishes them from aperiodic arrays and facilitates simplified analysis and design, often representing a discrete approximation of a continuous aperture distribution. Common configurations include uniform linear arrays (ULAs), where elements are aligned along a straight line, such as the z-axis, with constant inter-element distance d; uniform rectangular or planar arrays (URAs), featuring elements in a two-dimensional grid in the xy-plane; and uniform circular arrays (UCAs), with elements positioned equidistantly around a circle. These structures are foundational in phased array systems for applications requiring directional control.^[14] The radiation characteristics of periodic arrays are primarily described by the array factor (AF), which models the far-field pattern assuming isotropic elements, while the total pattern is obtained via pattern multiplication with the individual element pattern. For a uniform linear array of N elements spaced by d, with uniform amplitude excitation and progressive phase shift \beta, the array factor is given by

\text{AF}(\theta) = \sum_{n=0}^{N-1} e^{j n (k d \cos \theta + \beta)},

which simplifies to the closed-form expression

\text{AF}(\theta) = \frac{\sin(N \psi / 2)}{N \sin(\psi / 2)},

where \psi = k d \cos \theta + \beta and k = 2\pi / \lambda is the wavenumber.^[5] For planar arrays with M \times N elements spaced by d_x and d_y, the array factor extends to

\text{AF}(\theta, \phi) = \sum_{m=0}^{M-1} \sum_{n=0}^{N-1} e^{j (m k d_x \sin \theta \cos \phi + n k d_y \sin \theta \sin \phi + \alpha_{mn})},

allowing control over elevation and azimuth angles. These formulations enable electronic beam steering by adjusting \beta or \alpha_{mn}, without mechanical movement. Key characteristics of periodic arrays include high directivity, narrow beamwidth, and the potential for grating lobes—unwanted secondary beams arising when d \geq \lambda / 2, which degrade pattern quality and limit scan angles. For broadside configurations (\beta = 0), directivity approximates D_0 \approx 2N (d / \lambda) for linear arrays, increasing with N and d, while half-power beamwidth (HPBW) narrows as \text{HPBW} \approx 2 \sin^{-1}(0.443 \lambda / (N d)).^[5] Sidelobe levels can be controlled via amplitude tapering, such as uniform (resulting in -13.2 dB first sidelobe) or Chebyshev distributions for equiripple patterns with specified sidelobe levels. Mutual coupling between elements, though present, is more uniform in periodic setups, allowing compensation through periodic boundary conditions in simulations. Advantages of periodic arrays include ease of fabrication due to repetitive structure, computational efficiency in modeling large arrays via unit cell analysis, and scalability for high-gain applications like radar, where a 5-element ULA with d = \lambda / 2 achieves a directivity of approximately 5 and a pencil beam.^[5] They support rapid beamforming in phased arrays, with scan limits typically to \pm 60^\circ to avoid grating lobes.^[14] However, limitations encompass sensitivity to grating lobes, reduced bandwidth from element interactions, and scan blindness—abrupt pattern degradation at certain angles due to surface waves in finite arrays. For end-fire arrays (\beta = -k d), enhanced directivity via Hansen-Woodyard conditions yields D_0 \approx 1.8 \times 4N (d / \lambda), but requires d < \lambda / 4 to suppress grating lobes, limiting aperture efficiency.^[5] Representative examples include the Yagi-Uda array, a periodic linear structure with directors and reflectors for television reception, and large planar arrays in satellite communications, such as 8×8 configurations providing beamwidths under 15° in both planes. These designs, analyzed in seminal works like Mailloux's phased array handbook, underscore the role of periodicity in achieving low-cost, high-performance systems despite trade-offs in flexibility.^[15]

Aperiodic Arrays

Aperiodic antenna arrays consist of radiating elements arranged with non-uniform spacing, differing from the fixed-interval geometry of periodic arrays. This irregularity enables tailored beam patterns and addresses limitations inherent in uniform configurations, such as the emergence of grating lobes during wide-angle beam scanning. The term "aperiodic array" generally denotes an antenna system where element positions lack periodicity, allowing for enhanced control over the array factor and mutual coupling effects.^[16]^[17] Common types of aperiodic arrays include stochastic or random arrays, where element locations are determined probabilistically to yield predictable statistical properties in sidelobe levels and beamwidth;^[18] deterministic designs, such as minimum redundancy arrays (MRAs) that optimize virtual aperture for direction-of-arrival estimation;^[19] and geometrically inspired configurations like fractal or tiling-based arrays. Fractal arrays, drawing from self-similar structures such as the Sierpinski carpet or Penrose tilings, support multi-scale patterns that facilitate broadband and multi-band responses.^[20] Perturbed aperiodic tiling arrays, for example, adapt irregular motifs to curved surfaces or limited apertures while preserving low sidelobes.^[21] Clustered subarray approaches further enable scalable aperiodic layouts by grouping elements into non-uniform blocks, reducing control complexity in large systems.^[22] These arrays offer key benefits, including grating lobe suppression without excessive element spacing, lowered peak sidelobe levels (often below -18 dB via density tapering), and sparse element counts that cut hardware costs and weight compared to fully populated periodic arrays of equivalent performance. Synthesis techniques, such as the Gaussian method, derive positions through closed-form expressions based on a tapered probability density function, achieving near-optimal patterns with uniform amplitudes. Seminal developments trace to foundational array theory in Hansen (1983) and advanced position synthesis in Buttazzoni et al. (2017), which have influenced designs for 5G and vehicular radar.^[16]^[23]

Design and Synthesis

Geometry and Configuration

Antenna array geometry refers to the spatial arrangement of individual radiating elements, which fundamentally determines the overall radiation characteristics, including directivity, beamwidth, sidelobe levels, and scanning capabilities.^[24] Configurations encompass the spacing between elements, their orientation, and the number of elements, allowing for tailored performance in applications such as beam steering and pattern synthesis.^[24] These parameters are optimized to minimize grating lobes and mutual coupling effects while maximizing gain.^[25] Linear arrays consist of elements aligned along a straight line, typically the z-axis, providing one-dimensional control over the radiation pattern.^[24] Uniform linear arrays (ULAs) feature equal inter-element spacing, often set to λ/2 to avoid grating lobes, where λ is the wavelength.^[25] This configuration enables broadside radiation (maximum perpendicular to the line) or end-fire radiation (maximum along the line) by adjusting progressive phase shifts β between elements.^[24] For N elements, the array factor simplifies to AF(θ) = [sin(Nψ/2)] / [sin(ψ/2)], with ψ = kd cos θ + β, where k = 2π/λ and d is the spacing, allowing beam tilting via linear phase progression.^[24] Planar arrays arrange elements in a two-dimensional grid, such as rectangular or elliptical layouts, offering enhanced directivity and two-dimensional beam steering.^[24] Rectangular planar arrays, common in phased arrays, use M × N elements with uniform spacing in x and y directions, typically less than λ to prevent aliasing during scanning.^[24] The array factor separates into products of linear array factors, facilitating separable amplitude and phase distributions for pattern control.^[24] These configurations support wide-angle scanning but require compensation for scan blindness due to surface waves in printed implementations.^[24] Circular arrays position elements on a cylindrical or spherical surface, providing azimuthal symmetry and omnidirectional potential in the plane.^[24] Uniform circular arrays (UCAs) place N elements equally spaced along a circle of radius r = Nd/(2π), enabling constant performance across azimuth angles without pattern distortion during steering.^[25] The array factor involves Bessel functions for azimuthal dependence, J_0(ka sin θ), where a is the radius, supporting applications in direction-of-arrival estimation with isotropic response.^[24] Other configurations include conformal arrays, which adapt to curved surfaces like aircraft fuselages for aerodynamic integration, maintaining performance through non-uniform spacing.^[24] Sparse or aperiodic arrays reduce element count by irregular placement, minimizing sidelobes while preserving resolution, as in thinned linear arrays with spacing up to λ.^[24] Volumetric arrays extend to three dimensions, such as stacked planar layers, for full-sphere coverage but increase complexity in feeding and calibration.^[25] Selection of geometry balances aperture efficiency, cost, and application-specific needs like wideband operation.^[24]

Beamforming Techniques

Beamforming techniques enable antenna arrays to direct radiated or received energy toward specific directions by controlling the relative phases and amplitudes of signals at each array element, thereby shaping the overall radiation pattern.^[26] This process exploits constructive and destructive interference to enhance gain in desired directions while suppressing sidelobes or nulls elsewhere, a principle foundational to applications in radar, communications, and sensing.^[27] Conventional or fixed beamforming establishes predefined patterns using static weights, often implemented via phase shifters for narrowband operation or time-delay units for broadband scenarios to avoid beam squinting effects.^[26] For instance, the array factor for a uniform linear array under phase scanning is given by

AF(\psi) = \sum_{m=0}^{M-1} A_m e^{j(m\alpha + \psi)},

where \alpha is the progressive phase shift, \psi = (2\pi d / [\lambda](/page/Lambda)) \cos[\phi](/page/Phi), d is element spacing, [\lambda](/page/Lambda) is wavelength, and [\phi](/page/Phi) is the angle from broadside; this steers the main beam to \phi_0 where \alpha = -(2\pi d / [\lambda](/page/Lambda)) \cos\phi_0.^[26] Techniques like the Butler matrix, employing hybrid couplers to generate multiple orthogonal beams, exemplify fixed implementations for multibeam systems, as detailed in early phased array designs.^[27] Adaptive beamforming dynamically adjusts weights to optimize performance criteria, such as maximizing signal-to-interference-plus-noise ratio (SINR), in response to environmental changes like jamming or multipath.^[28] Pioneered by Howells' 1959 patent on automatic null steering and Applebaum's 1966 formulation of optimum weights \mathbf{w}_{\text{opt}} = \mathbf{R}_{xx}^{-1} \mathbf{r}_{xd}, where \mathbf{R}_{xx} is the input covariance matrix and \mathbf{r}_{xd} is the cross-correlation vector, this approach laid the groundwork for modern adaptive arrays.^[29]^[30] Algorithms like least mean squares (LMS), with update rule \mathbf{w}(n+1) = \mathbf{w}(n) + \mu \mathbf{x}(n) [d^*(n) - \mathbf{x}^H(n) \mathbf{w}(n)], enable real-time adaptation, though they trade off convergence speed against stability.^[26] In terms of implementation, beamforming is categorized as analog, digital, or hybrid.^[31] Analog beamforming applies phase shifts at radio frequency (RF) using hardware like variable phase shifters, offering low complexity and latency but limited flexibility for multiple simultaneous beams.^[32] Digital beamforming performs weighting in the digital domain post-analog-to-digital conversion, allowing precise, software-defined control and simultaneous beam formation, at the cost of higher power and computational demands; this evolved from early digital signal processing applications in the 1990s.^[33] Hybrid beamforming, prevalent in millimeter-wave systems, combines analog subarray precoding with digital baseband processing to mitigate hardware constraints while approximating fully digital performance, as analyzed in comprehensive surveys on 5G architectures.^[34] Recent advances as of 2025 incorporate machine learning (ML) and artificial intelligence (AI) techniques in antenna array design and synthesis, particularly for optimizing complex patterns, reducing computational costs, and enabling real-time adaptations in 5G/6G and IoT applications. ML methods, such as deep learning frameworks and reinforcement learning, accelerate synthesis by predicting optimal geometries and weights, achieving up to threefold faster design processes with high accuracy compared to traditional optimization.^[35]^[36]

Applications

Communications Systems

Antenna arrays play a pivotal role in modern wireless communications systems by enabling spatial multiplexing, beamforming, and diversity techniques that enhance capacity, coverage, and reliability. In these systems, arrays of multiple antenna elements allow for the directional transmission and reception of signals, compensating for path losses and multipath fading inherent in wireless channels. For instance, phased array antennas adjust phase and amplitude across elements to steer beams electronically, improving signal-to-noise ratios (SNR) and reducing interference in cellular networks.^[6] Multiple-input multiple-output (MIMO) configurations, a cornerstone of antenna array applications, exploit the spatial dimension of the channel to transmit multiple data streams simultaneously over the same frequency band. This approach, pioneered in seminal work demonstrating that capacity scales linearly with the minimum number of transmit and receive antennas in rich scattering environments, enables higher spectral efficiency without additional bandwidth. In practice, MIMO arrays in base stations and user devices support parallel data paths, as shown in layered space-time architectures where independent streams are decoded at the receiver using channel knowledge. Typical implementations in 4G LTE systems use 2x2 or 4x4 MIMO arrays to achieve up to 50% capacity gains over single-antenna systems.^[37]^[38] In 5G and beyond, massive MIMO (mMIMO) extends this paradigm by deploying large-scale arrays—often hundreds of elements at base stations—to serve multiple users concurrently with precise beamforming. This technique, introduced as a noncooperative cellular scheme with unlimited base station antennas, simplifies signal processing through channel orthogonality and pilot-based estimation, yielding multiplexing gains proportional to the number of antennas while mitigating inter-user interference. For example, mMIMO arrays operating at sub-6 GHz bands in 5G New Radio (NR) deliver up to 10-fold capacity improvements in urban deployments, with measured throughputs exceeding 10 Gbps in trials. At millimeter-wave (mm-wave) frequencies (e.g., 28 GHz), compact array designs like 8x8 patch configurations provide high gains (around 22 dBi) and wide-angle scanning (±50°), essential for point-to-multipoint access in dense networks.^[39]^[31]^[6] As of 2025, antenna arrays are central to 6G research and early trials, enabling ultra-massive MIMO at terahertz (THz) frequencies (0.1-10 THz) for data rates exceeding 100 Gbps and low-latency applications like holographic communications and digital twins. Large-scale arrays, often integrated with reconfigurable intelligent surfaces (RIS) comprising thousands of elements, support near-field beamforming and integrated sensing and communication (ISAC) for simultaneous data transmission and environmental sensing. For instance, 6G prototypes utilize fluidic or graphene-based tunable arrays to achieve adaptive beamforming over wide bandwidths, addressing propagation challenges at THz bands. These advancements, demonstrated in trials achieving over 200 Gbps in lab settings, promise enhanced connectivity for smart cities and extended reality (XR).^[40]^[41]^[42] Hybrid beamforming architectures, combining analog and digital processing in array systems, address the hardware challenges of mm-wave MIMO by reducing the number of radio-frequency chains while maintaining flexibility. These arrays support advanced features like multi-user MIMO (MU-MIMO), where beams are formed to null interference for simultaneous user serving, boosting overall system throughput. In satellite communications, geostationary arrays facilitate high-data-rate links with adaptive nulling against jamming, achieving link budgets over 50 dB. Overall, antenna arrays in communications systems prioritize low-complexity designs for integration into small cells and handsets, with ongoing research focusing on mutual coupling mitigation to preserve array performance.^[31]^[6]

Radar and Sensing

Antenna arrays play a pivotal role in radar systems by enabling precise control over the direction and shape of the transmitted and received beams through electronic means, primarily via phased array configurations. In these systems, multiple antenna elements are excited with controlled phase and amplitude differences to steer the beam without mechanical movement, allowing for rapid scanning and adaptability to dynamic environments. This capability is essential for detecting and tracking targets in real-time, as demonstrated in early developments like the U.S. Navy's AN/SPY-1 radar, which utilizes a planar array of over 4,000 elements operating at S-band frequencies to provide 360-degree coverage for missile defense.^[43]^[44] Phased array radars excel in multifunction operations, such as simultaneous surveillance, acquisition, and tracking of multiple targets, by dynamically allocating resources to different beam directions and waveforms. For instance, resource management algorithms optimize beam scheduling to maximize detection probability while minimizing interference, as explored in studies on scheduling for multifunction phased array radars. These systems achieve high angular resolution, often on the order of 1-2 degrees, through large aperture sizes, enhancing target discrimination in cluttered scenarios. In military applications, this translates to superior performance against fast-moving threats like aircraft and missiles, where electronic steering reduces response times to milliseconds compared to mechanical radars.^[45]^[46]^[47] In weather radar, antenna arrays facilitate high-volume coverage pattern scanning, enabling updates every 30-60 seconds versus minutes for traditional systems, which is critical for severe storm monitoring. The National Oceanic and Atmospheric Administration (NOAA) has advanced polarimetric phased array radar (PPAR) technology, incorporating digital beamforming to mitigate grating lobes and support dual-polarization for improved precipitation estimation and debris detection. Such arrays, often operating at S-band (2-4 GHz), provide volumetric data over wide areas, aiding in tornado warnings and flood forecasting with enhanced accuracy.^[48]^[49] Beyond traditional radar, antenna arrays are integral to sensing applications, where they enhance spatial resolution and signal-to-noise ratios for non-contact detection. In automotive radar, millimeter-wave (mmWave) arrays at 77 GHz enable advanced driver-assistance systems (ADAS) by forming virtual apertures through MIMO techniques, achieving azimuth resolutions below 1 degree for object classification and collision avoidance. For example, cascaded radar architectures with multiple transmit and receive elements create large virtual arrays, supporting long-range detection up to 200 meters while maintaining compact form factors for vehicle integration.^[50]^[51]^[52] In remote sensing, antenna arrays serve as feeds for parabolic reflectors in satellite systems, optimizing gain and sidelobe levels for Earth observation tasks like soil moisture mapping and ocean surveillance. Dual-band arrays operating in X and Ku bands, with orthogonal polarizations, improve data fidelity in environmental monitoring by reducing multipath effects and enabling simultaneous multi-spectral imaging. Additionally, in biomedical sensing, focused microwave arrays at sub-THz frequencies (e.g., 100-110 GHz) deliver high-gain beams (up to 14 dBi) for non-invasive tissue imaging, leveraging array beamforming to localize anomalies with sub-millimeter precision.^[53]^[54]^[55]

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[13]
[PDF] A Review on Mutual Coupling in Antenna Arrays and Decoupling ...
mutual coupling between antenna arrays is an undesirable effect which degrades the performance of many array signal processing algorithm. Many compensation ...
[14]
Aperiodic array synthesis using the Cross Entropy method
**Summary of https://ieeexplore.ieee.org/document/5525211:**
[15]
Periodic Arrays | SpringerLink
Most scanning array antennas are composed of rows and columns of periodically spaced antenna elements. Periodic arrays can be designed to provide extremely ...
[16]
Phased Array Antenna Handbook, Third Edition - Artech House
30-day returnsThe book explores design elements that demonstrate how to size an array system with speed and confidence. Moreover, this resource provides expanded coverage of ...Missing: periodic | Show results with:periodic
[17]
Gaussian Approach for the Synthesis of Periodic and Aperiodic ...
This paper presents a complete overview of the recently developed Gaussian approach for the synthesis of both periodic and aperiodic linear antenna arrays.
[18]
[PDF] Fast Full-Wave Methods for Aperiodic Antenna Arrays for Space ...
to the IEEE definition [15] of terms for antennas. As an example, the term “aperiodic array” is understood as “an array with non-uniformly spaced elements ...<|control11|><|separator|>
[19]
https://apps.dtic.mil/sti/tr/pdf/ADA292590.pdf
[20]
Fractal Array Antennas and Applications - IntechOpen
Fractal antennas find applications in advanced wireless communications, MIMO radars, satellite communications and space observations.
[21]
CEARL Research Group - Ultrawideband Aperiodic Arrays
Polyfractal arrays and perturbed aperiodic tiling arrays have recently emerged as categories of arrays that can lead to designs with ultrawideband performance.
[22]
Grating Lobe Suppression in Aperiodic Phased Array Antennas ...
Mar 4, 2015 · GL suppression is achieved using an aperiodic arrangement of subarrays. This approach provides a means to decrease GLs without a significant ...Missing: definition | Show results with:definition
[23]
https://doi.org/10.1109/TAP.2017.2765744
[24]
https://ia800501.us.archive.org/30/items/AntennaTheoryAnalysisAndDesign3rdEd/Antenna%20Theory%20Analysis%20and%20Design%203rd%20ed.pdf
[25]
None
Below is a merged summary of Chapter 6: Arrays from Balanis' *Antenna Theory* (3rd Ed.), consolidating all the information from the provided segments into a comprehensive response. To retain the maximum amount of detail, I will use a combination of narrative text and a table in CSV format for key characteristics, equations, and other detailed data. The narrative will cover the main topics, while the table will organize specific details such as array factor equations, directivity formulas, and limitations for easy reference.
[26]
None
Summary of each segment:
[27]
[PDF] chapter 3 antenna arrays and beamforming - VTechWorks
Array factor of 8-element phase-scanned linear array computed for three frequencies (f0, 1.5f0, and 2f0), with d=0.37λ at f0, designed to steer the beam to ...Missing: definition | Show results with:definition
[28]
A Review of Adaptive Antennas - SpringerLink
24, 1965. Google Scholar. S. P. Applebaum, “Adaptive Arrays,” Special Projects Lab., Syracuse Univ. Res. Corp., Rep. SPL TR 66 – 1, Aug. 1965. Google Scholar.
[29]
On the capacity of multi-antenna Gaussian channels - IEEE Xplore
Abstract: We investigate the use of multi-antennas at both ends of a point-to-point communication system over the additive Gaussian channel.
[30]
https://link.springer.com/chapter/10.1007/978-94-009-8447-9_23
[31]
Noncooperative Cellular Wireless with Unlimited Numbers of Base ...
Oct 7, 2010 · A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval.Missing: massive MIMO
[32]
A review of beamforming microstrip patch antenna array for future ...
This review article portrays a comprehensive analysis of the 5G beamformer Microstrip Patch Antenna array techniques for communication systems.
[33]
What is Phased Array Radar? | Keysight Blogs
Nov 24, 2020 · A phased array antenna is typically a computer-controlled array of antennas. Normally, when a signal is broadcast from multiple antennas there is a risk of an ...
[34]
Multifunction Phased Array Radar Advanced Technology ...
MIT Lincoln Laboratory (MIT LL) has been working in support of the Multifunction Phased Array Radar (MPAR) program to develop low-cost phased array radar.
[35]
https://link.springer.com/article/10.1007/s44291-025-00084-9
[36]
https://www.mdpi.com/2673-2688/6/10/248
[37]
Phased-Array Radar - an overview | ScienceDirect Topics
Phased array radar consists of an array of radiation elements that can control the amplitude and phase of each element to adjust the direction of the ...
[38]
https://ieeexplore.ieee.org/document/6770094
[39]
https://ieeexplore.ieee.org/document/5595728
[40]
https://ieeexplore.ieee.org/document/10930518/
[41]
Antenna Arrays and Automotive Applications - SpringerLink
In stock Free deliveryCovering antennas arrays operating in frequency range from UHF radio frequencies (300 MHz) up to automotive radar system microwave frequencies (above 70GHz) ...
[42]
High-Resolution and Large-Detection-Range Virtual Antenna Array ...
Mar 2, 2021 · This paper introduces a new generic scheme for a virtual antenna array (VAA) and its application in a train collision-avoidance system (TCAS).
[43]
https://www.keysight.com/blogs/en/inds/2020/11/16/what-is-phased-array-radar
[44]
Antenna Array Design for Wireless Communications and Remote ...
Antenna arrays have attracted growing attention in many applications ... sensing, making array antenna technology a cornerstone of electrical engineering.
[45]
A high-performance sub-THz planar antenna array for THz sensing ...
Jul 24, 2024 · The proposed antenna offers a peak gain of 13.90 dBi with less than 1 dB variation within the entire band of 100–110 GHz. The antenna holds the ...