How does beam steering work?

In a linear array with element spacing d, to steer the beam to angle theta from broadside, each successive element needs a phase shift of delta_phi = k*d*sin(theta), where k = 2*pi/lambda. This creates a progressive phase front that makes all element contributions add constructively at angle theta. For d = lambda/2 and theta = 30 degrees: delta_phi = pi*sin(30) = pi/2 = 90 degrees per element. Maximum scan angle is limited by: (1) Element pattern: cos(theta) gain rolloff, causing 3 dB scan loss at 60 degrees. (2) Grating lobes: at d = lambda/2, first grating lobe appears at theta_g = 90 degrees (horizon) when scanning to 90 degrees. For wider element spacing, grating lobes appear at smaller scan angles: theta_g = arcsin(lambda/d - sin(theta_scan)). Rule: d <= lambda/2 for grating-lobe-free scanning to 90 degrees.

What is an active electronically scanned array (AESA)?

An AESA integrates a T/R (transmit/receive) module at each element containing: PA, LNA, phase shifter, T/R switch, and optionally amplitude control. Advantages over passive phased arrays (which use a single PA/LNA and distribute power via a feed network): (1) Higher reliability: failure of one T/R module causes graceful degradation (slight gain reduction), not total failure. (2) Higher EIRP: distributed amplification avoids feed network losses. (3) Better NF: LNA at each element amplifies before lossy combining. (4) Arbitrary beamforming: each element has independent amplitude and phase. AESA is the dominant architecture for modern military radar (F-22 APG-77: 2000 T/R modules), 5G mmWave base stations (256-1024 elements), and LEO satellite terminals (Starlink user terminal: ~1200 elements).

How does a phased array differ from massive MIMO?

Phased array: all elements serve a single beam (or a few beams via subarray partitioning). The beam is steered by analog phase shifters. Signal processing is done after combining (at the subarray or array level). Typically 16-2000 elements. Massive MIMO: each element (or small subarray) has its own complete RF chain and ADC/DAC. Beamforming is done digitally in baseband. Can form many independent beams simultaneously to serve many users. Typically 64-256 elements in sub-6 GHz, 256-1024 in mmWave. In practice, 5G implementations are hybrid: analog beamforming within subarrays (phased array) combined with digital beamforming across subarrays (MIMO). This balances the flexibility of digital beamforming with the practical constraints of per-element digitization (ADC count, power, cost).

Antenna Systems

Phased Array

Q: What is an active electronically scanned array (AESA)?

An AESA integrates a T/R (transmit/receive) module at each element containing: PA, LNA, phase shifter, T/R switch, and optionally amplitude control. Advantages over passive phased arrays (which use a single PA/LNA and distribute power via a feed network): (1) Higher reliability: failure of one T/R module causes graceful degradation (slight gain reduction), not total failure. (2) Higher EIRP: distributed amplification avoids feed network losses. (3) Better NF: LNA at each element amplifies before lossy combining. (4) Arbitrary beamforming: each element has independent amplitude and phase. AESA is the dominant architecture for modern military radar (F-22 APG-77: 2000 T/R modules), 5G mmWave base stations (256-1024 elements), and LEO satellite terminals (Starlink user terminal: ~1200 elements).

Q: How does a phased array differ from massive MIMO?

Phased array: all elements serve a single beam (or a few beams via subarray partitioning). The beam is steered by analog phase shifters. Signal processing is done after combining (at the subarray or array level). Typically 16-2000 elements. Massive MIMO: each element (or small subarray) has its own complete RF chain and ADC/DAC. Beamforming is done digitally in baseband. Can form many independent beams simultaneously to serve many users. Typically 64-256 elements in sub-6 GHz, 256-1024 in mmWave. In practice, 5G implementations are hybrid: analog beamforming within subarrays (phased array) combined with digital beamforming across subarrays (MIMO). This balances the flexibility of digital beamforming with the practical constraints of per-element digitization (ADC count, power, cost).

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Phased array: multiple elements with phase-controlled steering, no mechanical movement. G_array = G_element + 10log(N). Beam steering: Δφ = k·d·sin(θ). d ≤ λ/2 for grating-lobe-free scan. AESA: T/R module per element (PA+LNA+phase shifter). F-22: 2000 T/R modules. 5G mmWave: 256-1024 elements. Hybrid analog+digital beamforming.

Array gain: +10log(N)

Spacing: d ≤ λ/2

Scan loss: cos(θ)

Understanding Phased Arrays

Phased arrays are the most important antenna technology of the 21st century. They enable 5G mmWave communications (beam tracking to mobile users), modern military radar (agile beam for tracking multiple targets), and satellite internet (Starlink user terminals). The ability to steer a beam electronically in microseconds, without any mechanical movement, transforms how wireless systems operate. Multiple beams, null steering, adaptive interference rejection: all become possible with a phased array.

Phased Array Equations

Array factor:
AF = Σ w_ne^{j(n·kd·sinθ + φ_n)}

Beamwidth:
θ_3dB ≈ 0.886λ/(Nd·cosθ_s)

Grating lobe condition:
d ≤ λ/(1+|sinθ_max|)

Phased Array Applications

Application	Elements	Frequency	Scan Range	Technology
5G mmWave BS	256-1024	28/39 GHz	±60°	SiGe/GaAs RFIC
5G mmWave UE	4-16	28/39 GHz	±45°	SiGe beamformer
AESA radar	1000-2000	X/S band	±60°	GaN T/R modules
Starlink terminal	~1200	Ku/Ka	±55°	Custom RFIC
Auto radar	12-48	77 GHz	±15°	SiGe transceiver

Key Equations

Friis transmission:
P_r = P_tG_tG_r(λ/4πd)²

Antenna gain:
G = η_ap × 4πA_eff/λ²

Beamwidth (3 dB):
θ ≈ 70λ/D degrees

Comparison

Elements	Gain	Beamwidth	Scan	Application
16	18 dBi	25°	±45°	WiFi MIMO
64	24 dBi	12°	±60°	5G gNB
256	30 dBi	6°	±60°	Satcom
1024	36 dBi	3°	±60°	Radar
4096	42 dBi	1.5°	±45°	AESA radar

Common Questions

Frequently Asked Questions

Beam steering?

Progressive phase shift per element: delta_phi = k*d*sin(theta). d=lambda/2, 30 deg: 90 deg/element. Max scan limited by cos(theta) gain rolloff. Grating lobes: d <= lambda/2 required for full hemisphere.

AESA?

T/R module per element: PA+LNA+phase shifter+switch. Graceful degradation, distributed amplification, better NF. F-22: 2000 modules. 5G BS: 256-1024 elements. Starlink: ~1200 elements.

vs massive MIMO?

Phased array: analog phase shifters, single beam, combined before digitization. Massive MIMO: per-element ADC/DAC, digital beamforming, many simultaneous beams. 5G: hybrid approach (analog subarray + digital across subarrays).

Related Terms

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