Phased Array Fundamentals for Satellite Engineers

If you work in satellite communications, phased arrays are no longer a "future technology." They're shipping in ground terminals, LEO user equipment, and hybrid antenna products today. But the jump from parabolic dishes to phased arrays introduces a set of tradeoffs that aren't always intuitive — even for experienced RF engineers.

This post covers the fundamentals that matter most in practice: beam steering, scan loss, grating lobes, and the architectural choices between analog, digital, and hybrid beamforming.

Why Phased Arrays?

A parabolic dish points mechanically. It has one beam, it's slow to reposition, and it can only track one satellite at a time. For GEO, that's fine — the satellite doesn't move relative to the ground.

For LEO and MEO constellations, everything changes:

• Satellites move. A LEO satellite crosses the sky in 5–10 minutes. You need to hand off between satellites continuously.
• Multiple simultaneous beams let you maintain links during handoff or talk to multiple satellites at once.
• No moving parts means higher reliability and lower maintenance, which matters for remote or maritime deployments.
• Conformal form factors enable flat-panel designs for vehicles, aircraft, and ships.

The cost? Complexity. A 256-element phased array has 256 phase shifters, potentially 256 amplifiers, and a beamforming network that must be calibrated and controlled in real time.

Beam Steering Basics

Each element in a phased array receives a progressive phase shift to steer the beam. For a uniform linear array (ULA) with element spacing $d$, the phase increment between adjacent elements to steer to angle $\theta$ from broadside is:

$\Delta\phi = \frac{2\pi d}{\lambda} \sin(\theta)$

This is the fundamental equation. Everything else — scan loss, grating lobes, sidelobe levels — flows from it.

Key intuition: You're not moving the antenna. You're constructively interfering in the desired direction by making the wavefront from each element arrive in phase at the far field.

Scan Loss

As you steer away from broadside, two things happen:

1. The projected aperture shrinks. At angle $\theta$, the effective aperture is $A_{\text{eff}} = A \cos(\theta)$. This is a geometric effect — the array looks smaller from off-axis.
2. Element patterns roll off. Each element has its own radiation pattern, typically modeled as $\cos^n(\theta)$ where $n$ depends on the element type.

The combined effect is typically $\cos^{1.2}(\theta)$ to $\cos^{1.5}(\theta)$ in power, depending on the element design. At 60° scan, you've lost 3–6 dB of gain compared to broadside.

Practical implication: System designers must account for scan loss in the link budget at the worst-case scan angle. For a LEO terminal that needs to track down to 20° elevation, you're scanning to 70° from broadside — and the array is barely functional. Most systems set a minimum elevation angle of 30–40° to keep scan loss manageable.

Grating Lobes

Grating lobes appear when the element spacing exceeds a critical threshold. For scan angle $\theta_{\max}$, the grating lobe condition is:

$d < \frac{\lambda}{1 + \sin(\theta_{\max})}$

At broadside only ($\theta_{\max} = 0$), the limit is $d < \lambda$. For ±60° scan, it tightens to $d < 0.535\lambda$.

Why this matters: Grating lobes are full-power copies of the main beam. They cause interference to adjacent satellites, violate regulatory EIRP density masks, and waste transmit power. You can't "window" them away like sidelobes — they're a spatial aliasing artifact.

Design tension: Smaller spacing means more elements per unit area (higher cost, more power, more complexity) but cleaner patterns. The half-wavelength rule ($d = \lambda/2$) is the standard compromise for arrays that need wide scan.

Sidelobe Control: Taper Tradeoffs

Uniform illumination gives the highest directivity but the worst sidelobes (−13.2 dB for a ULA). Amplitude tapering reduces sidelobes at the cost of directivity and beamwidth:

Taylor and Chebyshev are the workhorses for satellite antennas because they let you specify the sidelobe level directly while keeping beamwidth growth small. Taylor tapers are preferred when you need low close-in sidelobes but can tolerate higher far-out sidelobes (common for regulatory compliance).

Analog vs. Digital vs. Hybrid Beamforming

This is where architecture decisions get expensive.

Analog Beamforming

Each element has a phase shifter (and optionally a variable gain amplifier), and signals are combined in the RF domain before a single ADC/DAC chain.

• Pros: One RF chain, low power, simple baseband
• Cons: One beam at a time, calibration-sensitive, limited null steering
• Use case: Low-cost terminals, consumer LEO equipment

Digital Beamforming

Every element has its own full RF chain (LNA → mixer → ADC or DAC → mixer → PA). Beamforming happens in the digital domain.

• Pros: Unlimited simultaneous beams, adaptive nulling, per-element calibration, MIMO capability
• Cons: $N$ RF chains means $N\times$ the power, cost, and data throughput. A 256-element array at 2 GHz bandwidth needs ~1 Tbit/s of ADC data.
• Use case: Military systems, high-end ground stations where performance justifies cost

Hybrid Beamforming

Subarrays of elements share analog beamforming, with digital combining across subarrays. A 256-element array might have 16 subarrays of 16 elements each, requiring only 16 RF chains.

• Pros: Multiple simultaneous beams (up to the number of subarrays), manageable power and cost, good enough for most satellite applications
• Cons: Beam squint within subarrays for wideband signals, constrained pattern flexibility
• Use case: Modern ground terminals, hybrid parabolic+phased array products, 5G base stations

The industry trend is hybrid. Pure digital is too expensive for commercial ground terminals. Pure analog can't support the multi-beam requirements of LEO constellations. Hybrid gives you 80% of digital's capability at 20% of the cost.

Calibration: The Hidden Challenge

A phased array is only as good as its calibration. Manufacturing tolerances, temperature drift, mutual coupling, and component aging all introduce phase and amplitude errors that degrade the beam pattern.

What goes wrong without calibration:

• Beam pointing errors (the beam isn't where you think it is)
• Elevated sidelobes (regulatory violations, interference)
• Reduced gain (link margin erosion)
• Null fill-in (adaptive nulling doesn't work)

Calibration approaches:

• Factory calibration: Measure each element's response in an anechamber. Good starting point, doesn't track drift.
• Mutual coupling methods: Use inter-element coupling to estimate and correct errors in-situ. No external source needed.
• Over-the-air (OTA): Use a known satellite or beacon to calibrate the full array in its operating environment. Best accuracy, requires a reference signal.

For fielded systems, OTA calibration on a regular schedule (daily or per-session) is standard practice.

Putting It Together: Link Budget Impact

When you replace a parabolic dish with a phased array in a link budget, several line items change:

• Antenna gain becomes scan-angle-dependent. Your worst-case gain is at maximum scan, not broadside.
• G/T may degrade because the array's noise figure is set by the LNA at each element plus the combining losses — which can be worse than a single LNA behind a dish feed.
• EIRP per element is limited by the PA, and you're combining in space. Total EIRP scales as $N^2$ for coherent combining ($N$ elements, each contributing $N\times$ the single-element gain). But PA efficiency and thermal management at scale are real constraints.
• Polarization can be electronically switched or provide simultaneous dual-pol, which a dish needs an OMT and dual feeds to achieve.

The net result: a phased array often has 3–6 dB less peak performance than an equivalently-sized dish, but its ability to form multiple beams, steer electronically, and maintain links during handoff makes it the right choice for LEO/MEO ground terminals.

Summary

Phased arrays aren't magic — they're engineering tradeoffs with well-understood physics. The art is in choosing the right architecture for your link budget, regulatory constraints, and cost target.