Phased array

A phased array consists of multiple radiating elements (e.g., dipoles, patches, or horns) arranged in a regular lattice. By adjusting the phase of the signal at each element, constructive interference occurs in a desired direction, while destructive interference suppresses radiation in others. The beam direction is given by the equation d·sin(θ) = λ·Δφ/(2π), where d is element spacing, λ is wavelength, and Δφ is the phase difference between adjacent elements.¹

Beam steering is achieved by phase shifters or digital beamforming. In digital beamforming, each element has its own ADC/DAC and the phase shifts are applied in software, allowing multiple simultaneous beams and adaptive nulling.²

The concept dates to the late 19th century, with Karl Ferdinand Braun's 1905 patent on directional transmission using phase control. Early experiments in the 1920s by Marconi and others demonstrated beam steering. The first large-scale operational phased array was the AN/FPS-85, a U.S. Air Force space-tracking radar built in the 1960s. The 1970s saw the Aegis system's AN/SPY-1 radar, which uses phased arrays for naval defense.

Phased arrays can be classified as passive (PESA) or active (AESA). In a PESA, a single transmitter feeds all elements through phase shifters; in an AESA, each element has its own transmit/receive module, offering higher reliability and flexibility.² Other variants include conformal arrays that follow a curved surface and digital arrays that use direct sampling.

Phased arrays are used in military radar (e.g., Aegis, Patriot), air traffic control, weather radar (e.g., NEXRAD upgrades), satellite communications, and radio astronomy (e.g., the Allen Telescope Array). In 5G cellular networks, massive MIMO uses phased arrays for beamforming to improve spectral efficiency. They are also used in medical hyperthermia and microwave imaging.

One lesser-known application is in ground-penetrating radar for archaeological surveys, where phased arrays allow high-resolution subsurface imaging. Another is in wireless power transfer, where phased arrays focus microwave energy for space-based solar power concepts. The Murchison Widefield Array in Australia uses a phased array of dipole antennas to observe the early universe. Additionally, phased arrays have been used in non-lethal crowd control (e.g., the Active Denial System) which uses a focused millimeter-wave beam.

Key challenges include cost, power consumption, and thermal management, especially for AESA. Calibration and mutual coupling between elements also affect performance. Future trends include the use of metamaterials for low-cost beam steering and photonic beamforming for ultra-wideband operation. Research is also exploring reconfigurable intelligent surfaces as a passive alternative for 6G communications.

AESA

Active Electronically Scanned Array; each element has its own transmitter/receiver.

PESA

Passive Electronically Scanned Array; a single transmitter feeds all elements.

Beamforming

Technique to direct radiation in a specific direction using phase and amplitude control.

Mutual coupling

Electromagnetic interaction between adjacent antenna elements that affects performance.

Massive MIMO

Large-scale antenna arrays used in 5G for spatial multiplexing.