Phased Array
Understanding Phased Array Antennas
Phased array antennas have transformed radar and communications by enabling electronic beam steering at microsecond speeds. Unlike mechanically steered antennas that take seconds to reposition, phased arrays can switch beam direction in microseconds and form multiple simultaneous beams.
Phased Array Principles
- Element spacing: Typically lambda/2 to avoid grating lobes. At 10 GHz, this is 15 mm.
- Array gain: Increases with number of elements: G = 10 log10(N) + G_element.
- Scan volume: Typically +/-60 degrees from broadside. Gain decreases with scan angle (cos(theta) effect).
- T/R modules: Each element has its own transmit/receive module with phase shifter, LNA, PA, and switch.
Types
- Passive phased array (PESA): Single transmitter, passive phase shifters distribute power to elements. Simpler but cannot form multiple beams.
- Active phased array (AESA): Each element has its own T/R module. Can form multiple simultaneous beams. Higher reliability through graceful degradation.
Broadside array gain: G = N x G_element
Scan loss: G(theta) = G(0) x cos(theta)
Element spacing: d = lambda/2 (no grating lobes)
At 10 GHz: d = 15 mm
At 77 GHz: d = 1.9 mm
Number of elements: 64, 256, 1024, 4096 typical
Frequently Asked Questions
What is a phased array antenna?
A phased array consists of multiple antenna elements with individually controlled phase shifters that steer the combined beam electronically without physical movement. This enables rapid beam scanning, multiple simultaneous beams, and adaptive beam shaping.
How does a phased array steer the beam?
By applying a progressive phase shift across the array elements. The element closest to the desired beam direction transmits first, and each subsequent element transmits with a calculated delay, causing the wavefronts to add constructively in the desired direction.
What are the advantages of phased arrays over dish antennas?
Electronic beam steering (microseconds vs seconds), no mechanical wear, multiple simultaneous beams, adaptive null steering for interference rejection, low profile for aerodynamic applications, and graceful degradation (the array continues to work if some elements fail).