Electronically scanned array


Electronically scanned array

An electronically scanned array (ESA), or a phased array, offers many advantages over mechanically-scanned antennas such as instantaneous beam scanning, the availability of multiple concurrent agile beams and concurrently operating radar modes. T/R modules and passive electronically scanned subarrays are building blocks of an ESA. Many trade-offs are to be made in the design of an ESA and several of them are stated below:

# Active versus passive: Most high-performance and multi-functional ESAs are active, featuring a T/R module behind every antenna element. Transmit as well as receive path have a phase shifter and amplifiers in order to set beam position, to establish transmit power and to achieve the noise figure requirement of the receiver. The cost of T/R modules based on III-V MMIC technology is in the order of $1000 per module, and the accompanying brick construction approach and necessary cooling systems make active ESA (AESA) based radars with more than one thousand antenna elements bulky and expensive. In a passive ESA (PESA), a passive array is connected to a single high-power T/R module.
# Architectures: Real-beam ESA architectures include frequency scanning, commutating beam based on a monopulse TTD beam former and beamforming networks (BFNs) producing multiple spatially orthogonal beams [Mailloux, R. J.: "Phased Array Antenna Handbook", Artech House, 2005] [Brookner, E.: "Practical Phased Array Antenna Systems", Artech House, 1991] [Hansen, R. C.: "Phased Array Antennas", John Wiley & Sons, 1998] .
# Beam steering: phase shifting versus TTD phase shifting: The beam of an ESA is scanned preferably by applying a progressive time delay, Delta au, constant over frequency, across the antennas of the array. Invariance of time delay with frequency is the primary characteristic of a true time delay (TTD) phase shifter or a time delay unit (TDU). Usage of TTD phase shifters avoids beam squinting or frequency scanning. The scanning angle, heta, is expressed as a function of the phase shift progression, eta, which is a function of the frequency and the progressive time delay, Delta au, which is invariant with frequency:

k , d , cos{ heta} = eta left ( f ight ) = 2 , pi , frac{c}{lambda_0} , Delta au

heta = arccos { left ( frac{c}{d} , Delta au ight )}Note that heta is not a function of frequency. A constant phase shift over frequency has important applications as well, albeit in wideband pattern synthesis. For example, the generation of wideband monopulse Sigma/Delta receive patterns depends on a feed network which combines two subarrays using a wideband hybrid coupler.
# Construction: brick versus tile: Brick refers to a construction approach in which the RF circuitry (TTD phase shifters or T/R modules) is integrated perpendicular to the array plane. Tile, on the other hand, refers to a construction approach in which the RF circuitry is integrated on boards or wafers parallel to the array plane. Lamination or bonding of boards or wafers yields low cost, low weight and low profile multi-layer ESA panels. Tile construction is therefore favored whenever cost is the primary concern.

References


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