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A ''passive phased array'' or ''[[passive electronically scanned array]]'' (PESA) is a phased array in which the antenna elements are connected to a single [[transmitter]] and/or [[radio receiver|receiver]], as shown in the first animation at top. PESAs are the most common type of phased array. Generally speaking, a PESA uses one receiver/exciter for the entire array.
An ''active phased array'' or ''[[active electronically scanned array]]'' (AESA) is a phased array in which each antenna element has an analog transmitter/receiver (T/R) module<ref>{{cite book |last1=Sturdivant|first1=Rick|last2=Harris|first2=Mike|title=Transmit Receive Modules for Radar and Communication Systems |date=2015 |publisher=Artech House |___location=Norwood, MA |isbn=978-1608079797}}</ref> which creates the phase shifting required to electronically steer the antenna beam. Active arrays are a more advanced, second-generation phased-array technology that are used in military applications; unlike PESAs they can radiate several beams of radio waves at multiple frequencies in different directions simultaneously. However, the number of simultaneous beams is limited by practical reasons of electronic packaging of the beam formers to approximately three simultaneous beams for an AESA{{Citation needed}}. Each beam former has a receiver/exciter connected to it.
A ''digital beam forming (DBF) phased array'' has a digital receiver/exciter at each element in the array. The signal at each element is digitized by the receiver/exciter. This means that antenna beams can be formed digitally in a field programmable gate array (FPGA) or the array computer. This approach allows for multiple simultaneous antenna beams to be formed.
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