- Active phase arrays elements incorporate transmit amplification with phase shift in each antenna element (or group of elements). Each element also includes receive pre-amplification. The phase shifter setting is the same for transmit and receive.
- Active phase array do not require phase reset after the end of the transmit pulse, which is compatible with Doppler radar and Pulse-Doppler radar.
Passive Phase Array - Passive phase arrays typically use large amplifiers that produce all of the microwave transmit signal for the antenna. Phase shifters typically consist of waveguide elements that contain phase shifters controlled by magnetic field, voltage gradient, or equivalent technology.
- The phase shift process used with passive phase array typically puts the receive beam and transmit beam into caddy-corner quadrants. The sign of the phase shift must be inverted after the transmit pulse is finished and before the receive period begins to place the receive beam into the same location as the transmit beam. That requires a phase impulse that degrades sub-clutter visibility performance on Doppler radar and Pulse-Doppler radar. As an example, Yttrium iron garnet phase shifters must be changed after transmit pulse quench and before receiver processing starts to align transmit and receive beams. That impulse introduces FM noise that degrades clutter performance.
MICROWAVE FEEDER SYSTEM (DRIVER ELEMENT)
TYPES OF FEEDER
- Omnidirectional
- Cassegrain
- Gregorian
- Horn feed
Parabolic antennas are also classified by the type of feed, i.e. how the radio waves are supplied to the antenna. - The primary antenna is placed at the parabolic focus point.
- Reason: produce better transmission and reception. (enhance directivity and gain)
- The primary antenna has to be used together with the reflector to avoid the flaring of the radiation pattern and thus reduced the directivity.
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