Practical_Antenna_Handbook_0071639586
486 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s Branch feed { RF Microwave generators Antenna elements Phase shifters Corporate feed { Figure 20.31 Branch feed and corporate feed. Solid-State Arrays Some modern radar sets use solid-state arrays consisting of a large number of elements, each of which is capable of independently shifting the phase of a microwave input signal. In practice, both passive (Fig. 20.32A) and active (Fig. 20.32B) phase shifters are 0111057 FIG 18-32 found. In the passive implementation, a ferrite (or PIN diode) phase shifter is placed in the transmission path between the RF input and the radiator element (usually a slot). By changing the phase of the RF signal selectively, it is possible to form and steer the beam at will. A 3-bit phase shifter is capable of eight distinct states and thus allows the phase
C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 487 Binary control inputs RF input PIN diode or ferrite phase shifter Radiator element Figure 20.32A Phase shifter. Binary control inputs 0111057 FIG 18-33A PA RF port PIN diode or ferrite phase shifter T/R T/R ANT. LNA Figure 20.32B Phase-shift T/R element. to shift in 45-degree increments, while a 4-bit phase shifter (with sixteen states) allows 22.5-degree increments of phase shift. In addition to a phase shifter, the active element of Fig. 20.32B contains a transmit power amplifier (1 or 2 W) and a low-noise amplifier (LNA) for receiving. A transmit/ receive (T/R) switch isolates the 0111057 LNA input FIG 18-33B from the high-power RF emitted by the power amplifier, while a second T/R switch simultaneously selects whether the phase shifter is performing beamforming operations on the transmitted or the received RF. The total output power of this antenna is the sum of all output powers from all elements in the array. For example, an array of one thousand 2W elements forms a 2000W system. Slot Array Antennas A resonant slot (cut into a wall of a section of waveguide) is somewhat analogous, if not identical, to a dipole. Slot arrays are used for marine navigation radars, telemetry systems, and the reception of microwave television signals in the Multipoint Distribution Service (MDS) on 2.145 GHz.
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486 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s<br />
Branch<br />
feed<br />
{<br />
RF Microwave<br />
generators<br />
<strong>Antenna</strong><br />
elements<br />
Phase shifters<br />
Corporate<br />
feed<br />
{<br />
Figure 20.31 Branch feed and corporate feed.<br />
Solid-State Arrays<br />
Some modern radar sets use solid-state arrays consisting of a large number of elements,<br />
each of which is capable of independently shifting the phase of a microwave input signal.<br />
In practice, both passive (Fig. 20.32A) and active (Fig. 20.32B) phase shifters are<br />
0111057 FIG 18-32<br />
found.<br />
In the passive implementation, a ferrite (or PIN diode) phase shifter is placed in the<br />
transmission path between the RF input and the radiator element (usually a slot). By<br />
changing the phase of the RF signal selectively, it is possible to form and steer the beam<br />
at will. A 3-bit phase shifter is capable of eight distinct states and thus allows the phase
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