Page 187 - Radar Technology Encyclopedia

177 feed, constrained feed, dummy-snake

to produce the sum illumination function and a secondary line
that is coupled only to the difference channel (Fig. F10). The
discontinuity introduced by the center hybrid of the primary
line in the difference illumination is canceled by an opposite
discontinuity in the secondary line, so the desired smooth
functions can be obtained on both channels. To hold the elec-
trical tolerances over the operating band of the radar, each
element has to employ two precision couplers, which makes it
Connections to Electromagnetic
phase shifters lens
relatively costly to obtain low sidelobes as compared with
single-line feed. SAL
E Ref.: Barton (1988), p. 203.
Sum illumiation

Input Difference illumination
Primary line difference
Figure F8 Parallel feed networks (after Skolnik, 1990, p. 7.61).

Phase shifter
Secondary line difference
Primary line Secondary line
S
S

Difference port Sum port
S
Figure F10 A dual-ladder network for separate optimization of
sum- and difference-excitation of a linear array (after Barton,
S 1988, p. 206).
Figure F9 Series feed networks (after Skolnik, 1990, p. 7.60). A dummy-snake feed is a technique for implementing a fre-
quency-scanned monopulse antenna (Fig. F11). It has two
sion line from which energy is tapped through loosely cou-
halves of identical design to scan synchronously. Disadvan-
pled junctions to feed the radiating elements (Fig. F9a), and
tages that make it difficult to construct are difficulty in match-
the center-fed serial feed system, which can be used to form
ing the two halves and inability to obtain independent control
monopulse sum-and-difference beams (Fig. F9b).
of sum and difference patterns. SAL
When a mixer and power amplifier are used at each ele-
Ref.: Johnson (1984), p. 19.12.
ment to produce lower-power-level beam steering, the feed
configuration sometimes is called a mixer-matrix feed. A feed
often used as a series feed for economy and ease of fabrica-
tion is a slotted waveguide.
Second half
The main drawback of the constrained feed is complex-
ity, especially when a network for multibeam antenna has to
be implemented. SAL Dummy snake Aperture distribution Coupling distribution
Ref.: Barton (1988), p. 175; Skolnik (1980), p. 307, (1990), p. 7.58; Johnson
(1984), pp. 20.51–20.54.
First half
A dipole feed is a dipole placed at the focal point of a lens or
parabolic reflector to illuminate the aperture. The dipole feed
will normally use a small reflector to restrict its radiation to
D
the direction of the aperture.
Ref.: Sazonov (1988), p. 380; Silver (1949), p. 448.
S
A dual-ladder [Lopez] feed is a parallel, constrained feed for Figure F11 Dummy-snake frequency-scan monopulse antenna
monopulse phased arrays using the primary line with couplers (after Johnson, 1984, Fig. 19.9, p. 19.12).