Radar System Engineering

SEC. 15.10] COMPONENTS DESIGN 607
1. Azimuth angle of beams does not shift with a change in wavelength.
2. Multiple beams from separate transmitter-receiver systems can be
accurately set to the same azimuth angle. This property is very
desirable if the system is to be equipped for work with radar
beacons.
3. Open mesh or a grid structure can be used for the reflector surface
to reduce wind resistance. The linear-array system requires a
solid surface for the reflector, since the comparatively high gain
of the linear-array feed will give an objectionable back lobe
through
mesh or gridwork.
The ilfodulator.-Because of its simplicity and power-handling
capacity, an a-c resonance-charging line-type modulator using a rotary
FIG. 15 6.—Modulator of high-power ground radar.
gap as a switching device was chosen. The rotary gap is mounted
directly on the shaft of a 400-cps alternator, which excites the network
and is properly phased by mechanical adjustment (Fig. 15.6). This
modulator choice was important in fixing the value of the PRF, but,
as already remarked, the desirable value lies in the neighborhood of
400 pps in any event.
Separate transmitters are used on the upper-beam and the lowerbeam
systems; both are driven, through pulse transformers, from a
single modulator. The modulator is rated at 3000 kw.
R-j Components.—The high pulse powers used in this system demand
the use of waveguide for the r-f system. Each magnetron operates in a
field of 2S00 gauss at 28 kv pulse voltage, dra\ving a current of about 30
amp and putting 750 k\v of r-f power into the line at about 65 per cent
efficiency.
Crystal protection at this high power level is difficult, and a scheme
of “repulsing” the TR tube is used. Part of the 8-kv modulator pulse