Radar System Engineering

SEC. 9.10] R-F TRAA’SMISS1O.V LIVES 283
paraboloid about an axis passing through the feed, the paraboloid being a
few degrees “ drunk” in relation to this axis (A>-/APG-15); or by spinning
an electrically asymmetrical feed about the axis of the fixed paraboloid
(SCR-584, Fig. 9.13). All these methods and others are in wide use.
Of the complex motions, the spiral scan has been mentioned as
derived from conical scan, and more particularly from three varieties
just listed. This is exemplified in the first two cases by the Navy’s
AX/APS-6 nightfighter radar and the British AI Mark VIII. Helical
scan is used in the SCR-720 Army AI radar and in the SCR-584 antiaircraft
set. In the latter a conical scan is superimposed so that the
result is a sort of helical Palmer scan.
9.9. The Weight of Mechanical Scanners.-The design of a mechanical
scanner is largely dependent on its antenna. Although only a small
percentage of the scanner \veight is attributable to the antenna, a large
antenna necessitates a heavy scanner. Airborne scanners show this
relation very strongly. A survey of weights’ indicates that, very roughly,
the weight of scanners having simple scans is given by 0.09D2 pounds,
where D is the paraboloid diameter in inches. The formula for complex
scans is O.13D’. The survey shows that among airborne radars with
mechanical scanners the scanner weighs 13 to 106 lb, representing from
6 to 21 per cent of the weight of the set, with an average of about 14 per
cent. Surface-based scanners are in general larger than airborne scanners.
The weight runs from 75 to 5500 lb in shipborne antenna mounts,
and as high as 28,000 lb for land-based. The antenna mounts of surfacebased
radars represent 10 to 40 per cent of the total weight of the set.
9.10. R-f Transmission Lines .-The r-f energy generated at the
transmitter is radiated by the antenna, and the echo is led back from the
same antenna to the mixer. The same transmission line’ is used for both
the transmitted and the received energy. A rotary joint must be inserted
in the line whenever more than a few degrees of rotation of an antenna
are required. Such a joint is always installed with its axis coinciding
with the axis of the corresponding degree of freedom. Pressurization of
the rotary joints, when necessary, is accomplished, either by means of a
composition rubber ring. revolving snugly around a polished steel tube, or
by means of a polished carbon ring revolving in contact with a polished
steel annulus. Helical scanners have at least two rotary joints, i.e..
for azimuth and elevation, as have spiral scanners.
9.11. Data Transmission .-The continuous transmission of information
from the scanner to the indicator, giving the attitude of the antenna
in relation to its base, is known as ‘f data transmission. ” Often these
1W. L. Myers, USN”R,“ Weight Analysis of Airborne Radar Sets,” RL Report
No. 450, Jan, 1, 1945.
2Chap. 11contains a discussionof the transmission line and associated components.