Radar System Engineering

178 THE GATHERING AND PRESENTATION OF RADAR DATA iSEC. 69
station performance was never predictable from theory. It was always
necessary to “calibrate” a station by an elaborate series of test flights.
The maximum reliable range of a CH station was usually about 120
to 150 miles on single aircraft, but the range depended very much on the
skill of the operator and of the maintenance crew as well as on meteoro-
40,000 /
/
/
3 30,000
/
.s /
/
z 20,000-
,*
/
= 10,000-
/
.“ ~.
/ -/
O 10 ‘;O- ’30 40 50 60 70 80 90 100 110 120
Rangein miles
FIG. 6.15.—Polar diagram of typical CH station for reliable pickup of single fighter
aircraft along the line of sight. Solid line representsmain arrays; broken line represents
gap-fillingarrays.
logical conditions. As early as 1941 a Photographic Reconnaissance
Spitfire, flying at about 40,000 ft, was plotted by the CH station at
Canewdon, just north of the Thames estuary, as it flew almost all the way
to Genoa at a maximum range of over 450 miles.
Height-finding was performed by making use of the lobe pattern of
the receiving antennas. Figure 6.16
shows the vertical pattern produced
by two receiving antennas, respectively
at 220 and 90 ft above ground.
> m
~
If one compares the signals received
.:
in these two antennas as a function of
.= 9
angle of elevation, one obtains a curve
$ shown dotted in Fig. 6.16. It follows
c
that, by measuring the signal ratio,
o~4
it is sometimes possible to deduce
the angle of elevation of the target.
Elevationangle in degrees
FIG, 6.16.—Relative signalstrengths The comparison was performed by
from CH height-finding antennas. means of the goniometer already
Wavelength = 13.22 meters. (a) Two
dipoles, mean height 220 ft. (b) One described.
dipole at 90 ft. (c) Ratio of (a) to (b) Under normal circumstances, the
on an arbitrary scale.
field coils of the tzoniometer were
connected to two dipoles about 22o ft above the ground. By means of a
switch, one goniometer coil was connected to a dipole at 22o ft, the other
to a dipole at about 90 ft. By turning the goniometer to give minimum
signal, the angle of elevation of the target could be determined and hence
its height could be deduced (see Fig. 6.17).