Ground Controlled Intercept (GCI)
Type 7 Final GCI radar
The Type 7 was a metric radar operating in the 1.5 metre wave band used for Ground Control Interception (GCI). The usual operating frequency was 209 MHz, though later equipments operated on 193 and 200 MHz.
Early stations from 1940 were mobile. There were transportable and fixed versions of the Final Type 7 equipment which appeared in 1942. The fixed stations comprised an Operations Building ("Happidrome"), a Radar Well and an Aerial mounted over the Radar Well. The Radar Well housed the transmitter and the receiver. The peak power of the transmitter was 80 to 100 KW, the pulse length 3, 5 or 8 microseconds and the pulse recurrence frequency variable between 300 pps and 540 pps. A spark-gap common T and R switch was used. The original frequency was 209 MHz though later equipments used 193 MHz and 200 MHz also.
Continuous tracking of targets and fighters was essential to the interception procedure and the aerial system had to provide gap-free cover. To do this an array consisting of 32 centre-fed full wave dipoles was used, mounted in four stacks each with eight dipoles. A stack of eight dipoles could be used in various combinations. For transmission, the top four dipoles and the bottom four dipoles in each stack could be combined either in phase or in antiphase under the operator's control.
This achieved overlapping beam positions and provided adequate gapfilling. For reception the dipoles in each stack could be combined in four different ways providing beams at difference angles of elevation for height finding. Switching between beams was done automatically or on a pulse-to-pulse basis using a capacity switch in the feeders. This capacity switch set the pulse recurrence frequency of the equipment. If other radars were on the same site as a Type 7 radar they had to accept locking pulses from the Type 7 to synchronise their own pulse recurrence frequency to the Type 7.
Normally, for maximum detection range all the dipoles were in phase on reception; the range at which height finding was possible was necessarily less. The aerial could be rotated in either direction at any constant speed between 0.5 rpm and 8 rpm. The antennae beam width was 15o although a narrower beam width and greater range performance was provided for the ROTOR improvement plan by the addition of more stacks of dipoles on each end of the aerial.
Siting of the equipment needed great care. A regular shallow saucer shaped depression gave the best compromise between too much clutter and less accurate height finding. Performance tended to be variable from site to site and from day to day.
Range on a bomber aircraft was typically:
Height finding accuracy was within +/- 500 ft between angles of elevation of 2.5o and 20o. Bearing accuracy was within +/- 1.5o
Typical characteristics of the Type 7 were:
(source -"Watching The Skies")
Constructed by Dick Barrett