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Radar Type 7 GCI

(Continued)

Following the second world war the Type 7 remained in use at many R.A.F. radar  stations. In the late 1940's - early 1950's the Type 7 radar was extensively updated for use in the Rotor air defence radar reporting system. The following information is reproduced from SD 727, Part 2 , Section 3, Chapter 2:

Radars Type 7 Marks 4 and 5

Introduction

1. Radar Type 7 is a metre-wave searchlight azimuth-search radar designed for static installation, it is used for control (primarily at shorter ranges where clutter makes the Type 80 unusable), and as an alternative-frequency early warning radar in case of jamming. Mks. 2 and 3 were re-engineered versions of the earlier Type 7 (Final G.C.I.), and were produced for Operation “Rotor”. They have been modified to Mks. 4 and 5, which no longer have the height-finding facility. Each consists of an aerial system mounted on a column and driven by a motor; and below it an underground concrete well containing transmitters and receivers with control gear. They are designed to work with the displays and radar office equipment described in Chap. 4 of this section. They differ in that Mk. 4 is designed to work on the main radar site near the operations block, while Mk. 5 is designed for siting up to two miles remote from the operations block, on stations where the main site is not suitable for Type 7. Mk. 5 has a larger well than Mk. 4, containing a subsidiary master trigger rack and equipment for transmitting the information over long lines to the operations block (this extra equipment is described in Chap. 4, paras. 7 and 22 of this section). Type 7 has an I.F.F. Mk. 10 installation called Radar Type 79, which is described in para. 8.

Leading Particulars

2. The leading particulars of Radars Type 7 Mks. 4 and 5 are:-

(a) Radio Frequency. All Type 7 radars now work on 220 Mc/s ( l= 1.36 metres). The transmitter and receiver are tunable over the range 180 to 220 Mc/s, but the aerial system would require modification for any but a very small change of radio frequency.

(b) Pulse-Width and Bandwidth. The transmitted pulses are about 8 us wide, and the I.F. bandwidth of the receiver is about 0.5 Mc/s.

(c) P.R.F. The radar can be externally synchronized at any P.R.F. between 200 and 500 p.p.s., or free-run at 250 p.p.s.

(d) Output Power. The peak R.F.. power output of the transmitter can be varied up to a maximum of about 500 KW.

(e) Receiver Noise Factor. The receiver noise factor should not be greater than 8 db.

(f) Polarization. The radar generates horizontally polarized radio waves.

(g) Beam width. The half power point beam width in azimuth is about 3.6o, and the first appreciable side lobes are at about 20o from the line of shoot, all side lobes being at least 20 db down on the main beam.

(h) Scanning Rate. The turning gear allows continuous clockwise rotation only, at any rate up to about 6 r.p.m.

Aerial System and Turning Gear

3. The Type 7 aerial system is built on a metal framework 64 feet wide, 11. feet tall, and 5 feet deep; to this framework is attached a flat vertical steel-mesh reflector screen 11 feet high and about 0.11 l in front of the screen are mounted four stacks each of twelve full-wave horizontal dipoles made of copper tube. The framework is mounted (see Fig. 1) so that the array is at a mean height of 25 feet above the ground. The array is efficient only over a narrow frequency-band centred on 220 Me/s. The dipoles are energized through twin feeder lines, which are so designed that all the stacks are fed in phase.

4. The aerial framework is supported by rollers at the top of a fixed central column about 25 feet high, which is itself mounted vertically on a foundation above the transmitter well in such a way as to ensure that the arrays are at the correct height. The framework is rotated about the column by a 16 h.p. D.C. motor acting through a gear box and chain drive with an overall reduction of about 127:1. The speed of the motor is controlled by varying its armature current, supplied by a D.C. generator (a Ward-Leonard set) driven at constant speed, whose field current can be varied by a remotely controlled motor-driven rheostat to give aerial speeds from zero to between 6 and 8 r.p.m. The remote control panel has push-buttons for stopping and starting, and also for raising and lowering the speed (these drive the rheostat up or down); when the stop button is pressed the rheostat is automatically driven to its zero position, so that on starting it is necessary to use the RAISE button to bring the speed up to the required value (the aerial takes 15 seconds to reach 6 r.p.m. from rest). The aerial cannot be slaved in rotation to any other aerial, but carries two selsyn transmitters; one of these can be used in slaving other aerials to the Type 7, and the other is used to drive the rotating time bases on the P.P.I. displays in the operations block. Connections from the aerial to the well are made through a slip-ring and rotating R.F. coupling unit mounted in a cubicle on top of the column, and they run down inside the column.

Transmitter and Receiver

5. The Type 7 transmitter is normally triggered by pulses at 270 p.p.s., supplied from the master trigger rack in the operations block (Mk. 5 has a remote trigger rack in its well - see Chap. 4, para 7 of this section). The radar modulator develops 8 us pulses at this frequency, whose amplitude can be varied between 6 and 15 KV from a control unit in the transmitter cabinet. These pulses are applied to the R.F. oscillator, which consists of two air-cooled triode valves with resonant (Lecher) lines, tunable over the range 180 to 220 Mc/s. The output, R.F. pulses of peak power depending on the amplitude of the modulating pulses, is taken by a twin feeder from the top of the transmitter cabinet through the T.R. switch and up the column to the rotating coupling in the cubicle. The transmitter incorporates a monitor with which its performance may be checked and the incoming signals examined. The installation contains two identical transmitters, one as standby, with provision for easy changeover.

6. The aerial system is connected to the transmitter and the receiver through a T.R. switch unit, which employs spark gaps and ensures that transmitter and receiver are electrically isolated from one another. Received signals are first amplified by a low-noise R.F. amplifier and then mixed with the output of a conventional local oscillator at 175 Mc/s to give signals at the standard 45 Mc/s I.F.: the I.F. signals are then given about 60 db more amplification before being passed to the operations block. The transmitter and local oscillator are so stable that A.F.C. is not applied.

Range Performance

7. The range performance of Type 7 against a Meteor NF11 aircraft is illustrated in Fig. 2; the curve is of 1.5 : 1 signal-to-noise amplitude ratio, which has been found in practice to correspond roughly with 50 per cent. probability of paint. The V.P.D. shown in Fig. 2 is drawn for a flat site from a combination of theory and observation: much variation from it can be caused by differences between sites, and the pattern below about 2o has a fine structure of indentations that cannot be shown but may give unexpectedly high or low ranges. Interference lobes above the second have been left out, for the sake of clarity. The principles of Type 7 siting are essentially those given in para. 18.

I.F.F. Installation

8. It is not practicable to mount I.F.F. gear on the Type 7, and so an I.F.F. installation called Radar Type 79 is associated with it on the same site. In Stage I of Operation “Rotor” only the remote Type 7 had I.F.F., but it has now also been provided for Type 7 Mk. 4 (local). Type 79 consists essentially of a Type 14 radar cabin, mount, and turning gear with I.F.F. aerial and equipment but without any primary radar gear; its cabin and turning gear are described in Chap. 3, para. 32 of this section. It is synchronized in rotation with the Type 7, and receives 270 p.p.s. mode trigger pulses from a mode interlace unit in the well; video signals from its receiver go to a rack in the well where they are amplified for transmission to the operations block. The I.F.F. Mark 10 gear is described in Chap. 7 of this section.

Type 7  Mk4 Vertical Performance Diagram

The information above is reproduced from SD 727, Part 2 , Section 3, Chapter 2:


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 Updated 22/09/2003

Constructed by Dick Barrett

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©Copyright 2000-2003 Dick Barrett

The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.