The Chain Home radar system
For that purpose, four transmitter and four receiver towers were provided, each pair of towers being dedicated to one spot frequency. It was later decided to abandon the four frequency plan and to have simply a main and standby in the frequency band 20-30 MHz.
All CH Stations used horizontally polarized radiation. The decision to use horizontal rather than vertical was based mainly on three factors:-
1. An approaching aircraft has a predominantly horizontal aspect, favouring - for metric wavelengths -a horizontally polarized wavefront.
2. The signal phase change of Pi radians on reflection from the ground is constant for all relevant angles of elevation; this is particularly important for the formation of the vertical polar diagram required for height finding.
3. A horizontally polarized antenna is inherently symmetrical with respect to ground, permitting balanced, open-wire transmission lines capable of withstanding very high peak voltages to be used without undue complication. Also the symmetrical nature of the antenna simplifies both its design and installation.
OUTLINE DESCRIPTION OF TYPICAL 'EAST COAST' STATION
At the end of World War 2, there were approximately fifty early warning CH Stations (AMES Type 1), of which there were a number of variants, either in 24 hour operation or at standby around the coast of Britain. All used the same basic 'floodlighting/DF' principle but configured differently depending on their operational role; some were 'all-round-looking', there were a number of buried reserves to back up the Chain should the main station be disrupted by enemy action, the West-Coast Chain used different antennae masts and transmitters to the East-Coast; and there were variations in the siting arrangements.
The
CH described here is a typical East-Coast version, figure 2, of the type familiar
to most people because of the distinctive and massive appearance of the antennae
masts, and the vital role it played in winning the Battle of Britain. Of the
fifty CH Stations in Britain, twenty one were basically of this type and were
installed along the East and South-East coast from Ventnor in the Isle of Wight
to Netherbutton in the Orkneys. (See Appendix 1 for complete list).
Each station employed four (later stations were reduced to three) in-line, 360 feet steel transmitter towers spaced approx 180 feet apart, each tower being fitted with cantilevered platforms at 50, 200 and 350 feet. The transmitter 'curtains' were slung between towers and fed by strained 600 ohm transmission lines leading from the heavily protected transmitter building nearby. Two identical transmitters (Type T.3026) were used in a main and standby role, with rapid change-over arrangements in the event of failure of the operational transmitter.
- Typical operating conditions were:-
- Frequency:
- 20 to 30 MHz
- Peak Power:
- 350 kW
- (later 750 kW)
- p.r.f.:
- 25 and 12.5 p.p.s.
- Pulse Length:
- 20 us
Four 240 feet wooden receiver towers, usually placed in rhombic formation, carried the receiver arrays. These towers and the associated receiver building were some hundreds of yards from the transmitter buildings and in some cases, were in a separate com-pound; truly bistatic in fact!
The low p.r.f. of 25 p.p.s. was determined by the need for a long interpulse period to minimize the effect of long range 'scatter' (or clutter) being returned via the ionosphere as 'second, third or fourth time round' signals cluttering the display and masking the area under surveillance. A further requirement was the need to synchronize all CH Stations to the National Grid system to avoid mutual interference (known as 'running rabbits'). A sub-multiple of 50 Hz was therefore essential, plus the need for a long interpulse period to allow sufficient time space (40 ms) for adjacent stations to be allocated time slots which did not overlap. When ionospheric 'scatter' conditions were severe, the operator had a choice of an alternative p.r.f. of 12.5p.p.s., thereby increasing the interpulse period to 80 ms and extending the immunity range of the 'scatter' to over 6000 miles.
TRANSMITTER ANTENNAE SYSTEM
To provide the necessary 'floodlighting' the transmitter main array consisted of a vertical stack of eight half-wave dipoles. The dipoles were spaced by a half-wavelength and end-fed by an open wire transmission line with alternate feed points transposed to preserve in-phase excitation of the stack, figure 3a. Associated with each dipole was a 'tuned' reflector spaced by 0.18 wavelength. The mean height of the array was 215 feet, which produced a main elevation lobe resulting from ground reflection at about 2.6o, and a first gap at 5.2o. The horizontal 'beamwidth' of the dipole stack, influenced by the 0.18 wavelength spaced reflectors, was in the order of 100o.
(This article is taken from "The GEC Journal of Research", Vol. 3 No.2 1985 pages 73-83 and has been reproduced with the kind permission of the Editor. The copyright of the material remains with the owner.)
