Chain Home Radar - A Personal Reminiscence
The
Daventry transmitter was, of course, a direct descendant of these short wave
beam stations, and the CH system itself incorporated many features first seen
there. Thus Franklin invented coaxial cable as a means of distributing power
to his beam arrays: in CH, it was used to carry the receiver signals over l00m
or so from the tops of the receiver towers to the receiver itself. Another innovation
was the cooled anode transmitter (CAT) valve, in which the anode of the transmitter
valve was a massive water-cooled copper block. This was needed to overcome deficiencies
in the glass in valves with glass envelopes: under the influence of heat, high
voltages and high RE power, some glass-to-metal seals developed leaks. (An alternative
solution, adopted by the British Admiralty, was to use silica envelopes, despite
the difficulty of working this material).
Another 1920s innovation by Marconi which carried over directly into CH was the use of a goniometer (literally, angle measurer) for direction finding. In 1922, Marconi introduced an airborne direction finder, the Type 14 ADF, in which two large orthogonal coils, one running up and down the fuselage of an aircraft, the other across the wings, were connected via a goniometer to a receiver. By tuning in to various radio stations, bearings on them could be taken, and the position of the aircraft established. The goniometer was the key component, since it allowed the use of large coils fixed to the framework of the aircraft. Previously, direction-finding (D/F) stations on the ground used the aircraft's signals to fix its position, which was then radioed to it.
It
is clear that well before 1930 many of the key parts of the CH system were in
routine use. By 1935, when the query arose after the Daventry experiment 'where
to go from here?', it must have seemed natural and safe to develop a system
of which so many components were familiar and well-proven. The time-scale was
so tight that the less the development needed, the better.
Technical Outline of CH Stations
Neale's paper(3), published in 1985 under the editorship of the present author to celebrate the fiftieth anniversary of the birth of British radar (that is, the Daventry experiment), is by far the best overall technical account of a CH station, and should certainly be read, if possible, by those interested. Here only a much shorter account is given; if this leads readers to Neale's account (which may not be easily accessible to some), so much the better. Technical accounts of the transmitter(4) and receiver(5) have been given in post-war papers by the developers of these components.
The most striking features of on East Coast CH station were the towers: three steel towers, each 110m high and with three cantilevered platforms on each side (fig. 2) carried the transmitter arrays†, while four 73 m wooden towers carried the receiving arrays. The two sets of towers were perhaps 300m apart, and each group had a low building, surrounded by an earth-banked anti-blast wall, near the foot of the towers and housing the two transmitters, in the one case, and the two receivers and the operations room in the other. By comparison with the towers, the buildings were inconspicuous and bomb-proof, except for a direct hit. Even the towers, with their lattice construction, were difficult to damage by bombing or gun-fire from the air.
†One of these transmitter towers, perhaps the last to survive, can be seen at the MRC site at Great Baddow and is shown in fig. 3. It was originally installed at Canewdon in Essex, one of the earliest CH stations and was transferred to its present site in 1359 when the Canewdon station was demolished. The tower is therefore about 55 years old.
