The Battle of Britain Five Months That Changed History, May—October 1940 by James Holland (z-lib.org).epub

Ministry authorized the establishment of a twenty-station chain around
Britain’s coast.
Chain Home, however, had its limitations. It was rudimentary in many
ways, as Watson-Watt and his team were well aware. Transmitter antennae
floodlit the airspace directly in front of them with pulses of radio energy. If
these pulses hit an object – such as an aircraft – they would rebound and be
sent back, rather like an echo. These pulses were high-frequency beams of a
broad wavelength of 10–13.5 metres, which required large antennae capable
of enough power to achieve the floodlight effect. Thus, CH stations
required four 360-foot masts, 180 feet apart, with antenna wires strung
between them for transmitting the pulses, and then four different antennae
of 240-foot masts for receiving the echo-like reflections. This made them
pretty big and very obvious to any German who cared to look at them
through his binoculars on the other side of the Channel. Furthermore,
because the antennae were static, rather than rotational, they could only
transmit – and receive – on the section of the coast directly in front of them.
Nor did it work over land. They could detect what was coming towards
them over the sea, but once the raiders had passed, they could offer nothing
more.
A further limitation was that aircraft could fly under the masts
completely undetected. This, however, was resolved by the addition of a
second string of radar stations, known as Chain Home Low, which then
proved to have another benefit, namely a more accurate measure of the size
of an enemy raid. While the RAF had been developing Chain Home, the
Admiralty Research Laboratory had also been carrying out research for its
own needs and had developed coastal defence and gun-laying radars known
as CD, which could measure ships’ ranges accurate to about twenty yards
using a rotating antenna. These were much lower frequency and sent out on
a shorter wavelength of just 1.5 metres. These were copied and developed
for the RAF and became Chain Home Low (CHL). Much smaller, they
were effectively a searchlight of rays rather than a floodlight, and could be
manually rotated by an operator using cranks. A similarly hand-rotated
receiver would then pick up the echoes. The CHL programme was only
implemented in the autumn of 1939, and that thirty stations had been built
and were operating by June the following year was largely down to frantic
compulsory purchasing of land and a lot of red tape being cut; aircraft
production was not the only area where bureaucracy could be overcome