What began as a search for Nikola Tesla’s idea for a death ray led to the very practical creation of Chain Home radar system and victory in the Battle of Britain.
For the most part, radar is a form of wireless intercept. You force a metal object to transmit a signal. Actually, to reflect a signal on a frequency of your choosing. In 1935, some British scientists sent a memo to government outlining how this could be done. Fearing the worst, government established a crash program. Britain’s Chain Home radar system was built quickly and operating by 1938.
Although quite innovative, Chain Home radar system was not sophisticated. More brute force. To get it up and running fast, technicians re-purposed existing shortwave broadcast transmitters to send pulses in the 20 to 30 Mhz range. Transmission used curtain arrays of dipoles in a fixed direction – towards Europe. Old fashioned HF direction finders were used to receive reflected pulses from enemy aircraft. Over time, operators learned how to interpret the signals displayed on a scope. They could tell rough bearing, distance and height.
Chain Home was crude. But, good enough. The real secret of success was the centralized Filter Room that collected reports from radar operators. Home Watch volunteers provided additional sightings. The combined information allowed commanders to assign their limited fighter resources to where they could make the most difference.
Later, the Allies and Germans developed much more effective microwave radar systems. This included the Chain Home Low system which filled in the gaps at lower altitudes. The original Chain Home radar system was more of a proof of concept pressed into emergency service. It worked. As the man behind the system, Sir Robert Watson-Watt described later: “Give them the third best to go on with; the second best comes too late; the best never comes.” Proof, I suppose, that perfect should not be the enemy of good.
Here is a full length documentary about early radar, the Chain Home in particular.
Chain Home Radar System – How It Worked
Chain Home transmitters sent shortwave radio pulses. Each pulse was 25 micro-seconds long, and repeated every 12.5 or 25.0 times per second. Power was either 350 or 750 kilo-watts. Antennas were arrays of horizontal dipoles hung from three towers 110 meters high. The signal lobe was very broad, about 100 degrees, in a fixed direction.
On the receiving side were four 73 meter towers spread out in a square. These towers held Adcock direction finding arrays positioned at three different levels. Operators could configure different arrays (pairs of antennas at 90 degree offset) remotely using motor driven switches.
Each time a pulse was transmitted, a signal was sent to reset the receiver sweep on an oscilloscope. The timing of the returned pulses determined distance. Strength indicated number of aircraft. Operators found bearings by tuning for a null on the direction finding Adcock antennas using a radio goniometer. By switching between receiving antennas at different heights, they figured out altitude.
In effect, Chain Home was a giant radio floodlight with reflections provided by enemy bombers. Range was around 160 kilometers. Before the war, Germans sent a zeppelin to eavesdrop on the Chain Home towers. They did not find any signals. Their receivers were tuned to UHF. It never occurred to them that Britain would implement radar on shortwave.