Britain had successfully repelled invasion attempts and made the cost of the German bombing campaigns too high.
By Kurt Barling
BBC London's Special Correspondent
On 1 September 1939 Mickey Mouse put BBC Television to bed for the duration of World War II. The backroom boys hung up their white coats in Studios A & B at Alexandra Palace and mothballed the spanking new equipment including the iconic transmitter mast that dominated the North London skyline.
Not quite! Some bright spark in the Ministry of Defence had an idea that the transmitter might be useful to the war effort. At the beginning of the war, exactly what use that might be was still unclear?
The story of re-commissioning the transmitter to prosecute the war was a huge state secret. Many of the BBC engineers involved signed the Official Secrets Act and until late in their lives were mostly reluctant to make their story public. There are surprisingly few public records to help historians like Dr Jim Lewis piece together the story.
Lewis recalls that in the 1930s Alexandra Palace, or Ally Pally, as it is known locally, was state of the art radio wave science in its infancy. The scientific boffins within the intelligence agencies tasked Professor R.V. Jones with finding out how the transmitter might be used.
Based at Bletchley Park with some of the great Cambridge mathematicians, who eventually cracked the German Enigma codes, Jones scoured pre-war files of the Secret Intelligence Services to see if their was any evidence of a German secret weapon.
German radio beams
Intelligence leaked to the British Naval attaché in Oslo early in the war revealed that the Germans had developed advanced detection techniques for identifying aircraft at a range of around 80 miles.
Radio beams guided German bombers to their targets
More significantly the documents claimed that by using radio beams German bomber command was able to guide Luftwaffe bombers accurately to targets all over Britain, including London.
The so-called X-Gerat system gave Germany a massive advantage in navigational technology. Through a mixture of information from prisoner-of-war pilots, studying downed aircraft and radio wave expertise British scientists eventually deduced the Germans had developed a two beam system which could guide a bomber in a straight line and identify a target; the so-called Knickebein (crooked leg) system.
At the same time as this research was ongoing in early 1940, BBC engineer Tony Bridgewater was asked to return to Ally Pally to get the transmitter up and running on a care and maintenance basis; he was almost certainly unaware of what for. They would soon be pressed into service.
What's the frequency, Ally Pally?
Fear of a German sea borne invasion had made the BBC transmitter militarily useful as a means of jamming radio communications during the impending roll of the Panzers (the German tank divisions) towards London. In the wake of the German Blitzkrieg in the Low Countries and the defeat at Dunkirk these were very real fears.
By the end of 1940 intelligence reports showed that a new system was being developed because the British had clearly learned to jam that system. The Y-Gerat system was a groundbreaking a way of keeping ahead of British jamming capabilities.
By a stroke of good fortune, the Y-Gerat system was working within the same frequency spectrum (40-50MHz) as the sound and vision television transmitter at .You guessed it, Ally Pally!
The BBC mast under construction at Ally Pally
In October 1940 Wilfred Pafford, another engineer with the BBC since 1932, returned to head up operations at Ally Pally for Operation Domino. He was to remain as the engineer in charge until the end of the War.
The MoD decided to set up a listening station at Swains Lane in Highgate. In a set of huts attached to a huge relay transmitter (a huge mast is still in place) used for Outside Broadcasts before the war, a domestic EMI television was modified to listen to the radio traffic between the German command station in Kassel (and elsewhere in France) and the German navigators on bombing raids over Britain.
Incoming German bomber pilots would maintain the aircraft's correct bearing by following an instrument which monitored the path of a radio beam. When the German ground station had calculated the pilot was correctly positioned over the target a message from the ground station was sent instructing the bomb aimer to release his load.
From February 4th 1941 the date of the first bombing raid using the Y-Gerat system on Britain, by pure coincidence everything was in place to give the Swains Lane and Ally Pally teams a crucial opportunity to interfere with the information being sent to the bomb aimer.
The role of BBC engineers in WWII remains largely untold
From monitoring the incoming aircraft at Swains Lane it was possible to remotely activate the transmitter at Ally Pally to jam the German navigational technology for crucial periods.
In effect, BBC Engineers had devised a system which could 'capture' the German frequency momentarily and create a 'howl round' effect in the German navigational device when the transmitter at Ally Pally was switched on. Imagine the sound when a microphone is turned up too high at a concert: All that in the flying crew's headphones.
It has been said that the consternation caused amongst the flight crews by the 'howl round' effect was audible to the operators listening on their EMI television sets in Swains Lane!
Once the operators at Swains Lane had decided that the German navigator had missed his opportunity to identify the target, the Ally Pally transmitter would be turned to standby ready to be re-activated for the next bomb aimer.
Success under the radar
The jamming system may have been relatively crude but it's claimed that no more than 25% of bombers on Y-Gerat controlled air raids released their bomb loads.
The BBC transmitter can claim credit for undermining a crucial part of the technology which aimed to lay waste to many of our cities
Kurt Barling BBC London
It's estimated that this system of jamming went undetected until May 1941. So adept had the BBC engineers become that even when the Germans suspected their system was fallible it was simply a matter of retuning the transmitter when the German operators changed the frequency. Ally Pally's role remained undetected.
Within a month on June 22nd 1941 Nazi Germany's war emphasis changed, as it launched its invasion of Russia. Britain had successfully repelled invasion attempts and made the cost of the German bombing campaigns too high.
The BBC transmitter can claim credit for undermining a crucial part of the technology which aimed to lay waste to many of our cities.
Welcome back Mickey Mouse
Mickey Mouse was the first item on air when BBC Television began transmitting again in 1945. It was a very British way of saying welcome back and sorry for the interruption in service.
Wilfred Pafford has just celebrated his 101st birthday and now lives in a nursing home in Southern England, the last of the Battle of the Beam engineers.
Today, studios A & B remain largely empty and untouched since the BBC's departure in the early 1960s to Lime Grove Studios in Shepherds Bush, a reminder of the days of pioneering television. Jim Lewis is one member of an Alexandra Palace Television Group trying to maintain that legacy.
The transmitter mast still dominates the North London skyline and remains a symbol of the BBC. Less than two miles away a mast also remains at Swains Lane. The war stories of the BBC engineers that manned them remain largely untold.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.