The Bisham Gardens Mast

by Marc Haynes

It’s hard to think of a less-loved Highgate landmark than the spiky, metal telecommunications mast at the end of Bisham Gardens. The tallest of its kind in North London, visible from miles around, it's the village’s ugliest structure – but has a surprisingly exciting history.

Erected by the BBC in 1939 to transmit radio shows, the 130-foot tower (then wooden) was converted in the Second World War to combat the Luftwaffe.

In bombing raids, German pilots navigated via two radio beams from Germany, which crossed over their intended target in Britain. The first beam played a constant tone (if the pilots heard it, they knew they were on the correct course) but if hearing the second tone too, they knew they’d reached their target and would bomb it. The system was used to devastating effect for raids on Coventry and Birmingham.

The Bisham Gardens transmitter sabotaged the system ingeniously. When German planes were spotted, the mast broadcasted the second tone at low power. Believing it real, enemy pilots would follow the false signal - and quickly veer miles off-course.

Unfortunately, this method's success made Germany develop a new system, whereby planes were guided in along just one radio beam. But by chance, the night the Germans launched the new system, the MoD had converted Bisham Gardens into a listening station to supplement the Alexandra Palace state-of-the-art transmitter. In huts at the base of the Bisham Gardens tower, a modified EMI television listened to radio traffic between German High Command and their bombers.

Monitoring the incoming planes, British engineers quickly realised how the new German system worked. Once they received the frequency of the pulse from the bomber, they would order the Alexandra Palace transmitter to mimic it. Both real and fake pulses were sent back to German controllers, who couldn't tell where along the beam their planes were.

While the method may seem crude, it baffled the Germans, who concluded their planes were faulty! When the Nazis abandoned costly bombing raids in 1941, only a quarter of bombers guided into Britain by the pulse system had ever discharged their bombs.

The listening station was closed down in 1943. The credit it deserved for saving vast swathes of the country from the Luftwaffe was only revealed with the declassification of military documents in 2009.

The wooden mast blew down in a gale in 1944 (crashing directly onto neighbouring roofs) but when the BBC returned in 1945, intending to re-build it, the local authority rejected their plans. It was eventually replaced in 1950 with the metal structure seen today.

Supposedly used for taxi radios, the mast’s 2007 appearance in MI5 recruiting ads in the press suggests it may still play a role in protecting the nation today.