There’s a lot to see at the Bois Julien site near to Desvres, inland of Boulogne-sur-Mer as it was once the location of one of the largest German radio navigation systems created during the Second World War.
Bois Juilen was the home to a massive, 28m by 35m antenna which was part of the Bernhard radio navigation system for Luftwaffe aircraft.
Developed in late 1940, it was created to replace the Knickebein system which was jammed and overcome by Allied intelligence almost as quickly as it was introduced.
More than a dozen sites were constructed in the German occupied territories with Bois Julien designated as Bernard-3.

To the Germans, the system was known as FuSAn724 (Funksendeanlage) and the large antenna was a VHF transmitter which would send locational bearing data to radio receivers in night fighter aircraft helping to direct them to intercept Allied bombers.
The antenna, and the large wooden equipment and crew cabin incorporated into the structure, rotated along a set of rails mounted on top of the concrete ring at two revolutions per minute and the signal from it could be received over 300 miles away, depending on the altitude of the aircraft.

It was a structure so large and heavy at around 120 tons it required four electric trams running on rails on top of the 22m diameter concrete ring to turn the device.
The antenna was installed here in March 1941 and today you can still see the concrete ring in the corner of a small wood on the edge of farmland.
At the centre of the ring is the original flat-roofed, brick-built central equipment building which housed a power control panel and emergency shut off, transmitter units, printers similar to those installed in the receiving aircraft, and a slip ring assembly which ensured the electrical cables didn’t twist as the antenna rotated.

In the surrounding fields are the remains of barracks, air-raid shelters, machinery bunkers, and several heavily overgrown anti-aircraft gun emplacements.
Further to the north west, around 500m away from the main site – are the foundations for a large antenna which was constructed to monitor the output of the transmitter.
Much of the site lies on private land and permission must be sought before visiting.