Cobra Mist was a secret collaborative cold war project between the UK and the United States. It was originally planned to be built in Turkey, but after the US failed to reach agreement with the Turkish government the UK offered space on a nine mile long windswept shingle spit off the Suffolk coast.
With the southern part of the spit already being used for the testing of bombs, development of Britain’s Nuclear weapons and previous experimental radar work, much of which is still classified today, the isolated location on the northern end of the waterlogged salt marsh, an area already out of bounds to the public and surrounded by water on three sides, made the perfect place for another top secret experimental military installation.
Towards the end of the 1960’s work began building a large fan shaped radar system which it was hoped would detect enemy aircraft approaching from over the horizon as far away as 37,000 km. At the edge of the antenna array a large and imposing, windowless, two storey building was constructed on stilts so as to be above the 1953 flood level. This part of the top-secret installation would be home to the radar transmitting equipment on the lower floor, staffed by the British, and to the receiver and signals processing equipment, operated by the American Airforce, on the top floor.
Much like its soviet counterpart, Duga, the 150m high steel monster which now lies abandoned in the radioactive exclusion zone of the ill-fated Chernobyl Power Station, Cobra Mist, the hundred million dollar collaboration between the UK and the USA never actually worked. It was plagued with problems of unwanted noise, which were at one point suspected to have come from a Soviet vessel in the North Sea.
Cobra Mist was shut down almost as quickly as it had started on 30thJune 1973 and apparently some of the white antenna rods from the disassembled antenna array are still in use supporting runner beans in the gardens in Orford today.
After the military left Orford Ness, the Cobra Mist site was given a second lease of life and started to transmit radio signals once again, firstly into Northern Europe for the foreign office and later, after the site passed into private ownership, it transmitted the BBC World Service into southern Europe.
More nerdy information about the installation of the antennas for the transmitting station follows after the photo gallery.
In 1978 a new six mast array was built on the site of the previous fan shaped radar antenna. This was designed to broadcast on 1296KHz AM and was soon followed by a five antenna array designed to transmit on 648KHz three years later in 1981. A mono pole reserve antenna for when the 648KHz array went into maintenance was installed two years later in 1983.
The 1296KHz array replaced a six element vertical yagi installation at Crowborough, which was later shut down when all remaining services were transferred to Orford Ness. The six tower antenna system actually consists of two three element arrays, the centre element of each half being fed a 250 kilowatt signal down its own 50 ohm coax from the transmitter building, the route of which can clearly be seen on aerial photographs and on google earth today.
The 648KHz antenna system was originally designed by the BBC with six towers but Marconi won the contract to install the system in 1981 because they demonstrated they could achieve the required polar diagram with only five towers.
The 500 kilowatt signal for the 648KHz array is fed via two 50 ohm coaxes to the base of the central tower (ATH-3) where each one is then split and fed out to the remaining two antenna each side by a power division network based on power and phase relationship to achieve a broad beam forward direction towards Europe.
The single tower is a small omni directional reserve antenna with a maximum power of 250kilowatts fed by another 50ohm coax. This was used when maintenance was carried out on the main ATH 648 towers.