At the start of WW2 the military (Army, Navy and RAF) had only three, quite large, wireless intercept stations at Fort Bridgewoods, Flowerdown and Waddington, but a Post Office Radio Station in Smallford, St Albans (below) was already acting secretly as an intercept station for the Foreign Office and in 1938 it set up an experimental direction finding station which was quickly followed by two others at Cupar in Scotland and Stockland Bristol near Bridgwater. These three stations were funded, set up and manned by the Post Office to identify illicit transmissions within the UK under its peactime remit but probably with the prospect of future enemy agents in mind.
In december 1938 the Treasury Defence Committee requested the Foreign Office to urgently expand this network by building three intercept stations alongside the direction finding (D/F) sites. Three new intercept stations were built at Sandridge near St Albans, Cupar in Fife amd Brora, Sutherland on the north east coast of Scotland. Once built, they were run by the Post Office on behalf of the Foreign Office as Diplomatic Intercept stations. These three stations (along with the Post Office Station at St Albans) monitored a long list of diplomatic wireless links. A comprehensive list of UK diplomatic intercept stations, frequencies and target transmitter locations can be found in the book 'Churchill's Secret War' by Robin Denniston.
St Albans Radio was a Post Office point-to-point radio station for commercial traffic built in 1926. It has been reported as acting as a diplomatic intercept "Y" station from as early as 1932. It was from here that the first experimental direction finding station was set up and tested in 1938. It was also reported as receiving 50 National HRO wireless receivers in 1938, probably for their own intercept use as well as for the three new stations under construction.
The image below show what the station looked like from the air in 1946 with the building shown on the left at the end of a curved access road and two aerial
towers. Many thin slanting black lines can be seen, these were the supports for the many rhombic aerials used for interception.
This document was located in The National Archives and shows the setting up of an experimental DF station at Sandridge, St Albans to to locate illicit transmissions.
The first and largest of the new intercept stations was built alongside the Experimental D/F station near the village of Sandridge. The original architect's drawing for the main building is shown below.
A 1946 aerial image is below shows the original building and two towers visible only by their shadows, an annex is also visible which was added during ww2 as the demand for intercepts grew. The original d/f site in the field to the north is not visible, it had probably been demolished so that the field could be returned to farming. The intercept station remained and was used by the Diplomatic Wireless Service as a base for storage of mobile D/F vehicles.
INTERCEPT STATION AERIALS
The intercept stations were usually located in buildings containing a number of operator position with a "farm" of aerials on the surrounding land. The aerials used were mainly "rhombic" type and consisted of wires supported on telegraph poles laid out in a diamond shape. The long axis of the diamond which could be up to 300m (1000ft) was pointed in the direction of interest and intercept sites used many rhombic aerials of different sizes pointing in different directions. These aerials were not usually visible in aerial photographs.
The intercept stations were equipped with a large number of different aerials and the document on the right contains a list of new rhombic aerials required for the Foreign Office Y station at Sandridge giving frequency ranges and directions.
The link below shows an example of the aerial array of an intercept station near Forfar in scotland.
DIRECTION FINDING STATION AERIALS
These were designed to indentify the location of a wireless transmitters. Most used Marconi-Adcock systems which involved a small operators hut with four vertical monopole aerials located around the hut at North, South East and West positions. Short-wave (HF) stations were most common dealing with frequencies beteween about 2 to 30 MHz. These had monople aerials around 10m (30ft) high. There were alaso a smaller number of MF, below 2MHz stations and these had monopole aerials up to 30m (100ft) high. Sometimes they can be seen in aerial photographs as circles in the middle of fields. The circles probably outline the earth mat or earth radials laid out on the ground. The circles were also a different colour to the surrounding vegetation and solutions of salt or other chemcals may have been added to some sites to improve the earthing efficiency.
D/F requests were received from a controlling station by telephone line and the operator was given the frequency and the sound of the transmission. He listened to the telephone signal in one earpiece of the headphones and listed on the other earpiece as he adjusted the frequency of his receiver, when the two signals matched the bearing was obtained sent back down the telephone by morse code. The whole process was was usually achieved in seconds. Some operators were located in an underground metal tank to minimise any interference, for the same reason the D/F sites were almost always located in the middle of fields. At the control site they combined the results obtained from two or three D/F stations to plot the location of the transmitter.
At the end of WW2 traces of these stations could be seen from the air in the middle of farmers fields. The image below shows one such station in a field in Essex.
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