Looking back over the 65 years, one is struck by the quite remarkable specification of CH, given the short development period. The transmitter designed by the brilliant Dr.Dodds of Metrovick used his earlier invented commercially secret state-of the-art water cooled and continuously evacuated valves which provided a unique output power for those times of about 300 kilowatts (which was later doubled when 50 Hz full wave modulation was changed to true pulse modulation) on a choice of four predetermined frequencies between 20 & 60 MHz. The pulse length could be varied between 5 & 40 microseconds and the pulse repetition frequency (PRF) was a low 25.

Inside T Block showing the duplicated equipment

The theoretical range was about 200 miles and a reliable 150 was achieved when pulse modulation was adopted, with a claimed accuracy of 1 mile at 15000 feet. In addition, by transmitting different lobes of RF energy from aerials mounted at different points up the towers, the target height could be determined by comparison of the amplitude of the returns, to an accuracy of 500 feet. The enemy quickly became interested in these 350 foot towers and before the war started, sent an airship along the northern east coast to monitor the emissions. However the low PRF baffled them and fortuitously for us, they assumed wrongly that they were picking up radiation from defects in our 50Hz alternating domestic mains electricity system, this conclusion aided no doubt by the fact that all the stations transmitted in synchronism by using the mains waveform to achieve that.

RAF Ventnor was operational in an 'intermediate' form at the end of January 1939, the chosen site being at the top of St.Boniface Down, the highest point on the Island at about 750 feet above the sea. The steel towers were not completed, so initially two of the wooden 240 foot ones were used to support the transmitting aerials. Height finding was never possible with the CH kit at Ventnor as the ground sloped away steeply too close to the site to allow the correct formation of the required lobes of electromagnetic radiation. During construction of the four steel towers in July 1939 one of the workers fell to his death. The station was put on a war footing on the 24th August.

The wooden receiving aerial towers with transmitter towers to the rear

So how did this non-rotating equipment produce the required result? The Cossor designed receiving apparatus provided a cathode ray tube (CRT) which showed a horizontal bright line (the trace) above which was a scale calibrated in tens of miles. This trace in fact looked like a waving line of grass as the 'background noise' caused small vertical deflections in it. The trace originated from the left side of the CRT and was re-initiated by each transmitter pulse, successive pulses causing the trace to be repeatedly redrawn towards the right. Any echo received in the time between transmitter pulses of strength greater than that of the 'grass' was arranged to be displayed vertically downwards like a narrow finger and thus the range could easily be read off against the scale. The shape and amplitude of the 'finger' gave an experienced operator some indication of the number of aircraft in the approaching formation. The exact bearing was not immediately obvious, but at Ventnor was always going to be within 30 degrees of either side of 148 degrees (about SSE), the designed 'line-of-shoot'.

However, there were two receiver aerials on the tower and the signals from each were fed to the famous 'goniometer'. This was a hand control containing two coils, each provided with one of the aerial signal inputs. A third output coil fed the signal on to the display by means of the electromagnetic coupling between all the coils. By rotating the knob one input coil moved mechanically in relation to the other.
 
The effect achieved was exactly the same as if the receiving aerials themselves had swept across the sector centred on the line-of shoot. At one particular point of the knob's rotation, the interaction of the two input signals caused a 'null' point where the displayed echo reduced significantly in amplitude. A scale in degrees arranged in a ring around the knob allowed the bearing of the null-point and hence the target, to be read off, and this system provided an accuracy of bearing within 10 degrees.

 

   Interior of R Block showing one of the two display consoles

Next, a correction factor had to be applied and this was determined by reference to tables which had been drawn up when the 'calibration' of the equipment had been done during the commissioning of the station. This was necessary because local geographical features distorted the shape of the transmitted lobes, in effect squinting the transmitted lobe to left or right, and the degree of squint varied at different heights. Calibration was achieved by an aeroplane (usually a Cierva autogiro) flying an extensive pattern of courses and heights whilst in sight through a theodolite mounted on the top platform of a transmitter tower, with the aircraft flying in tight circles over a particular position while each plot was made. Simultaneously the radar operator recorded the plots too. Only the yet to be developed helicopter could have served the purpose better. In this manner tables of a whole series of visual and radar plots were compiled. Afterwards the visual and radar co-ordinates were compared and correction tables duly created. The correction figures obtained were projected out to cover the full range, and for all heights. Contrary to what might be read on other sites, the unarmed Cievra was not used to test the extent of the range of CH, which for the south coast radars would have been up to a hundred miles into occupied Fance.

So now a target's range and bearing could be ascertained, but what about its height? Well, again a goniometer was used, but this time connected to aerials mounted at different heights and again a correction factor from the tables had to be applied. CH could not see low flying aircraft nor anything above 20.000 feet. But no heights at all could be established at Ventnor as already mentioned.

The next step in this process was to report the data by voice via dedicated land-lines to a 'filter room' which was located in Stanmore, Middlesex. The Filter Room received information from all the radar stations in the south of the country and by cross referring the reports from the different 'lines of shoot' they could determine the bearing and height of the separate raids, with reasonable estimates of the number of enemy planes involved in each. As the Ventnor station looked to the south east, cutting across the lines of shoot of all the other south coast stations, its reports must have been of particular value in the cross referencing process. The edited data was assembled as markers on a large plotting table which showed a situation which was about four minutes late (about 15 miles of bomber flying ) compared with real time. Friendly aircraft were identified by an entirely separate system of HF direction finding which tracked the voice transmissions from the aircraft and these were also marked on the plotting table. The IFF system which made the echo from a friendly aircraft brighter and larger could not always be relied upon, especially as in the early days not every aircraft was so equipped. With the separate raids thus identified, Stanmore could then make the tactical decisions regarding the deployment and vectoring of the defending aircraft, either those already in the air or presently on the ground, towards their ever moving targets. Once the enemy aircraft had crossed the coast the CH radar could no longer see them and then the Royal Observer Corps reports to the Filter Room became the sole means of tracking the enemy.

RAF Ventnor continued to work effectively until 12th August 1940 when 15 Junkers 88 dropped about 50 bombs on the station at mid-day, leaving it badly damaged and off the air. This photo was taken from one of those bombers, the transmitter towers are in the foreground. Although the domestic site was also located within the compound at the top of St.Boniface, amazingly only one person was injured. Despite the danger from thirty delayed action bombs, the station was back in limited service within the hour. On the following Friday there was a second raid by 6 JU88 aircraft which deposited 15 bombs into the compound with the result that two unfortunate hits finally created damage that could not be fixed easily. (Today, near the parking spot close to R Block, there exists a strangely cratered area amongst the gorse. I believe these holes to be bomb craters.) The station remained unserviceable for two months but mobile equipment was installed at Bembridge to 'plug the gap' in the chain until the Ventnor repairs were completed.

The system of having to use correction charts before reporting plots to the Filter Room contributed to the four minute delay and and sometimes the human factor even introduced errors. This problem was solved by 'The Calculator'. Designed and installed by the Post Office and using relays and uniselectors, this little known and uncelebrated early form of computer automatically added the correction factors to the input plots and displayed the results visually as a grid reference. The machine could also correct heights in the same way and a mechanically linked teleprinter could send the data by telephone line to the remote Filter Room. Ventnor was equipped with its first calculator in June 1940 and received its second in April 1941.

The CH equipment received various updating in 1941. Increased power required new aerial co-axial to withstand the higher voltages. The aerial system was completely changed to the 'curtain array' which was slung between two of the towers, allowing one tower to be dismantled. A new twin screen console was installed equipped with 'electronic range markers'. Numerous effective anti-jamming devices were installed as from 1940 the Germans were using frequency modulation transmitters to cause interference.

In 1942 a new height finding system that did not require extensive calibration was brought into use, and is seen here to the right. Known as the Variable Elevation Beam (VEB), and working on 1.5 metre wavelength the system used a series of mechanically linked tilting dipole aerials which moved in unison and were mounted on one of the wooden CH receiver towers to look along the line of shoot. Recently developed automatic Transmit/Receive switching allowed the same dipoles to both transmit and receive. All the equipment for this device was housed in a bunker within the legs of the tower and the installation must have been a boon to RAF Ventnor as they were at last able to find their own heights. However VEB had but a short life as it was replaced in 1944 when 'nodding' centimetric height finders became the standard.

The compound was also host in 1941 to the experimental Gee transmitters for guiding our Bomber Command aircraft. The enemy jamming became worse and the Type 11 radar was introduced, only for use in dire emergency. It cunningly used the same frequencies as the German Wurzburg, on the assumption that the enemy would not jam their own radar. The Type 11 was a rotating machine feeding a Plan Position Indicator (PPI) display and had height finding capability and was intended to be used for Ground Controlled Interception. The GCI system observed both friend and foe, and allowed the operating staff to control directly a fighter pilot via VHF radio, thus to guide him to meet an evasive enemy, and a station of this type existed at nearby Blackgang. In 1942 a 'buried reserve' installation was completed at St.Boniface, with its own two wooden 120' masts (unburied!).

It should be mentioned that from 1941 a further reserve capability existed in the form of nearby RAF St.Lawrence. In response to enquiries a short page has been added about RAF Blackgang GCI which operated entirely independently of both RAF St.Lawrence and RAF Ventnor.

The Ventnor Chain Home became non-operational in March 1945, but the equipment remained there on a 'care & maintenance' basis. However the Type 14 and Type 13s on the site continued in use. In November 1947 one of the wooden receiver towers was dismantled after being severely damaged when an Avro Anson Channel Isles mail plane from Croydon, flying off course in very poor visibility, crashed into it. After that, viewed from first the south west and then the south east, the site appeared like this, and that was the state when the author was posted to Ventnor in 1957.

In 1950 the CH system was refurbished, but during the ensuing decade the ROTOR system was installed and the remaining CH towers were dismantled in the summer of 1957, this event being witnessed by the author. T Block, which is shown here behind the Type 80 building was removed in the sixties decade.

However in October 1962 the R Block area was sold to the GPO who retained it until 2005 when BT sold it together with a tall slim mast to an undisclosed buyer.

If you have been interested enough to read down to here then the Bawdsey Website is certain to be of interest.

And if you served at RAF Bawdsey at any time you should look at this Old Friends contact site.

And these two anecdotal sites about the the West Coast CH as installed in the Isle of Man.and Scarlett Point

FOOTNOTE :
When describing military installations it is both conventional and courteous to give the names of the Commanding Officers, but until now (March 2007) the author had no knowledge of any of those at Ventnor during the CH days. Now he is pleased to report that in 1942 Fl/Lt James Rowlatt RCAF served in that capacity and also as Technical Officer. Previously at Trevose Head, he later served similarly at Pevensey, Swingate, and St. Margaret's Bay. Amazingly Fl/Lt Rowlatt is still alive (aged 102) and well and lives in Canada. We salute you Sir! During his time at Ventnor he increased the air defences with Lewis and other guns as frequent opportunistic attempts to attack the site at low level were made by the enemy. Thanks to his son Les for providing this information, and it is hoped to hear more.