Because of the vulnerability of the Chain Home radar site at Ventnor, a site at St. Lawrence was chosen for a mobile reserve station that could be brought into use should Ventnor be rendered unusable following an air attack. There was already a buried reserve at Ventnor but as this was located on the technical site, it too could have been damaged in an attack. The mobile reserve was given the code number 10M.
Mobile reserve had no permanent installations and consisted of a convoy of trucks that could be set up at any required site.
These trucks formed an AMES Type 9 Mobile Radio Unit (MRU) and a truck and trailer with a portable 105 foot wooden mast complete with a Lutkin ‘all round’ looking array. A mobile reserve would be used during the construction of a permanent station or if that station was taken off the air following an attack.
In 1941, a remote reserve site was required for the Chain Home station at Southbourne, a few miles to the east of Bournemouth. No suitable site could be found in the vicinity so it was decided to construct the remote reserve at St. Lawrence; this was given the code number 11R.
The station was manned by personnel from RAF Ventnor with domestic accommodation provided at the Ventnor camp and at two hotels in St. Lawrence that were requisitioned. The station consisted of a transmitter and receiver block sited on top of a low cliff 44’ above sea level. The two blocks were approximately 200 yards apart and each had a 120’ wooden aerial tower alongside. The two blocks were completely mounded over with earth to give added protection. A standby set house was provided inland and this too was mounded over with earth.
By 1942, St. Lawrence had become a radar station in its own right operating 24 hours a day rather than acting as a reserve site for Ventnor and Southbourne.
There were no height finding capabilities at Ventnor so St. Lawrence was able to fulfill this role; the high hills behind the station blocked unwanted reflections making it an ideal site for height finding.
The station was proposed for conversion to CRDF (Cathode Ray Direction Finding) in the summer of 1943 but it wasn’t suitable. (Previously aircraft had been detected by an operator using a direction finding radio and aerial system to obtain a null [no signal] from which the direction could be calculated using a compass bearing. A vast improvement on this was the application of electronic principles using a cathode ray tube as a more accurate measuring device.)
With the development of the V1 flying bomb and V2 rocket, the station maintained a watch for these from the end of July 1943 until March 1944 although none had landed in Great Britain. Full watches were resumed on 13th June 1944 following the arrival of the first V1. St. Lawrence detected a handful of V1’s but never detected a V2.
St. Lawrence was once again active by D Day plotting more than 2000 outward aircraft between 12.00 - 13.00 on 24th July 1944. By the autumn of that year operations were beginning to wind down and in November 1947 the station was described as ‘non operational’ with the Type 1 Chain Home radar on care and maintenance.
The transmitter and receiver blocks are still extant on farmland close to the cliff top path; all entrances however have been sealed. The transmitter block is at SZ 53007599 and the receiver block is at SZ 52907595. Close to each block there are four concrete bases that supported the two 120’ aerial towers. The standby set house can also be found in woodland 100 yards to the north west of the technical blocks.
Sources: * Bob Jenner * ‘Radar on the Isle of Wight’ by Squadeon Leader Mike Dean, Published privately in 1994 by Historical Radar Archive