The Decca Type 80 was designed in the nineteen fifties for either Ground Controlled Interception purposes or as an Early Warning Search radar. It was to replace the aged Chain Home equipment and represented a huge leap forward in technology. Most CH stations were closed down after the war and there was a lull in R & D for economy reasons. However the Berlin Airlift in 1948 and the Korean war in 1950, both of which suggested world-wide Communist ambitions, prompted more urgent efforts. Thus at the RRE the Type 80 was derived from an experimental set up encoded as Green Garlic, which consisted of two Type 14 relectors bolted end to end on Type 7 rotating gear which also carried the transmitter, the modulator being on the ground. With a 1.5 MW magnetron and improved receiver the performance obtained was an enormous improvememt over any existing equipment.

With CH an error in reported target bearing was quite normal as the fixed dipole aerial system had a basic acceptance angle of about 60 degrees, which the goniometer in effect reduced to a 10% error. However the Type 80 could distinguish two aircraft a mile apart at 150 miles and had a reliable range of 250 miles. This was achieved by using a huge aerial reflector measuring no less than 75 feet wide by 25 feet high this being fed from a magnetron delivering massive 2 megawatt pulses of 2 (or 5) microsecond duration at a PRF of 270 per second. The aerial's narrow acceptance angle of a third of one degree meant that it had to turn at a slow rate to achieve good 'illumination' of the distant target and yet not have rotated too far on to miss receiving the reradiated returned signals. This massive structure was turned by two large 50 HP motors.

Front view showing the'leaky waveguide' in the dark coloured perspex tube positioned across the width of and at the focus of the reflector, with the much smaller IFF aerial mounted beneath.

 

Layout of the Modulator room i.e. the building straddled by the four huge legs.
(At Ventnor the equipment was not duplicated as shown in dashed lines in this diagram)

The power consumption of this beast totalled 160 kVA and interesting techniques were used to provide and control this energy. The 'annex' to the main building (LH part of diagram) contained a large motor alternator set fed from the Company's 3 phase supply and this buffered the radar's varying demand. The lower annex contained the induction regulator and the 12 phase transformer. The former was built like a large induction motor. However, it did not spin but behaved liked a variable transformer: the rotor was normally held stationary but allowed to move slowly round over about 270 degrees by a push-button controlled motor allowing the modulator to receive a gradually increasing voltage during the 'running up' process. The regulator fed a transformer which provided 12 output phases, these being taken to the anodes of a large mercury arc rectifier known as 'The Mekon', a very dramatic blue glowing and hissing device housed at chest height in a cabinet in the main room and providing the 600vdc diet for the modulator. This is what it looked like:-

This rare picture kindly supplied by Ron Cripps

(If you arrived here from 'End Game' Click to return to your place there)

The modulator was contained within the largest cabinet in the room. The front had a glass window and within could be seen the Pulse Forming Network, a 25:1 ratio transformer, a mercury pool switch and porcelain insulators everywhere, and the combined effect of these components was to brew up the enormous 25kV pulses to send up to the magnetron. That was in the rotating cabin so a bank of sliprings were mounted on an axial column which came down from the cabin and rotated with it. The 25kV pulses were applied to the end of a highly insulated conductor which went up the centre of the column. Also downstairs was the 'Emotrol' turning motor control cabinet. Details of the turning gear are provided on a separate page. One corner of the room was partitioned off to form a small office and this was equipped with just a bench, chair, and table. The literature consisted of Air Publications, a Signal Office Diary and the Meter Readings Book. There were about fifty meters built into the various cabinets, giving some indication of the state of health of the electronics the cabinets contained and their readings were required to be recorded every two hours. All the units had associated cooling fans and when 'on watch' their combined noise was difficult to live with. It was always a relief to get out of the door.

Outside a metal staircase led to the gantry in the centre of which rotated the cabin. This was about 15 feet square and to gain entry one had to leap aboard the thoughtfully provided running board and slide back the door. Walking across the floor during rotation was a strange experience somewhat reminiscent of inebriation. One side of the cabin was occupied with cabinets containing the transmitter, TR switch, receivers and test equipment. Another corner housed an air pump cabinet. The transmitting device was a water-cooled magnetron which on receipt of the high voltage pulse from the modulator immediately generated the 2 megawatt (for Type 80 Mk.3, the earlier Mk.1 had a 1 Megawatt magnetron) burst of radio frequency energy in the 3000MHz band and immediately stopped doing so when the modulation pulse ceased. The RF pulse was ducted through the top of the cabin by means of a rectangular waveguide and eventually was distributed at the focus of, and along the length of the reflector using the 'leaky waveguide' method. That is, via angled narrow slots cut into the smaller dimension of the waveguide. This part was enclosed in a Perspex tube as the waveguide was pressurised from an air pump to reduce any tendency to arc.

The TR switch was an extremely cunning arrangement of circles and short bits of waveguide known as 'the rat race'. This allowed both transmitter and receivers to use the common aerial array without the receiver being blown to pieces by the massive transmitter pulse.

In this diagram the output pulses from the magnetron arrive at the top left and leave for the aerial top right.

The very low amplitude received signals were fed to two receivers after being mixed with the output from the local oscillator klystron which differed in frequency from the magnetron by 13.5 MHz. The resulting received signals were then carried by this resulting intermediate frequency. In the diagram the klystron is shown bottom right and the IF output for the receivers departs to the right from the centre ring.

However, magnetrons have a tendency to drift in frequency so Automatic Frequency Control corrected the local oscillator frequency as necessary to cause an IF always of 13.5MHz. One receiver amplified the signals in a linear way, the other in a logarithmetic manner. The signals were then carried to the underground installation via co-axial cables and distributed to the consoles. The radar operator could choose either receiver remotely, but the linear one was the normal choice. Logrithmetic was used if the target returns were obscured by thunder clouds when as if by magic the dense patches on the display thinned and allowed the pinhead sized echoes to be seen. It was expected that this receiver would also have been effective against certain forms of jamming too.

Outside, back on the gantry, stairs led to the upper platform where the bottom of the reflector circled barely a foot above one's head and here were located the two turning motors.

The layout diagram indicates that some of the equipment might be duplicated. I am not cetain of the reason for this but it encourages the idea that the mighty Type 80 could be made even mightier! Indeed rumours existed of a similar gear but with another reflector fitted back to back, but so far I have no factual details of such a beast. However another variation of the general design was the the Type 82. This machine was introduced after I had left the service, so I do not have personal knowledge of it. However, I understand that it carried more than one reflector, each being smaller than that of the Type 80 and they all faced in the same direction. The Type 82 was used to control our Bloodhound Surface-to-Air guided missiles which defended our Thor ICBM sites and were installed at Watton, North Luffenham, and Lindholme. However I am told that they were withdrawn in the sixties decade when later equipment was able to cover a much greater range.

The Type 80 at Ventnor entered service on the twentieth of March 1956. It had but a short life there, being carefully dismantled during the summer of 1959 and removed in sections on Decca specifically adapted low loader lorries, possibly to be resurrected elsewhere. But many other Type 80 sites were also being closed down around that time as the current threat then was perceived to be less from the Bison bomber and more from the H bomb delivered by ICBM, and the main defence policy against that was 'deterrence'. The Fylingdale BMEWS presumably served mainly to provide the US with a short warning. Although the entire Ventnor site was transferred in March 1962 to the Ministry of Aviation for civil use, the Modulator room survives there to this day.