Type 85 air defence search radar

Radar Type 85 at Staxton Wold (photo - "Watching The Skies")

The AEI Type 85 radar formed part of the "Linesman" air defence system which was also used by civil air traffic control who called it "Mediator". Known as "Blue Yeoman" during development, the Type 85 was the principle radar head at "Linesman" sites. The Type 85 was an S-band radar that operated on a wavelength of 10 cm. It was a volumetric scan radar with twelve individual overlapping 1 degree beams that was designed to "burn through" EW jamming. The antenna reflector measured 60 ' wide by 21' 8" high (the same reflector that was used on the Type 84) and normally rotated at 4 rpm. The horizontal beam width of the antenna was < 0.5 degrees.

The smaller square aerial to the left of this Type 85 at R.A.F. Staxton Wold is a Marconi S723 "Martello" 3d radar, known to the R.A.F. as the Type 91.

The building that housed the Type 85 radar was known as the R12. The R12 building was huge and the thick steel reinforced concrete walls were designed to withstand nuclear blast. There were three levels in the building, the cellar could be used as a dormitory and held stores of emergency rations. The ground floor housed the Type 85's twelve (yes, 12!) water cooled klystron transmitters that each produced a peak power pulse of 4.5 Mw. In some parts of the band the klystrons could develop up to 8.0 Mw peak. The power developed by the klystrons was calculated by measuring the temperature differential of the coolant. The top floor housed the Type 85's 12 receivers and signal processing equipment, the "heights" computer, the RX12874 signal processing equipment and computers and the SSR (Secondary Surveillance Radar) equipment. You might be able to make out the SSR aerial just below the feed horn of the Type 85 aerial. There was also a display console cabin for the "scope dopes", one or two "Wendy houses", various workshops, offices, a crew room and kitchen and room 27, the Type 85's control room. There was also a considerable amount of wave guide from floor to ceiling and from wall to wall.

Radar Type 85 (photo - "Watching The Skies")

As mentioned above, the Type 85 radar had twelve klystron transmitters. Each transmitter changed frequency from one pulse to the next as an anti-jamming countermeasure. A total bandwidth of 500 MHz was covered, centred on 3 GHz (3000 MHz). This was done by providing klystrons to cover 60 MHz in each of 4 x 100 MHz bands. The middle band (band C, 2900 - 3000 Mhz) was unoccupied by the Type 85 radar (this band was still occupied by some Type 80 radars).

To improve the jamming resistance of the Type 85 the 4 x 60 Mhz bands for which klystrons were provided were split into parts of equal power and recombined with half the power from another band, e.g. 1/2A + 1/2D and 1/2B + 1/2E. In this way the frequency in a given beam would consist of two frequencies 300 MHz apart, changing from pulse to pulse either randomly or actively controlled by the amount of jamming received on that particular frequency.

There was also a facility called horn changeover where the output frequencies from the odd horns would be swapped with the frequencies of the even horns once per revolution, so that, within two rotations of the aerial, each beam would have used all possible frequencies in the total 500 MHz band. The Type 85 was designed to be resistant to the high power jamming produced by a Carcinotron. This method of frequency agility forced a Carcinotron jammer in to a broad band barrage mode, spreading the available jamming RF power over a greater bandwidth and thus diluting the jamming effect.

In normal, non jamming conditions, only four transmitters were used - one A band in use with one as standby and one B band in use with one as standby. When necessary all twelve transmitters could be fed into twelve beams as described, or could be combined to feed into 6 beams only. Jack Charlton tells me that they used bands A/D and B/E to combine or just A B in low power. For still greater concentration of transmitter power at a target under jamming conditions the aerial could be used in sector scan mode where it would rotate back and forth between two azimuth angles. Dave Sams states that The T85 aerial could rotate at speeds of up to 12 rpm and tripped out above this. Part of a scheduled service routine called for this to be tested but he never met anyone quite that brave (I tried taking an RX12874 PD head at R.A.F. Boulmer up to 8 rpm once but all that shaking and rumbling convinced me of the error of my ways and probably only served to shorten it's working life! - Ed.). Another little known feature mentioned by Dave Sams was that the Type 84 and Type 85 could also be run in a synchronous lock mode, ie looking in opposite directions.

Radar Type 85 at RAF Boulmer (photo - Dick Barrett)

One could not help but be impressed by room 27, the R12 control room, which was dominated by a huge mimic display that showed the configuration of the Type 85 radar. Containing some 2800 or so plaques, this mimic might have been made by a railway signal company, as the display looked like it had come from the signal box at London's Kings Cross railway station! To the right of this mimic was a smaller mimic for the RX12874 Passive Detection equipment. Set in a well below the Type 85 mimic there was 10 foot long (or there about) control console for the Type 85 from where the system was controlled. The control desk housed the control panels for the twelve transmitters, receiver control panels T1/T2 and PC and the remote noise factor control panel. A large suit above the well controlled the Type 85 power, configuration, sector scan, aerial reflector tilt and so on. Small knobs on the Type 85 mimic display actuated wave guide switches that directed transmit and receive signal paths through the system. The rest of the control room was taken up with PPI and other radar display consoles for the various radars around the site, so from here one could view the radar pictures from the Type 84, Type 85, HF200's and the RX12874 Passive Detection equipment. The aerial at the right hand edge of the colour picture is an HF200 height finder.

The pulse transmissions of all the radars on a site was controlled by a "No Break Trigger" (NBT) that ensured all the radar transmitters fired at the same time and Gerry Oakley tells me that it was a b*****d to work on!. Triggering the radars with the NBT considerably reduced local interference between the various radars on the site. If all the transmitters were allowed to "free run" you could get spiral patterns of dots on the PPI screen. These dots were known as "running rabbits" by U.S. radar people. At R.A.F. Boulmer the NBT was located on the top floor of the R12, just outside the crew room.

The site had its own power generation operated by the Department of Environment chaps.

Peak power:	4.5 Mw per Klystron (up to 8 Mw in some parts of the band)
Pulse length:	10 us, Pulse Compression
P.R.F.:	250 pps
Frequency	S-Band (500 MHz band width centred on 3 GHz)
Aerial rotation	4 rpm (capable of "sector scanning")
Beam system	12 overlapping 1^o vertical beams with a beamwidth of < 0.5^o

(Thanks to Martyn (Taff) Williams, Q-L-R-85 Neatishead 1975-1982, Boulmer 1990, Kenneth Allen, ex 87th entry, ex Chf/Tech LFitGR on Type 85's Dave Sams, Lew Paterson and Jack Charlton for their invaluable contributions)