Part 3 - The next generation

By: Chris Poole

The next Generation (Cavity Magnetron) Radars

While the AI Mark’s III & IV worked well, the scientists at TRE had already recognised that there were many limitations of a radar system operating on a 1.5 metre frequency and that radar would work better at shorter wavelengths, which would provide a finer resolution, a tighter beam, and would not be swamped by ground return signals.

To investigate and pursue the use of radar, which would operate in the 10 Centimetre band, a team was set up at TRE Worth Matravers. Co-incidentally, the Admiralty had also set up a special committee to investigate microwave radar that would operate on a ten-centimetre wavelength and they assigned the Clarendon Laboratory at Oxford to work on a microwave receiver, while a team from the physics department at the University of Birmingham was to work on a microwave transmitter.

John Randall and Henry Boot at the University of Birmingham made the real breakthrough. They were not at the heart of the transmitter development, all they were simply trying to develop was microwave detector circuits but to test their designs, they had to generate microwaves for their circuits to detect. Randall and Boot didn't know much about generating microwaves, so they set about learning how even though they were working on a shoestring budget.

The result was that they developed the Cavity Magnetron, which they powered up for the first time on the 21st February 1940.

Within a few days their magnetron was producing a power output in the order of 500 watts, which was enough to light up fluorescent tubes from some distance away. It is reported that they found this incredible and caused them to check and re-check their figures and the experimental set-up, but nothing was wrong. 

The result was that the magnetron – at a stroke – gave the scientists the leap forward in microwave technology, which they were seeking and over the following months intense development work took place to refine the magnetron and transmitter into something resembling an operational system, with a maximum output power of 15 kilowatts, which was something like three orders of magnitude greater than the output power available with any other device. 

The TRE received its first cavity magnetron on 19th July 1940. A microwave radar system operating at 9.1 centimetres was quickly assembled and tracked an aircraft on 12 August 1940. 

Rather than use the static aerial system from the Mark IV, a dish based scanning system was also developed.

AI Mark VII and VIII radars

Following the prototype work, the Mark VII and VIII radars were developed to a production stage and in early 1942 an initial order for 1000 sets was placed on EKCO, these units began to be delivered towards the end on 1942 and its first successful intercept and shoot down of a German aircraft is recorded in January 1943. In all EKCO manufactured circa 5000 Mark VIII sets and associated test equipment during the course of the war.

A new type of indicator display (spiral display) was developed as well as a new type of display screen, which meant that the all the information was shown to the radar operator on one screen.

Both the dish and screen were remarkable designs, the scanner dish was 28 inches in diameter and rotated at 200 RPM (although it was capable of 960 RPM) whilst tilting up/down and left/right, this creating a beam that was spiral on pattern. The screen displayed information in a circular format, which once used to, was easy to interpret.

This radar was fitted to both Beaufighter and Mosquitoes, with strict instructions NOT to fly over enemy territory in the early days although this was relaxed when night-fighters were sent out with the bomber stream from late 1943 onwards. It is easy to recognise either aircraft using Mark VII or VIII radar since they both had what became known (for obvious reasons) a thimble nose.

AI Mark VIII continued to be the mainstay of night-fighters throughout 1943 and 1944 only being superseded by the American developed SCR720 set, which used similar technology to the Mark VIII, but had twin scopes and was considered less susceptible to enemy jamming. This was adopted in the UK as AI Mark X.


Mosquito cockpit showing EKCO made AI Mark VIII

(Compare with units below) Photo copyright - IWM (CH16607).


Thimble nose Beaufighter fitted with AI Mark VIII


Example of Mark VIII display (scope) screen.

Radar operators had to learn how to interpret the AI Mark VIII screens since it presented information in a completely manner from the Mark IV system, nevertheless, once they got used to it, it was quickly recognised and accepted as being much simpler to understand than the Mark IV.

In the example shown, the inner ring represents zero range. The outer ring represents the aircraft altitude (4000 ft for example) and the horizontal band is due to ground return echoes although it conveniently also acted as an artificial horizon.

The arc off to the left is a target echo, which is about 20 degree’s off the aircraft heading and at a range of about 2 miles.

As the night-fighter turned towards the echo, the arc of the echo would increase until it became a full circle when the enemy aircraft was dead ahead. As the range decreased the circle would get smaller and once it intercepted the small inner ring, the enemy aircraft should certainly be within visual range and possibly also gunnery range.

AI Mark VIII inside view
AI Mark VIII display console - type 73.  (EKCO manufactured unit)

AI Mark VIII receiver type 184:
(EKCO manufacture)

AI Mark VIII scanner unit fitted to Beaufighter.

Note: The Nash and Thompson manufactured Scanner dish was hydraulically driven from pump on engine.


Some of the above photos of the AI Mark VIII are courtesy of the Dutch Signals Collection, which is a non profit organisation dedicated to collecting and preserving wartime radio and radar equipment. Their website can be found at www.qsl.net/pe1ngz/signalscollection.html 

I am also indebted to Charles Exton for allowing use of photos from his private collection.


Part 4: Ecko and the Bomber Fleet



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