The evolution of AI23, or ARI 5897 as it was officially designated, was in fact the genesis of modern, purpose designed Airborne Interception Radars. The fact that it was only a fraction of the weight and size of its predecessor, A118, was only part of the miracle so to speak; the unique feature of the set lay in the Radio Frequency Block. By a system of tape control milling machines. the control design of which was in-house Ferranti, the fourway feed from aerial was matched to this RF Block to provide a three channel output system:
1) A Sum Channel that added all four feeds in vector sumation
2) An Elevation Difference Channel that produced a vector summation of the sum of the two horizontal upper feeds minus the sum of the two lower horizontal feeds.
3) An Azimuth Difference Channel that produced a vector summation of the sum of the two starboard vertical feeds minus the sum of the two port vertical feeds.
The whole of this was achieved within the milled RF Block. All the waveguide couplings and splits. phase shifters, the input for the local oscillator. the connections and housing for the balanced mixers, the provision for an Automatic Frequency Control (AFC) Detector and phase adjusters for the Difference Channels were all there in the one assembly.
This was something that had never been achieved anywhere before and represented a tremendous leap in capability and reduction in the physical size that probably ranked with the transistor to the valve. The added bonus was that the result was not only space saving but produced a vastly improved front end performance. The decision was also taken not to follow the road to transistorisation at that time, but to stay with valve technology. This, in fact, proved to be a significant factor in the relatively low figure of obsolescence experienced during the service life of all the variants from the AI23 through to the A123D; the valves used at the Alpha were still available at the Omega.
The transmitter was built around the English Electric Valve Co. magnetron basic Type 4J50; the service types used were the CV2424, 2425 and 2426. The choice of magnetron allowed a measure of frequency selection. The radar was a Fixed Frequency Monopulse Tracker, operating in the X-band region around 8.5 to 8.7 Gcls at a transmitter power of 200KW peak. This high peak power endowed the radar with remarkable Air/Air detection range, figures in fact that modern Airborne Radars strive to match. The down side was that the world and his wife knew that you were there as soon as you flicked the switch to TX! The other remarkable thing was that the transmitter was part of the roll stabilised assembly; therefore this reduced the number of RF rotating joints required.
So, we had a radar set with a reduced weight, fully roll stabilised, high power, monopulse, with the ability to search. acquire and track targets, allied to an analogue computer that controlled the firing brackets and ballistic equations for the weapons. This was the first radar with this level of capability to go into squadron service anywhere in the world. The first aircraft, with the AI23 variant radar. carried the Firestreak missile. This was a pure pursuit device, therefore all the calculations were concerned with attacks from the rear sector. Later alc, fitted with the A123B. had additional kinematic computing capacity to include collision course attacks for the Red Top missile. since this had the capability to cover all angles of attack from head on to tail pursuit.