The AI MK X is a modified version of the American SCR 720 Radar. It required a two man crew, the operator giving instructions to the pilot over the intercom. It was used in aircraft like the Mosquito for nightfighter operations. The system radiates 0.75 microsecond pulses in the centimetric band at 9.1 cms. The peak power being approximately 70 KW. The aerial system, housed in a Perspex dome on the nose of the aircraft, consists of a small vertical dipole at the centre of a parabolic dish, the dipole being used for both transmission and reception.

The first British designed centimetric AI radar was the AI MK VII /MK VIII, this had a similar performance to the MK X although the scanning and display methods differ considerably. A British MK IX system was also developed that was more sophisticated, as it had ‘lock and follow’ capabilities, but problems in meeting production quantities and timescales required, prevented it from being adopted and the American MK X was used instead.

The MK VIII scanning system was what is termed a 'spiral scan'. In this system the dish is rotated about it axis and gradually deflected sideways, tracing out a spiral in the sky, out to an angle of about 45 degrees, The deflection then returns slowly to the zero position when the process is repeated.

In the MK X, the parabolic dish is rotated continually about its vertical axis. It is also slowly tilted up and down which effectively traces out a helix in the sky, much like looking out from the centre of a coil spring. The rear half of the scan, some 210 degrees, is blanked off, as its field of view is interrupted by the structure of the aircraft. The presentation of the information displayed to the operator is also different. In the MK VIII, a single tube with a circular display was used. The target range is measured from the centre of the tube with the target appearing as a segment of a circle, its angular position defining the azimuth and elevation and the length of the segment showing how much the target is off axis. As the target approached the axis of the aircraft, the segment gradually extended to a full circle.

The MK X has two tubes, the left one or the ‘C’ scope displaying the target as a spot on an azimuth/ elevation grid. The right one or the‘B’ tube has again an azimuth calibration on the horizontal axis but the vertical axis shows the range of the target. A range marker line, adjustable by the operator, can be moved up and down the trace to select a particular target. The control used to adjust the marker is calibrated in range, giving a more accurate reading from that obtained from the graticule markings. Only when this marker line overlays the target does the target appear on the left hand ‘C’ tube. The amount of vertical scanning or tilt can be selected by the operator and has 5 switched ranges. The maximum scan is +40 degrees to -20 degrees down to -5 degrees to +10 degrees. A fixed -5 degrees is used when homing onto a beacon. The range can also be selected from 2 miles, 5 miles, 10 miles up to 100 miles for use with a homing beacon.

The beam width is some 10 degrees with a vertical rotation speed of 360 RPM. (100 rpm for the beacon range). There are 12 scan lines up and 12 down for the +40/-20 degree range, giving a full scan time of 4 seconds.

Getting a working system up and running using modern circuits to simulate targets.

The possibilities of achieving this arose when I acquired a MK X indicator unit, or that is part of one. Only the inner chassis, the outer case, two cathode ray tubes with their deflection coils (magnetic deflection) and most importantly of all the orange Perspex graticule existed. All other components, the front overlay panel, chassis mounting sockets and controls were missing although apart from one broken one, the separately mounted control knobs still remained. Fortunately the original Indicator Unit contained very little circuitry, only two valves were used to DC restore the video signals and could easily be replaced by modern silicon diodes. As no units, other than the Indicator Unit was available, it was decided in the early stages to create, as near as possible, a display on the tubes near to that which would have been seen when the equipment was in service. No attempt was made to rebuild replica units or retain similar circuitry other than making the external appearance of the Indicator Unit as near as possible to that of the original. The original system required an 80v 1500 Hz and a 24v DC supply. Other high voltage supplies required were derived using transformers and rectifiers from the 1500 Hz supply. My own system uses supplies derived from the 240v 50Hz mains, the 5.6KV for the tubes being obtained from a scrap EHT supply unit from an old photocopier. All the deflection and target simulation signals were designed using a combination of analogue and discrete digital integrated circuits. The magnetic deflection used for the tubes proved to be a convenient interface with transistor circuits. The only high voltage supply that was required was for the tube focus coils and the drive circuit for the range deflection circuits. Due to the fast timebase used on the range timebases and the inductance of the range deflection coils, a high voltage transistor was required operating at about 200 volts.

The actual displays obtained from the simulation.

This first picture shows the front panel of the Indicator unit with the two tubes. The controls are for the display only and consist of brightness, focus, X & Y shifts for each tube, plus a graticule brightness control. The brightness on this display has been increased to show the scan lines. Due to the camera exposure time, only some of the lines can be seen, the less bright ones being due the tube afterglow. The brightness would normally be turned low such that only the target spot would be seen. The resolution, due to the limited number of scan lines is low and the target spot can appear on two elevation scan lines at the same time.

The second picture shows details of the LH Azimuth/Elevation display.(note -: I think this screen is the American SCR720 version with radial lines rather than the MKX with the box line structure as in the animated simulation)

The third picture is the RH Range/Azimuth tube showing the Range Marker Bar and two simulated targets. The Range Marker Bar is just intercepting the second target, the spot of which can just be seen on the LH tube. Electronic noise has been added to this trace to give it a realistic effect but it does not show well in the photograph. The two targets can be moved independently in all three axis i.e. range, azimuth and elevation controls from the simulator control box.
As I have never seen any photos of original traces from this equipment I can only assume that the results are close to the original. The technical specification of the timebases etc. are as detailed in the original manuals and by using the original tubes and graticule I think this must be a fair assumption.

INTERCEPTION ANIMATION The animation shows the two radar screens at the lower left and on the right. The demonstration shows the interception and destruction of the closest target in a flight of three, all at different ranges and each using evasive manoeuvres. The animation also demonstrates the ability to concentrate on one target by strobing a particular aircraft selected on the right hand range/azimuth screen and the position of only that aircraft is then displayed on the left hand elevation/azimuth screen.