AI-MK4 Airborne Interception Radar

Many people collect WWII aircraft radio, radar and other equipment but to get that equipment working again is the ultimate challenge. Radio equipment with its many variations are relatively easy projects to tackle but there is a vast amount of other equipment, i.e. radar, navigation aids, beam approach etc that can be made operational using simulated signals. Following the H2S and Gee projects, the next equipment chosen was the AI MK4. By using simulated signal only part of the system is required, in this case only the receiver and indicator. This is a great advantage as it is getting almost impossible to find this type of equipment nowadays and of course the power needed to operate it is greatly reduced


This was the first night fighter radar to enter service in late 1940. It was introduced as a response to the change of tactics that the Germans had taken, in moving away from daylight raids to bombing at night. It was initially installed in Blenheims but continued on, being installed in the Beaufighter and even into the early Mosquitoes until later radars were developed. The radar transmitted 20uS pulses from an aerial fitted on the nose of the aircraft, in a wide forward beam and at a frequency of 195 MHz. Four receiving aerials, each with a relatively narrow beam were fitted on the wings, two facing outwards, one for port and one for starboard and the other two for up and down directions, all being at an angle of about 35 degrees to the centre line of the aircraft. Depending on the position of the target, each aerial would receive a different signal level. The signal levels were displayed on the indicator unit after passing though a receiver. The equipment was installed in the position normally occupied by the rear gunner in the Blenheim. The operator sat facing the rear, looking into the indicator, the receiver being fitted vertically at the operator's side. Information as to the course that the pilot was to fly to intercept the target was passed to the pilot via the intercom system


The receiver was a fairly conventional superhet with two RF stages and a separate oscillator/ mixer. Tuning over a limited range of +- 1MHz was by means of a variable inductor. Normally four receivers would be required, one for each aerial but space and weight prevented this so that a two pole, 4 way motor driven switch was incorporated in the receiver that synchronously switched each aerial signal into the receiver whilst at the same time switching the output to the appropriate channel of the indicator unit. Sequences of port/starboard/up/down signals were thus sent to the indicator unit. The receiver also contained the power supplies for the indicator unit


The indicator unit contains two 3-inch cathode ray tubes, each tube having an identical timebase scan. On the right hand tube the trace is vertical and used for displaying port/starboard signals whilst the left hand tube is for up and down aerial signals. The trace or timebase is initiated at the same time that the transmitter pulse occurs and the receiver pulses appear someway along the timebase depending on the range of the target. On the right-hand tube, port signals deflect the trace to the left whilst the starboard signal deflects the trace to the right. A graticule, calibrated in miles, is fitted to the right hand tube only. Up/Down signals are treated in a similar manner on the left hand tube. When the target is on axis the signal levels on either side of the two traces are equal. If the target is off axis the signal level on that side will at first increase and then decrease if the target moves further off axis still.

Due to the low operating frequency (195 MHz) the aerials have limited directional capabilities such that strong echoes are received from the ground immediately below the aircraft. These die away with distance but still swamp any target signals. This results in the range of the equipment being limited to the height of the aircraft. The ground echo appears on the trace as a triangular shape, referred to as the "Christmas tree effect" and occurring at a distance equal to the height of the aircraft. Some breakthrough of the transmitter pulse also occurs at the beginning of the trace, limiting the minimum operating range, although the system works down to a few hundred yards allowing visual contact by the pilot to be achieved.

The system could also be used for homing on to a beacon. In this mode the transmitter is switched off which eliminates the ground echo pulse allowing the equipment to operate at ranges up to 20 miles. A switch on the underside of the indicator unit changes the timebase speed and a longer range calibrated scale on the graticule is used.


I was fortunate enough to obtain a receiver via an advertisement in a vintage wireless magazine, it was in poor condition but restorable. The indicator was more difficult and eventually proved impossible to find. In the end I managed to borrow an original from a friend and make an exact copy.

In order to reduce the weight and size of the equipment in the aircraft, an 80v high frequency supply, obtained from an engine driven alternator was used. In my case I used a static inverter, built in an old alternator control unit case, to perform the same function. The simulator has to produce the four target signals together with a ground echo return and the transmitter breakthrough pulse. The last two are common to the 4 channels but the target echoes must vary both in time delay and amplitude. A dual axis control stick is fitted to the simulator such that as the stick is moved either left or right, or up and down, the signals increase or decrease in the correct manner. The target signals are also arranged to appear to move towards the attacking aircraft at an approach speed that can be adjusted and the height of the ground return can also be set. A further switch changes the simulation to the homing beacon mode.


This animation demonstrates the relationship between the position and distance of the target to what is seen on the radar screens. The pilot of course can see nothing of the target until he is within a few hundred yards range and must identify the target aircraft visually as 'an enemy' before opening fire.

The left hand screen tells the radar operator if the target is above or below him. The echo first appears as a small yellow vertical line at the extreme right hand end of the trace, buried in the noise. In practice the radar screen is all green and the echo would not be seen in the noise but for demonstration purposes the trace has been changed to yellow to denote the target echo. The noise (the Christmas tree effect) is due to reflections from the ground immediately below and forward of the aircraft. Targets are lost in this noise and the range is therefore limited to the height of the aircraft. As the target echo appears out of the ground noise as the distance closes, the echo will move above the line if the target is high and conversely move below the line if the target is low. The radar operator gives verbal instructions to the pilot to bring the attacking aircraft on the correct course for interception, that is when the echoes on both screens are equi-distance either side of the axis. The right hand screen is virtually identical in operation, except the information displayed tells the operator whether the target is to the left or right.