IFF Mark II
It remained in use until 1943, when it began to be replaced by the standardised IFF Mark III, which was used by all Allied aircraft until long after the war ended.
To work with many types of radar, a complex system of motorised gears and cams constantly shifted the frequency through three wide bands, scanning each every few seconds.
This led to the introduction of the Mark III, which operated on a single frequency that could be used with any radar; it also eliminated the need for the complex gear and cam system.
Before Chain Home (CH) systems began deployment, Robert Watt had considered the problem of identifying friendly aircraft on a radar display.
This was a simple "reflector" system consisting of a set of dipole antennas that were tuned to resonate at the frequency of the CH radars.
When a pulse from the radar hit them, they would resonate for a short period and cause an additional signal to be received by the station.
When that turned out to be the case, the Royal Air Force (RAF) introduced a different system that consisted of a set of tracking stations using HF/DF radio direction finders.
The idea behind regeneration is to amplify the radio signal and send it into an LC circuit, or "tank", that resonates at a selected frequency.
[6] One problem with regeneration is that if the feedback is too strong, the signal will grow to the point where it begins to broadcast back out of the antenna and cause interference on other receivers.
[10] It was too easy to forget to adjust the gain during flight, especially in single-seat fighters, and it was estimated a usable signal was returned only about 50 per cent of the time.
[11] A solution was already under development in early 1939, similar to the Mark I but employing tuned circuits sensitive to many radar sets.
These changes eliminated the need for tuning or gain adjustments in flight, greatly improving the chance it would respond correctly to a radar.
The rapid expansion of the RAF precluded a significant proportion of its force being equipped by the time of the Battle of Britain in mid-1940.
In any case, the action took place mostly over southern England, where IFF would not be very useful as the CH stations were positioned along the coast and could see the fighters only if they were out over the English Channel.
Its installation on the Supermarine Spitfire required two wire antennas on the tail that slowed the top speed by 2 miles per hour (3.2 km/h) and added 40 pounds (18 kg) of weight.
They realised the importance of using a common IFF system and in early 1941 they decided to install Mark II in their own aircraft.
[14] In 1940, English engineer Freddie Williams had considered this problem and suggested that all IFF operations move to a single frequency.
[15] It remained the Allies' primary IFF system for the rest of the war; the 176 MHz common frequency was used for many years after.
