I recently read The Battle of Britain: The Greatest Battle in the History of Air Warfare by Richard Townshend Bickers. It had some interesting descriptions of the radios used in Spitfires and Hurricanes during the early war years.
“In the Battle of Britain radio assumed a totally new importance. During the 1930s it had been standard practice for defending fighters to mount a standing patrol across the expected line of approach of enemy bombers and at what was considered a likely altitude. In actual war this would have been far too costly in terms of engine hours, fuel, pilot fatigue and on many other counts, and with the Luftwaffe able to attack from an arc ranging from Norway to Spain the whole idea would have been impractical. Radio had been introduced by the British to enable ground controllers to transmit details of enemy attacks to squadrons on standing patrol, the source of the intelligence being the Observer Corps. Radio also enabled squadron and flight commanders to issue commands to their pilots and of course in actual combat it was needed almost constantly by everyone involved, both to shout warnings and instructions and to listen for others.”
“The ubiquitous TR.9D was a high frequency (HF) set. It was connected to a wire antenna (aerial) loosely slung between a rigid mast projecting vertically above the rear fuselage and the top of the rudder. Despite careful screening of the engine ignition system, interference (static) was invariably obtrusive and often deafening. Probably more than half ‘The Few’ were seldom able to understand the clipped bursts of noise that assailed their ears, and ‘say again’ became one of the most common transmissions.”
The TR.9D operated in lower HF, between 4.3 to 6.6 MHz. It was crystal controlled, and had a range of 5 miles air-to-air, and 35 miles air-to-ground. Power output was around 3 watts. The receiver was regenerative, unlike the more advanced super-heterodyne design used by the Germans during the Battle of Britain.The radio was mounted in the rear of the airplane, with a remote gain control and transmit switch for the pilot.
“In general the radio equipment was deplored. High frequency (HF) sets were obsolescent and slowly being replaced by very high frequency (VHF) equipment. Pilots’ dissatisfaction lay not so much in the range of HF as in the poor quality of reception, for it was susceptible to all manner of distorting interference. A further handicap was that an aircraft transmitter-receiver carried only one channel, so there was no communication between squadrons, whereas with the later VHF equipment there were four channels. Also, the system for fixing the position of an aircraft or formation imposed 14 seconds of radio silence on its HF every minute, which could mean missing vital messages and causing an aborted interception.”
That system of using the aircraft radio for position fixing was called Pip-Squeak.
According to the Dunford Radio Society: “Pip-Squeak was the code word for equipment installed in at least two key aircraft in each RAF Sector Station’s Flight or Squadron. When enabled, Pip-Squeak regularly keyed the Hurricane or Spitfire radio transmitter, usually a TR9D, for 14 seconds every minute, to allow ground-based direction finding stations to take bearings and thereby determine the position of the Squadron Leader’s aircraft and hence the general location of his Squadron.” According to Wikipedia, Pip-Squeak was named after a comic strip. The squeak referred to a 1,000 Hz tone issued by the radio for direction finding during the Battle of Britain.