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Incredibly rare, A26 Invader BC-608 Pip-Squeak Radio Transmitter Contactor Unit

During the lead up to and well into the opening months of the Second World War, radar was still very much in its infancy and Britain was frantically trying to establish a network of interlinked radio and radar air defence across the South and South East coast of of the country.

The British Chain Home air defence radar was to prove critical for Britain’s security during the Battle of Britain and the German Blitz on the nation’s key airfields, industries and ports. 

During the initial operational deployment of the British Home Chain Air defence, the limited radar coverage and organisational resources were predominantly directed at locating approaching enemy formations, rather than tracking friendly aircraft over British soil. Locating the ‘friendly’ interceptors to direct them to enemy formations required an interim system of radio location consisting of established radio direction finding (RDF) networks.

The British Royal Aircraft Establishment had already been working on a new airborne version of the primitive TR.9 radio to aid ground based Directional Finding (DF) operators as early as 1938.

Known as ‘Pip-squeak’, the new system consisted of a single transmitting amplifier coupled with two radio frequency oscillators which allowed the set to be switched quickly between tow broadcast frequencies.

Using the aircraft’s voice radio set to periodically send out a 1 kHz tone which was picked up by ground-based high-frequency direction finding HFDF, (Huff-Duff) receivers, observers could take three HFDF measurements to determine the location of friendly aircraft using triangulation.  

The Pip-Squeak was essentially a timer for communications radios that broadcasted a fourteen-second carrier wave at a 1 kHz tone (squeak) every minute, allowing the RDF stations to determine an aircraft’s position or ‘fix' - the forerunner to today’s airborne radio navigation direction finding system. 

Knowing the location of their fighter interceptors was essential for the RDF operators to direct them to the oncoming enemy aircraft which had been tracked using the Home Chain radar system. Knowing where their fighters were at any given moment also enabled operators to advise the fighters on any course corrections required.

With Southern and Eastern Britain divided up into sectors, each with their own RDF towers, interconnected control centres and fighter interceptor squadrons, defensive resources could be directed very effectively to any oncoming threat picked up by radar or coastal observation posts.

British aircraft carried the Type 4 TR.9D Pip-Squeak Contactors which were installed behind the pilot with a small ‘On-Off’ colour sectioned, clock-face control switch mounted in the cockpit. Frequencies were selected before the start of each mission using swappable crystal oscillators  Both the section leader and one other aircraft in the squadron would normally carry the unit.

Once in the air after a scramble, squadron leaders were asked to ready their Pip-Squeak clocks. The original system required the pilot to turn the ‘wind’ knob to rotate the single second hand counter clockwise to their coloured section. 

In the original system, this required them to turn the “wind” knob to rotate the single, second-hand counter-clockwise around the face of the clock. There were up to four sections of aircraft in each squadron, although most squadrons had two or three sections at any given time. Each section had its own position for the second-hand; red section had the 12 o’clock, yellow the 9, blue the 6 and green the 3 o’clock position.

Once the clocks were properly positioned, the sector controller would initiate a countdown, ‘ synchronise time, 5, 4, 3, 2, 1, mark'. At ‘ Mark’, the pilots would activate the clock, which would start the second-hand moving clockwise. 

When the second-hand reached the 12 o’clock position the oscillator was turned on, and it turned off again just before the 3 o'clock position, broadcasting for 14 seconds per minute. The system automatically switched the radio from voice to pip-squeak channel at the 12 o’clock location; unfortunately, if the pilot was talking he would be cut off for those 14 seconds of DF transmission.

Once synchronised  a given sector was activated and fighters from that sector were sent to intercept the enemy. With each ‘sector’ then transmitting in sequence, ground based DF operators knew where their fighter forces were and could determine where they should be directed.

The USAAF entered the war in Europe in December 1941 although in reality, they were already supporting their European Allies with their Lend Lease program supplying aircraft and aircraft parts from close to the beginning.

Unlike their British counterparts who used the VHF ‘changeable crystal’ radio systems, the USAAF originally used their HF based SCR-274 system which were incompatible with the Allies' VHF. Whilst commanders argued the pros and cons of each, an interim solution was put in place with the US producing their own version of the British ‘Pip-Squeak’, the BC-608 Contactor which was compatible.

Whilst only 500 BC-608 units were ever produced, it was a viable solution that filled the gap until the USAAF relented and installed their own version of their RAF’s VHF system. Eventually a true IFF (Identification Friend or Foe) transponder was developed which was used across all Allied aircraft.

Like the original British Pip-Squeak, the BC-608 variant was essentially a clock and was fitted in early versions of USAAF aircraft such as the P-39, P-40, P-51, P-38, P-47 and even the A26 Douglas Invader light bomber, mounted either in the main flying panel or on an externally mounted bracket.

Unlike the British ‘Clock’,  BC-608 was hand wound, preferably every 12 hours, although it could also run for 24 hours.

The A-26 Invader pilot would actuate the clock STOP-RUN switch before take off which would then activate the aircraft transmitter’s to send out a signal for 14 seconds of every minute, ie. In the 12 o’clock quadrant indicated. The transmitter switching could be disabled by the Contactor IN-OUT switch, leaving the clock to continue running. 

You are purchasing an original, battle hardened American BC-608 Contactor ‘Pip-Squeak which is one of only 500 produced during WWII and if you consider the extremely low survival rate of allied fighter pilots and their aircraft, there are likely to be less than 200 or so working examples in existence making this a rare WWII aviation artefact indeed.

This is a fully operational BC-608 ‘Pip-Squeak’ as installed in early versions of the Douglas A26 Invader. Its wind up mechanism still works as to its activate and de-activate switches which is remarkable for a instrument over 80 years old.

Mounted in its 100 yr old Mango Wood display stand with a highly detailed 1/72 scale, custom-made A26 Invader model perched atop its magnetic arm and detailed, printed and laminated Fact Sheet with a photo of the instrument in the Invader cockpit, this original Recovery Curios Aircraft Instrument Collectable would make the perfect gift for any aviation enthusiast.

This Douglas A-26 Invader Instrument comes complete with detailed Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit.

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Your Douglas A26 Invader BC-608, Pip-Squeak Radio Transmitter Contactor Unit, Original Recovery Curios Warbird Collectable includes:

  • Original Warbird instrument
  • Highly detailed hand-built and airbrushed 1/72 plastic scale model of the aircraft,* 
  • Hand-crafted and beautifully finished 100yr, Far North Queensland Mango Wood display stand
  • Detailed, 2-sided, printed and laminated Instrument Fact Sheet detailing aircraft and instrument
  • Removable Magnetic Display Arm

* Upsize to the more highly detailed 1/48 scale for just $45. Click on the 1/48 Scale Option at the top of the page.

Both the 1/72 and larger 1/48 scale custom built and airbrushed plastic models are available with 'wheels & flaps up or down' and 'canopy open or closed' options with a choice of two Squadron markings and camouflage.

Upon order placement you will receive an email asking for your preferred configuration.

Your complete Recovery Curios Original Instrument Collectable is securely packed and delivery normally takes between 4 - 6 weeks approx.

Did you fly, crew or maintain a Douglas Invader or have a friend, colleague or family member who did? Check out our PERSONALISED ORIGINAL INSTRUMENT COLLECTABLE OPTION here.