Transcript Slide 1
A short history of Radar up to 1945 How some mathematical calculations, plus a lot of science helped to win the war Chris Budd, G4NBG Where Radar Started The best equation of all Maxwell and the discovery of electromagnetic waves E .D , B D M, H J, t t .B 0. Radar before Radar Hertz: Practical demonstration of radio waves (50cm) and that they were reflected from metallic objects Marconi: Invention of radio communication (long wave) In 1899 he proposed used of CW Radio to detect ships in fog and demonstrated by Christian Huelsmeyer 1904 then used on the Normandie 1930s Set up of commercial radio stations Complaints by listeners of interference when aeroplanes flew near. Report on reflected radio waves by Post Office Engineers 1933. The British Invention of Radar Problem: vulnerability of UK to bombing attack: ‘The bomber will always get through’ Baldwin 1934: Defence committee set up: Tizard. Rowe, Blackett, Wimperis Q. 1935 : Could a bomber be destroyed by a radio ‘death ray’ Sir Robert Watson Watt (NPL), showed by calculation that this was not possible, as it required 5 GW of power BUT calculations (by Wilkins) showed that radio waves scattered by an aircraft could be detected. Worried about a factor of 10 This indicated that the aircraft and its range could be found 12th Feb 1935 'Detection and location of aircraft by radio methods’ Watson-Watt The basic physics behind the early radar Dipole aerial …. This is a transmitter and also a reflector of radio waves current I_0 Radiation pattern cos cos( ) i I0 2 i(tkr) E e , 2 0 c r sin( ) cos cos( ) 60I0 2 E r sin( ) The maths behind the memorandum: how maths won the war! 25m cos cos( ) 60I0 2 ET r sin( ) 6km A. Wilkins 30 MHz Field at target ET 14m V m1 per amp of antenna current Current in target wing I = 1.5 mA per amp of antenna current Received field E r 15V m per amp of antenna current 1 Amp = 15A .. So received field E 255V m which is detectable! 1 A question of power P: Transmitter power (100 kW) wavelength Power reaching aircraft at range r: GPA 4 r 2 Reflected power: Power reaching receiver: Power received: GP 2 4 r GPA 16 2 r 4 G 2 PA 2 Pr 64 3 r 4 If r = 10km then received power is in pico Watts! 26th Feb 1935: Daventry Experiment 49.8m Sir Hugh Dowding Heyford bomber £10 000 1935-1939 Orfordness, Bawdsey and pulsed radar E G Bowen .. Airborne radar 200MHz Pulsed radar gives range = c t Chain Home: Good Friday 1939 20 stations operational: 400kW 100 mile range … Gave 30 mins warning 13m Horizontal polarisation 350ft Estimation of height R h h 107R 0.88R2 h1 h2 elevation angle deg h height in feet R range in nMiles Curvature of earth correction 2 h1 sin sin( ) 2 h2 sin sin( ) Operator measures strength of two signals at antennae at two different heights to find Chain Home and the Battle of Britain July-Sept 1940. 15th Sept = Battle of Britain Day K. Park and H. Dowding 600 RAF vs. 2000 Luftwaffe Germans dismissed Radar thinking that a ground station could only control one aircraft at a time!! In contrast Radar was part of a major organisation Aircraft detected using a mixture of statistics and trigonometry Position combining the two Projected position using trigonometry Last known position of German aircraft Estimates of position from Radar stations Never in the field of human conflict was so much owed by so many to so few. Operations room 11 Group Uxbridge Problems with the original Radar Systems • 13m / 30 MHz wavelength gave poor resolution • lots of ground clutter • poor directional finding … RDF • too large to fit easily in an aircraft Solution .. Use much smaller wavelength eg. 10cm, 3GHz But .. Problems with existing Klystron valves (TRE) generating enough power at microwave frequencies The Birmingham Connection: The Cavity Magnetron Oliphant, Randall and Boot: 21/02/1940 University of Birmingham/GEC Kilowatts of power at centimetric wavelengths! E B v B2 Tizard Mission September 1940 British scientific secrets taken to America 15kW Magnetron no. 12 .. E G Bowen (Jet Engine and Atomic Bomb) Developed in the MIT radiation lab: 10cm airborne radar (Lawrence) Airborne Interception Radar (AI) Early 1.5m/200MHz radar AI mark IV German Ai radar Bowen!! 1 micro second pulse width .. 1 mile/speed of light H2S Radar April 1942 TRE Malvern: A Rowe Blumlein, Dee, Rowe, Lovell German Radar Freya Wurzburg Major use of Bletchley Intercepts!! Bruneval R V Jones Other uses of Radar Anti Submarine Radar Radar based navigation: Oboe Jamming: Window/Chaff What RADAR led to Radio Astronomy Hey: Radio interference from the sun Lovell: Jodrell Bank Microwave cooking Microwave communication and the mobile phone