75 years of radar a short history of radar 1935-1945 how some mathematical calculations, plus a lot...
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75 Years of Radar
A short history of Radar 1935-1945
How some mathematical calculations, plus a lot of science, won the war
Chris Budd
Some significant anniversaries this year
• 75 years: 1935 Invention of Radar
• 70 years: 1940 Invention of the Cavity Magnetron
Battle of Britain
Tizard Mission to the USA
• Wednesday: Statue for Keith Park• Invention of Radar
• Chain Home
• Battle of Britain
• Cavity Magnetron
• Airborne Radar
• German developments and how the UK found out
• Other uses of Radar
• What radar led to.
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∇×E = −∂B
∂t−M, ∇ ×H = −
∂D
∂t+ J,
∇.D = ρ, ∇.B = 0.
Where Radar Started
Maxwell and the discovery of electromagnetic waves
Radar before Radar
Hertz: Practical demonstration of radio waves and that they were reflected from metallic objects
Marconi: Invention of radio communicationIn 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 under Henry Tizard and A. RoweQ. 1935 : Could a bomber be destroyed by a radio ‘death ray’
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.
This indicated that the aircraft and its range could be found
Worried about a factor of 10
'Detection and location of aircraft by radio methods’
Watson-Watt
• Strength of radar reflection• Optimum wavelength• Range estimation using pulses• Position by three ranges• Cathode ray tube
12th Feb 1935
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
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Eθ =−i I0
2π ε0 c r
cosπ
2cos(θ)
⎛
⎝ ⎜
⎞
⎠ ⎟
sin(θ)e i(ωt−kr), Eθ =
60 I0r
cosπ
2cos(θ)
⎛
⎝ ⎜
⎞
⎠ ⎟
sin(θ)
The maths behind the memorandum: how maths won the war!
25m
50m wavelength6km
=18m
Height = 18m optimises ground reflection
Field at target per amp of antenna current
Current in target wing I = 1.5 mA per amp of antenna current
Received field per amp of antenna current
Amp = 15A .. So received field which is detectable!
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A. Wilkins
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ET =14mV m−1
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E r =15μV m−1
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E = 255μV m−1
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ET =60 I0r
cosπ
2cos(θ)
⎛
⎝ ⎜
⎞
⎠ ⎟
sin(θ)
26th Feb 1935: Daventry Experiment
Heyford bomber
Sir Hugh Dowding
£10 000
49.8m
1935-1939 Orfordness, Bawdsey and pulsed radar
Taffy Bowen .. Airborne radar
200Mhz
Pulsed radar gives range = c t
Chain Home: Good Friday 1939
350ft
13m Horizontal polarisation
20 stations operational
100 mile range … Gave 30 mins warning
Estimation of height
h€
α
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α
elevation angle deg
h height in feet
R range in nMiles
R
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h =107Rα + 0.88R2
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γ=sin
2π h1
λsin(α )
⎛
⎝ ⎜
⎞
⎠ ⎟
sin2π h2
λsin(α )
⎛
⎝ ⎜
⎞
⎠ ⎟
Operator measures strength of two signals at antennae at two different heights to find
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α
Curvature of earth correction
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h1
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h2
Chain Home and the Battle of Britain
July-Sept 1940. 15th Sept = Battle of Britain Day
Germans dismissed Radar thinking that a ground station could only control one aircraft at a time!!
K. Park and H. Dowding
600 RAF vs. 2000 Luftwaffe
In contrast Radar was part of a major organisation
Operations room 11 Group Uxbridge
Never in the field of human conflict was so much owed by so many to
so few.
Problems with the original Radar Systems• 12m 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. 10cmBut .. 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!
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v =E × B
B2
Tizard Mission
September 1940
British scientific secrets taken to America
15kW Magnetron no. 12 .. Taffy 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
1 micro second pulse width .. 1 mile/speed of light
H2S Radar April 1942
Blumlein, Dee, Rowe, Lovell
TRE Malvern: A Rowe
German Radar
Freya Wurzburg
Bruneval R V Jones
Anti Submarine Radar
Radar based navigation: Oboe
Jamming: Window/Chaff
Other uses of Radar
What RADAR led to
Radio Astronomy
Microwave cooking
Hey: Radio interference from the sun
Lovell: Jodrell Bank
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