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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
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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
The best equation of all
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.
This indicated that the aircraft and its range could be foundWorried about a factor of 10
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
6km
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!
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)
30 MHz
A question of power
P: Transmitter power (100 kW) wavelength
Power reaching aircraft at range r:
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GP
4π r2
Reflected power:
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GPA
4π r2
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GPA
16π 2 r4Power reaching receiver:
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Pr =G2PA λ2
64π 3 r4Power received:
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λ
If r = 10km then received power is in pico Watts!
1935-1939 Orfordness, Bawdsey and pulsed radar
E G Bowen .. Airborne radar 200MHz Pulsed radar gives range = c t
Chain Home: Good Friday 1939
350ft
13m Horizontal polarisation
20 stations operational: 400kW
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
€
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
Aircraft detected using a mixture of statistics and trigonometry
Last known position of German aircraft
Projected position using trigonometry
Estimates of position from Radar stations
Position combining the two
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
• 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!
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v =E × B
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) Bowen!!
Early 1.5m/200MHz radar AI mark IV
German Ai radar
1 micro second pulse width .. 1 mile/speed of light