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

'Detection and location of aircraft by radio methods’

Watson-Watt

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

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!

26th Feb 1935: Daventry Experiment

Heyford bomber

Sir Hugh Dowding

£10 000

49.8m

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

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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

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

H2S Radar April 1942

Blumlein, Dee, Rowe, Lovell

TRE Malvern: A Rowe

German Radar

Freya Wurzburg

Bruneval R V Jones

Major use of Bletchley Intercepts!!

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

Microwave communication and the mobile phone