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.
Where Radar Started
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 and
that they were reflected from metallic objects
Marconi: Invention of radio communication
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 under Henry Tizard and A. Rowe
Q. 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.
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
• Strength of radar reflection
• Optimum wavelength
• Range estimation using pulses
• Position by three ranges
• Cathode ray tube
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(tkr)
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
QuickTime™ and a
decompressor
are needed to see this picture.


cos cos( )
60I0
2

ET 
r
sin( )
A. Wilkins

6km
Height = 18m optimises
ground reflection
1
Field at target ET 14m V m
50m wavelength
=18m
QuickTime™ and a
decompressor
are needed to see this picture.
per amp of antenna current
Current in target wing I = 1.5 mA per amp of antenna current
Received field E r 15V m1
per amp of antenna current

1
Amp = 15A .. So received field E  255V m which is detectable!
26th Feb 1935: Daventry Experiment
49.8m
Heyford
bomber
Sir Hugh Dowding
£10 000
1935-1939 Orfordness, Bawdsey and pulsed radar
Taffy Bowen .. Airborne radar 200Mhz
Pulsed radar gives range = c t
Chain Home: Good Friday 1939
20 stations operational
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
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
• 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. 10cm
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 .. 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
Bruneval
Wurzburg
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