By Cornell Drentea
Whenever thinking of Radio, we usually think of one man: Guglielmo Marconi.
Radar, on the other hand resulted from the work of many men.
In 1793, the Italian scientist Lazarro Spallanzani, a professor at
Padua University studied the ability of blind bats to navigate using ultra sound. He observed
that bats flew well in the dark without the aid of vision. He then designed a unique experiment to demonstrate
the use of the bat’s ears and concluded that a bat would become disoriented without its hearing. He concluded
that the bats produced a continuous train of sound pulses and suggested that the rate of these pulses increased
as a bat approached objects. This was not proven until 1939 when professor Don Griffin
at Harvard University confirmed the phenomenon using novel sound recording techniques and instrumentation not
previously available. Although these studies did not result in any immediate conclusions, the concepts served
to create the first radars.
As early as 1864, the British physicist Clerk Maxwell developed a set of
equations which would govern the behavior of electromagnetic waves and the laws of reflections. In 1886,
the German physicist Heinrich Hertz experimented with spark transmitters and generated dampened
RF waves at a wavelength of 66 cm. He then discovered that the electromagnetic waves could be transmitted through
some types of materials while other materials reflected them. Thus, the newly discovered electromagnetic waves
were named Hertzian waves, after his name.
It wasn’t until 1903, when the German engineer Christian Hulsmeyer proposed
and developed an “obstacle detector” for ships. His experiments proved successful at a distance of one mile, but
did not result in a practical radar. Radar, as an anti-collision system was envisioned as a desirable tool especially
after the successful use of radio communications in the Titanic disaster in 1912.
Radar became practical because of several inventions happening almost coincidental at the turn of the 19th
century and the beginning of the 20th century. First, the sustained generation of un dampened or continuous
radio waves became possible with the invention of the thermionic valve, or the Audion as it was
named, by Lee De Forest (its inventor) in 1906. This was an
offshoot of the previous Fleming valve invention in 1904, and the Edison
effect invention in 1883.
The Audion made possible further developments in radio receiver technology with the invention
of the superheterodyne radio receiver by Edwin H. Armstrong in 1918,
an invention which is still with us today. The last major invention which ultimately made radar possible
was the early introduction of the oscilloscope in 1920, which in turn made possible for the first
time, displaying time intervals between events, and consequently distance on a cathode ray tube, another consequence
of the Audion. From this point on, it was just a matter of time before radar would become a major
part of our life.
After 1920, progress in radar was imminent. Serious considerations to the possibility of determining
distance by radio were given by Marconi in 1916. He noted the reflection of
short-wave Morse code radio communication signals and the possibility of using these signals
not only to communicate, but also to determine distance of objects via echoes. It was in June 1922
in New York at the American Institute of Electrical and Radio Engineers, that he professed the realization of
the radar in his key address note. He then predicted new types of marine radio apparatus that would project radio
waves and detect their reflections from metallic objects such as to “immediately reveal” the presence and bearing
of other ships in the dark or haze. Additional work in 1922 was done by Taylor
and Young at NRL who detected wooden ships using continuous wave
RF techniques at a wavelength of 5 meters. In 1924, a British physicist,
Sir Edward Victor Appleton used radio echoes to determine the height of the ionosphere, while
in 1925 in the US, Breit and Tuve used pulsed
radar techniques for the first time to do the same.
Additional work was done in USSR in 1934. This resulted in a crude early
warning radar system used during WW2 against the German aircraft to protect the cities of
Leningrad and Moscow. It was at the same time, in 1934, that
a patent was granted in the US to Taylor, Young and Hyland at NRL for a system
for detecting objects by radio and further interest in radar development was shown in the US
by the Naval Research Laboratory, US Army Signal Corps, RCA and AT&T Bell Laboratories. Further
radar developments took place in Germany in the 1930s with Rudolf Kuhnhold
and the electronic firm Telefunken who began experimenting with radio detection of ships.
Marconi’s initial work in maritime direction finding techniques helped pave the road to the
development of the first practical radar in Great Britain. This work has been attributed to the
British physicist Sir Robert Watson-Watt who in February 1935
demonstrated the first HF radar system which operated at 6 MHz and
detected aircraft at a range of 8 miles. By September 1935, the British
scientists demonstrated pulsed radar at 12 MHz . This detected aircraft at a range greater than
40 miles, and by March 1936, Great Britain demonstrated detection
of aircraft at 25 MHz at a range of 90 miles. In the mean time, in the
US, NRL experimented with the first radar echoes with half microsecond pulses using an even higher
frequency, 28.3 MHz at a distance of 2.5 miles. Soon after this, the range was
extended to 25 miles.
It was only in 1939 when radar was seriously considered for early warning defense in
Great Britain. A complex system was quickly built for the first time as a practical tool. The
earlier experiments with air defense of 1935 by Sir Watson-Watt paid off resulting
in the first practical early warning HF radar system in England. This was called “Chain-Home”.
The system was made of many pulsed radar stations built on 350 feet tall towers much like
a “chain” around the British Isles to protect England against German
aerial invasions. The “Chain-Home” system lined England’s entire South and East coasts.
Although this system served its purpose, the HF installations were rather large from a wavelength
point of view and RF power was limited by the early tube technology of the time, resulting in
It became immediately apparent that despite its complexity, “Chain-Home” was limited in its
performance. Something better was needed to overcome the short comings of the technology. In order to see with
higher resolution and further away, higher frequencies (shorter wavelengths) and higher power transmitting technologies
The year was 1939. Seeing the shortcomings of the “Chain-Home” system, the
British Government asked two scientists, professor John Randall and professor
Henry Boot of the Department of Physics at Birmingham University to come up
with a powerful microwave source to replace old tube technology. Only six months later, the two scientists invented
the resonant cavity magnetron in February 1940.
This magnetron generated 10 kilowatts of RF power at 10 centimeters wavelength, about a thousand
times more powerful than any other tube microwave source at the time.
However, the magnetron was a capricious device to manufacture, and Britain realized quickly
the inability of its industry already strangled by the German air attacks to manufacture magnetrons in the quantities
needed to produce new and better radar systems. It was clear that the magnetron’s versatility could provide aircraft
with unprecedented capability of seeing German U-boat periscopes at sea and tanks on land. The
magnetron could truly revolutionized radar technology.
Britain was facing its most desperate hours. Bombs were falling nightly over Liverpool and
London and a Nazi invasion was imminent. With its limited resources fully committed, a decision
was quickly made by Britain’s Prime Minister Winston Churchill to send the Magnetron
invention to the United States where vast industrial resources were readily available to produce
Merely escaping the German bombs and sailing from Liverpool, the first magnetron
secretly crossed the Atlantic in September 1940 aboard the Canadian
liner Duchess of Richmond. This was a most secret mission conducted by Sir Henry Tizard, Rector
of the Imperial College of Science and Technology and Chairman of the British Government’s key
scientific committee on air defense. This historic event is known as the Tizard Mission.
The British Magnetron arrives at Raytheon
The Duchess of Richmond arived quietly into Newfound Land’s Cape Race and Halifax
harbor on the morning of September 6, 1940. From here, the precious cargo left for Washington,
D.C. via railroad. For the next few days, Tizard met with US Government
officials including the Navy Secretary Franklin Knox and FDR.
Finally, Tizard met with his technical US counterpart, Dr. Vannevar
Bush a Scientist at MIT and also, co-founder of the American
Appliance Company also known as Raytheon (a name that means Light of Gods), a large
established electronics manufacturer in the US.
It is at this point that Raytheon enters the magnetron industry business. A meeting was quickly
arranged between Tizard and Percy L Spencer, Raytheon’s Chief Engineer.
Spencer was a brilliant self made engineer and an avid ham radio operator with a practical sense
of what can be achieved. He listened carefully to the manufacturing problems described by the British, and asked
to take the magnetron home over the weekend, to play with it in his ham shack. Permission was granted, and Spencer
came up with radical changes and performance improvements that made the magnetron manufacturable for the first
time. A contract was immediately awarded to Raytheon for a small quantity of magnetrons and by the end of
WW II, Raytheon was manufacturing over 80% of all the magnetrons in the
Also, thanks to Percy Spencer, the Magnetron found its way into the microwave oven. In
1945, Spencer discovered a melting chocolate bar in his shirt pocket while standing in front
of a magnetron powered radar. He immediately realized the value of this discovery. Inventor Spencer, who obtained
over 120 patents in his life time saw the practical application of the magnetron in the kitchen, and immediately
held a bag of corn seeds next to the magnetron powered radar transmitter and got popcorn. Raytheon
developed and marketed the first microwave oven ever using the magnetron in 1954. It was known
as the 1161 Radarrange. It stood five feet tall and weighed 750 pounds. At first,
it was used only by luxury restaurants and ocean liners, but in 1967, the Amana
division of Raytheon produced the first domestic kitchen microwave oven. Today, the magnetron
is present in every kitchen. Most Magnetrons today are produced in Japan or China.
From its inception in 1922 as the American Appliance Company to its new beginnings
in 1925 as Raytheon (Light of Gods), to the invention of the rectifier tube
(called the Raytheon) which allowed radio receivers to run on alternating current without needing a battery, to
the first guided missile, to space computers that made the historic lunar journeys possible, to today’s presence
in every aspect of the radio and radar, Raytheon has been an undisputed global leader of RF technology.