& App Notes
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
Lazzaro 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
James 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 limited performance.
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 were
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
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 it.
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
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 US.
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 Radar Range. 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.
Posted June 3, 2020