Your RF Cafe
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 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 needed.
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 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 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 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 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.