The Opana Radar Site is a National Historic Landmark and IEEE Milestone that commemorates the first operational use of radar by the United States in wartime, during the attack on Pearl Harbor . It is located off the Kamehameha Highway just inland from the north shore of Oahu , Hawaii , south of Kawela Bay . It is not open to the public.
120-514: In December 1939, the U.S. military established an Aircraft Warning Service (AWS) using radar to defend American territory. It employed the SCR-270 radar , the first United States long-range search radar created at the Signal Corps laboratories at Fort Monmouth , New Jersey , circa 1937. The radar's operating frequency was 106 megahertz and it had a maximum range of 150 miles, or greater if the equipment
240-433: A marine radar mounted on a recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent advances in aviation weather-avoidance radar and in marine radar have successfully replaced
360-428: A sulfur lamp , a magnetron provides the microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g., sulfur , metal halides , etc.). Although efficient, these lamps are much more complex than other methods of lighting and therefore not commonly used. More modern variants use HEMTs or GaN-on-SiC power semiconductor devices instead of magnetrons to generate
480-449: A force at right angles to their direction of motion (the Lorentz force ). In this case, the electrons follow a curved path between the cathode and anode. The curvature of the path can be controlled by varying either the magnetic field using an electromagnet , or by changing the electrical potential between the electrodes. At very high magnetic field settings the electrons are forced back onto
600-480: A higher incidence of cataracts in later life. There is also a considerable electrical hazard around magnetrons, as they require a high voltage power supply. Most magnetrons contain a small amount of beryllium oxide , and thorium mixed with tungsten in their filament . Exceptions to this are higher power magnetrons that operate above approximately 10,000 volts where positive ion bombardment becomes damaging to thorium metal, hence pure tungsten (potassium doped)
720-489: A key advance, the use of two cathodes, was introduced by Habann in Germany in 1924. Further research was limited until Okabe's 1929 Japanese paper noting the production of centimeter-wavelength signals, which led to worldwide interest. The development of magnetrons with multiple cathodes was proposed by A. L. Samuel of Bell Telephone Laboratories in 1934, leading to designs by Postumus in 1934 and Hans Hollmann in 1935. Production
840-422: A known point for the radar to find it, but could not be located at the agreed upon time. The radar operators then searched for the bomber and located it about ten miles (16 km) from its intended position. It was later learned that winds had blown the bomber off course, so what was to be a simple demonstration turned into an example of real-world radar location and tracking. Development of this system continued as
960-501: A microwave signal from direct current electricity supplied to the vacuum tube. The use of magnetic fields as a means to control the flow of an electric current was spurred by the invention of the Audion by Lee de Forest in 1906. Albert Hull of General Electric Research Laboratory , USA, began development of magnetrons to avoid de Forest's patents, but these were never completely successful. Other experimenters picked up on Hull's work and
1080-424: A more difficult problem for a wider array of radar systems. Neither of these present a problem for continuous-wave radars , nor for microwave ovens. All cavity magnetrons consist of a heated cylindrical cathode at a high (continuous or pulsed) negative potential created by a high-voltage, direct-current power supply. The cathode is placed in the center of an evacuated , lobed, circular metal chamber. The walls of
1200-514: A professor at Prague's Charles University , published first; however, he published in a journal with a small circulation and thus attracted little attention. Habann, a student at the University of Jena , investigated the magnetron for his doctoral dissertation of 1924. Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron. Most of these early magnetrons were glass vacuum tubes with multiple anodes. However,
1320-620: A pulse width from 10 to 25 microseconds, and a pulse repetition frequency of 621 Hz. With a wavelength of about 3 meters (nine feet), the SRC-270 was comparable to the contemporary Chain Home system being developed in Great Britain, but not to the more advanced UHF Würzburg radars being developed in Germany. This wavelength did turn out to be useful, as it is roughly the size of an airplane's propeller, and provided strong returns from them depending on
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#17328584814721440-480: A request for a long-range unit, this time from "Hap" Arnold who wrote to them 3 June 1937. Shortly thereafter the Signal Corps became alarmed that their radar work was being observed by German spies, and moved development to Sandy Hook at Fort Hancock , the coast artillery defense site for Lower New York Bay . After the move, work immediately started on the Air Corps request for what was to become known (in 1940) as
1560-462: A rod-shaped cathode, placed in the middle of a magnet. The attempt to measure the electron mass failed because he was unable to achieve a good vacuum in the tube. However, as part of this work, Greinacher developed mathematical models of the motion of the electrons in the crossed magnetic and electric fields. In the US, Albert Hull put this work to use in an attempt to bypass Western Electric 's patents on
1680-576: A second unit to the National Research Council in Ottawa ), who, unaware of its history, used it to image aurora for the first time in 1949. The technique was published in 1950 in Nature , and was a field of active research for some time. In 1990, after the radar had sat derelict for years, they received a phone call informing them of the historical nature of the radar, and requesting it be sent back to
1800-444: A second, The 621 Hz frequency is derived either from an internal oscillator or an external source, typically the oscilloscope. The keyed output stages consisted of two 450TH power triodes in series, with the final stage configured as a cathode follower . The receiver is a superheterodyne design, with a high-power 832 dual tetrode as its first RF amplifier and a RCA 1630 orbital-beam hexode electron-multiplier amplifier tube as
1920-508: A sector repetitively. Still later systems added additional controls to rotate the antenna at 5 RPM for use with a plan position indicator , like modern radars. The generator was driven by a LeRoi gasoline engine and could produce 15 KVA of electric power. After its use by the military, the Pearl Harbor unit (s/n 012) was loaned to the University of Saskatchewan in Saskatoon (along with
2040-404: A slower and less faithful response to control current than electrostatic control using a control grid in a conventional triode (not to mention greater weight and complexity), so magnetrons saw limited use in conventional electronic designs. It was noticed that when the magnetron was operating at the critical value, it would emit energy in the radio frequency spectrum. This occurs because a few of
2160-516: A small amount of funding and diverted some from other projects. A research team was organized under the direction of civilian engineer Paul E. Watson . By December 1936 Watson's group had a working prototype, which they continued to improve. By May 1937 they were able to demonstrate the set, detecting a bomber at night. This demonstration turned out to be particularly convincing by mistake; the Martin B-10 bomber had originally been instructed to fly to
2280-448: A small book and transmitted from an antenna only centimeters long, reducing the size of practical radar systems by orders of magnitude. New radars appeared for night-fighters , anti-submarine aircraft and even the smallest escort ships, and from that point on the Allies of World War II held a lead in radar that their counterparts in Germany and Japan were never able to close. By the end of
2400-460: A spark, short circuiting the line and creating a resonant stub that prevented most of the pulse energy from reaching the receiver. The oscilloscope ( A-scope ) display employed a five-inch diameter 5BP4 cathode-ray tube , the same type used in the first commercial RCA television set, the TRK-5, introduced in 1939. The sweep was normally generated from an internal 621 Hz oscillator that also drove
2520-477: A submarine periscope, which allowed aircraft to attack and destroy submerged submarines which had previously been undetectable from the air. Centimetric contour mapping radars like H2S improved the accuracy of Allied bombers used in the strategic bombing campaign , despite the existence of the German FuG 350 Naxos device to specifically detect it. Centimetric gun-laying radars were likewise far more accurate than
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#17328584814722640-418: A well-drilling derrick, and was mounted on the trailer for movement. When opened it was 55 feet (17 m) tall, mounted on an 8-foot (2.4 m) wide base containing motors for rotating the antenna. The antenna itself consisted of a series of 36 half wave dipoles backed with reflectors, arranged in three bays, each bay with twelve dipoles arranged in a three-high four-wide stack. (Later production versions of
2760-440: Is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output. Where there are an even number of cavities, two concentric rings can connect alternate cavity walls to prevent inefficient modes of oscillation. This
2880-436: Is a high-power vacuum tube used in early radar systems and subsequently in microwave ovens and in linear particle accelerators . A cavity magnetron generates microwaves using the interaction of a stream of electrons with a magnetic field , while moving past a series of cavity resonators , which are small, open cavities in a metal block. Electrons pass by the cavities and cause microwaves to oscillate within, similar to
3000-678: Is called pi-strapping because the two straps lock the phase difference between adjacent cavities at π radians (180°). The modern magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1-kilowatt input will generally create about 700 watts of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Large S band magnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75 kW. Some large magnetrons are water cooled. The magnetron remains in widespread use in roles which require high power, but where precise control over frequency and phase
3120-423: Is unimportant. In a radar set, the magnetron's waveguide is connected to an antenna . The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high-power microwave energy being radiated. As in all primary radar systems, the radiation reflected from a target is analyzed to produce a radar map on a screen. Several characteristics of the magnetron's output make radar use of
3240-644: The Battle of Britain , was at the school and discussed with the American generals the design and urgency of establishing the Hawaiian system, in particular emphasizing the need for thorough radar site coverage along the coasts. Despite the high-level attention and the excellence of the school in training on the use of the SCR-270 and its integration and coordination with fighter intercepts, the army did not follow through on supporting
3360-519: The Battle of Britain . The Japanese aircraft they detected attacked Pearl Harbor 55 minutes later, precipitating the United States ' formal entry into World War II . The northerly bearing of the inbound flight was not passed along in time to be of use. The US fleet instead fruitlessly searched to the southwest of Hawaii, believing the attack to have been launched from that direction. In retrospect this may have been fortuitous, since they might have met
3480-532: The Far East Air Force did not fare much better than the defending air force at Pearl Harbor. Though FEAF had five SR-270Bs, only two were functioning on 8 December 1941, one was a Marine Corps Air Warning detachment of the 4th Marine Regiment based at Cavite Naval Base . On 29 November, in response to the war warning sent to all overseas commands, the radar detachment went on continuous watch in three shifts. Even with correct detection of enemy flights from
3600-754: The General Electric Company Research Laboratories in Wembley , London , was taken on the Tizard Mission in September 1940. As the discussion turned to radar, the US Navy representatives began to detail the problems with their short-wavelength systems, complaining that their klystrons could only produce 10 W. With a flourish, "Taffy" Bowen pulled out a magnetron and explained it produced 1000 times that. Bell Telephone Laboratories took
3720-509: The Nobel Prize for Physics in 1905. In the USA it was later patented by Lee de Forest , resulting in considerable research into alternate tube designs that would avoid his patents. One concept used a magnetic field instead of an electrical charge to control current flow, leading to the development of the magnetron tube. In this design, the tube was made with two electrodes, typically with the cathode in
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3840-613: The Pearl Harbor Radar , since it was an SCR-270 set that detected the incoming raid about 45 minutes before the 7 December 1941, attack on Pearl Harbor commenced. Two versions were produced, the mobile SCR-270, and the fixed SCR-271 which used the same electronics but used an antenna with somewhat greater resolution. An upgraded version, the SCR-289 , was also produced, but saw little use. The -270 versions were eventually replaced by newer microwave units based on cavity magnetron that
3960-668: The SCR-268 , which eventually evolved into an excellent short-to-medium range gun laying system. In April 1937 a LtC. Davis, an officer in an Army Air Corps Pursuit Squadron in the Panama Canal Zone (CZ), sent a request for a "Means of Radio Detection of Aircraft" to the US Army's Chief Signal Officer (CSig.), bypassing normal channels of command. The SCR-268 was not really suited to this need, and after its demonstration in May they again received
4080-607: The Tizard Mission , where it was shown on 19 September 1940 in Alfred Loomis’ apartment. The American NDRC Microwave Committee was stunned at the power level produced. However Bell Labs' director was upset when it was X-rayed and had eight holes rather than the six holes shown on the GEC plans. After contacting (via the transatlantic cable) Dr Eric Megaw, GEC’s vacuum tube expert Megaw recalled that when he had asked for 12 prototypes he said make 10 with 6 holes, one with 7 and one with 8; there
4200-508: The University of Birmingham in the UK, John Randall and Harry Boot produced a working prototype of a cavity magnetron that produced about 400 W. Within a week this had improved to 1 kW, and within the next few months, with the addition of water cooling and many detail changes, this had improved to 10 and then 25 kW. To deal with its drifting frequency, they sampled the output signal and synchronized their receiver to whatever frequency
4320-404: The University of Birmingham , England in 1940. Their first working example produced hundreds of watts at 10 cm wavelength, an unprecedented achievement. Within weeks, engineers at GEC had improved this to well over a kilowatt (kW), and within months 25 kW, over 100 kW by 1941 and pushing towards a megawatt by 1943. The high power pulses were generated from a device the size of
4440-486: The anode . The components are normally arranged concentrically, placed within a tubular-shaped container from which all air has been evacuated, so that the electrons can move freely (hence the name "vacuum" tubes, called "valves" in British English). If a third electrode (called a control grid ) is inserted between the cathode and the anode, the flow of electrons between the cathode and anode can be regulated by varying
4560-490: The low-UHF band to start with for front-line aircraft, were not a match for their British counterparts. Likewise, in the UK, Albert Beaumont Wood proposed in 1937 a system with "six or eight small holes" drilled in a metal block, differing from the later production designs only in the aspects of vacuum sealing. However, his idea was rejected by the Navy, who said their valve department was far too busy to consider it. In 1940, at
4680-743: The "Radio Set SCR-270". Parts of the SCR-268 were diverted to this new project, delaying the completion of the -268. The non-portable version, the SCR-271-A, s/n 1 was delivered to the Canal Zone and began operation in October 1940 at Fort Sherman on the Atlantic end of the Panama Canal . It picked up airliners at 117 miles (188 km) in its initial test run. The second set was set up on Fort Grant 's Taboga Island on
4800-577: The AAF's operational radar at Iba, command disorganization resulted in many of the defending fighters in the Philippines being also caught on the ground and destroyed, as was the largest concentration of B-17s (19) outside of the continental US. The Iba set was destroyed in the initial attack on Iba on 8 December. After the first day, the effective striking power of the Far East Air Force had been destroyed, and
4920-601: The Army focused primarily on infra-red detection systems (a popular idea at the time), in 1935 work turned to radar again when one of Blair's recent arrivals, Roger B. Colton, convinced him to send another engineer to investigate the US Navy 's CXAM radar project. William D. Hershberger went to see what they had, and returned a positive report. Gaining the support of James B. Allison, the Chief Signal Officer, they managed to gather
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5040-405: The Army's maneuvers that summer. Several improved components followed as the Army offered additional contracts for eventual production. The original -270 consisted of a four-vehicle package including a K-30 operations van for the radio equipment and oscilloscope, a K-31 gasoline-fueled power-generating truck, a K-22B flatbed trailer, and a K-32 prime mover. The antenna folding mount was derived from
5160-567: The Bomber group advised him that whenever the radio station played Hawaiian music all night, a flight from the mainland was arriving, and using that for navigation homing. McDonald was relieved at about 7:40 and returned to his tent waking his tent mate up by saying "Shim the Japs are coming". Elliot and Lockard continued plotting the incoming planes until 7:40 when contact was lost. Shortly before 8:00 am they headed to Kawailoa for breakfast and only learned about
5280-617: The Pacific end of the Canal by December 1940, thus giving radar coverage to the vitally important but vulnerable Panama Canal. Westinghouse quickly ramped up production, and produced 100 by the end of 1941. Operators of sets that were sent to the Panama Canal, the Philippines, Hawaii and other strategic locations were all gathered for an air defense school at Mitchel Field , New York in April 1941. The school
5400-401: The SCR-270 used 32 dipoles and reflectors, either eight wide by four high (fixed) or four wide by eight high (mobile)). In use, the antenna was swung (rotated) by command from the operations van, the azimuth angle being read by observing with binoculars the numbers painted on the antenna turntable. The maximum rotation rate was one revolution per minute. The radar operated at 106 MHz, using
5520-533: The SCR-270's operation was the primary water-cooled 8 kW continuous/100 kW pulsed transmitting tube. Early examples were hand-built, but a contract was let to Westinghouse in October 1938 to provide production versions under the Westinghouse designation "WL-530" and the Signal Corps type number "VT-122". A pair of these arrived in January 1939, and were incorporated into the first SCR-270 in time to be used in
5640-407: The SCR-270, used separate antennas for transmit and receive, For maximum antenna gain at a given size it is desirable to use the same antenna for both functions. One obstacle is the need to protect the receiver from the high power pulses produced by the transmitter. This was solved by placing a spark gap across a "trombone" tuned section of transmission line. The high-voltage power pulses would create
5760-511: The SCR-270-D, namely "maximum range on a single bomber flying at indicated heights, when set is on a flat sea level site": Components of the SCR-270 system included the following: The transmitter used dual WL530 water-cooled triodes configured as a high power push-pull resonant-line oscillator. The grids of the WL530s were connected to the keyer output which provided a high negative bias voltage that
5880-630: The US for preservation. It is now located at the National Electronics Museum near Baltimore . A second unrestored unit is in the US Army Air Defense Artillery Museum collection at Fort Sill and will be undergoing restoration in 2020. 1942 view of an SCR-271 at the [Radar Installation and Maintenance School at Camp Evans] http://www.campevans.org/history/radar/wwii-radar-array-scr-270-and-scr-271-cs-2005-12-08l , Wall, NJ Cavity magnetron The cavity magnetron
6000-544: The US. There were only six B-17s in the group, so this could not account for the large size of the radar echo. The officer had little grasp of the technology, the radar operators were unaware of the B-17 flight (nor its size), and the B-17s had no IFF ( Identification friend or foe ) system, nor any alternative procedure for identifying distant friendlies such as the British had developed during
6120-483: The United Kingdom used the magnetron to develop a revolutionary airborne, ground-mapping radar codenamed H2S. The H2S radar was in part developed by Alan Blumlein and Bernard Lovell . The cavity magnetron was widely used during World War II in microwave radar equipment and is often credited with giving Allied radar a considerable performance advantage over German and Japanese radars, thus directly influencing
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#17328584814726240-553: The United States Army in the Dominion of Newfoundland in 1942 to protect NS Argentia , McAndrew AFB , Ernest Harmon AFB , and RCAF Torbay . The stations at Cape Spear ( Prime ) , Elliston Ridge ( Duo ) , St. Bride's ( Trio ) , Fogo Island ( Quad ) , and Allan's Island ( Cinco ) , were manned by the 685th Air Warning Squadron under operational control of Newfoundland Base Command at Pepperrell Air Force Base . Key to
6360-525: The air defense assets they had, and how vital radar was to those defenses. The vulnerability was well demonstrated in war games—in particular those of United States Navy Fleet Problem IX that annihilated the locks on the Panama canal, and Fleet Problem XIII , when the Pearl Harbor fleet was destroyed in a mock attack by 150 planes in 1932. At Midway Island in June 1942, an SCR-270 antenna and shack were located at
6480-451: The angle. Generally it had an operational range of about 150 miles (240 km), and consistently picked up aircraft at that range. A nine-man field operating crew consisted of a shift chief, two oscilloscope operators, two plotters, two technicians, and two electricians. The declassified US military document "U.S. Radar -- Operational Characteristics of Available Equipment Classified by Tactical Application" gives performance statistics for
6600-421: The anode, rather than external circuits or fields. Mechanically, the cavity magnetron consists of a large, solid cylinder of metal with a hole drilled through the centre of the circular face. A wire acting as the cathode is run down the center of this hole, and the metal block itself forms the anode. Around this hole, known as the "interaction space", are a number of similar holes ("resonators") drilled parallel to
6720-474: The areas around them. The anode is constructed of a highly conductive material, almost always copper, so these differences in voltage cause currents to appear to even them out. Since the current has to flow around the outside of the cavity, this process takes time. During that time additional electrons will avoid the hot spots and be deposited further along the anode, as the additional current flowing around it arrives too. This causes an oscillating current to form as
6840-403: The army return Bergquist from his fighter unit and tell him his job was to assemble the equipment when it arrived. The commander in charge of defending Hawaii, General Walter Short , had a faint grasp of the weapons and tactics that Army technologists (led by Hap Arnold ) were aggressively pushing them to adopt. Except in rare cases, there was little interest in assisting or even cooperating with
6960-517: The attack when they arrived. Elliot and Lockard rushed back to Opana and operated the radar until the attack ended. Today, a modern Navy telecommunications station occupies the top of the Opana Hill adjacent to the site. The station is a relay for the Department of State 's Diplomatic Telecommunications Service . The former site is a National Historic Landmark and IEEE Milestone . Since the 1941 radar
7080-417: The cathode, depositing their energy on it and causing it to heat up. As this normally causes more electrons to be released, it could sometimes lead to a runaway effect, damaging the device. The great advance in magnetron design was the resonant cavity magnetron or electron-resonance magnetron , which works on entirely different principles. In this design the oscillation is created by the physical shape of
7200-453: The cathode, preventing current flow. At the opposite extreme, with no field, the electrons are free to flow straight from the cathode to the anode. There is a point between the two extremes, the critical value or Hull cut-off magnetic field (and cut-off voltage), where the electrons just reach the anode. At fields around this point, the device operates similar to a triode. However, magnetic control, due to hysteresis and other effects, results in
7320-419: The cavities. In some systems the tap wire is replaced by an open hole, which allows the microwaves to flow into a waveguide . As the oscillation takes some time to set up, and is inherently random at the start, subsequent startups will have different output parameters. Phase is almost never preserved, which makes the magnetron difficult to use in phased array systems. Frequency also drifts from pulse to pulse,
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#17328584814727440-417: The central north shore ( Haleiwa ), Opana Point (northern tip), in the northwest at the highest point- Mount Kaala , and one in the southeast corner at Koko Head. However, initially no real communications system or reporting chain was set up. At one point the operators of one of the sets were instructed to phone in reports from a gas station some distance away. Although communications were eventually improved,
7560-430: The chain of command was not. And by explicit order of General Short, the radar stations were to only be operated for four hours per day and to shut down by 7am each day. The one operational radar set in the Philippines, by contrast, was put on continuous watch in three shifts in response to the war warning sent to all overseas commands in late November. SCR-270 serial number 012 was installed at Opana Point , Hawaii on
7680-448: The chamber are the anode of the tube. A magnetic field parallel to the axis of the cavity is imposed by a permanent magnet . The electrons initially move radially outward from the cathode attracted by the electric field of the anode walls. The magnetic field causes the electrons to spiral outward in a circular path, a consequence of the Lorentz force . Spaced around the rim of the chamber are cylindrical cavities. Slots are cut along
7800-456: The control of the ratio of the magnetic and electric field strengths. He released several papers and patents on the concept in 1921. Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves. However, in 1924, Czech physicist August Žáček (1886–1961) and German physicist Erich Habann (1892–1968) independently discovered that the magnetron could generate waves of 100 megahertz to 1 gigahertz. Žáček,
7920-440: The current tries to equalize one spot, then another. The oscillating currents flowing around the cavities, and their effect on the electron flow within the tube, cause large amounts of microwave radiofrequency energy to be generated in the cavities. The cavities are open on one end, so the entire mechanism forms a single, larger, microwave oscillator. A "tap", normally a wire formed into a loop, extracts microwave energy from one of
8040-408: The device somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter frequency. This instability results not only in frequency shifts from one pulse to the next, but also a frequency shift within an individual transmitted pulse. The second factor is that the energy of the transmitted pulse is spread over a relatively wide frequency spectrum, which requires
8160-404: The electrons in the flow experienced this looping motion, the amount of RF energy being radiated was greatly improved. And as the motion occurred at any field level beyond the critical value, it was no longer necessary to carefully tune the fields and voltages, and the overall stability of the device was greatly improved. Unfortunately, the higher field also meant that electrons often circled back to
8280-405: The electrons, instead of reaching the anode, continue to circle in the space between the cathode and the anode. Due to an effect now known as cyclotron radiation , these electrons radiate radio frequency energy. The effect is not very efficient. Eventually the electrons hit one of the electrodes, so the number in the circulating state at any given time is a small percentage of the overall current. It
8400-611: The example and quickly began making copies, and before the end of 1940, the Radiation Laboratory had been set up on the campus of the Massachusetts Institute of Technology to develop various types of radar using the magnetron. By early 1941, portable centimetric airborne radars were being tested in American and British aircraft. In late 1941, the Telecommunications Research Establishment in
8520-412: The extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar. The size of the cavities determine the resonant frequency, and thereby the frequency of the emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance , with changes in the supply current, and with
8640-413: The few devices known to create microwaves, interest in the device and potential improvements was widespread. The first major improvement was the split-anode magnetron , also known as a negative-resistance magnetron . As the name implies, this design used an anode that was split in two—one at each end of the tube—creating two half-cylinders. When both were charged to the same voltage the system worked like
8760-426: The fighter strength seriously reduced. The Marine unit was withdrawn to Bataan in January 1942, where it was successfully employed in conjunction with an SCR-268 antiaircraft gun-laying radar to provide air warning to a small detachment of P-40s operating from primitive fields. Key commanders responsible for the defense of installations vulnerable to air attack did not appreciate the need for and capabilities of
8880-427: The form of a metal rod in the center, and the anode as a cylinder around it. The tube was placed between the poles of a horseshoe magnet arranged such that the magnetic field was aligned parallel to the axis of the electrodes. With no magnetic field present, the tube operates as a diode, with electrons flowing directly from the cathode to the anode. In the presence of the magnetic field, the electrons will experience
9000-455: The functioning of a whistle producing a tone when excited by an air stream blown past its opening. The resonant frequency of the arrangement is determined by the cavities' physical dimensions. Unlike other vacuum tubes, such as a klystron or a traveling-wave tube (TWT), the magnetron cannot function as an amplifier for increasing the intensity of an applied microwave signal; the magnetron serves solely as an electronic oscillator generating
9120-523: The general lack of understanding at all levels of command of the capabilities of radar, with it often being regarded as a freak gadget "producing snap observations on targets which may or may not be aircraft." General Gordon P. Saville , director of Air Defense at the Army Air Force headquarters referred to the Watson-Watt report as "a damning indictment of our whole warning service". In the Philippines,
9240-485: The goal of setting up the air defense system. On his own initiative, Bergquist along with some other motivated junior officers built a makeshift control center without authorization, and only by scrounging. The first SR-270s became functional in July 1941 and, by November, Bergquist had only assembled a small team, but they were able to build a ring of four SCR-270-Bs around Oahu, with one unit in reserve. The radars were placed on
9360-556: The heart of your microwave oven today. The cavity magnetron's invention changed the world. Because France had just fallen to the Nazis and Britain had no money to develop the magnetron on a massive scale, Winston Churchill agreed that Sir Henry Tizard should offer the magnetron to the Americans in exchange for their financial and industrial help. An early 10 kW version, built in England by
9480-400: The interaction space, connected to the interaction space by a short channel. The resulting block looks something like the cylinder on a revolver , with a somewhat larger central hole. Early models were cut using Colt pistol jigs. Remembering that in an AC circuit the electrons travel along the surface , not the core, of the conductor, the parallel sides of the slot act as a capacitor while
9600-451: The junior officers who were trained at this session. Air defense required direct control of assets spread out over disparate units; anti-aircraft guns, radars, and interceptor aircraft were not under a unified command. This had been one of the primary problems identified by Robert Watson-Watt prior to the war, when a demonstration of an early radar system had gone comically wrong even though the radar system itself had worked perfectly. Dowding
9720-497: The keyer, but an external source could be used. The sweep signal passed through a calibrated phase shifter controlled by a large hand wheel on the front panel. The delay between the transmitted and received pulses could be measured accurately by placing the transmit pulse under a hairline on the screen and then adjusting the hand wheel so that the received pulse was under the line. Two high power WL-531 rectifier tubes provided adjustable plate voltage, up to 15 kV at 0.5 A, to
9840-425: The length of the cavities that open into the central, common cavity space. As electrons sweep past these slots, they induce a high-frequency radio field in each resonant cavity, which in turn causes the electrons to bunch into groups. A portion of the radio frequency energy is extracted by a short coupling loop that is connected to a waveguide (a metal tube, usually of rectangular cross section). The waveguide directs
9960-440: The magnetron with microwave semiconductor oscillators , which have a narrower output frequency range. These allow a narrower receiver bandwidth to be used, and the higher signal-to-noise ratio in turn allows a lower transmitter power, reducing exposure to EMR. In microwave ovens , the waveguide leads to a radio-frequency-transparent port into the cooking chamber. As the fixed dimensions of the chamber and its physical closeness to
10080-458: The magnetron would normally create standing wave patterns in the chamber, the pattern is randomized by a motorized fan-like mode stirrer in the waveguide (more often in commercial ovens), or by a turntable that rotates the food (most common in consumer ovens). An early example of this application was when British scientists in 1954 used a microwave oven to resurrect cryogenically frozen hamsters . In microwave-excited lighting systems, such as
10200-461: The microwaves, which are substantially less complex and can be adjusted to maximize light output using a PID controller . In 1910, Hans Gerdien (1877–1951) of the Siemens Corporation invented a magnetron. In 1912, Swiss physicist Heinrich Greinacher was looking for new ways to calculate the electron mass . He settled on a system consisting of a diode with a cylindrical anode surrounding
10320-471: The morning of 7 December 1941, manned by two privates, George Elliot and Joseph Lockard. Though the set was supposed to shut down at 7 that morning, the soldiers decided to get additional training time since the truck scheduled to take them to breakfast was late. At 7:02 they detected aircraft approaching Oahu at a distance of 130 miles (210 km) and Lockard telephoned the information center at Fort Shafter and reported "Large number of planes coming in from
10440-535: The most important invention that came out of the Second World War", while professor of military history at the University of Victoria in British Columbia, David Zimmerman, states: The magnetron remains the essential radio tube for shortwave radio signals of all types. It not only changed the course of the war by allowing us to develop airborne radar systems, it remains the key piece of technology that lies at
10560-406: The north, three points east". The operator taking his report passed on the information repeating that the operator emphasized he had never seen anything like it, and it was "an awful big flight." The report was passed on to an inexperienced and incompletely trained officer, Kermit Tyler , who had arrived only a week earlier. He thought they had detected a flight of B-17s arriving that morning from
10680-536: The older technology. They made the big-gunned Allied battleships more deadly and, along with the newly developed proximity fuze , made anti-aircraft guns much more dangerous to attacking aircraft. The two coupled together and used by anti-aircraft batteries, placed along the flight path of German V-1 flying bombs on their way to London , are credited with destroying many of the flying bombs before they reached their target. Since then, many millions of cavity magnetrons have been manufactured; while some have been for radar
10800-404: The original model. But by slightly altering the voltage of the two plates , the electrons' trajectory could be modified so that they would naturally travel towards the lower voltage side. The plates were connected to an oscillator that reversed the relative voltage of the two plates at a given frequency. At any given instant, the electron will naturally be pushed towards the lower-voltage side of
10920-524: The outcome of the war. It was later described by American historian James Phinney Baxter III as "[t]he most valuable cargo ever brought to our shores". Centimetric radar, made possible by the cavity magnetron, allowed for the detection of much smaller objects and the use of much smaller antennas. The combination of small-cavity magnetrons, small antennas, and high resolution allowed small, high quality radars to be installed in aircraft. They could be used by maritime patrol aircraft to detect objects as small as
11040-408: The plotters and notify Wheeler Field of the sighting. When Tyler again indicated that it was nothing, McDonald insisted that Tyler talk to Lockard directly. The information center staff had gone to breakfast and Tyler received the report. Tyler reasoned that the activity was a flight of Army B-17 Flying Fortress bombers, and advised the radar crew not to worry. Tyler told investigators that a friend in
11160-406: The radar display. The magnetron remains in use in some radar systems, but has become much more common as a low-cost source for microwave ovens. In this form, over one billion magnetrons are in use today. In a conventional electron tube ( vacuum tube ), electrons are emitted from a negatively charged, heated component called the cathode and are attracted to a positively charged component called
11280-429: The receiver to have a correspondingly wide bandwidth. This wide bandwidth allows ambient electrical noise to be accepted into the receiver, thus obscuring somewhat the weak radar echoes, thereby reducing overall receiver signal-to-noise ratio and thus performance. The third factor, depending on application, is the radiation hazard caused by the use of high-power electromagnetic radiation. In some applications, for example,
11400-420: The round holes form an inductor : an LC circuit made of solid copper, with the resonant frequency defined entirely by its dimensions. The magnetic field is set to a value well below the critical, so the electrons follow curved paths towards the anode. When they strike the anode, they cause it to become negatively charged in that region. As this process is random, some areas will become more or less charged than
11520-571: The same fate as the ships in Pearl Harbor had they attempted to engage the superior Japanese carrier fleet, with potentially enormous casualties. The radars on Oahu were put on round-the-clock operation immediately after the attack. After the Japanese attack, the RAF agreed to send Watson-Watt to the United States to advise the military on air defense technology. In particular Watson-Watt directed attention to
11640-448: The second RF amplifier stage. The local oscillator included a front panel tuning adjustment. The receiver sensitivity control was remotely located on the oscilloscope. The two RF and four 20 MHz IF amplifier stages could produce enough gain to fill the oscilloscope display screen with noise. A key innovation in the SCR-270 was a transmit-receive (TR) switch. The SCR-268 searchlight control radar, which shared much technology with
11760-474: The temperature of the tube. This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices, such as the klystron are used. The magnetron is a self-oscillating device requiring no external elements other than a power supply. A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage
11880-511: The temporary information center at Fort Shafter . Private Joseph McDonald took the call. McDonald found Lt Kermit Tyler when he entered the plotting room when he timed the message. Tyler told him that it was nothing. McDonald called back the Opana Radar site and spoke to Pvt Joseph Lockard . Lockard was excited, he had never seen so many planes. Infected with Lockard's excitement, McDonald returned to Tyler. McDonald suggested to Tyler to call back
12000-510: The transmitter, modulator, water cooler, receiver, oscilloscope, operator, generator and antenna. On December 7, 1941, the Opana Radar Site was manned by Private Joseph Lockard and Private George Elliot, who detected approaching aircraft at 7:02 am (past the end of the site's scheduled operating day). Since the truck to take them to breakfast was late, the pair continued to practice with the radar equipment. The men reported their findings to
12120-468: The transmitter. Because of pulsed nature of the transmitter, the small amount of filtration was needed. The RU-4 circulated triple-distilled cooling water through the WL530 high power triodes and cooled the return water with a blower. Triple-distilled water was used to minimize leakage current from the high voltage on the tubes' anodes. Later units incorporated an antenna steering control system that could sweep
12240-500: The triode. Western Electric had gained control of this design by buying Lee De Forest 's patents on the control of current flow using electric fields via the "grid". Hull intended to use a variable magnetic field, instead of an electrostatic one, to control the flow of the electrons from the cathode to the anode. Working at General Electric 's Research Laboratories in Schenectady, New York , Hull built tubes that provided switching through
12360-401: The tube. The electron will then oscillate back and forth as the voltage changes. At the same time, a strong magnetic field is applied, stronger than the critical value in the original design. This would normally cause the electron to circle back to the cathode, but due to the oscillating electrical field, the electron instead follows a looping path that continues toward the anodes. Since all of
12480-399: The two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency. While radar was being developed during World War II , there arose an urgent need for a high-power microwave generator that worked at shorter wavelengths , around 10 cm (3 GHz), rather than the 50 to 150 cm (200 MHz) that was available from tube-based generators of the time. It
12600-476: The vast majority have been for microwave ovens . The use in radar itself has dwindled to some extent, as more accurate signals have generally been needed and developers have moved to klystron and traveling-wave tube systems for these needs. At least one hazard in particular is well known and documented. As the lens of the eye has no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation. This heating can in turn lead to
12720-442: The voltage on this third electrode. This allows the resulting electron tube (called a " triode " because it now has three electrodes) to function as an amplifier because small variations in the electric charge applied to the control grid will result in identical variations in the much larger current of electrons flowing between the cathode and anode. The idea of using a grid for control was invented by Philipp Lenard , who received
12840-485: The war, practically every Allied radar was based on the magnetron. The magnetron continued to be used in radar in the post-war period but fell from favour in the 1960s as high-power klystrons and traveling-wave tubes emerged. A key characteristic of the magnetron is that its output signal changes from pulse to pulse, both in frequency and phase. This renders it less suitable for pulse-to-pulse comparisons for performing moving target indication and removing " clutter " from
12960-411: The western end of Sand Island. During the Battle of Midway , this radar was used to warn the island of incoming Japanese air attacks and to successfully direct the fighter interception that followed, but the island's radar did not play any significant part in the main carrier-action portion of the battle that followed. A series of five SCR-271-equipped early warning radar stations were constructed by
13080-475: Was a mobile unit, there is no physical evidence of the historic unit at the site. There is a commemorative plaque on the grounds of the Turtle Bay Resort at the foot of Opana Hill. SCR-270 radar The SCR-270 was one of the first operational early-warning radars . It was the U.S. Army 's primary long-distance radar throughout World War II and was deployed around the world. It is also known as
13200-473: Was actually being generated. In 1941, the problem of frequency instability was solved by James Sayers coupling ("strapping") alternate cavities within the magnetron, which reduced the instability by a factor of 5–6. (For an overview of early magnetron designs, including that of Boot and Randall, see .) GEC at Wembley made 12 prototype cavity magnetrons in August 1940, and No 12 was sent to America with Bowen on
13320-445: Was also noticed that the frequency of the radiation depends on the size of the tube, and even early examples were built that produced signals in the microwave regime. Early conventional tube systems were limited to the high frequency bands, and although very high frequency systems became widely available in the late 1930s, the ultra high frequency and microwave bands were well beyond the ability of conventional circuits. The magnetron
13440-605: Was at an elevated site. Under the command of Col. Wilfred H. Tetley the AWS established six mobile radar detector sites on O'ahu at Kawaiola, Wainaae , Kaʻaʻawa , Koko Head , Schofield Barracks , and Fort Shafter . On Thanksgiving Day in 1941, the Schofield Barracks radar set was moved to the Opana Radar Site, a location 532 feet above sea level with an unobstructed view of the Pacific Ocean. The set comprised four trucks carrying
13560-405: Was interrupted by 621 Hz pulses which drove the WL530s' grids to conduction, thereby allowing a pulse of RF to be produced. The transmission line to the antenna was connected to taps on the filament resonant lines. As described above, the keyer/modulator produced a grid bias voltage for the transmitter tubes that keeps them in cutoff except for brief positive pulses the keyer produces 621 times
13680-604: Was introduced to the US during the Tizard Mission . The only early warning system of the sort to see action in World War II was the AN/CPS-1 , which was available in mid-1944, in time for D-Day . The Signal Corps had been experimenting with some radar concepts as early as the late 1920s, under the direction of Colonel William R. Blair, director of the Signal Corps Laboratories at Fort Monmouth , New Jersey . Although
13800-562: Was known that a multi-cavity resonant magnetron had been developed and patented in 1935 by Hans Hollmann in Berlin . However, the German military considered the frequency drift of Hollman's device to be undesirable, and based their radar systems on the klystron instead. But klystrons could not at that time achieve the high power output that magnetrons eventually reached. This was one reason that German night fighter radars, which never strayed beyond
13920-530: Was no time to amend the drawings. And No 12 with 8 holes was chosen for the Tizard Mission. So Bell Labs chose to copy the sample; and while early British magnetrons had six cavities the American ones had eight cavities. According to Andy Manning from the RAF Air Defence Radar Museum , Randall and Boot's discovery was "a massive, massive breakthrough" and "deemed by many, even now [2007], to be
14040-409: Was one of the few devices able to generate signals in the microwave band and it was the only one that was able to produce high power at centimeter wavelengths. The original magnetron was very difficult to keep operating at the critical value, and even then the number of electrons in the circling state at any time was fairly low. This meant that it produced very low-power signals. Nevertheless, as one of
14160-468: Was taken up by Philips , General Electric Company (GEC), Telefunken and others, limited to perhaps 10 W output. By this time the klystron was producing more power and the magnetron was not widely used, although a 300 W device was built by Aleksereff and Malearoff in the USSR in 1936 (published in 1940). The cavity magnetron was a radical improvement introduced by John Randall and Harry Boot at
14280-607: Was the culmination of efforts begun in 1940, when the War Department created the Air Defense Command headed by Brig. Gen. James E. Chaney. Chaney was tasked by Hap Arnold to collect all information on the British air defense system and transfer the knowledge as quickly as possible to the US military. Air Marshal Dowding , one of the designers of the Ground-controlled interception (GCI) air defense system used during
14400-514: Was well aware of the importance of a unified command, but this knowledge did not result in changes within the U.S. Army structure. Army Major Kenneth Bergquist returned to Hawaii after attending the Mitchel Field school intending to set up a coordinated system, but when he arrived he found the local Army leadership was uninterested in the system, and he was reassigned to his former fighter unit. Only when incomprehensible equipment began appearing did
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