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94-531: A major design goal of the FPS-23 was to use the Doppler effect to filter out low-speed objects. Migrating birds flying by the similar Mid-Canada Line (MCL) stations had rendered that system useless during spring and fall. FPS-23 proved to be largely free of this problem, but instead was constantly triggered by light aircraft flying anywhere near the stations. As these were used for communications and crew rotations,

188-409: A "drift" tube, in which the faster electrons catch up to the slower ones, creating the "bunches", then through a "catcher" cavity. In the output "catcher" cavity, each bunch enters the cavity at the time in the cycle when the electric field opposes the electrons' motion, decelerating them. Thus the kinetic energy of the electrons is converted to potential energy of the field, increasing the amplitude of

282-513: A conventional Doppler shift. The first experiment that detected this effect was conducted by Nigel Seddon and Trevor Bearpark in Bristol , United Kingdom in 2003. Later, the inverse Doppler effect was observed in some inhomogeneous materials, and predicted inside a Vavilov–Cherenkov cone. Klystron A klystron is a specialized linear-beam vacuum tube , invented in 1937 by American electrical engineers Russell and Sigurd Varian , which

376-453: A cooling system. Some modern klystrons include depressed collectors, which recover energy from the beam before collecting the electrons, increasing efficiency. Multistage depressed collectors enhance the energy recovery by "sorting" the electrons in energy bins. The reflex klystron (also known as a Sutton tube after one of its inventors, Robert Sutton) was a low power klystron tube with a single cavity, which functioned as an oscillator . It

470-551: A filament. The electrons are attracted to and pass through an anode cylinder at a high positive potential; the cathode and anode act as an electron gun to produce a high velocity stream of electrons. An external electromagnet winding creates a longitudinal magnetic field along the beam axis which prevents the beam from spreading. The beam first passes through the "buncher" cavity resonator, through grids attached to each side. The buncher grids have an oscillating AC potential across them, produced by standing wave oscillations within

564-404: A function of the angle between his line of sight and the siren's velocity: v radial = v s cos ⁡ ( θ ) {\displaystyle v_{\text{radial}}=v_{\text{s}}\cos(\theta )} where θ {\displaystyle \theta } is the angle between the object's forward velocity and the line of sight from the object to

658-421: A half-cycle later, when the polarity is opposite, encounter an electric field which opposes their motion, and are decelerated. Beyond the buncher grids is a space called the drift space . This space is long enough so that the accelerated electrons catch up with electrons that were decelerated at an earlier time, forming "bunches" longitudinally along the beam axis. Its length is chosen to allow maximum bunching at

752-411: A klystron tube, by providing a feedback path from output to input by connecting the "catcher" and "buncher" cavities with a coaxial cable or waveguide . When the device is turned on, electronic noise in the cavity is amplified by the tube and fed back from the output catcher to the buncher cavity to be amplified again. Because of the high Q of the cavities, the signal quickly becomes a sine wave at

846-401: A klystron, an electron beam interacts with radio waves as it passes through resonant cavities , metal boxes along the length of a tube. The electron beam first passes through a cavity to which the input signal is applied. The energy of the electron beam amplifies the signal, and the amplified signal is taken from a cavity at the other end of the tube. The output signal can be coupled back into

940-417: A laser light beam causes bunching of the electrons. Then the beam passes through a second undulator, in which the electron bunches cause oscillation to create a second, more powerful light beam. The floating drift tube klystron has a single cylindrical chamber containing an electrically isolated central tube. Electrically, this is similar to the two cavity oscillator klystron with considerable feedback between

1034-408: A negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum as the electron beam re-enters

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1128-595: A non-contact instrument for measuring vibration. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface. Dynamic real-time path planning in robotics to aid the movement of robots in a sophisticated environment with moving obstacles often take help of Doppler effect. Such applications are specially used for competitive robotics where

1222-516: A pulse radar will only see them near to the station, whereas a forward scatter system will see them over a much greater area. Originally this was seen as a major advantage of the concept, allowing it to cover long ranges using much less power. But when the first experimental versions of the MCL stations were set up, a problem was immediately noticed. Birds, normally only seen as radar angels at short range, if at all, could now be seen at long distances from

1316-738: A resonator). During the Second World War, Hansen lectured at the MIT Radiation labs two days a week, commuting to Boston from Sperry Gyroscope Company on Long Island. His resonator was called a "rhumbatron" by the Varian brothers. Hansen died of beryllium disease in 1949 as a result of exposure to beryllium oxide (BeO). During the Second World War, the Axis powers relied mostly on (then low-powered and long wavelength) klystron technology for their radar system microwave generation, while

1410-401: A signal that was not the original frequency, by comparing the original to the received signal, the Doppler shift could be measured to reveal the speed of the target, which allowed it to filter out any slow-moving targets like birds. The system was set to filter out anything under 125 miles per hour (201 km/h). Another advantage of Fluttar over MCL was that by using multiple Doppler filters in

1504-439: A stationary observer and a wave source moving towards the observer at (or exceeding) the speed of the wave, the Doppler equation predicts an infinite (or negative) frequency as from the observer's perspective. Thus, the Doppler equation is inapplicable for such cases. If the wave is a sound wave and the sound source is moving faster than the speed of sound, the resulting shock wave creates a sonic boom . Lord Rayleigh predicted

1598-559: A string of three towers would be built between the main stations. But some solution would be needed for the bird problem. The first attempt was made by the Air Force Cambridge Research Laboratory , who surmised that using lower frequencies in the VHF range might mitigate the problem due to the lower Rayleigh scattering cross-section as the wavelength would be much greater than a bird. Tests showed that

1692-571: A very small scale; there would not be a noticeable difference in visible light to the unaided eye. The use of the Doppler effect in astronomy depends on knowledge of precise frequencies of discrete lines in the spectra of stars. Among the nearby stars , the largest radial velocities with respect to the Sun are +308 km/s ( BD-15°4041 , also known as LHS 52, 81.7 light-years away) and −260 km/s ( Woolley 9722 , also known as Wolf 1106 and LHS 64, 78.2 light-years away). Positive radial speed means

1786-471: Is an effective tool for diagnosis of vascular problems like stenosis . Instruments such as the laser Doppler velocimeter (LDV), and acoustic Doppler velocimeter (ADV) have been developed to measure velocities in a fluid flow. The LDV emits a light beam and the ADV emits an ultrasonic acoustic burst, and measure the Doppler shift in wavelengths of reflections from particles moving with the flow. The actual flow

1880-520: Is computed as a function of the water velocity and phase. This technique allows non-intrusive flow measurements, at high precision and high frequency. Developed originally for velocity measurements in medical applications (blood flow), Ultrasonic Doppler Velocimetry (UDV) can measure in real time complete velocity profile in almost any liquids containing particles in suspension such as dust, gas bubbles, emulsions. Flows can be pulsating, oscillating, laminar or turbulent, stationary or transient. This technique

1974-568: Is electrically insulated from the cavity walls, and DC bias is applied separately. The DC bias on the drift tube may be adjusted to alter the transit time through it, thus allowing some electronic tuning of the oscillating frequency. The amount of tuning in this manner is not large and is normally used for frequency modulation when transmitting. Klystrons can produce far higher microwave power outputs than solid state microwave devices such as Gunn diodes . In modern systems, they are used from UHF (hundreds of megahertz) up to hundreds of gigahertz (as in

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2068-417: Is emitted from a position farther from the observer than the previous cycle, so the arrival time between successive cycles is increased, thus reducing the frequency. For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect in such cases may therefore result from motion of

2162-405: Is fired at a moving target – e.g. a motor car, as police use radar to detect speeding motorists – as it approaches or recedes from the radar source. Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source. As each wave has to move farther, the gap between each wave increases, increasing the wavelength. In some situations, the radar beam

2256-422: Is fired at the moving car as it approaches, in which case each successive wave travels a lesser distance, decreasing the wavelength. In either situation, calculations from the Doppler effect accurately determine the car's speed. Moreover, the proximity fuze , developed during World War II, relies upon Doppler radar to detonate explosives at the correct time, height, distance, etc. Because the Doppler shift affects

2350-458: Is fully non-invasive. The Doppler shift can be exploited for satellite navigation such as in Transit and DORIS . Doppler also needs to be compensated in satellite communication . Fast moving satellites can have a Doppler shift of dozens of kilohertz relative to a ground station. The speed, thus magnitude of Doppler effect, changes due to earth curvature. Dynamic Doppler compensation, where

2444-423: Is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all. There are often several regions of reflector voltage where the reflex klystron will oscillate; these are referred to as modes. The electronic tuning range of the reflex klystron is usually referred to as the variation in frequency between half power points—the points in

2538-413: Is the speed of the mobile station, λ c {\displaystyle \lambda _{\rm {c}}} is the wavelength of the carrier, ϕ {\displaystyle \phi } is the elevation angle of the satellite and θ {\displaystyle \theta } is the driving direction with respect to the satellite. The additional Doppler shift due to

2632-455: Is used as an amplifier for high radio frequencies , from UHF up into the microwave range. Low-power klystrons are used as oscillators in terrestrial microwave relay communications links, while high-power klystrons are used as output tubes in UHF television transmitters , satellite communication , radar transmitters , and to generate the drive power for modern particle accelerators . In

2726-526: The Taylor's series expansion of 1 1 + x {\displaystyle {\frac {1}{1+x}}} truncating all x 2 {\displaystyle x^{2}} and higher terms: 1 1 + v s c ≈ 1 − v s c {\displaystyle {\frac {1}{1+{\frac {v_{\text{s}}}{c}}}}\approx 1-{\frac {v_{\text{s}}}{c}}} When substituted in

2820-657: The cardiac output . Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements. Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift ( when the received signal arrives). Velocity measurements of blood flow are also used in other fields of medical ultrasonography , such as obstetric ultrasonography and neurology . Velocity measurement of blood flow in arteries and veins based on Doppler effect

2914-464: The kinetic energy in a DC electron beam into radio frequency power. In a vacuum, a beam of electrons is emitted by an electron gun or thermionic cathode and accelerated by high-voltage electrodes (typically in the tens of kilovolts). This beam passes through an input cavity resonator . RF energy has been fed into the input cavity at, or near, its resonant frequency , creating standing waves , which produce an oscillating voltage, which acts on

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3008-414: The oscillations . The oscillations excited in the catcher cavity are coupled out through a coaxial cable or waveguide . The spent electron beam, with reduced energy, is captured by a collector electrode. To make an oscillator , the output cavity can be coupled to the input cavity(s) with a coaxial cable or waveguide . Positive feedback excites spontaneous oscillations at the resonant frequency of

3102-434: The resonant frequency of the cavities. In all modern klystrons, the number of cavities exceeds two. Additional "buncher" cavities added between the first "buncher" and the "catcher" may be used to increase the gain of the klystron or to increase the bandwidth. The residual kinetic energy in the electron beam when it hits the collector electrode represents wasted energy, which is dissipated as heat, which must be removed by

3196-564: The AN/FPT-4 transmitter was placed in the middle, with the AN/FPR-2 receivers at the stations on either side. The AN/FPS-23 stations became active in 1957, but soon discovered problems of its own. Annoyingly, although birds flying between the stations were indeed filtered out, it turned out that birds liked to congregate in warm locations, like the Diesel generators at the stations. The signal

3290-465: The Allies used the far more powerful but frequency-drifting technology of the cavity magnetron for much shorter-wavelength centimetric microwave generation. Klystron tube technologies for very high-power applications, such as synchrotrons and radar systems, have since been developed. Right after the war, AT&T used 4-watt klystrons in its brand new network of microwave relay links that covered

3384-473: The Doppler shift. Doppler shift of the direct path can be estimated by the following formula: f D , d i r = v m o b λ c cos ⁡ ϕ cos ⁡ θ {\displaystyle f_{\rm {D,dir}}={\frac {v_{\rm {mob}}}{\lambda _{\rm {c}}}}\cos \phi \cos \theta } where v mob {\displaystyle v_{\text{mob}}}

3478-562: The Extended Interaction Klystrons in the CloudSat satellite). Klystrons can be found at work in radar , satellite and wideband high-power communication (very common in television broadcasting and EHF satellite terminals), medicine ( radiation oncology ), and high-energy physics ( particle accelerators and experimental reactors). At SLAC , for example, klystrons are routinely employed which have outputs in

3572-471: The FPS-23 system ultimately proved to be as ineffective as the MCL and the system was shut down in 1963. In the early 1950s, Canada undertook the development of a pioneering radar system as part of the Mid-Canada Line (MCL). This system was based on continuous wave radars that broadcast a signal between separate transmitter and receiver stations. When an aircraft passed through the space between

3666-697: The Fluttar system would have to be much larger and more powerful to provide the same range performance. Fluttar was an inexpensive system compared to the main DEW radars. It used a 1 kilowatt, continuous wave, klystron amplifier as a source, and as it was not pulsed, the high-voltage circuitry was much simpler. It could operate from 475 to 525  MHz . Towers were separated from 40 to 70 miles (64–113 km) and were 100 to 400 feet (30–122 m) tall, depending on local terrain. The main DEW stations were normally about 100 miles (160 km) apart, so typically

3760-510: The Varians were probably unaware of the Heils' work. The work of physicist W. W. Hansen was instrumental in the development of the klystron and was cited by the Varian brothers in their 1939 paper. His resonator analysis, which dealt with the problem of accelerating electrons toward a target, could be used just as well to decelerate electrons (i.e., transfer their kinetic energy to RF energy in

3854-427: The amplifier. No two klystrons are exactly identical (even when comparing like part/model number klystrons). Each unit has manufacturer-supplied calibration values for its specific performance characteristics. Without this information the klystron would not be properly tunable, and hence not perform well, if at all. Tuning a klystron is delicate work which, if not done properly, can cause damage to equipment or injury to

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3948-448: The cavities. The simplest klystron tube is the two-cavity klystron. In this tube there are two microwave cavity resonators, the "catcher" and the "buncher". When used as an amplifier, the weak microwave signal to be amplified is applied to the buncher cavity through a coaxial cable or waveguide, and the amplified signal is extracted from the catcher cavity. At one end of the tube is the hot cathode which produces electrons when heated by

4042-411: The cavity, excited by the input signal at the cavity's resonant frequency applied by a coaxial cable or waveguide. The direction of the field between the grids changes twice per cycle of the input signal. Electrons entering when the entrance grid is negative and the exit grid is positive encounter an electric field in the same direction as their motion, and are accelerated by the field. Electrons entering

4136-503: The coloured light of the binary stars and some other stars of the heavens). The hypothesis was tested for sound waves by Buys Ballot in 1845. He confirmed that the sound's pitch was higher than the emitted frequency when the sound source approached him, and lower than the emitted frequency when the sound source receded from him. Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France,

4230-613: The contiguous United States . The network provided long-distance telephone service and also carried television signals for the major TV networks. Western Union Telegraph Company also built point-to-point microwave communication links using intermediate repeater stations at about 40 mile intervals at that time, using 2K25 reflex klystrons in both the transmitters and receivers. In some applications Klystrons have been replaced by solid state transistors. High efficiency Klystrons have been developed with have 10% more effiency than conventional Klystrons. Klystrons amplify RF signals by converting

4324-430: The cosmological redshift is that it is indeed a Doppler shift. Distant galaxies also exhibit peculiar motion distinct from their cosmological recession speeds. If redshifts are used to determine distances in accordance with Hubble's law , then these peculiar motions give rise to redshift-space distortions . The Doppler effect is used in some types of radar , to measure the velocity of detected objects. A radar beam

4418-454: The direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations is that the ultrasound beam should be as parallel to the blood flow as possible. Velocity measurements allow assessment of cardiac valve areas and function, abnormal communications between the left and right side of the heart, leaking of blood through the valves (valvular regurgitation), and calculation of

4512-404: The effect is sometimes called "effet Doppler-Fizeau" but that name was not adopted by the rest of the world as Fizeau's discovery was six years after Doppler's proposal). In Britain, John Scott Russell made an experimental study of the Doppler effect (1848). In classical physics, where the speeds of source and the receiver relative to the medium are lower than the speed of waves in the medium,

4606-530: The electron beam, such a tool can be pulled into the unit by the intense magnetic force, smashing fingers, injuring the technician, or damaging the unit. Special lightweight nonmagnetic (or rather very weakly diamagnetic ) tools made of beryllium alloy have been used for tuning U.S. Air Force klystrons. Precautions are routinely taken when transporting klystron devices in aircraft, as the intense magnetic field can interfere with magnetic navigation equipment. Special overpacks are designed to help limit this field "in

4700-458: The electron beam. The electric field causes the electrons to "bunch": electrons that pass through when the electric field opposes their motion are slowed, while electrons which pass through when the electric field is in the same direction are accelerated, causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities. The beam then passes through

4794-415: The environment is constantly changing, such as robosoccer. Since 1968 scientists such as Victor Veselago have speculated about the possibility of an inverse Doppler effect. The size of the Doppler shift depends on the refractive index of the medium a wave is traveling through. Some materials are capable of negative refraction , which should lead to a Doppler shift that works in a direction opposite that of

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4888-413: The field," and thus allow such devices to be transported safely. The technique of amplification used in the klystron is also being applied experimentally at optical frequencies in a type of laser called the free-electron laser (FEL); these devices are called optical klystrons . Instead of microwave cavities, these use devices called undulators . The electron beam passes through an undulator, in which

4982-445: The following effect in his classic book on sound: if the observer were moving from the (stationary) source at twice the speed of sound, a musical piece previously emitted by that source would be heard in correct tempo and pitch, but as if played backwards . A siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from

5076-506: The frequency of a signal is changed progressively during transmission, is used so the satellite receives a constant frequency signal. After realizing that the Doppler shift had not been considered before launch of the Huygens probe of the 2005 Cassini–Huygens mission, the probe trajectory was altered to approach Titan in such a way that its transmissions traveled perpendicular to its direction of motion relative to Cassini, greatly reducing

5170-439: The frequency will decrease if either source or receiver is moving away from the other. Equivalently, under the assumption that the source is either directly approaching or receding from the observer: f v w r = f 0 v w s = 1 λ {\displaystyle {\frac {f}{v_{wr}}}={\frac {f_{0}}{v_{ws}}}={\frac {1}{\lambda }}} where If

5264-410: The gates at one point or another. This caused alarms to ring out throughout the station, which had to be turned off or ignored, rendering the system ineffective. It was declared obsolete in 1963, and the intermediate stations were closed. Doppler effect The Doppler effect (also Doppler shift ) is the change in the frequency of a wave in relation to an observer who is moving relative to

5358-401: The grids at a point in the cycle when the exit grid is negative with respect to the entrance grid, so the electric field in the cavity between the grids opposes the electrons motion. The electrons thus do work on the electric field, and are decelerated, their kinetic energy is converted to electric potential energy , increasing the amplitude of the oscillating electric field in the cavity. Thus

5452-409: The input cavity to make an electronic oscillator to generate radio waves. The power gain of klystrons can be high, up to 60 dB (an increase in signal power of a factor of one million), with output power up to tens of megawatts , but the bandwidth is narrow, usually a few percent although it can be up to 10% in some devices. A reflex klystron is an obsolete type in which the electron beam

5546-433: The interaction depends on the resonance condition, larger cavity dimensions than a conventional klystron can be used. This allows the gyroklystron to deliver high power at very high frequencies which is challenging using conventional klystrons. Some klystrons have cavities that are tunable. By adjusting the frequency of individual cavities, the technician can change the operating frequency, gain, output power, or bandwidth of

5640-429: The klystron immediately influenced the work of US and UK researchers working on radar equipment. The Varians went on to found Varian Associates to commercialize the technology (for example, to make small linear accelerators to generate photons for external beam radiation therapy ). Their work was preceded by the description of velocity modulation by A. Arsenjewa-Heil and Oskar Heil (wife and husband) in 1935, though

5734-532: The klystron was under development. The klystron was the first significantly powerful source of radio waves in the microwave range; before its invention the only sources were the Barkhausen–Kurz tube and split-anode magnetron , which were limited to very low power. It was invented by the brothers Russell and Sigurd Varian at Stanford University . Their prototype was completed and demonstrated successfully on August 30, 1937. Upon publication in 1939, news of

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5828-483: The large birds seen in the arctic made perfectly good reflectors even at these frequencies. Development of another solution began at the MIT-backed Lincoln Laboratory . Although similar to the MCL in layout, it worked along different principles. Instead of the signal being scattered along the line between the two stations, the antennas were aimed about 15 degrees "forward" of the line between

5922-768: The last line, one gets: ( 1 + v r c ) ( 1 − v s c ) f 0 = ( 1 + v r c − v s c − v r v s c 2 ) f 0 {\displaystyle \left(1+{\frac {v_{\text{r}}}{c}}\right)\left(1-{\frac {v_{\text{s}}}{c}}\right)f_{0}=\left(1+{\frac {v_{\text{r}}}{c}}-{\frac {v_{\text{s}}}{c}}-{\frac {v_{\text{r}}v_{\text{s}}}{c^{2}}}\right)f_{0}} For small v s {\displaystyle v_{\text{s}}} and v r {\displaystyle v_{\text{r}}} ,

6016-410: The last term v r v s c 2 {\displaystyle {\frac {v_{\text{r}}v_{\text{s}}}{c^{2}}}} becomes insignificant, hence: ( 1 + v r − v s c ) f 0 {\displaystyle \left(1+{\frac {v_{\text{r}}-v_{\text{s}}}{c}}\right)f_{0}} Assuming

6110-472: The line between the stations, the signal might be present only for a few minutes over a period of months or years. To ensure such fleeting signals were not missed by the operators, the Fluttar system used "alarm gates" that triggered when a signal of a particular type was seen. These stayed on until reset by the operators. The problem occurred when the small aircraft would fly from station to station, and during their progress would invariably trigger almost all of

6204-403: The modulation forces alter the cyclotron frequency and hence the azimuthal component of motion, resulting in phase bunches. In the output cavity, electrons which arrive at the correct decelerating phase transfer their energy to the cavity field and the amplified signal can be coupled out. The gyroklystron has cylindrical or coaxial cavities and operates with transverse electric field modes. Since

6298-471: The observer. The Doppler effect for electromagnetic waves such as light is of widespread use in astronomy to measure the speed at which stars and galaxies are approaching or receding from us, resulting in so called blueshift or redshift , respectively. This may be used to detect if an apparently single star is, in reality, a close binary , to measure the rotational speed of stars and galaxies, or to detect exoplanets . This effect typically happens on

6392-442: The observer. Astronomer John Dobson explained the effect thus: The reason the siren slides is because it doesn't hit you. In other words, if the siren approached the observer directly, the pitch would remain constant, at a higher than stationary pitch, until the vehicle hit him, and then immediately jump to a new lower pitch. Because the vehicle passes by the observer, the radial speed does not remain constant, but instead varies as

6486-442: The oscillating field in the catcher cavity is an amplified copy of the signal applied to the buncher cavity. The amplified signal is extracted from the catcher cavity through a coaxial cable or waveguide. After passing through the catcher and giving up its energy, the lower energy electron beam is absorbed by a "collector" electrode, a second anode which is kept at a small positive voltage. An electronic oscillator can be made from

6580-415: The oscillating mode where the power output is half the maximum output in the mode. Modern semiconductor technology has effectively replaced the reflex klystron in most applications. The gyroklystron is a microwave amplifier with operation dependent on the cyclotron resonance condition. Similarly to the klystron, its operation depends on the modulation of the electron beam, but instead of axial bunching

6674-404: The receiver, the approximate velocity and direction of travel (north or south) could be determined. The only downside to this approach was that it did not rely on the forward scattering of the signal, so it did not take advantage of the very large effective signals available to the MCL system, which can be orders of magnitude larger than the backscatter used in traditional radars. As a consequence,

6768-407: The recession. When the source of the sound wave is moving towards the observer, each successive cycle of the wave is emitted from a position closer to the observer than the previous cycle. Hence, from the observer's perspective, the time between cycles is reduced, meaning the frequency is increased. Conversely, if the source of the sound wave is moving away from the observer, each cycle of the wave

6862-475: The relationship between observed frequency f {\displaystyle f} and emitted frequency f 0 {\displaystyle f_{\text{0}}} is given by: f = ( c ± v r c ∓ v s ) f 0 {\displaystyle f=\left({\frac {c\pm v_{\text{r}}}{c\mp v_{\text{s}}}}\right)f_{0}} where Note this relationship predicts that

6956-511: The resonant cavity, thus ensuring a maximum of energy is transferred from the electron beam to the RF oscillations in the cavity. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a variation in frequency. This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission

7050-444: The resonant frequency, and may be several feet long. The electrons then pass through a second cavity, called the "catcher", through a similar pair of grids on each side of the cavity. The function of the catcher grids is to absorb energy from the electron beam. The bunches of electrons passing through excite standing waves in the cavity, which has the same resonant frequency as the buncher cavity. Each bunch of electrons passes between

7144-420: The satellite moving can be described as: f D , s a t = v r e l , s a t λ c {\displaystyle f_{\rm {D,sat}}={\frac {v_{\rm {rel,sat}}}{\lambda _{\rm {c}}}}} where v r e l , s a t {\displaystyle v_{\rm {rel,sat}}} is the relative speed of

7238-399: The satellite. The Leslie speaker , most commonly associated with and predominantly used with the famous Hammond organ , takes advantage of the Doppler effect by using an electric motor to rotate an acoustic horn around a loudspeaker, sending its sound in a circle. This results at the listener's ear in rapidly fluctuating frequencies of a keyboard note. A laser Doppler vibrometer (LDV) is

7332-462: The signal was continuous, not pulsed, the transmitter was simpler and cheaper. The original idea had been to mount the systems on telephone poles and overhead power line towers covering relatively short distances, but the need to build thousands of such systems led to this idea being abandoned. The telephone poles were replaced with tall towers, the distance between them increased from a few miles to about 90 kilometres (56 mi). A string of 90 stations

7426-425: The source approaches the observer at an angle (but still with a constant speed), the observed frequency that is first heard is higher than the object's emitted frequency. Thereafter, there is a monotonic decrease in the observed frequency as it gets closer to the observer, through equality when it is coming from a direction perpendicular to the relative motion (and was emitted at the point of closest approach; but when

7520-420: The source of the wave. The Doppler effect is named after the physicist Christian Doppler , who described the phenomenon in 1842. A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during

7614-433: The source, motion of the observer, motion of the medium, or any combination thereof. For waves propagating in vacuum , as is possible for electromagnetic waves or gravitational waves , only the difference in velocity between the observer and the source needs to be considered. Doppler first proposed this effect in 1842 in his treatise " Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels " (On

7708-1183: The speed of the wave, the relationship between observed frequency f {\displaystyle f} and emitted frequency f 0 {\displaystyle f_{\text{0}}} is approximately where Given f = ( c + v r c + v s ) f 0 {\displaystyle f=\left({\frac {c+v_{\text{r}}}{c+v_{\text{s}}}}\right)f_{0}} we divide for c {\displaystyle c} f = ( 1 + v r c 1 + v s c ) f 0 = ( 1 + v r c ) ( 1 1 + v s c ) f 0 {\displaystyle f=\left({\frac {1+{\frac {v_{\text{r}}}{c}}}{1+{\frac {v_{\text{s}}}{c}}}}\right)f_{0}=\left(1+{\frac {v_{\text{r}}}{c}}\right)\left({\frac {1}{1+{\frac {v_{\text{s}}}{c}}}}\right)f_{0}} Since v s c ≪ 1 {\displaystyle {\frac {v_{\text{s}}}{c}}\ll 1} we can substitute using

7802-483: The star is receding from the Sun, negative that it is approaching. Redshift is also used to measure the expansion of the universe . It is sometimes claimed that this is not truly a Doppler effect but instead arises from the expansion of space. However, this picture can be misleading because the expansion of space is only a mathematical convention, corresponding to a choice of coordinates . The most natural interpretation of

7896-438: The station. During migration, the system was completely swamped with returns that rendered it essentially useless. The same basic forward-scatter concept was perfectly suited to fill the gaps between the DEW line stations. Because the radars were very simple, they could be run unattended, forwarding data to the main stations. Desiring better low-altitude coverage, the new system would be spaced about 25 miles (40 km) apart, so

7990-421: The stations, some of the signal was reflected off the aircraft and back to the receiver. This produces a heterodyne effect that is easily detectable using simple electronics. Today, this style of operation is known as a forward scatter bistatic radar . Because the beam was not steered, unlike a conventional scanning radar, the antennas did not move and the physical design was greatly simplified. Additionally, as

8084-433: The technician due to the very high voltages that could be produced. The technician must be careful not to exceed the limits of the graduations, or damage to the klystron can result. Other precautions taken when tuning a klystron include using nonferrous tools. Some klystrons employ permanent magnets . If a technician uses ferrous tools (which are ferromagnetic ) and comes too close to the intense magnetic fields that contain

8178-426: The two cavities. Electrons exiting the source cavity are velocity modulated by the electric field as they travel through the drift tube and emerge at the destination chamber in bunches, delivering power to the oscillation in the cavity. This type of oscillator klystron has an advantage over the two-cavity klystron on which it is based, in that it needs only one tuning element to effect changes in frequency. The drift tube

8272-404: The two stations. When an aircraft entered this area, it would scatter back to the receiver as before, but in this case the aircraft's motion would shift the frequency of the signal. This effect had first been noticed in television signals when aircraft flew overhead, which is where it gained the name "flutter" for the way the image shifted back and forth on the screen. Because the system received

8366-532: The wave incident upon the target as well as the wave reflected back to the radar, the change in frequency observed by a radar due to a target moving at relative speed Δ v {\displaystyle \Delta v} is twice that from the same target emitting a wave: Δ f = 2 Δ v c f 0 . {\displaystyle \Delta f={\frac {2\Delta v}{c}}f_{0}.} An echocardiogram can, within certain limits, produce an accurate assessment of

8460-561: The wave is received, the source and observer will no longer be at their closest), and a continued monotonic decrease as it recedes from the observer. When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt. When the observer is far from the path of the object, the transition from high to low frequency is gradual. If the speeds v s {\displaystyle v_{\text{s}}} and v r {\displaystyle v_{\text{r}}\,} are small compared to

8554-458: Was constructed across Canada. Unlike a pulse radar where the signal from the station travels out and back, in a forward scatter radar like the MCL, it travels an almost straight line from the transmitter to receiver. For this reason, the forward scatter signal is subject only to one inverse square law reduction in power, compared to the normal radar equation where power drops with the fourth root of range. Thus with small targets and weak returns,

8648-461: Was reflected back along its path by a high potential electrode, used as an oscillator. The name klystron comes from the Greek verb κλύζω ( klyzo ) referring to the action of waves breaking against a shore, and the suffix -τρον ("tron") meaning the place where the action happens. The name "klystron" was suggested by Hermann Fränkel , a professor in the classics department at Stanford University when

8742-408: Was so strong that it overwhelmed the filters. A more annoying problem turned out to be the aircraft that flew from station to station for maintenance. The DEW line was designed to detect a Soviet attack, whose occurrence would likely be a one-time event if it occurred at all. In the case of the gap-filler stations, where the detection only took place during the brief period while the aircraft transited

8836-409: Was used as a local oscillator in some radar receivers and a modulator in microwave transmitters in the 1950s and 1960s, but is now obsolete, replaced by semiconductor microwave devices. In the reflex klystron the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun . After passing through the resonant cavity they are reflected by

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