Английская Википедия:Doppler radio direction finding
Шаблон:Short description Шаблон:Tone Doppler radio direction finding, or Doppler DF for short, is a radio direction finding method that generates accurate bearing information with a minimum of electronics. It is best suited to VHF and UHF frequencies, and takes only a short time to indicate a direction. This makes it suitable for measuring the location of the vast majority of commercial, amateur and automated broadcasts. Doppler DF is one of the most widely used direction finding techniques. Other direction finding techniques are generally used only for fleeting signals, or longer or shorter wavelengths.
The Doppler DF system uses the Doppler effect to determine whether a moving receiver antenna is approaching or receding from the source. Early systems used antennas mounted on spinning disks to create this motion. In modern systems, the antennas are not moving physically, but electrically, by rapidly switching between a set of several antennas. As long as the switching occurs rapidly enough, which is easy to arrange, the Doppler effect will be strong enough to determine the direction of the signal. This variation is known as pseudo-Doppler DF, or sometimes sequential phase DF. This newer technique is so widely used that it is often the Doppler DF seen in most references.Шаблон:Citation needed
Direction finding
Early radio direction finding (RDF) solutions used highly directional antennas with sharp "nulls" in the reception pattern.Шаблон:Sfn The operator rotated the antenna looking for points where the signal either reached a maximum, or more commonly, suddenly disappeared or "nulled". A common RDF antenna design is the loop antenna, which is simply a loop of wire with a small gap in the circle, typically arranged to rotate around the vertical axis with the gap at the bottom.Шаблон:Sfn Some systems used dipole antennas instead of loops. Before the 1930s, radio signals were generally in what would today be known as the long wave spectrum. For the effective reception of these signals, very large antennas are needed. Direction finding with rotating antennas is difficult at these wavelengths due to the size of the antennas.Шаблон:SfnШаблон:Sfn
A great advance in RDF technique was introduced in the form of the Bellini-Tosi direction finder system, which replaced the rotation of the antenna with the rotation of a small coil of wire connected to two non-moving loop antennas. The loop antennas were similar to those used in earlier systems but fixed in position, set at right angles to each other to form a cross-shaped arrangement. Each antenna will produce a different output whose relative strengths depend on how close the signal is to either antenna's null. These signals were sent to two coils of wire, the field coils, also arranged at right angles. These re-created the original signals in a much smaller space, about the size of a soda can. By rotating a small loop antenna, the sense coil, in the space between the two crossed field coils, one could perform DF. In effect, it recreated the traditional technique at a much smaller scale, allowing the main antennas to be built at any size.Шаблон:Sfn
Robert Watson-Watt introduced the next major advance in direction finding as the "huff-duff" system, a nickname for high-frequency direction finding. Huff-duff also used crossed antennas, often an Adcock antenna,Шаблон:Sfn but sent their output to the two channels of an oscilloscope. The relative strengths and phases of the two signals deflected the X and Y locations of the oscilloscope's electron beam by different amounts, causing an ellipse or figure-8 to appear on the screen, with the long axis indicating the direction of the signal.Шаблон:Sfn The readout was essentially instantaneous and proved able to easily detect even short transmissions. Huff-duff was used in about one-quarter of all successful U-boat sinking.[1]
Both of these systems have drawbacks. The Bellini-Tosi system still has moving parts, albeit small ones, but has the more major limitation that it requires the operator to hunt for the signal, which may take several minutes. Huff-duff provides a direct and immediate indication of the signal direction, but only at the cost of requiring an oscilloscope or similar display system with an equally fast response. Both require two closely matched receivers and amplifiers, and often a third for the "sense" antenna, if used.Шаблон:Sfn
Doppler effect
If one places an antenna on a moving platform like the roof of a truck, the movement of the truck will cause the Doppler effect to shift the frequency of the signal upward as it moves towards the signal, or downward as it moves away. When the truck is driving at right angles to the signal, or not moving at all, there will be no shift.Шаблон:Sfn If the truck is driven around a circular track, there will be times when it approaches the signal, moves away from it, or moves at right angles. This will produce a rising and falling frequency shift of the target signal, producing a frequency modulated (FM) signal known as the Doppler sine wave.Шаблон:Sfn The FM signal has the same frequency as the rotational speed of the vehicle.Шаблон:SfnШаблон:Sfn
The magnitude of the shift is a function of the wavelength of the signal and the angular velocity of the antenna:
Where Шаблон:Math is the Doppler shift in frequency (Hz), Шаблон:Math is the radius of the circle, Шаблон:Math is the angular velocity in radians per second, Шаблон:Math is the target wavelength and Шаблон:Math is the speed of light in meters per second.Шаблон:Sfn Converting to more common units:
- To convert Hz to radians per second, multiply by 6.28 (2 pi)
- To convert MHz to Hz, multiply by 1 million
- Eliminating the constants gives (6.28 × 1000000) / 300000000 = Шаблон:Sfrac ~= 48
Such that:
Where Шаблон:Math is the frequency of rotation in Hz and Шаблон:Math is the target frequency in MHz.Шаблон:SfnШаблон:Efn
Consider the example truck hunting an FM radio station at 101.5 MHz while driving around a Шаблон:Convert wide pad (50 m radius) at Шаблон:Convert. The circumference of the pad is 2π×50 or 314 m, and its velocity in m/s is 25,000 / 60 / 60 ~= 7 m/s, so the truck completes one circuit in 314 / 7 = 45 seconds. Шаблон:Math is therefore Шаблон:Frac. Feeding that into the formula above, the frequency shift is:
This amount of frequency shift is too small to be accurately measured. To improve the detection odds, the product Шаблон:Math must be increased. For this reason, Doppler DF systems normally mount their antennas on a small disk that is spun at high speed using an electric motor. Performing the same calculation using an antenna mounted to a Шаблон:Convert diameter disk spinning at 1000 Hz results in:
Which is easily detected. Nevertheless, such a rotation speed, 60,000 rpm, demands precision systems. Because the antennas have to move at very high speeds, this technique is only really useful for higher frequency signals where the antennas can be shorterШаблон:Efn and the higher Fc produces a larger dividend.Шаблон:Sfn
Early examples of Doppler DF systems date to at least 1941,[2] and they were used in the United Kingdom for hunting out German early warning radars, which operated at 250 MHz in the 1.25-meter band. By 1943, examples were available that worked in the UHF region, used to find the German Würzburg radars operating at 560 MHz.Шаблон:Sfn
A significant advantage of this technique is that it requires only a single receiver, amplifier, and the appropriate FM demodulator. In contrast, huff-duff and B-T systems require two closely matched receivers, one for each antenna pair, and often a third for a sense channel.Шаблон:Sfn Widespread civilian use of the technique did not start until the introduction of practical circuits for the quadrature detector and phase-locked loop, both introduced after the war, which greatly simplified the reception of FM signals. Its use roughly follows the spread of FM radio, which also used these techniques.Шаблон:Sfn
Pseudo-Doppler
To further simplify the system, it is possible to simulate the movement of the antenna with a small amount of additional electronics. This is the pseudo-Doppler direction finding technique.Шаблон:Sfn
Consider a pair of omnidirectional antennas receiving a signal from a target transmitter. As the signal propagates past the receiver, the amplitude of the signal at the antennas rises and falls. At long distances from the transmitter, well into the "far field", the wavefronts can be considered to be parallel.Шаблон:Sfn If the two antennas are arranged perpendicular to the line to the target, the phase difference between them is zero, whereas if they are arranged parallel to the line, the phase difference will be a function of the distance between them and the wavelength of the signal.Шаблон:Sfn
For this example, consider the two antennas to be located Шаблон:Frac of the target wavelength apart and aligned parallel to it. If the two antennas were sampled instantaneously, the difference in phase between them would always be the same, 90 °. But if one instead switches the input from one antenna to the other, there will always be some inherent delay during which time the signal continues to move past the two antennas. In this case, if the original sample was taken when the peak of the wavefront was at the nearer antenna and the system then switches to the farther one, the phase will not be 90 ° but somewhat smaller, because the wavefront approached the second antenna during that time.Шаблон:Sfn
Now consider a series of such antennas arranged around the circumference of a circle, and a switch that connects to the antennas in turn in a clockwise fashion. If the target signal is at the 12 o'clock position, then the phase shift will be increased when the switching is moving "forward" between the 7 and 11 o'clock positions and reduced when moving "away", between 1 and 5. When switching between antennas perpendicular to the line to the signal, 11 to 1 and 5 to 7, the shift will be a constant value.Шаблон:Sfn
The signal from the antennas is sent into a single receiver, resulting in a series of pulses whose amplitude depends on the phase at the instant of sampling. That signal is then smoothed to produce a sine wave.Шаблон:Sfn That sine wave is modulated exactly as it would be in the case of a single moving antenna. In the case of the moving antenna, the frequency shifts because the antenna is moving through the wavefront as it passes, whereas in the pseudo-Doppler case this is accomplished by timing the samples to simulate the movement of a single antenna. The direction to the target transmitter can then be determined in the same fashion as the moving-antenna case, by comparing the phase of this signal to a reference signal. In this case, the reference is the clock signal triggering the switch.Шаблон:Sfn
Because it has no moving parts and can be built using simple electronics, the pseudo-Doppler technique is very popular. Whilst not quite as fast as to take a measurement as the huff-duff system, in modern systems the measurement is so rapid that there is little practical difference between the two concepts. Pseudo-Doppler has a significant advantage that the antenna system is much simpler, using monopole antennas, and if the switching system is located on the antenna, only a single wire runs back to the receiver and thus only one amplifier is required.Шаблон:Sfn Because this technique is so widely used it is often referred to simply as Doppler DF, the "pseudo" rarely being added.Шаблон:Sfn
The main disadvantage of the technique is a requirement for more signal processing. Because the "movement" in pseudo-Doppler proceeds in steps, the resulting signal is not smooth as it is in the case of a moving antenna. This results in a signal with considerable numbers of sidebands that have to be filtered out. The switching system also introduces electronic noise, further confusing the output.Шаблон:Sfn Modern signal processing can easily reduce these effects to insignificance.Шаблон:Sfn
Notes
References
Citations
Bibliography
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