Direction finding of radio signals. How it works?
In the previous section , the possibility of receiving a local oscillator signal from a working radio receiver was considered. We now consider a more general question: how is the radio signal being detected? With what accuracy?
What is true and what is myth, let's try to figure it out.
Note: I don’t have access to a real direction finder for real tests. All information was found in open sources.
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Most probably, the obvious, and historically appeared first, is a method of direction finding signals using directional antennas. It was used, in particular, during the USSR for sports radio direction finding, called “fox hunting”. The cover of the magazine of the Radio of that time shows how approximately it looked:
We are now more interested in not the sports, but the technical side of the issue. As can be seen from the photo, the receiver contains 2 antennas: one frame, the other whip. Schematically, the signals from the antennas are combined so that the radiation pattern in the form of a cardioid is obtained (a scheme from the site unradio.ru):
As you can see, the radiation pattern is very broadband, but it allows you to “detect” the direction to the maximum signal. The accuracy of determining the maximum is not particularly high, which, incidentally, was offset by the speed and physical training of the athlete.
If we talk about modern devices, then something similar can be seen, for example, in the ARK-NK3I wearable direction finder , which, as can be seen from the description, is also equipped with a frame antenna. Detailed descriptions could not be found, but it can be assumed that the accuracy of taking a bearing with such a device is roughly comparable to the aforementioned cardioid.
With directional antennas, everything is more or less clear, it is also clear that they need to be rotated at least, or to go with them, which of course is inconvenient. Of much greater interest are the phase methods that allow you to take the bearing on the signal using a fixed antenna.
(antenna finder 3000, photo from irga.sut.ru/sp.html )
There are different subsets of phase methods, consider for example the principle of quasi-Doppler direction finder. Imagine a signal coming from a certain direction and an antenna rotating in a horizontal plane.
Obviously, due to the Doppler effect, while the antenna is moving towards the source, the frequency will be higher, in the opposite direction, respectively, lower. By analyzing the maximum and minimum frequency oscillations, you can easily determine the direction. Of course, nobody rotates the antenna in real life - a fixed array of antennas is used (approximately as in the photo), the switching of the signal from which is performed by electronic switching. By comparing the phases of the signals, you can determine the direction to the radiation source.
By the way, such devices can also be used by radio amateurs, for example, for the same “fox hunt”. For $ 400, you can purchase a ready-made Doppler Direction Finder Kit:
There are also simpler schemes containing not more than 20 parts. In them, as a receiver, a ready-made radio station is used, and it is only necessary to finish the module for switching antennas.
However, we will return to stationary direction finders. Probably the main question that interests the users is the accuracy and frequency range of direction finding. For example, consider the ARC-SP Stationary Direction Finder, which is described on the website bnti.ru:
- Working frequency range: 20 - 3000 MHz
- Field sensitivity in the range of 20-1000 MHz: no more than 12 µV / m
- Instrumental accuracy (SKO), no more: 2 ° (20-1000 MHz)
- Minimum duration of the signal being recalled, once at the 5 MHz processing band: 30 ms
- Continuous recording of radio signals in the band: up to 24 MHz, data rate for continuous recording of a radio signal in the 24 MHz band: 102.4 MBytes / s
From this description, you can select a number of useful facts:
- The operating frequency range extends up to 3GHz, which covers all practically possible signal sources.
- A very short signal is really enough for bearing.
- The maximum bandwidth of the recorded signal is 24 MHz, this is due to the maximum sampling rate of the available ADCs. The description on the site dates back to the 2012m year, given some progress, we can assume that now the ADC is available at 60 or even 100 MHz. But more is unlikely, and one can definitely say that no one writes the entire radio broadcast , it is too complicated and expensive. Thus, the backdating of the signal by recording is almost impossible, unless the signal was recorded in the recording by accident.
- The declared accuracy is not more than 2 °, which, on the one hand, is quite good, on the other hand, it is clearly not enough to search with accuracy “to the apartment”. Moreover, as follows from the principle of operation, in the vertical plane the signal is not bearing at all, so it is also impossible to know the height of the source (or floor).
Speaking of accuracy, on the same site you can find a screenshot of the Radio Explorer program where the accuracy of the 3000 direction finder is visible:
At what maximum distance is it possible to detect the radio signal? Far enough, because Direction finder antennas are usually placed on the tallest buildings in the city. On ess.ru, we managed to find an article published in 2006, which contains the following table (direction finding of a 5W radio station):
As you can see, the maximum range was 27 km
As for automotive direction finders, their description (including photos of installation and installation, as well as operator jobs) can be found in the same article .
I hope that we managed to dispel some of the direction finding myths, to confirm some of them. All data for the article were taken from open sources, 5-10 years old. Something was probably improved, but obviously not by orders of magnitude, and the laws of physics in this area did not seem to have changed in 10 years.
I would like to note another point. Despite the fact that modern technologies do not allow the intruder to be charged with an accuracy of the apartment, room and floor, a second after pressing the PTT button, you should not deceive yourself. Practice shows that malicious offenders are still caught, it is only a matter of time.
What is true and what is myth, let's try to figure it out.
Note: I don’t have access to a real direction finder for real tests. All information was found in open sources.
')
Principles of radio wave direction finding
Directional antennas
Most probably, the obvious, and historically appeared first, is a method of direction finding signals using directional antennas. It was used, in particular, during the USSR for sports radio direction finding, called “fox hunting”. The cover of the magazine of the Radio of that time shows how approximately it looked:
We are now more interested in not the sports, but the technical side of the issue. As can be seen from the photo, the receiver contains 2 antennas: one frame, the other whip. Schematically, the signals from the antennas are combined so that the radiation pattern in the form of a cardioid is obtained (a scheme from the site unradio.ru):
As you can see, the radiation pattern is very broadband, but it allows you to “detect” the direction to the maximum signal. The accuracy of determining the maximum is not particularly high, which, incidentally, was offset by the speed and physical training of the athlete.
If we talk about modern devices, then something similar can be seen, for example, in the ARK-NK3I wearable direction finder , which, as can be seen from the description, is also equipped with a frame antenna. Detailed descriptions could not be found, but it can be assumed that the accuracy of taking a bearing with such a device is roughly comparable to the aforementioned cardioid.
Phase methods
With directional antennas, everything is more or less clear, it is also clear that they need to be rotated at least, or to go with them, which of course is inconvenient. Of much greater interest are the phase methods that allow you to take the bearing on the signal using a fixed antenna.
(antenna finder 3000, photo from irga.sut.ru/sp.html )
There are different subsets of phase methods, consider for example the principle of quasi-Doppler direction finder. Imagine a signal coming from a certain direction and an antenna rotating in a horizontal plane.
Obviously, due to the Doppler effect, while the antenna is moving towards the source, the frequency will be higher, in the opposite direction, respectively, lower. By analyzing the maximum and minimum frequency oscillations, you can easily determine the direction. Of course, nobody rotates the antenna in real life - a fixed array of antennas is used (approximately as in the photo), the switching of the signal from which is performed by electronic switching. By comparing the phases of the signals, you can determine the direction to the radiation source.
By the way, such devices can also be used by radio amateurs, for example, for the same “fox hunt”. For $ 400, you can purchase a ready-made Doppler Direction Finder Kit:
There are also simpler schemes containing not more than 20 parts. In them, as a receiver, a ready-made radio station is used, and it is only necessary to finish the module for switching antennas.
However, we will return to stationary direction finders. Probably the main question that interests the users is the accuracy and frequency range of direction finding. For example, consider the ARC-SP Stationary Direction Finder, which is described on the website bnti.ru:
- Working frequency range: 20 - 3000 MHz
- Field sensitivity in the range of 20-1000 MHz: no more than 12 µV / m
- Instrumental accuracy (SKO), no more: 2 ° (20-1000 MHz)
- Minimum duration of the signal being recalled, once at the 5 MHz processing band: 30 ms
- Continuous recording of radio signals in the band: up to 24 MHz, data rate for continuous recording of a radio signal in the 24 MHz band: 102.4 MBytes / s
From this description, you can select a number of useful facts:
- The operating frequency range extends up to 3GHz, which covers all practically possible signal sources.
- A very short signal is really enough for bearing.
- The maximum bandwidth of the recorded signal is 24 MHz, this is due to the maximum sampling rate of the available ADCs. The description on the site dates back to the 2012m year, given some progress, we can assume that now the ADC is available at 60 or even 100 MHz. But more is unlikely, and one can definitely say that no one writes the entire radio broadcast , it is too complicated and expensive. Thus, the backdating of the signal by recording is almost impossible, unless the signal was recorded in the recording by accident.
- The declared accuracy is not more than 2 °, which, on the one hand, is quite good, on the other hand, it is clearly not enough to search with accuracy “to the apartment”. Moreover, as follows from the principle of operation, in the vertical plane the signal is not bearing at all, so it is also impossible to know the height of the source (or floor).
Speaking of accuracy, on the same site you can find a screenshot of the Radio Explorer program where the accuracy of the 3000 direction finder is visible:
At what maximum distance is it possible to detect the radio signal? Far enough, because Direction finder antennas are usually placed on the tallest buildings in the city. On ess.ru, we managed to find an article published in 2006, which contains the following table (direction finding of a 5W radio station):
As you can see, the maximum range was 27 km
As for automotive direction finders, their description (including photos of installation and installation, as well as operator jobs) can be found in the same article .
Conclusion
I hope that we managed to dispel some of the direction finding myths, to confirm some of them. All data for the article were taken from open sources, 5-10 years old. Something was probably improved, but obviously not by orders of magnitude, and the laws of physics in this area did not seem to have changed in 10 years.
I would like to note another point. Despite the fact that modern technologies do not allow the intruder to be charged with an accuracy of the apartment, room and floor, a second after pressing the PTT button, you should not deceive yourself. Practice shows that malicious offenders are still caught, it is only a matter of time.
Source: https://habr.com/ru/post/397175/
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