Radio direction finding

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DF antenna in a forest on Bramberg in Lucerne , Switzerland

Radio direction finding ( english - radio direction-finding ) is - in accordance with Article 12.1 of the ITU Radio Regulations of the (Radio Regulations) International Telecommunication Union (ITU) - defined as " radiolocation that the reception of radio waves used to determine the direction in which a Radio station or an object. »

DF radio stations are purely passive and only evaluate waves that are emitted elsewhere by other devices. A receiver and one or more directional antennas are required for radio direction finding. Application is among other things the radio compass .

Procedure

Principle of radio direction finding using triangulation

The minimum bearing method is used for long, long and medium wave , in which a ferrite or loop antenna is rotated until the signal has a minimum intensity. In this case the axis of the coil points to the transmitter. Since the display is ambiguous, the signal is superimposed on that from a rod antenna with omnidirectional characteristics in order to enable the direction to be clearly defined.

In the VHF and microwave range, a maximum bearing is usually carried out with the help of direction-dependent Yagi or parabolic antennas . In order to minimize the bearing error, the antenna gain should be as high as possible. To determine the location of a transmitter, as shown in the figure on the right for the transmitter at point C , at least two DF antennas with known locations at A and B are required. The location of the transmitter C can be determined from the determined angle of the two DF antennas . If more than two DF antennas are available at different locations, the accuracy of the bearing can be improved.

Another variant is the Doppler direction finding , mainly in the VHF range, which, however, is associated with a higher technical effort: One of four or more antennas fixed in a circle is always connected to the receiver one after the other, so that an electronic rotation of a virtual receiving antenna results (approx. 300 revolutions per second). If the rotation runs counter to the radio signal, the reception frequency increases slightly; if the rotation moves away, the frequency of the reception signal is minimally lower. This virtual rotation modulates the antenna rotation frequency on the radio signal to be tracked due to the Doppler effect . In the receiver, this approximately sinusoidal LF signal is obtained by means of frequency demodulation and compared with the (known) antenna control signal: The phase positions of both signals are directly related to the direction of the incident wave. The direction information obtained in this way can be displayed with a precision of a few degrees, for example on a round display. Under certain circumstances it is also possible to calculate an elevation angle for the direction . The more receiving antennas are arranged in a circle, the more precise the determination of the direction will be.

The phase position of the signals from two or more antennas can also be evaluated for direction finding . With a phase detector , the phase signal can be obtained and used directly for tracking the antenna ( English tracking ). If the absolute frequency of the received signal is measured with sufficient accuracy, the relative speed between transmitter and receiver can be determined using the Doppler effect .

The time of arrival of a time-discrete transmission signal can also be determined at several receivers at different locations. This means that not only the direction but also the distance to the transmitter can be determined. Conversely, the time of arrival of several time-coded and synchronized transmitter signals can be measured (for example when determining the location using a GPS receiver ).

You can also determine your own location by aiming for several radio beacons or so-called radio beacons (fixed direction finding transmitters / in aviation, for example, VOR - VHF omnidirectional radio range ) (has been used for a long time on ships and aircraft for radio navigation ). .

In principle, RADAR methods are also part of the radio direction finding method - here, however, the radio echo of the object is evaluated.

DF antennas

DF antenna Galeta Island
DF receiver with loop antenna
Military DF antenna

For radio direction finding u. a. the following antenna types are used:

application

Military / intelligence

  • Finding ships, vehicles and aircraft based on their radio traffic or based on interference emissions
  • Locating mobile broadcasting stations
  • Determination of your own position by receiving the GPS satellite transmitter (((restricted before the year 2000) can also be used for civil purposes)
  • Finding RADAR location and fire control systems
  • Location of agent radio

Executive

  • Finding the location of cell phones. Several cellular base stations act as "DF receivers" in which they calculate the approximate distance to the cell phone with the help of a certain transmission parameter ( timing advance ).
  • Location of illegal transmitters
  • Locating sources of radio interference

Civil

  • Tracking / observation of animals equipped with tracking devices
  • Measuring a vertical wind profile using balloon probes (trajectory tracking)
  • Tracking (Engl. Tracking ) of satellites and rockets on the basis of their radio signals
  • Own location determination on board ships and aircraft by receiving stationary tracking transmitters ( radio navigation )
  • own location, for example with the Galileo satellite navigation system (GPS receiver)
  • so-called amateur radio arrows as an amateur radio sport and for further training (finding hidden transmitters)
  • Locating thunderstorms in meteorology based on the electromagnetic waves they generate

literature

Web links

Individual evidence

  1. ↑ VO Funk, 2012 edition, Article 1.12, definition: radio direction-finding / radio bearing.
  2. ↑ Belt pillar. In: German Spy Museum. Accessed June 7, 2020 (German).