In the basic VDF system an arrangement of two pairs of dipole aerials (known as an Adcock Aerial) is used in conjunction with a single omni directional aerial.
If the communications transmitter on an aircraft is tuned to the VDF frequency and the transmitter is activated, the aerials at the VDF unit will detect the incoming transmission and each aerial element will feed a signal to the VDF receiver.
Since the aerial elements will all be at slightly different distances from the source of the signal, each will detect a slightly different phase of that signal at the same instant.
The value of these detected phase differences will be directly related to the direction of the incoming signal.
The phase differences are used to drive the bearing indicator.
On some VDF units a simple digital read out gives the bearing.
A ground DF station can give true or magnetic bearings.
It is common to use the so-called «Q-codes» to represent bearings.
These codes originate from the old days when telegraphy was used for communication, where it was convenient to transmit short codes instead of full text messages.
Therefore, the Q codes are not abbreviations.
A number of codes were used, but in aviation, only a few exist today.
Listed below are the codes that are relevant to direction finding.
QTE True bearing from the station (no wind)
QDR Magnetic bearing from the station (no wind)
QDM Magnetic bearing to the station (no wind)
QTF Position of a station taken by bearings from D/F stations (no wind)
QUJ True bearing to the station (no wind)
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