Doppler radio direction finding

Doppler radio direction finding, also known as Doppler DF, 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.