Editing Direction finding (section)

===Post-war systems===
Several developments in electronics during and after the [[Second World War]] led to greatly improved methods of comparing the phase of signals. In addition, the [[phase-locked loop]] (PLL) allowed for easy tuning in of signals, which would not drift. Improved [[vacuum tube]]s and the introduction of the [[transistor]] allowed much higher frequencies to be used economically, which led to widespread use of VHF and UHF signals. All of these changes led to new methods of RDF, and its much more widespread use.

In particular, the ability to compare the phase of signals led to phase-comparison RDF, which is perhaps the most widely used technique today. In this system the loop antenna is replaced with a single square-shaped [[ferrite core]], with loops wound around two perpendicular sides. Signals from the loops are sent into a phase comparison circuit, whose output phase directly indicates the direction of the signal. By sending this to any manner of display, and locking the signal using PLL, the direction to the broadcaster can be continuously displayed. Operation consists solely of tuning in the station, and is so automatic that these systems are normally referred to as [[automatic direction finder]].

Other systems have been developed where more accuracy is required. [[#Pseudo-doppler DF technique|Pseudo-doppler radio direction finder]] systems use a series of small dipole antennas arranged in a ring and use electronic switching to rapidly select dipoles to feed into the receiver. The resulting signal is processed and produces an audio tone. The phase of that audio tone, compared to the antenna rotation, depends on the direction of the signal. Doppler RDF systems have widely replaced the huff-duff system for location of fleeting signals.