Auto Direction Finder
Auto Directional Finders (ADF)
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A radio direction finder (RDF) or automatic Direction Finder (ADF) is a device for finding the direction to a radio source. Due to radio's ability to travel very long distances and "over the horizon", it makes a particularly good radio navigation system for ships and aircraft that might be some distance from their destination.

Post-war systems

Several developments in electronics during and after the 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 wouldn't drift. Finally, improved vacuum tubes and the introduction of the transistor allowed much higher frequencies to be used economically, with led to widespread use of VHF and UHF signals. All of these changes led to new methods of RDF, and 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, who's 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 radio direction finder systems use a series of small dipole antennas arranged in a ring and use electronic switching to rapidly select pairs of dipoles to feed into the receiver. The resulting signal is processed and produces an audio tone, who's frequency is dependant on the direction of the signal. Doppler RDF systems have widely replaced the huff-duff system for location of fleeting signals, as it does not require an oscilloscope.

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VHF Portable Radios
Source: WikiCommons
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