Radio direction finding basics

What it is

Radio direction finding (RDF) means working out where a signal is coming from by measuring the direction it arrives from. One reading gives a direction, not a place; the skill is combining several. Amateurs use it for fox hunts, for tracking down interference, and for finding a stuck transmitter or a lost beacon. The antennas that make it possible are in Direction-finding antennas; this article is about the technique.

Bearings and a fix

A bearing is a line on the map, not a point. Take one, move to a different place, take another, and the lines cross at the transmitter. A third bearing from a third place tells you whether to trust the first two: if all three agree, you have it.

Real bearings are never exact, so each is better thought of as a wedge whose width grows with distance: an error of 5° is about 90 m sideways at 1 km. Where the wedges overlap is where the transmitter might be.

Two lessons fall out of this. A smaller reading error shrinks the overlap proportionally. And the angle between the bearings matters as much as their accuracy. With a 5° error, a 90° crossing leaves an uncertain region about a quarter of your distance across; at 30° it is more than half, a long thin smear. Bearings taken from nearly the same direction say almost nothing about distance. So move sideways, not forwards.

Taking a good reading

• Stand clear. Hold the antenna out at arm's length. Your body, a vehicle or a metal fence close to it distorts the pattern and bends the bearing.
• Match polarization. Hold the antenna so it matches the transmitter's polarization, usually vertical on VHF. A mismatch weakens the signal and can blur the bearing; see Polarization.
• Read from open, high ground where you can. Buildings, trees and hills reflect and block signals.
• Take more than one reading at each place, turning the antenna through the whole circle. If the signal seems to come from several directions, believe the strongest or most repeatable.
• Agree on north. A compass points to magnetic north, while many maps are drawn to true north. The difference, called declination, depends on where you are. Correct for it, or use one reference throughout.

When bearings lie

Problem	What you see	What to do
Reflections (multipath)	Bearing changes when you move a few steps	Take several readings; trust the ones that agree
Front-or-back ambiguity	Two opposite directions both look right	Use a sense antenna, or take a second bearing from elsewhere
Receiver overload	Strong signal from every direction	Add attenuation until the meter moves again
Sky-wave on HF	Direction wanders, bearing makes little sense	Hunt on VHF or UHF where the signal travels in a straight line

Near the transmitter, a bearing from one spot is no longer enough. Switch from plotting lines to simply walking toward the strongest signal, reducing sensitivity as you go; Fox hunting and ARDF covers that final approach.

Related

• Direction-finding antennasLoops, Adcock arrays and Yagis for finding a transmitter.
• Fox hunting and ARDFCompetitive transmitter hunting.
• The Yagi-Uda beamDriven element, reflector and directors.
• Radiation patternsReading azimuth and elevation plots, beamwidth and front-to-back.
• PolarizationThe orientation of a wave's electric field, and why antennas must match it.
• Line of sight and the radio horizonWhy VHF and UHF reach about as far as you can see, and a little further.