Direction-finding antennas

What it is

A direction-finding (DF) antenna tells you which way a signal is coming from. Take a bearing, move, take another, and the lines cross at the transmitter. Hams use it for fox hunts, for tracking down interference, and for finding a stuck transmitter or an emergency beacon.

Finding the transmitter

One bearing gives a line; two give a point; a third confirms it. Take each from a clear spot, with the antenna held well away from you and metal objects, and aim as accurately as you can. See Fox hunting and ARDF for the sport and Radio direction finding basics for the technique.

Peak or null?

A Yagi or quad shows the strongest signal when it points at the transmitter, but that peak is broad: you can swing it many degrees before the signal level drops noticeably. A small loop does the opposite. It is blind straight through the loop, a null, and the signal drops sharply on either side. So a loop gives a much more precise bearing, but only the direction, not how close you are, and it has two nulls 180° apart. You can't tell front from back.

A sense antenna fixes that. A whip or other omnidirectional element, added with the right phase and level, merges with the loop's figure-8 into a cardioid, which has a single null. Cardioids are popular for DF because that one null is sharp and has only one direction. A loop usually has an electrostatic shield, a metal tube around it with a thin gap, to keep unbalanced capacitive pickup from the surroundings from filling in the null.

Types of DF antenna

Antenna	How it finds the bearing	Notes
Small loop, with sense antenna	sharp null	simple; shield improves the null
Adcock array	two vertical elements, signals subtracted: figure-8 for vertical polarization	does not respond to the horizontal parts of a wave, so it avoids the polarization errors that spoil loops
Yagi or quad with an attenuator	peak, sharpened by reducing receiver gain	the usual hunt tool on VHF and UHF
Doppler array	switches or spins an antenna and measures the tone-shift in the signal	no moving parts to aim; often built into vehicle systems

In practice

• Reflections off buildings and terrain bounce signals in from false directions. A bearing that shifts as you move a few steps is multipath, not the target.
• Near the transmitter the signal can overload the receiver. Turn gain down, or use an attenuator, so the strongest direction stays obvious.
• A signal that arrives by sky-wave comes down from the ionosphere at a steep angle, so a nearby HF transmitter can show no clear direction at all. Hunts are usually done on VHF or UHF, or on ground-wave HF.
• Two bearings from the same place are no use. Move far enough that the lines cross at a sensible angle.

Related

• Fox hunting and ARDFCompetitive transmitter hunting.
• Radio direction finding basicsBearings, triangulation and null techniques.
• The Yagi-Uda beamDriven element, reflector and directors.
• Receive loops and active antennasSmall receiving antennas for noisy locations.
• 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.