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twin-dipole TDOA:


TDOA (Time Difference of Arrival) systems use two dipoles, usually spaced a little less than 1/2 wavelength
apart, connected to a single receiver by a coax that uses PIN diodes to isolate one of the two antennas
at a time. The control box typically sends a DC square wave up the antenna to switch between the two
elements, using a 1khz or similar clock.

Whenever the two antennas are not equidistant from the transmitter, the switching frequency (1khz etc)
can be heard on the receiver as an FM modulation. The louder the sound, the stronger the separation.

When both antennas are equidistant from the transmitter, the two antennas receive an identical, in-phase
signal, and no tone is heard on the receiver. This can occur in one of two directions, 180 degrees
apart, so you don't initially know whether it's in front of you or behind you, unless your receiver has
some way to tell you which way to go.

This is a very common tool to use, because it's extremely cheap and easy to build, using very common parts,
requiring just a 555 timer, a voltage regulator, 9v battery, and a handful of diodes and resistors.
It's also easy and effective to use, and is mostly immune to overload, negating the need for an attenuator.

The lack of front/back sense can be a drawback however, as can multi-path, which brings the tone back into
the null and will create a second not entirely quiet null. It does have the subtle advantage of providing
a purely auditory feedback, not requiring the operator to watch the radio. This frees up the hunder to
observe their surroundings more closely and visualize their bearings as they listen to the mixing tone.

Some advanced TDOA receivers reprocess the receiver's audio in a way simular to a doppler, and can show the
magnitude of phase difference on a meter, and may even display a left / right indicator.


Handi-Finder

misc bi-directional toners

Ramsey DF-1

twin-dipole antennas

WB2HOL tdoa



last updated 11/30/2023 at 20:19:05