[FoxHunt] TDOA questions.

[Jay Hennigan]

On 8/5/10 5:12 PM, C.Whitaker wrote:
> de WB2CPN
> Would someone point me to a URL that would tell
> me what you guys are talking about.  I can get the
> Time Domain, and I have part of one here, but what
> is TDOA?

Time Difference Of Arrival.

In its simplest form, two antennas separated by less than 1/4 wavelength
and diode switched 50% duty cycle at an audible rate.  Think of two
vertical dipoles separated by a cross member like a letter H.

With an FM receiver, if both antennas are equal distance from the
transmitter, the signal will hit both at the same time and no tone will
be induced.  If either is closer to the source (the "H" isn't
perpendicular to the transmitter), then the signal will hit one antenna
before the other causing a phase shift at the switching frequency.  This
will result in an audible tone in the receiver.  By rotating the antenna
until the tone disappears one can tell that the antenna is facing
directly toward (or away from) the transmitter.

The basic form has a 180 degree ambiguity.  You don't know if the
transmitter is directly ahead of or behind the unit.  Handi-Finder is of
this type.  Simple, no connection to the radio other than the antenna
fitting.

More sophisticated designs can resolve the front-back ambiguity by
getting feedback from the audio of the radio to determine which antenna
gets the signal first.  The "BMG" units are of this type and have a
left-right indicator.

One can argue that the switched-antenna Doppler units are a
sophisticated form of TDOA.

The original Little L-Per although similar in appearance actually isn't
TDOA but a two-element Yagi that toggles driven element and reflector.

--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
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