[FoxHunt] TDOA questions.

[Larry Benko]

Jay,

I am pretty sure the maximum separation for a TDOA is 1/2 wavelength and 
not 1/4 wavelength.  The resulting signal which produces an FM modulated 
signal due to the switching between 2 identical signals which have 
different phase shifts (this is the time difference part) repeats every 
1/2 wavelength.  In other words a difference of 0.1 wavelength produces 
exactly the same FM signal as a 0.9 wavelength difference (and 1.1, 1.9, 
etc.).  Likewise a difference of 0.45 wavelength appears the same as 
0.55, 1.45, 1.55 etc.

I have built several TDOAs years ago and always used slightly more than 
a quarter wavelength separation.  I no longer use any of them since they 
perform poorly if the fox signal is horizontally polarized.  The ability 
to have your hunting equipment able to be turned/adjusted to 
discriminate between polarizations is very important unless your group 
has rules only allowing vertical polarization.  The other thing that 
causes problems with a TDOA setup is if the fox transmitter is FM 
modulated near the same rate the TDOA is switching antennas (usually 500 
to 1000Hz).  Then the resulting 2 tones you hear can be confusing.  
Finally the TDOA will never be more sensitive than a dipole and 
generally is less sensitive due to the small noise created by constantly 
switching antennas.  I would put the TDOA in the easy to use for 
non-modulated, vertical polarized, medium or strong signal fox hunting 
category.  Otherwise I would not use it.

I have some info at http:www.w0qe.com in the Transmitter Hunting tab.

73,
Larry, W0QE

Jay Hennigan wrote:
> 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/
> Your local telephone and internet company - 805 884-6323 - WB6RDV
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