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Author Topic: Vehicle mounted TDOA antennas  (Read 13996 times)
G7MRV
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« on: March 08, 2012, 12:37:25 AM »

I have a TDOA unit based on this design http://theleggios.net/wb2hol/projects/rdf/tdoa1.htm used with a handheld and portable antenna

However, I have the opertunity whilst adding permanent mounts to my car, to add a pair of Panorama M8 antenna mounts. These do not have any 'normal' antenna use intended for them, and I can position these either side of the roof, accurately measured and with identical whips, I also have the oppertunity to cut the feed coax for them to preceise electrical 1/2wave, allowing me to use just one set of PIN diodes.

I cannot see any problem with using such a fixed antenna system with the TDOA unit, but before i take the plunge and drill two extra holes, has anyone done this before who can offer any advice or hints?

I appreciate that such a system will give me a direction indication relative to the vehicles direction of travel
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WB6BYU
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« Reply #1 on: March 08, 2012, 01:05:30 PM »

I've used such an approach with mag mount whips and/or quarter wave whips attached
to the roof rack rail on my car.  They aren't perfect, of course, but the are quite convenient
for a FORWARD/AFT indication.  In a town with streets neatly laid out on a square grid you
simply drive straight as long as the display reads FORWARD, then make a right turn when
the needle centers.  If it reads AFT turn around.  This can quickly get you close to the
transmitter, as long as you don't get confused by reflections, etc.

Sometimes it is helpful to also have LEFT/RIGHT indication - you can do this with the same
receiver and just switch in a second part of antennas.

I find the TDOA still works better as a rotatable unit, however, as that allows you to
spin it around and count the number of times that the needle crosses the center.  If this
happens more than twice in 360 degrees you are in an unreliable site and shouldn't trust
bearings.  For hunting where the street layout is less tidy, I'd consider instead putting
a pair of dipoles on a rotatable mast instead of mounting them to the roof.
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G7MRV
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« Reply #2 on: March 10, 2012, 08:53:56 AM »

Thanks for the reply. It occured to me only after posting that i'd asked the same question before quite some time ago, i think it was you who replied that time Cheesy Its only now im in a position to try it.

  For hunting where the street layout is less tidy, I'd consider instead putting
a pair of dipoles on a rotatable mast instead of mounting them to the roof.

Ha! Here in the UK, most village and town layouts, unless they are so-called 'new towns' are anything but tidy!  I intend to build a VHF loop to keep in the boot (trunk) for DF on foot.
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