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1-3 of 3 messages
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Mobile mount antennas for TDOA?
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by G7MRV on August 22, 2008
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Hi, some time ago i build a TDOA RDF. The design calls for a pair of vertical dipoles, mounted n fractions of a wavelength apart, and uses PIN diodes at the antennas and at the feedpoint of the phasing to switch between them. However, i wish to use this system mobile. Is there a way i can create suitable 1/4wave whips using small magmounts that would work? Obviously there would be no DC ground connection but would that matter? or would i be better using gutter mounts where i can directly connect to the car for grounding the PIN diodes?
IM also going to create a portable dipole array for the set, since a colleage today gave me a surplus FM BC 4el yagi to convert. This will be usable from anywhere i go, but would have to be set up, so not truly mobile!
Regards
Martin G7MRV
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RE: Mobile mount antennas for TDOA?
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by G7MRV on August 22, 2008
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For info, this is the system i built
http://home.att.net/~jleggio/projects/rdf/tdoa1.htm
the antenna calls for a resistor to ground for each dipole, and a spacing apart of <1/2wavelength. This spacing probably rules out using gutter mounts due to the width of the car roof.
I would have to manufacture the magmounts myself to ensure i can work on them, probably from rare earth magnets and plastic 35mm film cans (amazingly useful things for radio!)
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RE: Mobile mount antennas for TDOA?
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by WB6BYU on August 22, 2008
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The exact spacing isn't that critical for a TDOA, as long
as the antennas are not TOO far apart (which introduces
an extra set of nulls into the pattern.)
You can use a pair of mag mounts - I found some thin
commercial ones with magnets about 1cm diameter. The
two coax cables (which have to be the same length) both
connect to the switch box. You don't need a DC ground
to the mounts - the coax shield will suffice. Actually
you can put the switches in a common box and plug the
cables into it rather than mounting the switch at the
base of the antenna. This isn't always a perfect approach,
as can be seen from some of the discussion about switching
in Doppler array antennas, but is generally adequate.
The antennas do, however, have to be very similar, with
little or no phase shift between them. That is why a
full sized antenna is better than a loaded one. If the
antenna is adjustable you can use one as a reference
and adjust the other for a null in the proper direction.
An example of that approach is here:
http://web.telia.com/~u85920178/antennas/vhftest1.htm
I also built a pair of whips using welding rods and
steal corner brackets attached to the roof rack on my
car. These worked fairly well, too. I generally used
them positioned fore and aft, rather than side-to-side,
so the TDOA told me whether I was going towards or away
from the signal. This is useful when the streets are
arranged in square blocks: take a convenient street and
go in whichever direction is towards the transmitter.
When the direction changes, turn on the next cross-street.
If you are going the wrong way, turn around. This gets
you close to the transmitter very quickly, and in the
same distance that a zig-zag pattern across the map would
require.
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