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I have a lot to do with GPS antennae in my working day.
For a good explanation of GPS, go to:
http://www.trimble.com/gps/
A question was asked:
- 'What is the difference between the active and passive
GPS antennas?'
The answer is that an active antenna has a built-in
amplifier to boost the (relatively very weak) signals
picked up from the satellites. Most GPS receivers
and antennae use amplification, and this is acheived
by the GPS receiver putting out a small amount of
power up the antenna cable, at about 17 volts. This
does not interfere with the signals coming down as
they are at around 1.5 GHZ
Another question asked was:
- 'What about remote mouting an antenna?'
The antenna simply needs to be able to see the sky, and
glass, paper and thin wood are transparent to GPS signals.
This also why receivers work OK in cockpits and dash-boards
of cars. This means that you can easily mount an antenna in the
body of a Europa. Because the cable might be relatively long
it is a good idea to have an active antenna and a GPS receiver
capable of driving it.
It is also important to realise that the 'horizon' of a GPS
antenna matters, so mount your antenna as flat as possible
in normal use. Putting an antenna at a 30 degree angle can
make a lot of difference for the worst if only a few satellites
are visible for some reason at any time.
Lastly
' Where would I buy one, and how much should they cost?'
Well, for UK builders, go to TDC
http://www.tdc.co.uk/antennas/antenna_gps.htm
I recommend the active mag-mount GPSM, expect to pay
about 60.00 GBP plus VAT and shipping. Check the cable
and connector fits your GPS first. Don't let anybody tell you
that you need an 'aviation' antenna unless you are building a jet.
I would suggest a mag-mount one would be a good idea, and
you can build in a small steel 'shelf' the size of a playing card
into the top of the inside of the fuselage with about 20mm
clearance on top. You can then magnetically 'stick' the
antenna to it, and it is free for removal/replacement at any
time (the magnet is very strong indeed!).
Simon
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