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Thanks
guys; I was not sure about the electrical properties of carbon fibre, but I’ve
had a look on google as well now.
If
the carbon fibre is a good ground plane, then it may not be necessary to grind
the resin away to make a physical connection. What ought to work is to use a
disc of a specific radius flat against the skin but electrically isolated. The
centre of the disc will then appear to be connected to the carbon fibre at the
specific frequency.
The
radius needs to be a quarter wavelength, but corrected for the the presence of
the resin. In practice going about 15% lower than a quarter wavelength should
work out.
The
disc can go on the inside of the skin. Other shapes than a disc are also
possible. The key is that a quarter wave transmission line open at the far end
appears as a short circuit at the near end.
Sent: Wednesday, November 6, 2013 12:31 PM
Subject: [LML] Carbon is a good ground plane
I
agree with Kevin. I also have no xpdr issues using the carbon as a ground plane.
The nav/com/xpdr/gps frequencies are plenty high to insure good capacitive
ground coupling to the carbon. The resistance of the carbon is plenty low for RF
purposes.
John... Don't know where your xpdr antenna actually is. If it way forward
at the firewall station the problem may be interaction with the propellor. Just
a guess at this point. The prop blades passing near the xpdr have been
identified as the issue on other installations I saw in the early days. Mine is
located just behind the spar box on my IV with no ground plane or problems.
Jack Morgan
On Nov 6, 2013, at 6:01 AM, Lancair Mailing List wrote:
Subject: TXPDR Antenna and
such
Date: November 5, 2013 11:58:13 AM
EST
For
what it is worth, I'm using the carbon as my ground plane.
I've been on
a couple of cross country flights and the only thing ATC is having trouble
with is the noise in the cockpit. I suspect interior and such will
absorb some of that noise when I get to it. Otherwise they are getting
5x5 on the signal quality (or at least that is what they are telling me when I
ask).
Both the TXPDR and communications seem (thus far) pretty clear
trouble free.
I decided not to put on an additional ground plane
because the resistance of the carbon, while worse than copper or aluminum, is
still pretty decent. I took a chance and it seems to be working out so
far.
Perhaps at some point I'll be proven wrong, but I think it is
worth discussing and trying to quantify.
Kevin
N969RJ
(flying)
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