You have raised some interesting questions. I have some recent experience
with an antenna mounted on a carbon fiber structure - in this case an
antenna in a different frequency range on a superonic jet fighter but
none-the-less there are some lessons here. I should point out that I am not
an avionics person but work along side a couple of them.

First you are right, carbon is conductive to a degree, if you get a meter
and measure the resistance between two points in the structure (you need to
touch fibers with the probes for best results) you will get a resistance
which indicates that it is a conductor albeit not that good. I don't
remember exact numbers but I think 50-100 milli ohm over a few feet of
length (but don't shoot me if I got those numbers wrong). I can get the
exact numbers if you are interested. So placing the ground plane on the
inside is probably not going to work. It is this conductivity that allows
many carbon aircraft to be certificated for lightning strike with no copper
mesh in the laminate. The carbon conducts the lightning current away from
the strike and disipates the current within the laminate itself.

If carbon fiber is conductive then the obvious question is "what is its
performance as a ground plane". We have searched the literature and there is
little if any data out there. Consequently we tested two large panels one
with copper mesh (Exmet expanded copper foil) bonded in place with a film
adhesive and another panel with just carbon fibre. The performance of the
carbon only panel was good enough that we chose not to install copper mesh.
I do need to caution you though; the panels had an aluminium honeycomb core
and the effect of this compared to the carbon is not clear but the core was
not earthed to the antenna base where as the carbon was.

There are several things coming out of this.

First the testing we did was recommended by an antenna design specialist
because she didn't know what the effects of carbon, aluminium core etc were
and could not predict what would happen. Neither could the OEM of the
antenna. So I suggest you test your installation, possibly without the foil
tape at all to start with - just to see what happens. Testing is the only
sure way to know. A rudimentary test can be done on the cheap.

Second instead of the foil tape you are proposing to use, bond in some of
the expanded copper mesh from Exmet or Astrostrike .. this is what the
aerospace industry does when they have this sort of a problem. This stuff is
cheap and for what you need you may even get a free sample if you play the
game right.

Third ... I don't know about the Legacy but the Lancair IV already has
copper mesh in some parts of the airframe for lightning protection. Is there
any in the area where you are proposing to mount the antenna ?? If not could
you relocate the antenna to an area where there is some ???

Fourth ... you are not the first person to mount a comm's antenna on a
Legacy .. just take the lead from the others. If I works why bother. All
this stuff is black magic and even those who are supposed to know are
usually only guessing !!!






"Dennis Mountains" wrote in message
...

"Orval Fairbairn" wrote in message

news ...

Why not attach it to the INSIDE of the fuselage? That's where all of the
grounding takes place -- and -- you don't have to cover it up!

I would use the 4 strips of the 1/4" tape, at 45 deg from each other and
a piece of thin copper sheet about 1" dia, soldered to the center,
joining all the radials.

1/4" wide is plenty wide for the frequency band you are using -- just
make sure that each radial is about the same or greater length as the
antenna.

Thanks for the reply!

I have heard that carbon fiber does not allow radio signals to pass
through
it, so that for a ground plane to be effective, it would have to be
outside
the carbon barrier. As a practical matter, I'm sure some radio signals
get
through, but if it were a significant amount, I'd put the antenna inside!

Another benefit of putting the foil on the outside is that I have clear
space to put the foil. Inside the fuselage, there isn't 22" of clear
space
around the antenna, because the antenna is near the aft spar.

Some Legacy builders have installed short ground plane radials inside the
fuselage, some have put them outside, others have painted the belly with
special metal paint, and others have done nothing about ground planes. So
far as I know, every method tried that uses an external antenna seems to
work, so I'm likely worrying over nothing anyway. But the search for an
answer is, by itself, a pretty interesting process!

Thanks again,
Dennis Johnson