Narrow Foil Strips for Ground Plane

I would be interested to see some pictures of the ground plane inside the
Diamond Star if you have them handy. The relative locations of the carbon
and the foil would be of particular interest if the pictures show that.

I have seen a similar metallic strip along the bottom of the fuselage in
other composite aircraft that all the antenna earth to.

The use of mesh in lieu of foil just makes it easier to bond in place (and
more reliable) because the adhesive impregnates the mesh. The mesh is cheap
at a few dollars / sq ft. They also make aluminium and possibly monel mesh -
although I won't use aluminium on a carbon structure because they are widely
separated on the galvanic scale. Both Exmet and Astroseal (Astrostrike) can
be found on the web.


"Steve Chalfin" wrote in message
om...
"Dennis Mountains" wrote in message
...
Hi,

I'm building a Lancair Legacy (all carbon fiber) and am planning to put
copper foil strips on the belly to serve as a ground plane for the
belly-mounted com antenna. I'm going to use 1/4" wide copper foil with
an
adhesive back, attached to the outside of the fuselage and covered with
a
thin layer of fiberglass (not carbon fiber) to protect the foil. I'll
solder it together at the center and attach it somehow to the outside of
the
BNC connector.


Why not put foil on the inside of the fuselage as a ground plane?
That's what Diamond did on my Star. The entire 'scuppers' of the
fuselage is lined with metal (but not copper) foil, as a ground plane
for the Com 2, DME, Marker, and Transponder antennas. Although not
entirely made of carbon fiber, the Star has a lot of it in that area
(spar carry through structure). I can e-mail some pics to you if you
are interested.

Com 1 and GPS antennas on the 'roof' have metal plate ground planes,
also located inside.

Steve

I remember him sayin a laser disc would work fine. If yer plane is
composit it could be inside if metal no need and wood and fabric
probably just under the fabric but i'm not so sure on that!

In article ,
"Stephen Mitchell" wrote:

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

Didn't the carbon/aluminum honeycomb have some major corrosion issues?
That is why you NEVER use a lead pencil on aluminum.

"Dennis Mountains" wrote in message
...


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!

Whoa! You are using the foil for a ground plane not an antenna. It is
perfectly OK for your foil to be inside the carbon because it is your antenna,
sticking outside, that does all of the radiating. Your foil will capacitively
couple to the carbon and the two should combine to make should should be a great
ground plane. Depending on the conductivity of your plane's skin, length of the
foil may not be critical. If possible, dummy everything together, borrow an SWR
meter and a radio, and make sure that everything is OK while you can still
change things without a major hassle.

Vaughn

Commercially built airplanes are full of carbon epoxy / alumnium honeycomb
panels.

They separate the alumnium honeycomb from the carbon via a film adhesive
which has a fiberglass scrim in it. Corrosion is not normally a problem.

You are right though ... carbon is at +0.1 Volt and alumnium at something
like -0.75 Volt on the galvanic series so you have a battery !! Normally
you would not want any more than 0.3 Volt difference.


"Orval Fairbairn" wrote in message
news ...
In article ,
"Stephen Mitchell" wrote:

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

Didn't the carbon/aluminum honeycomb have some major corrosion issues?
That is why you NEVER use a lead pencil on aluminum.

"Vaughn" wrote in

Whoa! You are using the foil for a ground plane not an antenna. It
is
perfectly OK for your foil to be inside the carbon because it is your
antenna,
sticking outside, that does all of the radiating. .

Vaughn


Whoa! This is going to be fun, when Jim W. gets back with some replies.
--
Jim in NC

Ever since we did the very first copper-foil-inside-the-airframe design for
Burt's prototype Vari-EZ, we've offered to do FREE designs for any manufacturer
who wanted to use our experience. FREE is the operative word here, and we don't
even require them to use OUR foil or OUR toroids. About seventy of the
companies out there offering plans out there have chosen to use our FREE service
to do the antenna design. We even do free designs for factory builts and give
the factory our support and advice in the process, whether or not they buy our
components.

Lancair and RV are the only ones who told us to go pound sand. So be it.
You've paid tens of thousands of dollars for their designs and their parts and
those cheap *******s won't even give you the plans for an antenna design?

Please, sir, don't come on these newsgroups pleading poormouth and how-to-do-it.
Write or call Lancair and ask them why they don't understand how to do INTERNAL
antennas in carbon fiber. There are ways. They are too lazy or too cheap to
investigate them or too proud to ask us how to do them. According to Rutan, you
save between a knot and a knot-point-five per 100 knots of airspeed for each
COM/NAV antenna you get off the outside of the airframe.

Don't believe me? Go look at an F-117 Stealth Fighter. How many external
antennas do you see on that carbon composite airframe? Zero in my picture.
Got the idea?

Don't come back here asking for professional help. Go ask your
designer/supplier.

Jim







"Dennis Mountains"
shared these priceless pearls of wisdom:

-Hi,
-
-I'm building a Lancair Legacy
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

It is sad that Jim reacted the way he did .. I was hoping to learn
something. Just a pity he took his frustrations out on a builder who is
unaware of what transpired between Lancair and Jim ...........



"Jim Weir" wrote in message
...
Ever since we did the very first copper-foil-inside-the-airframe design
for
Burt's prototype Vari-EZ, we've offered to do FREE designs for any
manufacturer
who wanted to use our experience. FREE is the operative word here, and we
don't
even require them to use OUR foil or OUR toroids. About seventy of the
companies out there offering plans out there have chosen to use our FREE
service
to do the antenna design. We even do free designs for factory builts and
give
the factory our support and advice in the process, whether or not they buy
our
components.

Lancair and RV are the only ones who told us to go pound sand. So be it.
You've paid tens of thousands of dollars for their designs and their parts
and
those cheap *******s won't even give you the plans for an antenna design?

Please, sir, don't come on these newsgroups pleading poormouth and
how-to-do-it.
Write or call Lancair and ask them why they don't understand how to do
INTERNAL
antennas in carbon fiber. There are ways. They are too lazy or too cheap
to
investigate them or too proud to ask us how to do them. According to
Rutan, you
save between a knot and a knot-point-five per 100 knots of airspeed for
each
COM/NAV antenna you get off the outside of the airframe.

Don't believe me? Go look at an F-117 Stealth Fighter. How many external
antennas do you see on that carbon composite airframe? Zero in my
picture.
Got the idea?

Don't come back here asking for professional help. Go ask your
designer/supplier.

Jim







"Dennis Mountains"
shared these priceless pearls of wisdom:

-Hi,
-
-I'm building a Lancair Legacy
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

"smjmitchell" wrote in message
u...
It is sad that Jim reacted the way he did .. I was hoping to learn
something. Just a pity he took his frustrations out on a builder who is
unaware of what transpired between Lancair and Jim ...........



My thoughts exactly! For me, part of the fun of building my own airplane is
the opportunity to learn new skills as well as the intellectual challenge of
understanding new concepts. I would like to thank all of those on this
group who have helped me, on this thread as well as so many other threads.

Thanks,
Dennis Johnson

Oookay, then, who has the time to build up a "typical" skin out of the same
carbon material that you are using for the airframe -- say a sheet 4' x 8' or so
and ship it over to me so that I can do the testing required to answer the
question. Seems to me that the only folks with those sorts of resources are the
ones that are taking the money for the plans and parts.

Or am I wrong? Somebody want to make me a sheet of glued-up material? That's
how this whole thing started, you know. Bellanca wanted inside-the-wing
antennas and they crated up a whole WING and sent it out to me to destroy. That
is how we got the data in the first place.

Any takers?

Jim



"smjmitchell"
shared these priceless pearls of wisdom:

-It is sad that Jim reacted the way he did .. I was hoping to learn
-something. Just a pity he took his frustrations out on a builder who is
-unaware of what transpired between Lancair and Jim ...........
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com