Narrow Foil Strips for Ground Plane

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.

I'm planning to make four radials, each 22" long, connected at the center of
the antenna and oriented at 90° to each other.

I'm using narrow 1/4" copper foil for two reasons: 1) I already own it,
left over from my wife's stained glass hobby, and 2) since I'll be sticking
it on the outside, the narrow strips are more likely to stay attached to the
fuselage when covered with the fiberglass. I think a wider strip is more
likely to separate from the fuselage and create a bubble that would be
objectionable.

Here are my questions:

1. I'm planning to make each of the four radials from three 1/4" wide
strips run side-by-side, with 1/8" space between the strips. I hope that
this will provide epoxy bonding areas between the strips but still make the
antenna think that each radial is a single piece 1" wide. Any idea how the
performance of this might compare with a solid 1" wide strip?

2. Is there any benefit to soldering a foil strip across the ends of the
three individual strips making up each radial to bond them together at the
end opposite the center?

3. I'm assuming that 1" wide radials are significantly better than1/4" wide
radials; is that true? Maybe I'd be just as well off to make each radial
out of a single 1/4" wide strip?

4. The Com antenna is a Comant 122, which has a streamlined metal base a
couple of inches in diameter. Should the length of the ground plane radials
be 22" from the BNC connector at the center of the base or should it have
22" of length extending beyond the base?

Thanks for your help,

Dennis Johnson

I have NO experience with copper foil or antenna...but I did have an
experience with foil window security tape developing a hairline
break...perhaps due to temperature related expansion/contraction. I would
think your application would encounter temperature related considerations
made more acute by attachment to dissimilar materials...just a thought.
"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.

I'm planning to make four radials, each 22" long, connected at the center
of
the antenna and oriented at 90° to each other.

I'm using narrow 1/4" copper foil for two reasons: 1) I already own it,
left over from my wife's stained glass hobby, and 2) since I'll be
sticking
it on the outside, the narrow strips are more likely to stay attached to
the
fuselage when covered with the fiberglass. I think a wider strip is more
likely to separate from the fuselage and create a bubble that would be
objectionable.

Here are my questions:

1. I'm planning to make each of the four radials from three 1/4" wide
strips run side-by-side, with 1/8" space between the strips. I hope that
this will provide epoxy bonding areas between the strips but still make
the
antenna think that each radial is a single piece 1" wide. Any idea how
the
performance of this might compare with a solid 1" wide strip?

2. Is there any benefit to soldering a foil strip across the ends of the
three individual strips making up each radial to bond them together at the
end opposite the center?

3. I'm assuming that 1" wide radials are significantly better than1/4"
wide
radials; is that true? Maybe I'd be just as well off to make each radial
out of a single 1/4" wide strip?

4. The Com antenna is a Comant 122, which has a streamlined metal base a
couple of inches in diameter. Should the length of the ground plane
radials
be 22" from the BNC connector at the center of the base or should it have
22" of length extending beyond the base?

Thanks for your help,

Dennis Johnson

Do a Google search for RST Engineering.
Jim Weir wrote an article for KITPLANES awhile back on this subject.
He has archived those articles on his website.

In article ,
"Dennis Mountains" wrote:

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 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.

"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

"john smith" wrote in message
...
Do a Google search for RST Engineering.
Jim Weir wrote an article for KITPLANES awhile back on this subject.
He has archived those articles on his website.

Hi and thanks for the reply.

I looked at Jim Weir's site and didn't see an answer to my specific
question, but I'll take another look.

Thanks,
Dennis Johnson

"Stan Premo" wrote in message
...
I have NO experience with copper foil or antenna...but I did have an
experience with foil window security tape developing a hairline
break...perhaps due to temperature related expansion/contraction. I would
think your application would encounter temperature related considerations
made more acute by attachment to dissimilar materials...just a thought.
"Dennis Mountains" wrote in message
...

Hi,

Thanks for the reply. I may be wrong, but the foil that I remember using
for a burglar alarm was much thinner than the copper foil my wife uses for
stained glass, which I plan to use. I "stress tested" it and it seems quite
durable. Since copper is such a ductile material, it should tolerate the
expansion/contraction cycles it will encounter.

Thanks,
Dennis Johnson

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

"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

"Stephen Mitchell" wrote in message
...
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 !!!



Hi and thanks for the lengthy reply! Your experience with supersonic
aircraft sounds pretty interesting. I'll check into Exmet and Astrostrike,
but since I already have the copper foil...

As far as I know, the Legacy doesn't have any copper mesh in the layups.

As I said in a previous post, I just talked to a Legacy flyer who didn't do
anything for a ground plane, other than thoroughly sanding the carbon fiber
under the antenna mount so that the antenna base would be in direct contact
with the carbon. He reports that people hear him fine. But I'm always
reluctant to put too much faith in anecdotal stories.

I have a friend who may be able to help me rig up some kind of test; thanks
for the idea.

Thanks again,
Dennis Johnson