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Composite planes, Metallic paint, Antennae

 
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nuckollsr(at)cox.net
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PostPosted: Sat Mar 04, 2006 8:21 am    Post subject: Composite planes, Metallic paint, Antennae Reply with quote

At 03:51 PM 3/3/2006 -0500, you wrote:

Quote:


Bob: I am sure you have answered this one 700 times but please once more
for the not yet informed: I have a Lancair IV which is carbon
and fiberglass.
The aft upper fuse is fiberglass, under which is an antennae farm for com,
gps, elt, etc. This is all pretty standard, built generally IAW the kit
guidance. How it is time to select paint and I am considering SW Acry Glo
metallic Urethane. The question: will metallic urethane on the aft
upper fuse
fiberglass section significantly degrade antennae performance?
thank you,


Surface coatings chosen have no measurable effect on antenna
performance. Since carbon composite materials have a sheet resistance
on the order of 1000 times that of aluminum, they also make poor
ground planes under resonant monopole antennas. The goal is to
achieve a long lived, low resistance path between the antenna's
mounting base and the ground plane . . . which on your airplane
will need to be on the inside surface of the skin.

Make a solderable mounting base plate from brass or copper. They
may be thin. Copper hobby foil or brass shim stock (.010") are good
choices. bond your base plate to the underside of the antennas
mounting surface and then work to get good conductivity between
antenna and base plate via the bolts.

In the figure posted at:

http://www.aeroelectric.com/Pictures/Antenna/Antenna_Installation.gif

I've marked the critical conduction points with an (*). Here's
where all the magic happens. All other surfaces may be left as
supplied or as finished for corrosion protection. There's no
value in cleaning the entire surface of the base or large interfacing
surfaces between airframe, antenna and doubler (in this case your
doubler is the copper or brass sheet).

Then you need radials made from shim stock or copper tape. Wider the
better up to 2" or so. Same length as height of antenna. 4 is about
the minimum, no measurable differences beyond 8. They don't have
to lay flat (they might run up and over a doubler or other surface
feature). Solder these to the base plate.

This is described in more detail in the antenna chapter.

Bob . . .

< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >


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PTACKABURY(at)aol.com
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PostPosted: Sat Mar 04, 2006 8:39 am    Post subject: Composite planes, Metallic paint, Antennae Reply with quote

Bob: Thanks for your prompt response to my question, BUT: I didn't
communicate my concern well and therefore need to retransmit. I am not interested
in a ground plane, I understand that one from your excellent chapter in the
"Connection" and used that info when installing my EXTERIOR antennae. My
concern is the INTERIOR antennae buried in the upper aft sections of the Lancair
IV under the fiberglass skin. Again this is the recommended place for com,
gps, etc and has been used successfully by many builders for years so I am not
concerned about that either. HOWEVER, I am considering a metallic paint and
wonder if the small amounts of metallic in the paint when applied over the
fiberglass will degrade the performance of these buried antennae. I don't
think metallic paints are used on radomes but I wonder if this is really a
concern for my installation. thanks for giving me a second chance, paul


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aadamson(at)highrf.com
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PostPosted: Sat Mar 04, 2006 8:45 am    Post subject: Composite planes, Metallic paint, Antennae Reply with quote

http://www.highrf.com/gallery/Feb-06/DSCN1305

Here is my solution. Couple of notes.

3M makes a copper tape that has acrylic glue on the back and the glue is
conductive. They use it in EMI solutions. I first lined the Comm antenna
indent in the fuselage with said tape, then I ran the 22" radials out from
that area. I then got some copper flashing and made a conductive backing
plate. My original plan was to use some solder paste and put the backing
plate down and "wick" in some solder around the edges.... HOWEVER, I tested
my approach on a sample piece and determined that any soldering iron heat
(and I use a Metcal), would degrade the laminate So I'm going to use a
little conductive copper no-ox compound. Just a little as it goes a long
ways. One other note before I stuck down the radials I did sand down to the
exposed carbon so that at least I'd have some sort of electrical bond
between the radials and the bottom of the fuse.

I later came back and put a 2 bid of 1.7oz glass over each radial just to
provide protection.

That backing plate will be drilled for nutplates and rivets and once it's
down and held in place, I'll come back and drill and countersink a screw
from the outside (flush), and put a nut and washer on the inside. This stud
will then be ran back to the central ground.

Lot of discussion of this approach on the Lancair list. Seems some believe
that the ground plane needs to be on the outside and not inside....Oh, well,
I didn't want to "paint" the groundplane with copper/silver paint as I've
heard it will deteriorate over time... I figured this was next best.

Alan

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aadamson(at)highrf.com
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PostPosted: Sat Mar 04, 2006 9:37 am    Post subject: Composite planes, Metallic paint, Antennae Reply with quote

Just one comment, Radar uses a much higher frequency than Comm or Nav, etc.
So while the metal flake paint messes with radar returns due to it's
"reflectivity". I doubt it would do much to the comm antenna

Alan

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jimw_btg(at)earthlink.net
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PostPosted: Sat Mar 04, 2006 7:29 pm    Post subject: Composite planes, Metallic paint, Antennae Reply with quote

Alan,

Jim Wickert here. It is quite well known that foil internal antennas do function well under carbon fiber cloth. they do however function very well under glass substrate. Now with regard to paint. Per a quite an experienced individual Jim Weir VP RST Engineering group who as published "The Reference Text, The RST-820 Antenna which deals strictly with copper Tape Antennas, see quote>

"These antennas have worked in glass and fiberglass airplanes with almost every kind of paint and dope (including the aluminum coat used on fabric as a UV protectant) that we could find. A couple of times in the past two years there have been reports that there is a new German metallic paint (sorry, I don't know the brand) that makes ALL hidden antennas (not just ours) work poorly if at all."

I know of builder that has his plane flying with a silver metallic paint and he has no problems. Jims Ref manual is about $5.00 and well worth the price of admission for any laying out antennas in composite planes

Take care, happy building

Jim Wickert
Vision Vair #159
Some will have it some will not!!

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PostPosted: Sat Mar 04, 2006 8:17 pm    Post subject: Composite planes, Metallic paint, Antennae Reply with quote

Jim, thanks for the post. Can I ask for some clarification? Your quote
.. "It is quite well known that foil internal antennas do function well
under carbon fiber cloth." Is that correct, or should there have been a
*NOT* before "function"? Are you trying to say, they do or do not work on
carbon?

Second and a point of clarification. The Lancair actually uses an "outside
antenna". It uses a bent whip from ComAnt. But I wanted to improve the
"ground plane" only, so I added those 4 "radials" of 22" and the base plate
to provide the counterpoise and conductive attach points for the external
antenna.

I know of one other Legacy that is completed this same way and he says his
Comm antenna works very well. I'm cautiously optimistic.

But I think I'll order that $5 book just the same.

Thanks and can ya clear up the opening quote for me?

Alan

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PostPosted: Mon Mar 06, 2006 7:02 am    Post subject: Composite planes, Metallic paint, Antennae Reply with quote

Alan,
Sorry fat fingers "DO Not" they are quite functional under glass and kelvar but not Carbon Fiber. Take care.

Jim Wickert
Vision Vair #159

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