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Composite planes, Metallic paint, and Antennae

 
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nuckollsr(at)cox.net
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PostPosted: Sat Mar 04, 2006 9:52 am    Post subject: Composite planes, Metallic paint, and Antennae Reply with quote

At 11:38 AM 3/4/2006 -0500, you wrote:

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

No problem. Attenuation effects of the various paints is a crap
shoot. We've studied the effects of various coatings on our
nose radomes on the bizjets and yes, there are some paints
that you don't want to use. The manufacturer of the paint is
unlikely to know and the dealer is almost sure not to know
whether your paint of choice represents a hazard to GPS
performance. Antennas at lower frequencies are less likely
to be affected.

For the most part, paints with metal flecks in them don't seem
to offer much attenuation. Some builders have reported adequate
GPS performance when antennas are under a fabric skin with
several coats of aluminum prep. I don't recall any conversations
about VHF antennas under paint. But these data are all subjective.

The #1, always-works, foolproof method to evaluate your paint
is to set your GPS up for operation with the antenna in the
clear and get a reading on relative signal strengths. Take
a large plastic mixing bowel painted with your proposed coatings
and cover the antenna. Drops of 20% or less are probably not
going to noticeably affect your radio's performance. If the
paint is good for GPS, it's probably fine for other frequencies
too.

The #2, usually-works method is to find someone who has used
the same combination of materials and see what they report. The
risk here is that subjective measures of performance can yield
wildly variable results. In deference to the Repeatable Experiment,
I highly recommend #1.
Bob . . .


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nuckollsr(at)cox.net
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PostPosted: Sat Mar 04, 2006 9:56 am    Post subject: Composite planes, Metallic paint, and Antennae Reply with quote

At 11:42 AM 3/4/2006 -0500, you wrote:

Quote:

<aadamson(at)highrf.com>

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.

No auxiliary grounding of antenna bases is required or recommended.
I've seen some composite aircraft where the interiors had spider-webs
of wire supposedly installed to replace the lack of "grounds" offered
by metallic aircraft. There is no advantage in doing this and may
even make RF interference problems worse. The only things that need to
connect to your ground plane is the base of the antenna and the outer
braid of the coax feedline.
Quote:
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

Absolutely not. Looks ugly, hard to do, doesn't work any better . . .

Quote:
....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.

Aside from the extra ground wire, what you describe should be just fine.

Bob . . .


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aadamson(at)highrf.com
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PostPosted: Sat Mar 04, 2006 10:39 am    Post subject: Composite planes, Metallic paint, and Antennae Reply with quote

Great, I haven't done that "extra ground" and can always do it later if
deemed necessary... For now, I'll leave it out of the design and try my
current approach (minus the extra ground wire).

Thanks Bob,
Alan

PS for those interested, Mouser carries that tape and It think it's a model
1181 or something like that. Google for 3m copper tape and you'll find it,
there is a smooth version and a "waffled" version. For obvious reasons, I
went with the smooth.


--


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PTACKABURY(at)aol.com
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PostPosted: Sun Mar 05, 2006 9:24 am    Post subject: Composite planes, Metallic paint, and Antennae Reply with quote

Bob: Thanks for the second prompt response. After considering your
recommendation for an experiment, a logical approach indeed, I am reconsidering my
paint choices. Since I have six internal antennas in my Lancair, both in the
upper aft fuse and each wing tip and since I would like each to operate as
well as possible, I now think using any metallic coatings may be a bad idea.
Why introduce even a 20% degradation in performance if it can be avoided? So
rather than metallic, I think I will use pearl (plastic additives rather than
metallic) to achieve the desired effect. It is a bit more difficult to
apply and touch up, but it shouldn't effect nav/com reception--should it? paul


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