Today's Message Index:
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1. 08:23 AM - Re: Paint question (Robert L. Nuckolls, III)
2. 09:00 AM - Re: Paint question (Robert L. Nuckolls, III)
Message 1
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At 11:54 AM 6/20/2018, you wrote:
>Bob,
>
>You=99ll like the antenna you have pictured, I
>believe.=C2 I have been using a home-brewed
>version of that antenna now for several years
>and I love it as a dual-band antenna.
Pleased to hear it. I've been aware of this design
for some years but never had an opportunity/need
to try it. Some of our local volunteer fire crews are
interested in outdoor antennas for their radios that
will cover both the MURS/GMRS frequencies which are,
like 2M/70CM, harmonically related.
Best yet, this is an antenna they can build themselves.
Basic cut, drill, thread and assemble. I don't have
to get into the hammer-n-tongs loop!
Bob . . .
Message 2
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| Subject: | Re: Paint question |
At 09:46 AM 6/20/2018, you wrote:
>At 07:56 AM 6/20/2018, you wrote:
>>Hi Bob,
>>Long time no contact... I=99m happy you=99re still in this
game!
>>
>>Regarding this topic, I wonder how much the the
>>conductive bits are in contact with each other.
>>I don=99t know how isolated each bit of
>>conductive material is in such a paint, but I
>>could imagine there could be billions of
>>contacts between neighboring bits of flake or
>>powder such that there would be a random
>>resistive path across the painted surface.
I've been doing some 'asphalt contemplation' on a simple
test setup to evaluate the effects of various paints/coatings
on antenna performance.
I was trying to imagine how conductive particles suspended
in a non-conductive 'solution' would achieve a
microscopic version of the gas-tight contact needed
for reliable connection between conductors. It seems
that surface tension would cause each particle to
be totally enveloped, thus prevented from making
physical contact with other particles.
Not sure about what happens as the solution becomes
a solid when solvents evaporate and the paint dries.
I've seen volume resistivity measurements on some
coatings/fillers, like potting compounds. These
are always VERY high . . . including those designed
for thermal conductivity.
I'm thinking that there are three potential effects
of paint . . . the dielectric effects you hypothesized
which would probably be limited to a lowering of
resonant frequency, the shielding effects which
block and or re-direct the wave fronts of interest
and attenuation/dissipation effects that simply
turn the RF energy into heat.
I'm recalling a bit of a fire drill on the ELT
transmitters for Beechjets where a new version
of the ELT kept tripping off due to high SWR
on the VHF antenna. Seems the older version happily
existed with a pair of antennas tucked under
the fairing at the root of vertical fin leading
edge.
Emacs!
Oookkaaay . . .
Now, let's lay the VHF antenna back so that it
sorta conforms to the inside of the fairing . . .
except . . .
Fairing was also part of an air-intake ductwork
for the A/C . . . so the antennas wound up
looking like this:
Emacs!
There were metallic braces inside the
fairing along with bond straps that tied
the braces to airframe . . . for lighting
effects. Added on top of all this was a
composite fairing material . . .
capped off with paint often chosen by
the customer hence of uncontrolled pedigree.
What's a poor ELT transmitter to do?
Emacs!
I couldn't find anywhere in the archives where
this installation had been measured for
performance . . . a condition that didn't
raise its ugly head until a new ELT complained
about an 'unsatisfactory' antenna.
I suggested we design a new, top loaded vertical
for the VHF antenna that remained vertical with
better separation from the effects of overhead
structures.
Emacs!
Further, we could fine tune the antenna to accommodate
any residual effects from proximity of other structures.
This got the flying fuzz all in a dither . . . a new
antenna would violate the TSO on the ELT . . . but
abusing the TSO'd antenna did not . . . go figure.
I think they wound up widening the SWR trip tolerances
on the ELT transmitter.
Not one of my happier experiences with the bureaucracy.
Bob . . .
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