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nuckolls.bob(at)aeroelect
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 Posted: Tue Jun 22, 2021 7:48 pm Post subject: Archer style wingtip antenna fabrication questions |
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At 12:03 PM 6/22/2021, you wrote:
| Quote: | I have an Archer wingtip comm antenna I intend to install in one wingtip (can always add a 1/4 wave whip if needed), and I'm thinking about building the nav version detailed in Bob's 'Antennas & Feedlines chapter for the other wingtip (figure 13-12 in my document).
How critical is the material for the .032" x .8" x 3.25" bakelite insulator that forms what I'm guessing is a matching capacitor? Does the bakelite participate in any way in operation, or is it simply a way to get a fixed air gap?
Bakelite in that 'thinness' doesn't seem to be available without paying more for shipping than the 3-postage-stamp size needed would cost. What about substituting something like the thin HDPE cutting board stock, available at discount stores? Two layers would be about .034" thick. |
It IS critical . . . yes it forms the dielectric
for a capacitor used to series resonate the feeder
strut.
Bob must have conducted experiments to optimize
the characteristics of that capacitor with respect
to the matching strut for optimal mid-band impedance
match.
If it were my project, I'd use ANY available dielectric
material and conduct experiments over a ground plane
on the table to achieve (1) antenna resonance and
(2) best match to the feed line.
When I was a kid, a 'gamma match' was commonly
used to bring a feed line and antenna radiator
into functional harmony. The attached figure
illustrates an adjustable feeder strut paired
with a variable capacitor to feed a grounded
quarter-wave antenna.
My 10M, 2-element beam had such a feature that
was tuned with the aid of an SWR bridge. Today,
a vector network analyzer is the instrument
of choice and can cost less than a good SWR
meter.
After getting close on the bench, you can
confirm on the airplane to see if 'tweeking'
is called for. I've had readers assemble the
Archer wing tip antennas per drawings but found
it useful to trim dimensions per measured results.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jun 23, 2021 8:30 am Post subject: Archer style wingtip antenna fabrication questions |
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| Quote: |
If any dielectric is worth a shot, I think I'll try my flexible cutting board idea for a start. I've got a pack of them on the shelf, left over from another project (bearing surface for the round top tube of an aluminum hangar door). Package says 43% HDPE/57% EVA, and it's ~0.017" thick, so 2 layers will be pretty close in thickness, at least. How critical is tuning, for a receive-only Nav antenna? I do have a NanoVNA; I just need to learn how to use it.  |
The reason that any dielectric can be considered is
based on the physics of capacitors. The value of a
capacitor is proportional to area of the 'plates',
space between the plates, and dielectric constant
of the material.
Here's a comprehensive tutorial on the topic:
http://www.phys.uri.edu/gerhard/PHY204/slides8-phy204.pdf
In cases like this, one can estimate the value
of any particular capacitor fabrication with
the goal of duplicating Bob Archer's design
goals. It would be REALLY nice if we had
an Archer Original to measure the value of
his productions.
| Quote: | | Thanks for the offer on the dielectric, but let me give the cutting board a shot first. |
No problem. I thought about the cutting sheets
too but wondered if their flexibility might
compromise the mechanical integrity of Bob's
original design goal. But we're in experimental
waters here.
| Quote: | BTW, since this is a Nav antenna and can lie flat on the bottom of the wingtip,
I'm thinking I may try aluminum flashing (~0.008"), and slightly 'break' the
edges to stiffen it. Any issues with that, if width/length dimensions are maintained? |
Skin effects for current flow due to operating
frequency tell us that thickness of a conductor
has no practical influence on performance. It's
surface area and then only to a depth of .001"
or so. The primary concern for materials selection
are conductivity (aluminum and copper are good)
and mechanical robustness suited to the environment.
Bob's antennas were no more 'robust' than needed to
avoid falling apart or breaking.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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