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Ceengland
Joined: 11 Oct 2020
Posts: 391
Location: MS
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 Posted: Tue Jun 22, 2021 9:08 am Post subject: Archer style wingtip antenna fabrication questions |
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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.
Thanks,
Charlie
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Charlie |
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user9253
Joined: 28 Mar 2008
Posts: 1924
Location: Riley TWP Michigan
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| Posted: Tue Jun 22, 2021 12:14 pm Post subject: Re: Archer style wingtip antenna fabrication questions |
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Charlie, It looks like the critical dimension is the 0.31 air gap. My concern is that a softer plastic will flow over time resulting in the screws loosening up.
It looks like the angle part of the antenna is fastened to an aluminum wing rib. The directions are not specific. A Bakelite sheet on eBay costs $5 including shipping. Item #: 281793575532 from China. You might have to wait up to a month for shipping. It is 1mm thick which converts to 0.039". I would not sand it down. The Bakelite could possibly contain asbestos.
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Joe Gores |
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Ceengland
Joined: 11 Oct 2020
Posts: 391
Location: MS
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| Posted: Wed Jun 23, 2021 5:06 am Post subject: Archer style wingtip antenna fabrication questions |
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On Tue, Jun 22, 2021 at 10:52 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | 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 . . .
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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. 
Thanks for the offer on the dielectric, but let me give the cutting board a shot first.
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?
Charlie
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Charlie |
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johnbright
Joined: 14 Dec 2011
Posts: 165
Location: Newport News, VA
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| Posted: Wed Jun 23, 2021 9:01 am Post subject: Re: Archer style wingtip antenna fabrication questions |
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| Ceengland wrote: | ... I do have a NanoVNA; I just need to learn how to use it.
Charlie |
MegawattKS is a retired electrical engineering professor from Kansas State University in Manhattan, KS. He has YouTube videos about the NanaVNA.
https://www.youtube.com/user/MegawattKS
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John Bright, RV-6A, at FWF, O-360
Z-101 single batt dual alt SDS EM-5-F.
john_s_bright@yahoo.com, Newport News, Va
N1921R links |
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