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Eric Page
Joined: 15 Feb 2017
Posts: 242
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 Posted: Mon Feb 08, 2021 5:03 pm Post subject: Antenna Grounding |
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I think I know the answer to this question, but I'd like confirmation: does an antenna ground plane have to be electrically bonded to the airframe?
The aircraft is a Kitfox, which has a steel tube fuselage. In an effort to keep the outside of the plane as clean as possible, I'm trying to put as many antennas as I can inside the fabric envelope. For example, the comm antenna...
https://www.aircraftspruce.com/catalog/avpages/antennasystems.php
...is mounted inside the vertical stab. Other Kitfox builders have reported good performance with this antenna in the tail, despite a couple of steel tubes in the vicinity.
Since the transponder and ADS-B antennas...
https://www.aircraftspruce.com/catalog/avpages/ted_transponder.php
--and--
https://www.aircraftspruce.com/catalog/avpages/monopole11-13561.php
...are so short, they'll easily fit inside the wings. I can fabricate a ~3" diameter ground plane from aluminum sheet for each of them, but there's no way to electrically ground those plates to the airframe if they're suspended inside the wings. Is that a problem, or is a good bond from coax shield to ground plane all that's needed? Thanks.
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Mon Feb 08, 2021 7:51 pm Post subject: Re: Antenna Grounding |
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No, the ground plane does not have to be grounded to the airframe.
Com antennas should be vertically polarized. The ANTENNA VHF-5 will work in a wing,
but not very well. It might work well vertically in the tail.
How about this antenna for $45?
https://www.aircraftspruce.com/catalog/avpages/stainlessantenna.php
You could make your own antenna.
http://www.aeroelectric.com/articles/Antennas/DIY_Comm_Ant/DIY_Comm.html
But for $45, it will be easier to buy one.
Mount it vertically inside of the fuselage. It can either point up or down.
Make a ground plane out of 4 wires going out in 4 directions.
Keep the first half of the antenna away from airframe tubes.
Use a SWR meter to check how well the antenna works.
What engine are you using in your Kitfox?
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Joe Gores |
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Mon Feb 08, 2021 7:56 pm Post subject: Re: Antenna Grounding |
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Did you mean 3 inch radius for the transponder antenna? 3 inch diameter is too small.
The radius should be at least as long as the antenna.
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Joe Gores |
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user9253
Joined: 28 Mar 2008
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Location: Riley TWP Michigan
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Eric Page
Joined: 15 Feb 2017
Posts: 242
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| Posted: Tue Feb 09, 2021 10:00 am Post subject: Re: Antenna Grounding |
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| user9253 wrote: | | No, the ground plane does not have to be grounded to the airframe. |
OK, that's what I thought.
| Quote: | | Com antennas should be vertically polarized. The ANTENNA VHF-5 will work in a wing, but not very well. It might work well vertically in the tail. |
Indeed. I installed the VHF-5 vertically, inside the tail.
| Quote: | | Did you mean 3 inch radius for the transponder antenna? 3 inch diameter is too small. |
Whoops! Yes, I meant radius.
Thanks, Joe.
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nuckolls.bob(at)aeroelect
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| Posted: Wed Feb 10, 2021 3:41 pm Post subject: Antenna Grounding |
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| Quote: | | ...are so short, they'll easily fit inside the wings. I can fabricate a ~3" diameter ground plane from aluminum sheet for each of them, but there's no way to electrically ground those plates to the airframe if they're suspended inside the wings. Is that a problem, or is a good bond from coax shield to ground plane all that's needed? Thanks. |
no ground to aircraft needed for the UHF antennas.
However, their RADIUS is equal to the seated height
of the antenna so closer to 6" diameter for ADSB
and 5.75" for the xponder antenna. Orient them
so that the antennas extend down from the ground
plane.
It's hard to argue with customer satisfaction
for any particular antenna configuration . . .
but I'd really like to see a sweep of the
antenna mounted in the vertical fin. At the least,
proximity of metal to the antenna might well
call for physical 'shortening' of the antenna.
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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Eric Page
Joined: 15 Feb 2017
Posts: 242
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| Posted: Thu Feb 11, 2021 9:04 am Post subject: Re: Antenna Grounding |
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| nuckolls.bob(at)aeroelect wrote: | ...RADIUS is equal to the seated height of the
antenna so closer to 6" diameter for ADSB and
5.75" for the xponder antenna |
Yeah, the 3" diameter was a brain fart on my part. I meant radius.
| Quote: | ...I'd really like to see a sweep of the
antenna mounted in the vertical fin. At the least,
proximity of metal to the antenna might well
call for physical 'shortening' of the antenna. |
I have a poor-man's VNA, so I'll see what I can do. I ordered some RG-400 last night, so it'll be a week or two before I can mock up a test. If I can make the VNA show something that looks sensible, I'll post it here.
According to the manufacturer's data, this antenna cannot be shortened.
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