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bobair(at)me.com
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 Posted: Sun Jan 30, 2011 7:04 pm Post subject: Comm Antenna & SWR: Grounding Issue |
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Bob N., et al,
I have run a bunch of additional tests of my antennas in various configurations to see what effect they had on VSWR readings. The raw data and VSWR results are pretty variable (to my eyes) and not very encouraging.
But my immediate issue is the one that Bob N. focused on shortly after my original post -- the quality of the ground between my antenna base and my ground plane. At the time I did the original tests, it tested out at ~0.1 Ohms on my digital VOM.
But I dug out the four-wire ohmmeter that I build years ago based on Bob N's directions in the AeroElectric Connection and soldered in a fresh "D Cell" battery. The meters (especially the voltmeter) fluctuated quite rapidly and over quite a range so I'm not sure my data are accurate (although it did come close to the calculated resistance of a hunk of 18 AWG wire that I used to check the four-wire ohmmeter for accuracy).
As best I can tell, the resistance between my antenna mounting screws and the ground plane are in the 250 milli-Ohm range. This seems to be way too high. Is it? I believe you said that resistance should be in the very low milli-Ohm or even micro-Ohm range.
I've got one of your newer low resistance testers on order and as soon as I get it, I'll measure the resistance again.
I'll probably just have to pull the ground plane and re-do the mechanical mounting again and hope I get a lower resistance.
Would it make sense to get some conductive epoxy and bond a ground strap from the antenna base to the ground plane? I know soldering copper to aluminum doesn't work and corrodes almost immediately. I've seen some silver-filled epoxies rated as low as 0.001 Ohm-cm but that stuff is expensive. Could I solder a small ground strap to the antenna's metal base without damaging the antenna? I've got room in the hole for the BNC connector to run a small ground strap up from the antenna base to a ring terminal that I could clamp under one of the mounting screws. Would conductive gel under the hardware help?
Since I've got a glass skin between the antenna base and the washer stack that leads to the ground plane, I didn't "brighten" the underside of the ground plane -- just the top under each of the four machine screws. I'm thinking if I brightened up both sides of the ground plane, there might be some improvement because of the additional contact between the hardware stack, the machine screws and the underside of the ground plane.
All comments and thoughts are welcome.
Best regards,
Bob Falstad
GlaStar N248BF
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glastar(at)gmx.net
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| Posted: Mon Jan 31, 2011 1:05 am Post subject: Comm Antenna & SWR: Grounding Issue |
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Bob,
I would pull the foam on a 1/2 radius out, fill them with resin/filler
mix like the other cage holes, then drill them out to add a metal sleeve
over the screws, that one would touch the base of your antenna and the
groundplane (probably on two holes already enough), make sure the
sleeves a re a tad longer then the fiber ull thickness. Then mount the
antenna again (tarque screws)and use some rtv around the corner to seal.
That is what I did with my belly antenna and it works fine.
br Werner
On 31.01.2011 03:58, Bob Falstad wrote:
| Quote: |
I'll probably just have to pull the ground plane and re-do the mechanical mounting again and hope I get a lower resistance.
Would it make sense to get some conductive epoxy and bond a ground strap from the antenna base to the ground plane? I know soldering copper to aluminum doesn't work and corrodes almost immediately. I've seen some silver-filled epoxies rated as low as 0.001 Ohm-cm but that stuff is expensive. Could I solder a small ground strap to the antenna's metal base without damaging the antenna? I've got room in the hole for the BNC connector to run a small ground strap up from the antenna base to a ring terminal that I could clamp under one of the mounting screws. Would conductive gel under the hardware help?
Since I've got a glass skin between the antenna base and the washer stack that leads to the ground plane, I didn't "brighten" the underside of the ground plane -- just the top under each of the four machine screws. I'm thinking if I brightened up both sides of the ground plane, there might be some improvement because of the additional contact between the hardware stack, the machine screws and the underside of the ground plane.
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kuffel(at)cyberport.net
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| Posted: Mon Jan 31, 2011 8:48 am Post subject: Comm Antenna & SWR: Grounding Issue |
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Bob,
Another issue you might want to keep in mind is the
characteristics of the surface under a belly mounted
antenna. Rebar in concrete or conductive soil near the tip
(as opposed to the base) of a whip antenna can screw up
performance until you are in the air.
A reverse example of this is up in Alaska (again) the
permafrost is very non-conductive. You could spread out an
HF antenna on the ground, tune it exactly to frequency and
raise it up 50 feet in the air with no change in
performance. Or more related to your possible situation,
Seattle's soggy soil is so conductive you can mount a
vertical antenna in the ground and have it perform well
without any of the normally required ground radial wires.
Tom Kuffel
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