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Author Topic: Coax above or below ground?  (Read 832 times)
KB0TXC
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Posts: 78




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« on: July 27, 2013, 05:57:37 AM »

Hi,

As I am getting near finishing installing all of the ground image plane radials for my 4BTV, (120 40 foot 14 gauge stranded insulated wire and four 10 foot bare No. 4 copper terminated with (and bonded to) ten foot ground rods for lightning), now my next question is this: Should I bury the coax (in PVC conduit) below the ground plane, or should I elevate the coax above the ground plane?

I have read in an old QST article about a parasitic vertical array that with elevated coax, the SWR is much better than if the coax is below the radials.

Please advise this HF antenna neophyte.

Best,

Joe KB0TXC
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AC4RD
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Posts: 1236




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« Reply #1 on: July 27, 2013, 06:50:38 AM »

Joe, just a thought, but you might want to just run the coax along the ground loosely for now.  That way if there are any problems that might be related to the feedline, you don't have to dig it up to check on it.  I use verticals often, but I rarely leave any one antenna up unmodified for more than a few months--maybe a year, tops.  I always just run the coax on the ground; seems to work perfectly well.

That's a heck of a radial system!   Sounds like you'll have a great antenna when it's done.  Verticals can work REALLY well--let us know when you have it working!
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K7JQ
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Posts: 282




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« Reply #2 on: July 27, 2013, 08:20:43 AM »

If you use a feedline current choke at the vertical feed point (preventing the shield of the coax from acting like a radial, especially if you have too few or too short radials), then it doesn't matter if the coax is buried or not. However, with a great radial system like  you have described, it's largely a moot point.
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K3GM
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Posts: 1754




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« Reply #3 on: July 28, 2013, 05:54:55 AM »

Joe, with a large radial field like what you have, be prepared for some major tuning issues.  You are going to have a large feedpoint impedance mismatch with al those radials.  It will require adjusting the traps, and possibly the addition of an UnUn transformer.  The work you put into it though will be worth it.  An antenna analyzer is a good tool to help you with these issues.
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