Seems like this problem should be an easy one to model with AO or similar
modeling program.
Tell the program the antenna is in free space, then define the antenna and
then the screen in the Z-lane below it. The diagonal version would be
easiest to model. Anyone want to give it a try?
73s
Bob, W5LT
-----Original Message-----
From: Jim Lux [mailto:jimlux@earthlink.net]
Sent: Tuesday, July 26, 2005 9:34 AM
To: Mark .; towertalk@contesting.com
Subject: Re: [TowerTalk] Practical Ground Screens for NVIS antennas
At 05:17 AM 7/26/2005, Mark . wrote:
>Thank you all for your thoughtful and informative replies. Having a
>large spool of light wire with which I can easily lay down a screen is
>the easiest way to go, much like the pattern of cooling coils in AC heat
exchangers.
>This avoids the tedious placement of individual wires.
>
>I remember the 0.05 wave max spacing rule from past discussions of
>conventional ground radials. On 80m, our local Virginia Phone net and
>emergency net meets on 3947. During the day, there's also a 40m freq...
>hmmm. I would have to size the spacing for the smallest wavelength (40m
>band).
>
>lessee.... (11811/7.2)*.05 = 82 inches. A nominal 6 foot spacing
>ought to do it for both 40 and 80m. I'm not quite sure how much overlap
>(lengthwise) to use. A halfwave on 80m is about 135 ft in wire.
Approach it geometrically as a start (even though it's probably not valid
electromagnetically).. if you have your antenna up 10 ft, and you're looking
at a 45 degree takeoff angle, go out by 45 degrees (i.e. 10 ft out on the
ends).. "resonant length" is actually going to be MUCH shorter than the
wire in the air.
Realistically, you're looking at "shielding" the earth from the antenna, and
so, you want to shield where it's doing the most good. This is probably a
magnetic field situation, so the shielding would be best deployed where the
current is highest (i.e. the middle). Just guessing, though. An
interesting analysis problem: With a limited number of feet (or pounds) of
wire, are you better off "shielding" the middle real well with lots of back
and forth, or longer runs with bigger gaps between the wires. Or, the miles
of cheap aluminum (higher resistivity, but more strands in parallel,
effectively) better or worse than fewer miles of copper.
This is sort of like optimizing radials for a vertical antenna, where the
true optimum is something other than N identical radials laid out
symmetrically. (i.e. some sort of branching scheme that keeps the "radial to
radial distance" the same as you go out is probably optimum, but a pain to
install and/or analyze).
There's probably some literature out there from the 30's or 40's talking
about designing coax shields that would be applicable.
As for arrangement, probably either would work, and it might be hard to tell
the difference in a practical sense (a 6 foot wiggle in a 130 ft span is not
very big). Go with what makes it possible to put the most wire out quickly.
The other idea that comes to me is that if you have a place where you often
operate NVIS (like a standard field location for emcomm), you could get some
inexpensive wire, lay it out, and just leave it there.
Ahem.. apropos of recent discussions about the ham literature, this would be
an excellent thing to write up for QST, especially if you do some tests of
the various configurations. You would need to find a suitable set of
beacons (or some moderate distance hams willing to transmit a carrier at
constant power for long enough to make measurements). Got an pre-Eagle
Scout around looking for a project? Let them round up a crew oftrusty
assistants, a few miles of wire, hie thee to a park, and spend a few days
making coiling and uncoiling wire in various patterns and making comparative
measurements. It's at least as useful to society as building fire rings at
a campground. Might even get some of them interested in licensing, too.
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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