[TowerTalk] Practical Ground Screens for NVIS antennas

[Jim Lux]

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.