[TowerTalk] Practical Ground Screens for NVIS antennas

[Jim Lux]

At 07:04 AM 7/25/2005, Mark . wrote:
>Hi All,
>All this talk lately of NVIS antenna work goes along with some antennas and
>masts I have been building lately to enjoy casual, portable hamming, with
>some emphasis on cheap antenna plans for a cheap emergency 80 trunk antenna.
>So far, results have been quite good using 18ga lamp cord or speaker cord,
>using a 30' or less of the unzipped portion as a feedline, to a 4:1 current
>balun and then connected to the mobile HF rig.
>
>I once read that an old trick for boosting the efficiency of horizontal
>antennas over questionalble ground was to run a series of radials along the
>ground parallel to the antenna.
>
>For instance, if the antenna is a flat-top or inverted Vee, run 4-5 radials
>parallel to it along the ground, spaced 10-15  few feet apart.
>
>This is pretty easy to accomply by unrolling precut wires from a homemade
>spool (you can get free plastic spools by asking for the empty ones at the
>home stores from their rotary wire supply racks).
>
>What is an effective placement for such wires under the antennas? Any rules
>of thumb for length and orientation?


In general, you'd want to orient the wires in the direction of current 
flow, which would be parallel to the radiating element.



>If separate wires, do they need to be approximately 5% longer than the
>driven antenna to act as reflectors?

Nope.. they're laying on the ground, near a dielectric, so at best, you'd 
be guessing as to the propagation velocity.  So, you just consider them as 
"ground resistance reducers".  Lay out what ever is convenient.



>It would be convenient to just have a long wire on a spool and lay out a
>zig-zag pattern around temporary stakes, such that you could just walk
>around and pay out the line from the spools. Winding them up would be a
>snap, too.

That would work.


>Thanks for your ideas.
>
>--...MARK_N1LO...--

You may find that there's just not all that much difference.  My experience 
with making a "counterpoise" for tesla coils (where the wavelength is 
hundreds of meters) is that you need fairly good coverage to make a difference.

Imagine a single wire on the ground.  The main antenna induces a current in 
the wire, but that wire is very strongly coupled to the earth, so it's 
pretty lossy, overall.

The wires essentially have to "shield" the ground under the wires (just 
like the shield in coax).  I'm sure someone has analyzed the problem in 
excruciating detail, so a bit of research might find a good estimate of the 
performance.