> I have seen more than one reference to adding solid plates or disks
> under verticals. ON4UN's Low Band book states "a solid metal plate of
> reasonable size right under antenna (vertical) can result in notable
> decrease in ground resistance, and consequently in ground loss".
>
> So - what constitutes a "reasonable size" plate/disk to have this
> notable decrease in ground resistance and loss? 1 foot? 3 feet? 10
> feet?
The comprehensive RCA funded study's conclusion was that adding a
screen does nothing for efficiency if the installation has even a
modest radial ground system. You can find measured comparisons in the
famous study "Ground Systems as a Factor in Efficiency", but the
conclusions were that a screen or radial system has to be at least
1/8th wl or so in radius to become reasonably efficient, and small
screens don't do much either by themselves or when used in
conjunction with even a modest radial system.
The idea that a few wide strips used for radials are better than even
very fine wires covering the same footprint of area is not correct.
The distance out is important, and the distance between wires is
important. The individual conductor thickness or surface area is
almost entirely meaningless once the radial becomes so thick or wide
it is mechanically reliable. Other then life of the system, you'd
never know the difference between 50 #24 gauge copper wires or 50 six-
inch-wide strips on 160 meters...as long as they were the same
length.
I wouldn't think twice about using #20 or smaller wire, if it was a
temporary system. I would never even consider using anything larger
than #10 wire for myself, let alone strap or flashing for radials,
unless it was somehow cheaper.
Broadcast stations almost always use ground screens near towers, but
the primary reason is for physical installation reasons rather than
actual efficiency. It is easier to work around the antenna and to
ground all the equipment near the towers if a screen is used near the
towers.
It would take a huge screen (certainly over 1/8th wl in radius) to
equal or improve on a system with thirty 1/4 wl radials. Use a screen
if it is easier to install or cheaper, not to make things "better" or
to replace or augment a reasonable size radial system.
73, Tom W8JI
W8JI@contesting.com
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