K9YC:
> It has long been well understood that return current is greatest
near the base of the antenna, so copper near the tower is more
important than copper at a distance in reducing I squared R losses.
This is why AM broadcast stations often use a heavy copper screen
that looks like monster chicken wire (40 ft x 40 ft is a number that
comes to mind) in the immediate area of the tower, with radials
terminated to the perimeter of the mesh.
I believe this might be another broadcast myth (like
supposedly needing 120 1/4 wl radials) and which is NOT mandated by
the FCC. About 10 years ago I decided to make a ground screen for my
180' Topband vertical using ~16 50' X 4' rolls of welded galvanized
fence wire (with rectangular openings about 2" x 4"...not the smaller
gauge woven hexagonal chicken wire). I couldn't tell any significant
difference in performance with this laid over my existing radial
field (about 110 radials of 100150') although I didn't actually make
before and after measurements.
I later read an edition of ON4UN's book which indicated you
could achieve a nearly perfect nearfield ground using 1/4 wl radials
spaced ~.025 wl apart at the tips. Doing the math for 160 this
implies ~13 feet apart at the tips. No matter how long your radials
(even if differing lengths), you simply maintain the 13' spacing at
their tips for an optimum ground. For 1/4 wl radials, this implies
60 (radial circumference / .025 wl spacing = 60 radials). For longer
radials, it implies more.
If you accept this logic (which I do because several
including W8JI, N6LF and others point to ~60 as an optimal number of
1/4 wl radials), then there's little point in a ground screen near
the antenna. Assuming the approximate dimensions of a ~40' square
ground screen (I'll assume a circular radius of 20'), your 60 radials
above are only 2' apart (0.004 wl) at 20' out, so the improvement
from a ground screen should be minimal. Of course the radial spacing
becomes even closer as you near the base.
My galvanized fence wire has long since rusted out and quite
frankly I now wish I had never put it down because it's now a hazard
to trip over. If anyone is serious about ground screens I believe
you are well advised to use real copper which will last forever (but
I hate to think what that might cost). IMHO you are better advised
to forget any sort of ground screen and spend your money on at least
60 1/4 wl copper radials which will last forever.
73, Bill W4ZV
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