On Tue,3/1/2016 7:29 PM, Brad Rehm wrote:
You certainly can use the galvanized roof as a ground screen.
Certainly can is FAR too optimistic a statement. Poor connections
between segments of the roof will do more than cause SWR shifts, they
will create intermod, which could make you unpopular in nearby places
like FL. Also, the roof, 16 ft above ground, shortens the radiator by
that amount.
It might be
a good idea to measure resistance between sheets to get an idea of how well
they're all tied together electrically. If they're not, you might see some
odd shifts in VSWR when you send RF through it. Another potential problem
is that RF may be coupled into the building wiring when you transmit.
That's likely to happen with ANY vertical that is next to the building,
but the problem will be minimized if we give the antenna a good radial
system, ground screen, or counterpoise.
This is why you'll want to bring grounds up to the roof in as many places as you
can.
Gee, misconceptions keeps getting perpetuated! A connection to the
EARTH does NOT make an antenna work better. We use radials, ground
screens, and counterpoises to SHIELD the antenna and it's field from the
earth, NOT to make a connection to it. We want the antenna field and
drive current to return to the radials or ground screen or counterpoise,
not the lossy earth. The ONLY reason for an earth connection is
LIGHTNING protection (and that's a very good reason).
Almost 40 years ago, Rob Sherwood showed that galvanized screen (like
fencing) can be an effective ground screen when the space for radials is
very limited. I described this in considerable detail in this slide
presentation. I suggest that you begin by studying the entire slide
show. The ground screen discussion begins with slide #55.
http://k9yc.com/160MPacificon.pdf
73, Jim K9YC
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