On 4/29/18 5:36 AM, Steve Walter wrote:
Ive been casually planning my first tower for years and only recently
discovered that Rohn guyed towers use EPA (effective projected area)
spec and most antenna manufacturers use something else. Whats that
all about?
EPA is really "equivalent flat plate area" because it takes the
projected area (what you'd see as a shadow) and multiplies it by the
drag coefficient of 1.2 (flat plate), 0.85 (sphere), angled body (1.0)
etc (there's been some fiddling with the coeffcients over the years, and
the length/width ratio also affects it)
then you can plug the EPA into the standard drag formula without needing
to worry about Cd
drag(lb) = EPA* v(mi/hr)^2/400
is an easy approximation
http://lists.contesting.com/_towertalk/1999-10/msg00447.html
I naively assumed that EPA and simple square footage antenna specs
were the same. I have not looked at all manufacturers but what clued
me in was JK Antennas listing three different windload numbers for
their antennas.
I sort of understand the flat plate calculation and the .67 multiplier
for round elements but where does the 1.5 multiplier for EPA come
from?
Something that has significantly higher drag than you'd expect from just
the projected area. Round tubes and wires are a good example where the
drag coefficient varies substantially with the diameter (and with wind
speed too). The drag of those wires on the Wright Flyer is orders of
magnitude higher than the drag of the single streamlined strut on a
modern small plane, even though the projected area of the wires is
smaller than the projected area of the strut.
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