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?
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?
In my case an 80' Rohn 45 has an EPA of 25.9 sq ft (90 mph Rev G/ 70
mph Rev F) and the JK Big Tri has an EPA of 22.2 sq ft. I had been
using the Rev C 14.8 spec for the Big Tri which is in the same
ballpark as other similar antennas as the C31XR and OB16-3 (is the
square boom calculation done here?)....but now I see that is wrong
when selecting a Rohn guyed tower to match...Am I correct?
Other questions: The Rohn catalog shows the top guys on an 80' 45G at
65'. Does this mean that if the guys were at the top the wind load
would be 31.9? 80/65 x 25.9.
Is it correct to assume that a second Big Tri at 40' on my 45G would
impart an EPA at the top of the tower of 11.1 sq ft? 40/80 x 22.2.
Theres obviously a wide safety margin built in thats keeping a lot of
overloaded towers up in the air.
Am I over-thinking this?
Steve WA3A
--
Steve Walter
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