[TowerTalk] Antenna wind load vs tower specs.

jimlux jimlux at earthlink.net
Sun Apr 29 09:56:29 EDT 2018

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


> 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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