[TowerTalk] Antenna wind load vs tower specs.

kstover at ac0h.net kstover at ac0h.net
Sun Apr 29 14:11:55 EDT 2018

I'm planning one now and am using the Projected Area number on the JK site.
JK Mid-Tri = 16sqft
JK 12/17 Jr =  7.8sqft (I use 8)
JK 402T = 9.3sqft ( I use 9.5)
20 foot out the top 3" mast 5 sqft
All that is 38.5 sqft and I round it up to 39.
Using the RevG numbers from the JK site gets me to 40 sqft of antennas plus
5 sqft of mast. So, 45 sqft total.
It's not going on a Rohn tower. I'm getting a AN Wireless HD70 which is
rated to carry up to 52.5  sqft at 100mph (RevG, Exposure Category B, soil
type clay).


-----Original Message-----
From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of Steve Walter
Sent: Sunday, April 29, 2018 7:37 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Antenna wind load vs tower specs.

I've 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.  What's 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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