Thanks Kurt for the very interesting history of
how antenna square footages came to be. If we walk
through TIA-222-G it will illustrate Kurt's point that
just giving a square footage isn't that helpful:
====
Force on an antenna element is worst case of:
F = 1.6W + 1.2D (lbs) (no ice)
F = 1.2D + 1.0Di + 1.0Wi (with ice)
F = 1.2D + 1.0E
W is the wind force, D is dead load (weight)
Wi is wind force with ice present
Di is weight of ice
E is seismic loading
To pass 222-G, the antenna must barely survive (at
the yield point) when force F is applied.
For now let's ignore weight, ice and seismic, so
the worst case is the first one, F = 1.6W.
W = qz Gh (EPA)
qz is the wind velocity factor (varies as a
function of height, terrain, etc.)
qz = 0.00256 Kz Kzt Kd V^2 I (lb/ft2 )
I = importance factor (risk category, usually 0.85
for ham towers)
Kzt = terrain factor
Kz = velocity pressure coefficient (varies based
on height above grade and exposure)
Kd = wind direction probability factor
V = design wind speed (50 year storm)
Gh is a "gust factor"
EPA is the Effective Projected Area
(EPA) = Ka[(EPA)N cos2(θ) + (EPA)T sin2(θ)]
Ka = 1 for round members.
(EPA)N = EPA of the windward face = Ca * Aa
(EPA)T = EPA of the side face = Ca * Aa
Aa = projected area of either the windward or
side face
Ca = force coefficient
(the side face of an antenna element is the tip
and can be ignored, IMHO, unless it's very stubby)
Ca depends on a laminar flow test using C:
C = (I Kzt Kz)^0.5 (V)(D)
D = diameter of the element
For aspect ratio (length / diameter) >= 25 ,
If C < 32, Ca = 1.2 when airflow is subcritical
If C > 64, Ca = 0.6 when airflow is supercritical
There's a transition equation to use if 32<C<64 that
ramps Ca from 1.2 down to 0.6
====
If you've managed to wade through all of this the
main takeaway is that both the drag coefficient and
the wind force are dependent on local site conditions,
which drive the Kz, Kzt, qz and V factors.
Note the drag coefficient drops in high wind speed
situations, like ridgelines and hilltops. But the high
Kzt and Kz factors that drop the drag also drive the
wind force qz up and will generally swamp out whatever
benefit the lower drag coefficient gives.
For an antenna if we know what Aa is and all of the
design assumptions, then that square footage would be
useful and fully 222-G compliant. One manufacturer who
"gets it" is AN Tower. If you look at their design
loading tables it gives the allowable Aa for given
wind speeds, terrains and exposures.
73 Jim K6OK
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