Steve,
I don't think that there is anything that was meant to be contradictory in
those statements. "Flat" projected area was likely an emphasis added by
the author to distinguish it from the areas with a shape factor built-in.
However, Rohn differentiates "flat member antennas" from "round member
antennas" in their use of the projected area. EIA considers beam antennas
to be a "Discrete Appurtenance" (2.1.4.1 of 222-E) - an appurtenance whose
load can be assumed to be concentrated at a point. In 2.3.10.4 of 222-E,
the force coefficient (Ca) applied to the projected area (ft^2) of a
discrete appurtenance (Ac) shall be determined from Table 3. Table 3 shows
a coefficient of 2.0 for flat members (aspect ratio > 25) and a coefficient
of 1.2 for cylindrical members (aspect ratio > 25). The aspect ratio is
the length/width ratio in a plane normal to the wind direction. If you
multiply Rohn's number in the oval by 1.2, and the number in the rectangle
by 2.0, they are the same.
Our web site now has the projected area spec's for the previously listed
antennas on the HF-beam page and a separate page describing the new and old
spec's.
The new "Projected Area" spec's should not cause anyone to mistrust their
current tower/antenna system. However, if you are installing a new system,
I would definitely use the new spec's.
73 Roger WB0DGF
Telex Communications, Inc.
http://www.hy-gain.com/windarea.htm
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