First of all, I'm a relatively new subscriber to the reflector and wish to
thank all of you the for insights I'm gaining in monitoring the messages. I'm
now just beginning to plan a future antenna installation and I'm finding the
information incredibly valuable.
It looks like Rohn guyed towers (55G) are going to make the most sense for what
I want to do; however, I'm now confused about what Rohn is trying to specify in
their catalog and how it relates to various revisions of the EIA-222
specification. I understand the concept of sticking with a particular revision
of the standard (including its wind speed chart) to analyze a particular
alternative; I'm just not sure what Rohn is specifying.
I'm reviewing the specifications of Rohn 55G guyed towers in the 2004 Revised
Rohn Catalog. There are drawings showing reference guying plans for various
heights at 70mph, 90mph, and 110mph Basic Wind Speed. The drawing notes
indicate: "Tower designs are in accordance with approved National Standard
ANSI/EIA-222-E-1991 (No Ice)." and "Equivalent flat-plate antenna areas, based
on EIA RS-222-C."
So, my questions are:
* Are these windspeeds 70, 90, and 110, equivalent to the "fastest-mile" speeds
for 222-E/F? (This is a question in my mind as, even though 222-C as well as
222-E/F are based upon "fastest mile" speeds, the windspeed specifications for
most regions, for some reason, apparently changed very significantly in the
downward direction between revisions of the specification. For example, in the
222-C spec, Zone A equates to about 86.6mph using the pressure formula in the
spec. compared to 70mph or 80mph for the same regions in the -E/F specs.)
* How do the flat-member antenna square-foot ratings and the round-member
antenna square-foot Rohn ratings relate to the equivalent areas calculated
using the methods of 222-E/F? (For a given projected area, the calculations
yield much larger numbers for -E/F than for -C.) The REAL QUESTION is whether
the tower will stand up to calculations per 222-C at the rated windspeed, OR,
whether calculations should be per 222-E/F at the rated windspeed?
A couple final questions:
* Is there any published or readily available data on how antenna capacity
varies with windspeed for these reference designs? For example, a 140' 55G
tower/guy design for 110mph has a flat-plate capacity of 5.5 ft^2. The same
design should have significantly more capacity at, say, 90mph. But, how much?
(Just as an exercise, I tried scaling based upon the change in windspeed^2;
but, this clearly produces very conservative results.)
* Is there anything available (free) on the internet showing the calculation
methods specified in the -G spec? I've seen discussion of "3-sec gust" versus
"fastest mile"; but, haven't seen anything on the prescribed calculation
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