Sometimes the "boys out west" get lucky. I recall operating at
K0RF's place in the early 80s in the ARRL CW contest when I worked a
number of both JAs and EUs at HIGH NOON LOCAL on 40M. 3L Yagi at 100ft
at the time.
Back in the late 60's or early 70's, I remember hearing W6HJN (nr San Diego)
working EU on 40 at 1400 or 1500 local. Of course he had the largest 40m Yagi
I've ever seen then or since - 5el on a 90' boom at 100'! The boom was two
back-to-back aluminum towers! My rig at the time was Heath Apache/S-85 to a
trap vertical!
73 Bill
N6CQ@paonline.com
>From David & Barbara Leeson <0005543629@mcimail.com> Wed Sep 20 20:45:00 1995
From: David & Barbara Leeson <0005543629@mcimail.com> (David & Barbara Leeson)
Subject: Rohn 25 capacity
Message-ID: <65950920194556/0005543629NA1EM@MCIMAIL.COM>
Stan, W7NI, asks if there is a standard modelling program for unguyed tower
strength.
For an analysis of unguyed tower, see Chapter 7 of Physical Design of Yagi
Antennas, pg. 7-18. The first reference for this chapter is the NCJ mast
article by Stan Griffiths, W7NI. Chapter 2 treats wind and ice loading
issues. The spreadsheets available with the book include a mast-strength
calculation that works for unguyed tower sections as well.
Rohn gives the following data for tower sections, with no indication of
safety factor:
Max bending moment Wind force/ft @ 50 lb/sq ft Weight/section
25G 5130 ft-lb 12.70 lb/ft 40 lb
45G 9610 14.63 70
55G 17160 16.72 95
With this information, plus the wind area of top-mounted antennas, mast and
rotator, and an estimate of the maximum wind speed and ice loading at your
site, you can calculate the strength of unsupported tower the same way as for
masts. You can start with the antenna area and calculate the maximum safe
windspeed for a given tower height, or you can estimate the highest expected
windspeed and calculate the allowable height for a given top loading.
Because of aerodynamic interference among the legs and cross braces, tower
sections have a surprisingly large equivalent wind area that must be included
in the calculation. Cables, rotators and ice can also add a significant area.
As Stan mentioned in his posting, there are a number of EIA and other
standards that relate wind force to mast and antenna area, including factors
treating wind statistics, height above ground, antenna streamlining shape,
general location and, in some cases, terrain roughness (for example,
buildings and trees), ground slope and ice loading. Standards include
EIA-222-C through EIA-222-E, the Uniform Building Code (UBC), the British
Standards Institution's CP3 and American Society of Civil Engineers Manual
74. They all include a formula that shows wind pressure or force is
proportional to the square of the windspeed.
Older Rohn catalogs include EIA's RS-222-C (now EIA-222-C), which includes a
map of general windspeed areas in the U.S. RS-222-C, which was in effect
when I wrote the Yagi book, makes simplifying assumptions about drag
coefficients and height over ground, and doesn't consider terrain variations.
EIA-222-D introduced a new map that is the same one that is in the current
UBC, and EIA-222-E updated the treatment of cylinders in yaw (that is, at an
angle to the wind) to reflect the concept that wind force on a cylinder
results primarily from the distribution of surface pressure, which is
perpendicular to the surface and hence to the axis of the cylinder.
EIA-222-E and the other more recent standards use drag coefficients
normalized to 2.0 for a large flat plate and 1.2 for a long cylider rather
than 1.0 (flat) and 2/3 (cylinder) as in RS-222-C; this has been a source of
some confusion. CP3 has an excellent treatment of terrain, which I
incorporated into the book, and the new ASCE 74 has the best overall
description of wind and wind forces.
All these standards are based on a basic windspeed that is the fastest wind
expected over a 50-year period in a general locale, measured with about a 30
to 50-second averaging time at 33 ft. height over flat, open ground (at an
airport, for example). Gusts are about 30% faster, but a large structure
doesn't respond fully to the shortest gusts. By the time you have accounted
for the different wind maps and factors of the various standards, you
generally calculate about the same force in the same locale no matter which
standard you use.
Aside from terrain factors, an issue of note is that the updated aerodynamic
modelling of cylinders in yaw in EIA-222-E and ASCE 74 yields a smaller
equivalent area for Yagi antennas than is typically published in
manufacturers' data, most of which is still based on RS-222-C. Typical
published antenna specifications estimate area by the RS-222-C method as 2/3
the square-root of the sum of the squares of the element and boom cross
section area; using 222-E you just take 0.6 times the larger of the element
or boom area. The more recent standards' treatment of cylinders in yaw also
reduces calculated requirements for boom strength, although both science and
direct experience suggest it's an oversimplification to ignore vertical
gusting, peak gust statistics and column loading of booms.
The biggest safety factor arises from the fact that houses are typically in
built-up areas, where the dissipation due to other structures and trees
results in a significantly lower windspeed. The development of wind-power
systems has resulted in very sophisticated terrain modelling approaches.
However, if you have surviving tall trees in your neighborhood and your
roofing doesn't blow off, you probably don't experience 70 mi/h winds. Also,
it appears that maximum ice loading and maximum windspeed do not occur
together with high probability. Any current newspaper will remind that there
are exceptions in hurricane, tornado and mountain wind areas.
As you can see, the deeper you dig the more complicated the wind-loading
problem gets, but accepted simplifying assumptions let you engineer a
free-standing tower or mast to survive in your area with a reasonable safety
factor. You can use the spreadsheets available with the Yagi book to make
this calculation, although for a ground-mounted tower you would need to
include the variation of windspeed with height. Tony, K1KP, has the right
idea; it's a good idea to get professional help in this area.
I ran Tony's final selection through the mast spreadsheet, updated to use
the 222-E definition of area (5.7 sq ft) but retaining the 222-C wind force
formula and drag coefficient of 2/3, ignored the ice and the coax and came up
with the following estimates for Rohn 25 unguyed above the bracket support:
Equivalent area, sq ft
Boom Elements Published
A4 1.8 5.7 6
# Sections Windspeed, mi/h
1 135
2 88
3 67
If you use the published equivalent of 6 sq ft, which I assume is a 222-C
definition (root of sum of squares), you get 86 mi/h, so the difference
between using 222-E and 222-C here is well below the level of other
uncertainties (1/4" vs. 1/2" ice, effects of other structures and terrain,
etc.).
You can also get a slightly different answer if you consider whether the
wind is perpendicular to the tower face or not, and of course this doesn't
include the area of the rotator, nor does it consider the decrease in antenna
area if you are lucky enough to have the wind across the boom rather than the
elements. If it were my installation, I would add another house bracket
halway up, but that's probably belt and suspenders. The usual disclaimers
apply; I don't have any control over your engineering ability, materials or
workmanship, so if it falls on you it's your fault, not mine.
73 de Dave, W6QHS
>From Robert Penneys <penneys@UDel.Edu> Wed Sep 20 23:59:23 1995
From: Robert Penneys <penneys@UDel.Edu> (Robert Penneys)
Subject: Team score SSB Sprint
Message-ID: <199509202259.SAA02300@strauss.udel.edu>
OK, guys, this is it... the scores so low we can't even be N.E.R.D.S.
The Back Order Boys (HRO DE startup team)
WN3K 14 x 9 = 126 Very low power, virtually no antenna, mike ng
N1IFL 0 x 0 = 0 Mike ng, maiden appearance, young and strong, watch out
This is a team with borrowed complicated radios, no manuals, 8 PM hunt
for adapters, hey, it's only a hobby...
IC-775 for John, 850 for me, next time 870, JST-245, something with a lotta
menus. Rules, no more than 10 min before contest plugging in radio and trying
to figure out how to disable tuner, find sub-menu, etc. Stay tuned, has
big ongoing potential. So far, nobody else will play. We'll show them.
We will submit all logs no matter how humiliating.
Bob
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