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40 meter eu (and ja !

Subject: 40 meter eu (and ja !
From: bill.lumnitzer@paonline.com (bill.lumnitzer@paonline.com)
Date: Wed Sep 20 16:22:06 1995

     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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