Mike:

I think you are dealing at a much more sophisticated level than I am. I
guess the wind loading was not my primary concern, so much as the weight
of the wire, and what size lag screw to use going into a 2X4 truss. I'm
a software guy, not a mechanical guy, and I know just enough to worry
that I don't know what I am doing! I see little tips on the 'net like
"...using a chimney mount subjects the antenna to products of combustion
and shortens antenna life..." which sounds quite reasonable when I read
it, but had not occurred to me before. An example of my concerns: I
have a lot of trees. Say I hang a long random wire, perhaps 100' of #14
copperclad steel. Say we have a wind storm-the loading on the wire from
the wind doesn't concern me as much as a large branch breaking and
hitting this pretty strong(?) steel wire, and... So do I arrange for
some sort of mechanical equivalent of a fuse that will break and drop
the wire before it tears the tripod (and a piece of roof) off? What are
the mechanical considerations? If someone hung a wire antenna, and gave
no thought to static charge/lightning concerns, I would think them
negligant. What are the equivalent mechanical concerns? I am not
finding much about this in the ARRL Antenna book. I'm sure you guys are
commonly thinking of a lot of "common sense" stuff when you are on the
roof that I am not even aware of.



Mike Kaliski wrote:
"skippy" wrote in message
...
I would like to experiment more with antennas, but am concerned about
roof issues (creating leaks) and mechanical issues like how do I
calculate the side loading for a 50 ft. random wire running from 1
5-foot tripod on my roof.

I googled around, and looked at some antenna books at the public lib,
but have has little success. Any suggestions? I looked at FAQ's, but
haven't found much.

Thanks.

Skippy

Wind loading is unlikely to be a factor with random wire systems. It is
mainly a factor when considering antenna towers fitted with an array at the
top. Wind load increases by a factor of four every time the wind speed
doubles. So if you had a tower and antenna array with a side on area of say
5 square metres and a wind load of say 20 Kg force with a 20mph wind, that
sideways force would be 80 Kg with 40 mph wind and 320 Kg with an 80 mph
wind.

320Kg of sideways force applied along the length on an antenna tower would
be enough to start to shift even very large chunks of concrete used in a
base.

Unless you have a very weak roof structure, the load imposed by a wire on a
5 foot pole is unlikely to be an issue.

Searches of the Internet show hundreds of different formulae available to
calculate windloads, however most of them appear to relate to building
regulations and the effect of wind loading on walls, roofs and other
structures. The reason you cannot find a definitive answer is that the
presure exerted by the wind depends not only on it's speed, but also the
temperature, cold air is denser than warm air and applies a greater sideways
presure to any structure in the way.

From the RSGB Radio Communication Handbook 6th Edition typical wind drag at
100mph is 5.81 pounds per foot length of 2 inch tubing, 0.17 pounds per foot
length of 14 swg wire, 135 pounds for a typical three element 14MHz yagi
antenna and 260lb for a typical cubical quad antenna.

Using a spreadsheet, these figures seem to translate into a wind loading of
approximately 35 pounds per square foot at 100 mph or approximately 170 Kg
per square metre at 160 kph if you prefer metric measures.

So very approximately, sideways wind load works out to the following:-

Windspeed Wind Load per Square Foot Wind Load per Square Metre
12.5 mph 1/2 pound
2.6 Kg
25 mph 2 pounds
11 Kg
50 mph 9 pounds
43 Kg
100 mph 35 pounds
170 Kg

These are very approximate figures and are only suitable for a ball park
estimate of wind loading on your antenna systems. When calculating the
strength of support guys needed to resist wind pressure add a safety factor
of six or more.

Mike G0ULI