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


Hi Mike.
Well I have used such an arrangement on 2 house roofs in my time.
The first on a crrugated asbestos fibre roof! Yes it was along time ago!
Fortunately it terminated a G5RV ant very well and I got DXCC and WAC
with 100 watts back in the 70's.
No problems with leaks though it was well braced.
Later I put a wooden cradle on the peak of a ceramic tile roof for the
centre of another G5RV (my favourite GP antenna) that was not in such a
good QTH but it worked fine for more than 20 years without damage to the
roof at all, perhaps the cradle (which I still have spread the stresses
well.
I have also put a steerable vertical array for 20 M on an "iron" roof
mounted on insulators and had no problems there either. Of course the
corrugated iron had to be carefully bonded together as it was the ground
plane with the verticals about 1/8 wave apart fed with tapped coax to
steer the beam around in about 60 degree steps.
So with a bit of care and common sense I think you will be OK. The only
kind of roof that would really bother me is a good old Pommy slate one.
They are both very fragile and extremely slippery when even slightly wet.
As to random wires, I have never been a fan of those since I got a nasty
RF burn about 48 years ago when I was only running 40 watts on 7 MHz.
I like an antenna with an impeadance a bit below infinity at the
entrance to the shack!
&#'s Cliff Wright ZL1BDA ex G3NIA.