"skippy" wrote in message
. ..
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:
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 Sq Foot Wind Load per Sq 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


Skippy

The wind loading IS the primary concern with vertical antenna systems
involving masts and tower supports. The sideways forces exerted by a long
wire antenna are going to be in the order of a few pounds provided you don't
try to tighten up the antenna wire so that it is absolutely rigid.

For a 50ft wire a sag in the middle of a couple or three feet should be
enough to keep the tension low. Add an extra couple of feet if you are
fastening the far end to a tree or something that might move a bit in high
winds. You can use a couple of shackles at one end of the wire to insert a
weak link. The shackles are fastened to the wire about 2 feet apart. A one
foot link or thinner wire is then inserted between the shackles. If the
antenna is struck by a branch, the weak link breaks and allows the antenna
some extra slack to move out the way. Hopefully this is enough to allow the
antenna to remain operational until you can replace the weak link.

A bracket secured into a 2"x4" truss with four 1-1/2" or 2" long wood screws
should provide all the anchorage you will need. Use 2-1/2" long screws if
going into the 4" thickness of the truss. If you decide to bolt completely
through the truss, make sure the diameter of the holes will not weaken the
wooden truss. Fit a couple of extra lengths of wood either side of the truss
to help spread the load and reinforce the area where the holes have been
drilled.

Feed the wire into the house across the top of an old spark plug and earth
the body of the spark plug to ground with a thickish cable and an earth rod.
Stick the spark plug in a gash plastic box to keep the dirt out of the spark
gap. You can use one of the commercial neon tube lightning protectors if you
wish. If you have a 1 megohm resistor lying about, connecting this between
the antenna and earth will hardly affect receiver (or transmitter)
performance but will help to earth static build up from the antenna.
Obviously, the antenna should be disconnected from the receiver during
storms but the spark plug and resistor method offer some protection from
nearby lightning strikes. For a receiver only system, a couple of silicon
diodes connected back to back across the antenna input provide a great deal
of protection for the receiver front end by effectively shorting out any
voltage over 0.6 of a volt or so.

If you are using a rope and pulley method for hauling up either end of the
antenna, it is likely that the rope will break before the main antenna wire,
so you might want to make sure that the rope can be replaced easily if it
breaks. For example, fit a second lightweight parallel cord through the
pulley, so it can be used to pull up a new support rope without resorting to
getting the ladder out.

The ideal is to arrange the antenna so that easily fixed bits break before
the hard to get at pieces. Hence the 'weak' wire link, a backup system to
put new bits of rope up in the air and possibly some extra bracing around
where the support bracket is fixed to the roof if you are concerned about
stuctural integrity in this area.

It really is just common sense and you don't need an engineering degree to
erect a really robust antenna system. Now GPS - that really is rocket
science :-)

Regards

Mike G0ULI