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" 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

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.

cliff wright wrote:
I like an antenna with an impeadance a bit below infinity at the
entrance to the shack!

1/4 wavelength away from that very high impedance
is a very low impedance.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
cliff wright wrote:

I like an antenna with an impeadance a bit below infinity at the
entrance to the shack!


1/4 wavelength away from that very high impedance
is a very low impedance.
Ah yes indeed Cecil. BUT what is the impeadance at the ATU at the
frequency you are using, for a given length of random wire?
A lot of house lots or "sections" (as we call them here in NZ) these
days tend to give you about 60 Ft/20 metres in the clear and that gives
you a Really high impeadance when end fed on 7 Mhz!!!
A good old centre fed ladder feeder is so much more controllable and
less lossy, that's why I'm a Z match fan too.
ZL1BDA

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

"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