Richard Clark wrote:
On Thu, 3 Mar 2005 11:40:09 -0600, (Richard
Harrison) wrote:
Table 2 on page 20-3 of the 19th edition of the "ARRL Antenna Book"
recommends 8 pounds tension for #18AWG hard-drawn copper wire.
Hi Richard,
From "Reference Data for Radio Engineers," the breaking load for #18
Hard-Drawn Copper is 85 pounds. The weight for 88 feet of this wire
(at 4.9 pounds per 1000') would be 0.43 pounds.
Many issues are involved in tensioning. For calculations see Ed
Laport`s
"Radio Antenna Engineering" page 346.
Simple Trig would reveal that with an 88 foot catenary with a 5 foot
sag would develop roughly an angle of 6.5 degrees below the
horizontal. My earlier post employing arctan was in error. It is
the
sine of the depressed angle used as a divider into half the weight
(we
are presuming this is a symmetrical dipole) that describes the
tension. The sine of 6.5 degrees is 0.113. The tension in the wire
then becomes 0.215/0.113 pounds (1.9#).
I think that if you do a really rigorous job of it you'll discover that
a hyperbolic function sneaks into the mix.
Trying to flatten the catenary is where tension becomes increasingly
stressful. If you attempted to hold the catenary to within 1 foot of
flat, then this would be a depression of 1.3 degrees, resulting in a
divisor of 0.022. The tension in the wire then becomes 0.215/0.022
pounds (9.5#).
If we were to add in the weight of the transmission line drop, RG-58
comes in at roughly 0.029 pounds per foot or RG-11 as much as 0.096
pounds per foot.
Let's ballpark the last dipole with 1 foot sag to hold up 40 feet of
RG-11 (whatever the practicality, and irrespective of the center
being
held up, we are considering only a two point suspension). The added
weight of the cable comes to 3.84 pounds. The tension in the wire
then becomes 2.135/0.022 pounds (97#).
The wire snaps.
73's
Richard Clark, KB7QHC
w3rv