On Wed, 23 Aug 2006 11:17:37 -0700, Richard Clark
wrote:


A parabolic approximation is better than the triangular approximate,

More properly a "catenary" for sags which is a curve of constant
tension; and for our antenna use, this is a classic application. The
difference is slight in this regard as both involve hyperbolic
transcendentals, but the point of constant tension is more to be
noted. When we add the kicker of center feed weight, the curve is
obviously pulled out (which suggests linear analysis).

I think the case of a concentrated load such as a balun and feedline,
along with a distributed load is also exactly solved by a catenary,
just the ends are at unequal heights. The three dimensional nature of
the problem when horizontal wind forces are considered as well as
vertical weight forces makes solutions messy.


From my 1912 copy of "Standard Handbook for Electrical Engineers"
comes coverage of stringing power lines and calculating sag for a
given tension. Tables and calculations are not remarkably different
from my first approximation. This volume states "the working stress
should not be over one fourth this [ultimate breaking strength]."

Now that it is so easy to calculate the catenary, it is the way to
go.. but on shallow catenarys of equal height, the solution is not
much different to a parabolic one. There is still a computational
advantage to the parobolic approximation.

Standards here (aka "codes" in your country) stipulate the safety
factor to be used, and it is 3.5 for standing rigging, 5 for running
rigging. That does apply to guy wires, and seems appropriate to
antenna wires that may not also be guy wires.

I note that the ARRL takes a different approach. The ARRL Antenna
Handbook 18th edition has some information on sagging wire antenna
spans on pages 20-2ff. Similar information may be in other editions.

There is no explicit discussion of wind loading, and the design guides
(tables, nomographs, text) lead the reader to a design based on
weight loading alone and with a Safety Factor of 10 or 5 depending on
the chosen tension.

I guess that approach supports the maxim that "if your antenna didn't
blow down last season, it wasn't gib enough", or am I mythtaken!


Try using a rope runner.

Do you mean a rope as a carrier for the conductor... runs into some
other issues like differential stretch, and huge wind resistance.

True.

The
structure may stay in the sky, but the wire might be fractured anyway.

How do you come by the conclusion that the wire is destroyed, but the
rope remains?

If the rope stretches more easily than the metal conductor, it may
(depending on how it is supported) increase the tension in the metal
conductor and break it.

Owen
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