Many thanks, Jim;
Your calculations and experience appear quite conclusive. We don't need
to worry too much about the weight of common coax overloading the
tensile strength when it is supported at a single point, but we need to
prevent the increased tensile load due to wind from exceeding its
strength by tying it to a structure to support the horizontal load.
That may require a clever solution on telescoping structures. It's been
many years since I used a telescoping tower. If I remember correctly
(and I'm not sure of that!), I suspended the coax inside the sections
and carefully handled it to make sure it wasn't damaged while lowering
or raising or lowering the tower. I'm sure that coax was subject to
flapping in the wind, within the confines of the tower. I used it for
only a couple years before replacing it with a freestanding tower with
all coax and control cables secured to a tower leg at short intervals to
prevent flapping.
73 de Red
Jim Lux wrote:
> So, 100 ft of RG213 will weigh 11 lbs, which is a lot less than the
> breaking strength of 160 lbs. Granted, you probably want to stay away
> from the absolute limit.
>
> Wind loads might well be a bigger problem. A 1/2" diameter pipe 100
> ft long could have quite a bit of wind drag. In fact, this is the
> real limiter, I think. A quick back of the envelope shows that the
> wind load on a 1/2" diameter tube at 80 mi/hr is about 0.7 lb/ft. The
> analysis goes just like a catenary except the wind load is used
> instead of gravity. Say you've got a 100 ft run, and you allow it to
> move a foot (like sagging 1ft in a 100ft span).. The tension in the
> coax would be 870 lb. If you allow 2 meters (6 ft) of displacement,
> the load goes down to about 70lbs, still high. And, in that situation,
> the cable would probably flap around.
>
> Moral of the story.. the weight of the cable isn't a big deal, wind
> loads on the coax is. So you need a fairly reasonable grip to hold it
> at top, but you need to tie the cable to the tower to take the lateral
> loads.
>
> And, now actually having run some numbers, it seems to match practical
> experience at field day. The disasters with coax are almost always
> wind induced.
>
>
> Jim, W6RMK
>
>
>
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