At 09:32 AM 3/20/2007, Red wrote:
>Dear TTs;
>
>I'm sure there is a limit to how much coax may hang from one point on
>the coax, before the weight or wind overstresses the coax and either
>breaks it or alters its characteristics. I expect the limit varies with
>the type of coax and with its manufacturer.
>
>Does anyone have any data regarding such limits?
Found some data in a Times Microwave catalog. LMR400 (and Commscope
WBC-400 is similar) is rated with a tensile strength of 160 lbs, and
weighs 0.068 lb/ft. RG-213 style coaxes run about 0.11 lb/ft, and
the strength is probably about the same.
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|