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#6
March 4th 05, 03:04 AM
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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 |
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