[TowerTalk] cable on a reel using a formula?

[Jim Jarvis]

In the amount of time it has taken to discuss this thread,
you could have pulled off the cable, measured it using the
'arms-length' method, and re-spooled it, 5 times over.
Unless it's really heavy, of course.

There are three approaches (and a punchline at the end):

1)  You COULD calculate the volume of the entire spool,
and subtract from that the volume of the central core, 
and divide the result by the cable length per unit volume.
But then, you'd have to pull off some cable to determine
the length v. volume.   And, you'd still have error due to
the dead space between cable wraps.

2)  If you knew the weight of the bare spool, you COULD 
weigh the full spool, subtract the two, then weigh a length
of cable, and divide.

3)  Or, you can count the number of turns along the long 
axis of the spool, and the number of turns deep on the spool.
The formula then becomes:

(Core Diameter x Pi x # turns in a layer)+
((Core Diameter + cable diameter)x Pi x # turns/layer)+
iterate this the # of layers, incrementing by cable diameter
each time.

You can rough this out by rounding Pi to 3, and core diameter
to the nearest inch.  An example for half inch coax would be:

Dcore = 4"
Pi= 3
turns = 10
layers = 5
Dcable = .5

(4 x 3 x 10)+ 4.5 x 30 + 5 x 30 + 5.5 x 30 + 6 x 30  

or,

120+135+150+165+180 = 751 inches
751/12 = 62.5'

-0-

Which method you use really depends on what information you
have at hand, what accuracy you require, whether you can actually
physically handle the cable, and how much time you have at your
disposal.

-0-

For science-nerds, the punchline is, "Assuming a chicken to be a
perfect sphere..."   

n2ea
jimjarvis at ieee.org