Reg Edwards wrote:
Does all the above apply to the diameter of a pop bottle?

Yes 4" dia. and 2 turns per inch using RG-213.

And what is the diameter over the plastic jacket of the coax?

0.405" for RG-213, approximately 1/2".

The length of the coil will be N times the jacket diameter, assuming
the N turns are close-spaced.

Yes, that's why there are 2 turns per inch for RG-213.

Don't forget, mean coil diameter will be pop bottle diameter plus coax
jacket diameter. It does make a difference.

Maybe a 10-20% difference. Doesn't make much difference to a
rule of thumb. :-)
--
73, Cecil http://www.qsl.net/w5dxp