Reg Edwards wrote:
Suppose you havn't got a pop bottle but only a length of plastic drain
pipe of different diameter.

Then I use my EXCEL spreadsheet which allows variation
in the number of turns per inch and diameter of the coil.
If you would like a copy, it can be downloaded from

http://www.qsl.net/w5dxp/Corum1.xls

You will need a different rule of thumb for every contingency.

2 liter pop bottles are in abundance in the US. If a coil
form is used for a coax choke, that's the most likely.
--
73, Cecil http://www.qsl.net/w5dxp