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terrykc · 6th Oct 2018, 3:14 pm

Provided that you know the relative permeability of the dielectric (εr), you can calculate the characteristic impedance (Zo) by measuring the centre conductor diameter (d) and the internal diameter of the screen (D).

The formula is: Zo = (138 x log(D/d))/√εr

For a foamed polyethylene or semi air-spaced dielectric, I doubt the relative permeability will be much above that for air which, of course, is 1 but it might be interesting to try the formula out with some known cable samples to find out.

Getting an accurate internal diameter for a braided outer will, of course, be impossible but it will be the same as the outside diameter of the dielectric.

6th Oct 2018, 3:14 pm	#4
terrykc Rest in Peace Join Date: Nov 2010 Location: North Hykeham, Lincolnshire, UK. Posts: 515	Re: Determining Zo for coax cable Provided that you know the relative permeability of the dielectric (εr), you can calculate the characteristic impedance (Zo) by measuring the centre conductor diameter (d) and the internal diameter of the screen (D). The formula is: Zo = (138 x log(D/d))/√εr For a foamed polyethylene or semi air-spaced dielectric, I doubt the relative permeability will be much above that for air which, of course, is 1 but it might be interesting to try the formula out with some known cable samples to find out. Getting an accurate internal diameter for a braided outer will, of course, be impossible but it will be the same as the outside diameter of the dielectric.