More on transmission lines

PTFE has a more constant (!) dielectric constant over a reasonable frequency range (not that the 4 ohms delta of Kynar from 1MHz to 100MHz is a particularly significant amount), and apparently PTFE’s constant is about 2.1. Farnell sells wire which has a ‘Tefzel’ insulation which according to DuPont website is about 2.5 for around 1MHz-100MHz. An example is order code 1202477 which is 30AWG too.

Plugging the data into the spreadsheet gives the following curve:

impedance_vs_twists_tefzel.jpg

It can only be used between 5 and 14  twists per inch to stay within the 20-45 degree pitch angle range, so it is basically useful for 84-125 ohms characteristic impedance (recall that Kynar would have allowed lower impedances to be achieved).

Trying the exercise with 26AWG Tefzel wire (e.g. Farnell 1202499 ) gives a range of 68-100 ohms.

Earlier, it was mentioned that the impedance formula relies on ‘hard’ or ‘soft’ insulator determination. It makes quite a big difference in the impedance calculated. For example it could be 15 ohms difference or more, if the wrong type is used in the formula.

This web page seems quite good to list the dielectric constants of soft insulators.

This entry was posted in dielectric constant, relative permittivity, transmission lines. Bookmark the permalink.

Leave a Reply