Co-ax V 300 ohm flat twin.

[ppppenguin]

As the impedance is related to the log of the ratio of conductor diameter and spacing multiplied by a figure derived from the relative permittivity of the insulating material, varying any one of the three parameters can have quite an effect on the result.

The log function reduces the effect of size and separation. That's why co-ax is only available over a fairly narrow range of impedances. Much below 50R and the inner gets very close to the outer. Much above 90R and the inner gets very thin.

Twisted pair seems to end up in the 100R to 120R range unless you explicitly do something very different. I think Cat5 etc is 110R, which is also the impedance specified for AES digital audio. I wonder why