Topband: Zo of an individual CAT5 twisted pair

[Jim Brown]

On 8/12/2013 9:32 AM, James Rodenkirch wrote:
> Does it matter which twisted pair of wires I use?

The Zo of all pairs is the same, to manufacturing tolerances, but each 
pair has a different twist ratio (called the "lay"). This is done to 
minimize crosstalk between pairs. A higher twist ratio (more turns per 
inch) improves interference rejection of the line, so the one with the 
greatest number of turns per inch would be the one to use. I don't 
remember which one that is, but it's sort of obvious when you open up 
the cable.

BTW -- Belden CAT5/6/7 cables have pairs with each pair molded together, 
so they tend to be better transmission lines (more uniform over their 
length). Not a big deal for loss on 160M, but uniformity does minimize 
interference rejection.

73, Jim K9YC