Topband: CAT5 for Flag Antenna Feedline ?

[Jim Brown]

On 9/8/2013 10:30 PM, Bob K6UJ wrote:
> I got the impression from the post
> that CAT5 is immune to common mode noise.

I've not heard that, nor can I think of a theoretical basis for saying 
so. CAT5 is very good twisted pair, with a high twist ratio, Zo = 100 
ohms, and its loss is low enough to make it work fine on 160M.  The 
primary advantage of twisted pair is that it rejects DIFFERENTIAL mode 
noise. Rejection is greatest when both ends of the line are balanced.

One possible source of your confusion may be reading, but not fully 
understanding, my work, and Neil Muncy's work, on SCIN, whereby certain 
deficiencies in the construction of a cable shield convert common mode 
current to a differential voltage on the signal pair. I have on several 
occasions observed that high quality unshielded twisted pair, like 
CAT5/6/7, would have much better noise rejection than that sort of 
cable.  Read about it in several tech papers and tutorials on my 
website. No need to burden the list with it.

73, Jim K9YC