Topband: Zo of an individual CAT5 twisted pair

[Jim Brown]

On 8/12/2013 9:46 AM, James Rodenkirch wrote:
> Well, Jim, here is what N4IS stated (bolded words my emphasis) - looks like he found the twisted pair to reduce the noise
Yes, but there's a bit more to it. Twisted pair, by its nature, 
minimizes noise pickup.  That rejection is maximized if the terminations 
at both ends of the line are balanced -- that is, each side of the line 
has equal impedance to ground -- and if there is minimal common mode 
coupling at each end. This would be achieved by connecting the twisted 
pair to the low-Z side of the RX antenna transformer, and to a 1:1 
transformer in the shack. Both transformers should be wound to minimize 
capacitive coupling between windings, which is what mimizes the common 
mode coupling (by maximizing the common mode Z).  That's the same thing 
we're doing when we wind a lot of turns of any cable through a ferrite 
core to form a high-Z common mode choke.

On 8/12/2013 9:38 AM, Jim Garland wrote:
> I believe that if you parallel two twisted pairs of a CAT5 cable, you'll
> have a 50 ohm transmission line.(Each twisted pair is 100 ohms).  I've tried
> this with CAT5e cable and find it works well, even for transmitting, up to
> about 100W.
Yes. But to clarify (to others, not you) we must be placing two 
transmission lines in parallel by using the solid color conductors as 
one side of the line, and the corresponding "color/white" conductors as 
the other side of the line. If we did the opposite -- use both sides of 
a pair as one side of the line and both side of another pair as the 
other side, we would have a parallel wire line of unknown Zo and with 
lousy noise rejection properties.

As to power handling and loss -- at these frequencies, its all copper 
loss, and CAT5 is pretty small cable. We don't care about modest losses 
in RX antennas, but it's a different story for TX.

73, Jim K9YC