Topband: Zo of an individual CAT5 twisted pair

[James Rodenkirch]

AhSo!  Unnerstud'!!

> Date: Mon, 12 Aug 2013 10:33:09 -0700
> From: jim at audiosystemsgroup.com
> To: topband at contesting.com
> Subject: Re: Topband: Zo of an individual CAT5 twisted pair
> 
> 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
> _________________
> Topband Reflector