AhSo! Unnerstud'!!
> Date: Mon, 12 Aug 2013 10:33:09 -0700 > From: [email protected] > To: [email protected] > 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 _________________ Topband Reflector
