Topband: Zo of an individual CAT5 twisted pair

[Jim Brown]

On 8/12/2013 2:10 PM, JC N4IS wrote:
> 50/75 BALUN

Thanks for the detailed post, Carlos. BUT -- please let's use the right 
words to describe things so that people understand what you're 
describing and how it works. I strongly suspect that at least some of 
those things you are calling a "balun" are really a simple transformer 
-- that is, a primary and a secondary with magnetic coupling between 
them, and probably on a ferrite or powdered iron core. If it's a 
transformer, let's call it a transformer. Likewise, if we have a common 
mode choke formed by winding a coil of the transmission line, it is a 
common mode choke, not a "balun."  Using the word "balun" confuses 
things, because that word is used to describe at least a dozen very 
different things that I know of.

When we use the word "balun," it's a magic box that few hams really 
understand. When we use the right word, most hams have a chance of 
understanding what it does in a circuit. :)

Yes, there are arrays of common mode chokes that can be used to 
transform impedance, and there are transmission line transformers of 
various sorts that can do that as well.

BTW -- your discussion of phasing between elements of an RX array causes 
me to add an important post script to my advice that a perfect match is 
not required. When ANY passive network is used to produce phase shift, 
the source and termination impedances DO matter. The tricky part, 
though, is knowing what the input Z of the RX is, and if you're doing 
something like a phased array using phasing lines that end at the RX 
input, it might be a good idea to actually measure input Z and the 
antenna Zs with a VNA.

73, Jim K9YC