Topband: Zo of an individual CAT5 twisted pair

[JC N4IS]

Yes, that's a complicated matter. The name and the function can get very
confused if you don't know what you are doing. Any transformer can change
the voltage from the primary to the secondary and the impedance follow the
square of turn ratio. How you connect the transformer is an application. How
you build the transformer is an art!

For broadband RX antennas you want the transformer to be broadband. For
isolation from the primary to the secondary you want low capacitance. An
autotransformer could be used as BALUN, balances input and unbalanced
output, it could be broadband, but has no isolation. 

One example, you take a FT140-77 core and build a primary 12 turns in one
side and 4 turns on the other side, you have a voltage  transformer but it
will perform very bad as a  BALUN, or a BALBAL or UNUN depending your
application. However if you build 3 times 4 turns for the primary and add 4
turns on secondary in between the primary, you can get the same voltage
transformer but It will work as a broadband impedance transformer from 1 MHz
to 10 MHz with no adding reactance if the load is a pure resistor or low
inductance resistor.

I did try to explain it with text, I used pictures, I posted diagrams but
people come back to me saying the antenna is not working. When I check what
the guy did, he was using the wrong "transformer". 

Jim I'm with you again, very few hams really understand it.

Regards
JCarlos
N4IS



-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Shoppa,
Tim
Sent: Tuesday, August 13, 2013 8:08 AM
To: 'jim at audiosystemsgroup.com'; 'topband at contesting.com'
Subject: Re: Topband: Zo of an individual CAT5 twisted pair

A transformer that is connected such that it is UNbalanced on one side and
BALanced on the other, and connected that way on purpose, is not a balun?

Tim N3QE

----- Original Message -----
From: Jim Brown [mailto:jim at audiosystemsgroup.com]
Sent: Tuesday, August 13, 2013 03:16 AM
To: topband at contesting.com <topband at contesting.com>
Subject: Re: Topband: Zo of an individual CAT5 twisted pair

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
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Topband Reflector
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Topband Reflector