Balun vs reactance
 

Hi to all..

How does a 4:1 or 1:1 balun affects reactance? For example, what would
be the new value Z=a +/-Jb of, say, Z=3000 - J4500 Ohms after
transformation by the 4:1 or 1:1 balun ?

Thanks de Pierre VE2PID

Balun vs reactance
 

ve2pid wrote:
How does a 4:1 or 1:1 balun affects reactance? For example, what would
be the new value Z=a +/-Jb of, say, Z=3000 - J4500 Ohms after
transformation by the 4:1 or 1:1 balun ?

Depends upon the design of the balun and needs to be
measured under operating conditions. That's one reason
a lot of us prefer 1:1 chokes. *If* the choking impedance
is high enough, it will perform the balun function.

What I do with impedances like 3000-j4500, with a 16.4:1
SWR on the 600 ohm open-wire line, is to extend the
feedline until that impedance is transformed to
600/16.4 = 36.6+j0 ohms and install my 1:1 choke at
that current maximum point. Here's how I do that.

http://www.w5dxp.com/notuner.htm
--
73, Cecil http://www.w5dxp.com

Balun vs reactance
 

ve2pid wrote:
Hi to all..

How does a 4:1 or 1:1 balun affects reactance? For example, what would
be the new value Z=a +/-Jb of, say, Z=3000 - J4500 Ohms after
transformation by the 4:1 or 1:1 balun ?

Thanks de Pierre VE2PID

The impedance that your transmitter or matching network has to deal with
is the differential impedance, that is, the impedance between the two
conductors. A properly designed, constructed, and applied 1:1 balun will
effect no transformation at all of the differential impedance. A 4:1
balun will make it either four times greater or four times less. Both
types of baluns, again if properly designed, constructed, and applied,
will greatly increase the common mode impedance, that is, the impedance
you'd see if you connected the two feedline conductors together and
measured the impedance to ground.

In reality, any transformer will usually behave reasonably well only
when terminated in an impedance near its design impedance. For example,
a 4:1 balun or transformer designed to transform 200 to 50 ohms will
work well only if the high-Z end is connected to around 200 + j0 ohms.
(Such a transformer might have a high-Z winding impedance of 1000 - 2000
ohms, and it might well be resistive as well as reactive.) If you get
very far from 200 + j0, you'll find that the transformation ratio
changes and the transformer adds reactance and sometimes resistance. The
error can get dramatic with even moderate excursions from the design Z,
depending on the transformer design. It would be a difficult job to make
a transformer other than air core with a resonant winding which would do
a decent job of transforming the impedance you quote. So the short
answer is that it's anyone's guess what a 4:1 balun would do to that
impedance. A 1:1 current balun can be made from a section of
transmission line made into a coil, wound on a core, or with cores put
around it. The impedance will be transformed according to well known
transmission line effects, which depend on the length and Z0 of the line
used for making the balun.

Roy Lewallen, W7EL

Balun vs reactance
 

On Jan 31, 12:40 am, Roy Lewallen wrote:

....

In reality, any transformer will usually behave reasonably well only
when terminated in an impedance near its design impedance.

And, I might add, in the frequency range for which it's designed.
That may be obvious for something like trying to apply an audio
transformer to RF balun service, but it may not be so obvious if you
just pick up a "ham" 4:1 balun and think it's going to work for your
particular frequency range, if it's not designed for that range.

You'll _probably_ have better luck using a balun designed for, say,
50:200 ohms over 1-50MHz with a 3000-j4500 load at 20MHz than if you
try the same load impedance at 2MHz. That's because the transformer
reactance must be high enough at 1MHz to work at 50:200 OK, so there's
at least some hope the reactance will be enough larger at 20MHz to not
screw things up too badly. You will also want to watch out for core
saturation: run at lower power if the load is higher impedance. You
wouldn't want to push the balun too far toward the high end of its
frequency range, either, because there may be stray capacitances
limiting the response at the high end.

As Roy says, it's hard to say what your particular balun will do when
presented with a load so far away from the (presumed) 50:200
impedances it was designed for, unless you measure it with that load.

Cheers,
Tom