On Aug 31, 3:07*pm, Ian White GM3SEK wrote:
Dan wrote:
On Aug 28, 2:26*am, Ian White GM3SEK wrote:

In other words, people with limited antenna opportunities are often the
ones who need a balun - or more accurately, a common-mode choke - the
MOST.
Technically I would have to disagree with calling even a 1:1 balun the
same thing as a common mode choke. *A CM choke is an EMI prevention
device intended to filter out RF components generated in a circuit,
away from the feed of a power source, usually an electrical mains.

That is too far narrow a definition *of a "common mode choke",
especially the reference to electrical mains. The term is widely applied
to transmission line for both digital data and analog RF signals.

A common mode choke is used in RF applications, very true, but it
serves a filtering purpose,
not a conversion of unbalanced to balanced energy transfer or vice
versa. A common mode choke that operates well will turn
unwanted RF into heat or cause it to dissipate in its core or a
resistor etc..


A
balun is intended to change the feed from an unbalanced transmission
line to a balanced output, for example, for connection to a balanced
transmission line or to an antenna such as a dipole. With the balun,
we wany NO reduction in RF current flow.

What exactly do you mean by that?

You do not want the balun to operate hot (ir to dissipate heat as you
do with a CM choke filter). You strive for 100% transfer of energy and
settle for the best
you can get. With a CM choke, you try to filter and dissipate unwanted
back-RF. Any back RF from your balun
should be converted to unbalanced transfer back to the source. You
reduce back-RF by matching impedances (which can also involve baluns
but not the 1:1 application discussed here). If you try to filter it
the unwanted back-RF, you will also end up filtering the forward
energy transfer. Of course, that would be an undersirable situation.


And also, what exactly do you mean by "balanced" in the context of a
feedline?

For a 2 conductor feedline, the V in each conductor is 180 degrees out
of phase with each other. Same with I. One conductor is +90 degrees
and the other is -90 degrees with respect to earth. At any given
instant and location the summation of both conductors with respect to
each other is equal to the magnitude it would be on the inner
conductor on the unbalanced (coax) with respect to ground (shield).
Since magnitude of the V on each conductor of the balanced line are
equal and opposite in phase, the term "balanced" is appropriate. Same
with I.



I agree that the effect is
the same, semantically, ie one side effect of the use of a balun is
less CM interference from coming down a balanced feedline but it is
there for a different reason.

Not in my station. My motivation for using common-mode chokes is
*specifically* to control any incoming and outgoing interference that
may be caused by common-mode currents on the feedline.

Of course. But it is not due to filtering unwanted RF, it is due to
the conversion of balancing your energy so
that the coax properly acts as a shielded unbalanced line with no
energy in the shield and all energy in the inner conductor
(assuming perfect conditions). Your dipole will try to balance when
fed as a dipole directly from a coax.. Without the balun, any
reflected energy will
partially come down the shield to ground causing interference. The
balun simply unbalances the reflected energy, if any, to that it all
returns through
the inner conductor eliminating RFI if the radio and the shield are
properly earthed.


When the common-mode component of the feedline is reduced, it will also
be accompanied by an improvement in "balance" on the antenna, because
the two things go together (or at least, they do for some definitions of
that word).

But think of your dipole as a balanced transmission line. That's what
it is, with a lot of loss (into radiation resistance).
You WANT common mode on THAT that lossy transmission line and you do
not want it filtered away.

But "balance" is never my primary goal because I don't find
the concept helpful, either when deciding what to do next or when
evaluating the results.

I say "balanced" is your primary goal though you do not realize it.
You want to balance the energy propagation in your dipole "lossy
transmission line", when feeding it with an unbalanced coax. The balun
should accomplish that. Anything reflected is not good but at least it
is reflected "unbalanced" inside the grounded shield causing less EMI.