Dan wrote:
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..


Common-mode chokes, and filters in general, do NOT aim to "turn unwanted
RF into heat"! That is a total misunderstanding of the whole concept.

An ideal common-mode choke would dissipate zero heat energy, and a
successful real-life choke will dissipate only a tiny fraction of the
available RF power.

When you insert a common-mode choke, you are inserting a large impedance
into the pathway of the common-mode current. The RF current
distribution throughout the entire antenna/feedline/ground system will
adjust to take account of this new impedance. As a result, most of the
common-mode current will be DIVERTED away from its former pathway, and
will flow instead in the antenna.

The details are complicated, but the concept that the choke DIVERTS
common-mode current away from the feedline is reasonably accurate. (By
contrast, the concept that it "turns unwanted RF current into heat" is
just plain wrong.)

If the choke is doing its job, the new value of common-mode current
(I_cm) flowing through the choke will be much less than the previous
value. The power dissipation in the choke will then be (I_cm)-squared x
R, where R is the resistive part of the choke's impedance at that
frequency. Note that I_cm is the small amount of common-mode current
that remains *after* having inserted the choke - not the value before!
The practical outcome is that a higher choke impedance will give *lower*
heat dissipation in the choke itself.

If a common-mode choke is getting hot, it isn't working. Unfortunately
there are many chokes that don't have a high enough impedance to handle
the full range of real-life situations. If a choke is not able to
suppress the common-mode current to a low enough value, then in some
situations it will get hot [1, 2]. But PLEASE don't imagine that is
how common-mode chokes are intended to work!

[1] http://www.w8ji.com/Baluns/balun_test.htm

[2] http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf

Also see other pages and publications from the same authors.



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.


Sorry, but that is so confused I can't even begin to unpick it... except
by pointing to "you try to filter and dissipate unwanted back-RF". In so
many different ways, that is NOT what we're trying to do. Tug on that
loose strand, and the whole thing unravels.



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.

Yes.

One conductor is +90 degrees
and the other is -90 degrees with respect to earth.

No. Earth and 90 degrees don't come into this at all.

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.

Yes... but this definition of "balanced" also REQUIRES that the
common-mode current is zero. The two are locked together, so if
"balance" is your aim, the practical way to achieve it is to force the
common-mode current to a lower value.

We have some direct leverage on common-mode current, because it's a
real, measurable thing. But "balance" is only a concept, and there isn't
any *direct* leverage that we can apply to it.

So even though a "common-mode choke" and a "current balun" are two
different names for the same physical device, it does make a difference
which name you choose. Think "common-mode choke", and you can see the
levers that will make your antenna/feedline system perform the way you
want it to. Think "balun", and all you see is a label that covers those
levers up.


[...]
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.

Again, that is all so misconceived - at every turn, it clashes with
obvious, measurable physical reality; or else it contradicts itself.
I'm sorry, but you really do need to do a clean wipe and start again
with a good textbook.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek