On Tue, 20 Jan 2009 20:15:38 GMT, Owen Duffy wrote:

Hams have a affinity for powdered iron cores in RF applications, mislead
by the thought that low loss material naturally produces a better
solution. Thing is that it is a huge leap from low loss *material* to a
low loss *solution*.

Here is a recent design for a Guanella 1:1 balun for 6m and 2m on a #61
ferrite co http://www.vk1od.net/balun/G1-1-FT140-61/index.htm .

The article contains a graph of the material characteristics, which
shows that at 2m, µ' has fallen to 10% or so of µi, and µ'' has risen,
so an inductor will be more resistive than inductive.

But, does that make it a poor TLT? Not at all, it has very high common
mode or choking impedance (a critical performance parameter that is
rarely reported for commercial baluns).

Would such a balun wound on a powdered iron core work better? Probably
not. I say probably because I have been unable to find manufacturer's
loss data for powdered iron materials above about 5MHz. The probability
is that a powdered iron core would yeild a balun with a choking
impedance that is lower and more inductive than resistive and may likely
be *more* lossy. (I have a prospective project to measure a couple of
powdered iron cores at 6m to further explore this, but they will not be
lab grade measurements.)

The key is in thinking about I^2*R. The lossy ferrite balun has very
high Z, so very low I, very high R, and the product of I^2*R is
relatively low. A powdered iron choke will have lower Z, so higher I,
low R, and the product I^2*R may be quite higher than the ferrite.

Higher choking impedance can reduce loss, even if the higher choking
impedance is by way of a lower Q inductor. The characteristic is a
curve, concave down and the trick is to choose a design (whether it is
powdered iron, air cored, or ferrite cored) for acceptable loss, and
that often means an operating point that is well on the right hand side
or the left hand of the curve maximum.

Another facet of ferrite TLTs operated in their lossy region, is that
they remain useful well above the device self resonance, whereas when
choking depends on a high Q impedance, it rapidly falls above resonance.
(Self resonance is ignored in most models of balun performance that I
have seen.)

Owen

"A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing" is
an very good tutorial written by Jim Brown, K9YC, for understanding
and fixing RFI problems. Chapter 6 (page 23) Includes an excellent
explanation of balun operation and constrution information. A must
read for anyone designing and/or building baluns

http://audiosystemsgroup.com/RFI-Ham.pdf

Danny, K6MHE