From: Roy Lewallen on Sat, Dec 3 2005 1:48 am

wrote:
From: Roy Lewallen on Fri, Dec 2 2005 4:20 pm
wrote:


I didn't know that anyone else but Micrometals made powdered iron cores
suitable for RF.

I have a few left-over Arnold Magnetics powdered-iron toroidal
cores that were used by RCA Corporation back in the 70s. For
the high end of HF for maximum Q.
. . .

A look at Arnold Magnetics' current catalog doesn't show anything
suitable for HF or above(*). Do you know of any company other than
Micrometals that currently sells powdered iron cores suitable for HF?

At least a half dozen companies off-shore (to the USA). There's
probably three dozen (give or take) in the world that advertise
but their product lines vary according to which kind they are
able to sell.

I'd guess that while low frequency powdered irons are still pretty
widely used in power supplies, the market for RF powdered iron cores
must be relatively small.

It's just a specialized area of electronic components that has
been around for at least seven decades. "Small" is a sometimes
a subjective thing...depending on one's collection of catalog
data...:-)

In 1950 I had a chance to visit two companies, one in my home
town of Rockford, IL, (Greenlee Tool Co. division of Greenlee
Inc), one a small BC-SW radio factory in a suburb of Stockholm,
Sweden where my uncle-in-law was boss and only engineer. The
radios used Philips IF cans which had powdered-iron "pot" cores
and excellent voice bandwidth, sharp skirt response, etc. Those
Philips cans weren't advertised at all in the popular press but
they were relatively cheap in large quantities. Of course,
Greenlee chassis punches had been widely advertised and much
talked about by home-brewers back then. The actual production
area for the Greenlee chassis punches (sold all over the USA)
was about the size of my home office (13' x 13')...and they
also made files in that same cubicle. From the ads one would
have thought they had thousands of square feet of production
space! :-) Advertising can be deceptive. shrug

Powdered-iron and ferrite material production is done in huge
batches prior to molding (powdered) or sintering (ferrites).
It is relatively cheap per end item to produce. The QC needed
costs much more per unit item than many other components (such
as axial-lead resistors) so the manufacturers want to sell that
in large lots. The only ones buying large lots are the
manufacturers...who plan ahead for large-run (thousands of
units) production. [you know this, of course, Roy, I'm
mentioning it for others]

Most production lines shun making their own toroidal inductors
because they are time-consuming compared to axial or radial-
lead inductors. That's a specialized area and it is generally
cheaper for equipment makers to buy them from inductor-making
specialist companies. For home-brewers making as many as a
dozen toroidal inductors by hand for one project, it is quite
easy to do with normal dexterity. Doing it for hundreds of
equipment units in a factory run makes production costs go up
since the number of toroids needed may be ten times that. So,
despite the clear advantages of a toroid form and closeness of
magnetic field that is natural to it, many designers opt for
the cheaper solenoidal form inductors. Some get the idea that
solenoidal forms are "better" and toroidal forms should be
"avoided" which is contrary to what they do.

Getting into ferrites, the microwave area has long been a user
of specialized GHz-range ferrites for isolators, phase-shifters,
attenuators, dividers, etc., purchased in certain stock sizes
and ground to fit necessary physical shape. The makers of those
materials tended to keep advertising to the specialized micro-
wave side of "radio." Those parts just wouldn't work for the
HF spectrum.

(*) That is, materials having low loss at HF and therefore suitable for
use as cores for inductors in tuned or relatively high-Q circuits. Like
ferrites, powdered irons having high loss at HF can be very useful as
broadband transformer cores, chokes, and in EMI suppression.

Cable TV spread in North America has been a great boon to Asian
component making and aiding consumers with low, low prices. A
good example is the little balun in most TV receivers made for
decades allowing 75 Ohm or 300 Ohm "antenna" connection. Cheap
and works over nearly a decade of frequencies. The same with
"splitters" and "line amplifiers" (for several TV sets). It is
difficult to get ads from the component makers themselves but
those are starting to appear in the industry trade press. With
the explosion of PC-making came all kinds of relatively cheap
EMI-suppression material and gadgets around cables and the like.
Whatever their material is tends to be secondary if it works.

The search for the ultimate high-Q coils started around 1950,
popular in the amateur radio press with the SSB "revolution"
needing it for IF selectivity. Was a BIG thing then as I recall
but that wound down as mechanical and quartz filters became
available with their "textbook" filter responses. That led to
a general feeling of "high Q is always better" for nearly every
tuned circuit or low/high pass filter. Corners on bandpass
filters HAD to be sharp according to "CW" (Conventional Wisdom).
Not really so in practice. A few hours of experimentation
on computers with CAE analysis modeling actual Q losses will
show many misconceptions on the "necessity - always - of high Q."
The same with "chokes" and even sharp tuned circuits for tubes
(which need a high impedance tuned circuit for maximum gain).

To me, a toroidal form is superior for holding IN the magnetic
field, thus enabling a toroid to be snugged down to a substrate
with little effect on inductance or Q. That also allows toroids
to be placed closer together with much less mutual interference.
One just can't do that with solenoidal forms without individual
inductor shielding. Philips has a couple of great application
notes on broadband toroidal transformers and "how to do them"
written back in the early 70s...reissued on their big website
around Y2K. [links are buried in some archive CDs, not handy]

Please pardon a few "editorial comments" in here...it's a very
quiet Saturday and I'm trying to procrastinate away from hitting
the crowded stores for gifts. :-)