On 6/3/13 7:04 AM, Chris wrote:
Jim.
What is the "right choice" for an impedance transformer core (for a 4;1, 9:1
etc. )? What mix?
Depends on the frequency.. Typically you're looking for a powdered
metal, not a ferrite. mix #2 is good for lower bands up to around 10 MHz
mix 4 or 6 looks like it would work up to 40 MHz
micrometals.com is a big supplier of these things
http://www.micrometals.com/materials_index.html
As a practical matter, a lot of ham applications use #2 for everything
(for instance, LDG tuners use #2 for their inductors) This works out ok
in practice because usually, you're only putting L into the system at
low frequencies (e.g. because the antenna is physically small compared
to a wavelength, and looks capacitive), so the higher loss at higher
frequencies doesn't raise an issue.
I think the W1JR transformers are wound on #2 cores, as well.
In summary.. look at the fair-rite website for ferrite materials for
choke and EMI applications; look at micrometals for powdered metal
materials for inductors and transformers.
Both of them make enormous quantities and sell through the usual
distributors (Newark, Mouser, etc.) as well as direct (if you buy
enough!). Specialty retailers also sell them (Amidon, Palomar). I seem
to recall that Amidon is sourcing their own materials. They've changed a
lot since the days when Amidon himself was selling cores out of his
garage in North Hollywood. The company split and has reformed a number
of times.
There are also, of course, other suppliers of these materials from
offshore, and the prices can be a lot lower, however, as with all "made
to be inexpensive" things, you might suffer from lower quality: the mix
might not be as consistent, they might have painted the wrong color on
the core, so what you think is a #2 is actually something else. (each
manufacturer uses their own color codes.. a factory might make cores for
multiple brands, and mismarking is a possibility; as is the plant
manager making a special off the book production run on the second
shift; and all the other things that go on in a loosely supervised
manufacturing environment.
If you've got a way to test them, then they're probably ok.
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