[TowerTalk] One more dipole-balun question

[Jim Lux]

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.