On Mon, 21 Feb 2005 11:19:19 -0800, Jim W7RY wrote:
>Here is an excerpt from their page on this product:
>Ferrite material: seven Fair-Rite P/N 2643102002 (850 Initial
Perm, 2900
This core has a resonant frequency of about 400 MHz. Below that
frequency, a single turn (that is, the coax going through it) will
look inductive, and above it will look capacitive. It's going to
have fairly low loss at HF, so there shouldn't be a lot of
dissipation.
I'm looking at a plot of the Z of one core, and the product you
are citing uses seven. Their impedance adds in series. One core is
roughly 85 ohms at 10 MHz, 170-180 at 30 MHz. The parallel
equivalent R component is roughly 270 ohms at resonance, and I
would guess it's in the same order of magnitude at HF.
Another point -- any resistive component will cause heating with
common mode current flow, while any reactance can resonate with
the common mode length of a feedline (which is why Tom's
applications note on baluns talks about feedline length). Now,
let's say that the resonance is occuring -- now, the only thing
limiting common mode current (that is, shield current) is the
resistive component that the balun contributes! I would expect the
resistive component to be pretty small on this balun.
This product doesn't look like it would have enough Z to make a
decent balun below 20 meters, but perhaps Tom can shed some light
on this. Tom --what do you think the common mode Z of a balun
ought to be? And what do you think about the tradeoff between R
and X (that is, dissipation vs: interaction with the feedline's
lengh)?
If I were using these parts to make a balun, I would use at least
twice as many of them. BTW -- I just measured the Palamar RG8
balun kit -- it is a #31 part, but the same size, I think. They
use six of them, and it doesn't have much Z below 10 MHz either!
Here's a tutorial I'm writing for sound contractors on ferrites
that you might find helpful. It's in progress, and not complete.
I'm still adding and revising it as i learn more.
http://audiosystemsgroup.com/SAC0305Ferrites.pdf
Jim Brown K9YC
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