On Sat, 31 Jul 2010 04:47:38 -0700, Bill, W6WRT wrote:
>We're getting pretty heavily into semantics here.
And perhaps some misunderstandings as well. There is no such thing
as a pure inductance. EVERY real inductor has stray R and C. EVERY
real inductor has a self resonance. EVERY real inductor has loss.
The key to making things work the way we want them to is to
understand and CONTROL R and C so that losses and resonances
either don't give us trouble or work for us.
Here's a common example (and a VERY important one). ALL ferrite
chokes used for RFI suppression are intentionally made LOSSY at
the frequency where we want suppression. We are NOT usuing their
inductance (which for most ferrites is present only at low
frequencies), we are using their RESISTANCE. And to get that
resistance, we are using them near their SELF RESONANCE!
Example -- go to the Fair-Rite catalog (its online) and look at
the impedance curve of a cylinder or clamp-on core. Look for one
made of #31 or #43 material. You will see a graph that starts low
at low frequencies, climbs linearly through the HF spectrum, and
rises to a resonant peak at some frequency, typically several
hundred MHz. Clamp that onto a cable and it will look like a small
inductor at 10 MHz (that linear rising curve), and a 300 ohm
resistor at 200 MHz. Want to move that resonance (and the high
resistance) down to 50 MHz? Wind 2-3 turns through the same core.
N-squared causes both R and L to increase, C increases, the
resonance moves down, and the impedance at resonance goes up. We
make these VHF materials work at HF by winding enough turns to
move the resonance down to the HF range.
Chokes using #31 and #43 typically have Qs on the order of 0.5.
Now, there ARE lower loss ferrite materials, but they are not
often used for suppression because their Q is far too high. Fair-
Rite #61 and #67 are examples. These are good core materials for
INDUCTORS to handle power.
Hope this helps.
73, Jim Brown K9YC
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