[Amps] More parasitic choke questions

[Jim Brown]

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