Topband: FT-240-77 Toroid Impedance R + jX Components

[Jim Brown]

On Mon, 27 Dec 2004 14:10:47 -0800, Chuck Hutton wrote:

>I've seen a peak like yours when sweeping cores, but can't reconcile the
>Fair-Rite charts with that. I think their chart doesn't mean what I think 
it
>means. Can you explain?

I'll take a stab at it, Chuck. I've also measured a bunch of ferrite 
chokes, both single turn and multiple turns. It is VERY important to 
understand that these are not transformers, they are chokes, and what is 
being measured is the series equivalent impedance of R, X, and Z. The X 
starts out as L at low frequencies, then becomes C as the stray 
capacitance of the coil dominates. This resonance is quite visible in 
Michael's data. 

The complex Z of these chokes will vary with the material and the number 
of turns. See Fig 22 on page 181 of that same Fair-Rite catalog you 
referenced. This is the more relevant plot. 

I find Michael's data quite consistent with mine -- although I haven't 
measured any 77 material, I have measured the rather similar #78. Also, I 
suspect that Michael's instrumentation is considerably better than mine. 

Remember that with suppression, you want lots of R but relatively little X 
(because you don't want the X to resonate with the cable that you are 
choking, causing current to increase rather than decrease). Transformers 
are different, especially if you are putting power through them -- you 
want lots of X but not much R. 

To convince yourself of this, wind a 1:1 balun with the toroids that 
Amidon sells made from different materials and hook them up to your rig. 
These baluns will get hot pretty quickly with 100 watts at frequencies 
where the R is high, but not when R is low. Obviously, you want that power 
in the antenna, not the balun. 


Jim Brown  K9YC