Topband: Ferrite Common Mode Chokes

[Tom W8JI]

I don't know what you are talking about Jim, are you talking about core 
internal resonance? If so, I'm not talking about that.

> Not quite.. A strong resistive component is present only around resonance, 
> and the resonance of a single pass through a #31 or #43 core is in the 
> range of 150 - 300 MHz, depending on the dimensions of the core. This is 
> clearly shown by Fair-Rite data sheets.

The Fair-rite data sheets show the reactance of that material and the 
resistance crossing at about 3 MHz. This is completely independent of the 
number of turns.

For example, a 73 material loss tangent and reactance crosses near the top 
of 160 meters. If I take ANY 73 mix core and measure conductor impedance 
going through a core window, with external leads calibrated out, R and X 
will be equal (Q=1 ) at the high end of 160. This means the core contributes 
equal parts reactance and resistance at that frequency, with resistance 
progressively increasing in proportion to reactance (Q lowering below 1) 
higher up.

43 materials have a Q=1 around mid HF.

If I have a choke that is 100 j100 and worse case series tune the core with 
0 -j100 from common mode impedance, the result is 100 j0.

I just don't follow what you are saying, because defeating the choke can 
only happen if resistance is low.

This is some cores people have used for many years actually DO work, and a 
certain mix (and authoritatively dictated impedance) is absolutely not a 
requirement.  I've worked with this stuff daily for 30-40 years, and the 
first thing I look at in broadband systems is core Q=1 frequency. This is 
where loss and reactance cross. It is also how I sort cores, and verify a 
core is labeled correctly. Many of us would be surprised at the number of 
cores that get mis-sorted or mislabeled in distribution.   I've seen bags of 
100's of 73 mix cores, from normal good distribution, that were 61 or 43.

73 Tom