Topband: Ferrite Common Mode Chokes

[Jim Brown]

On 5/21/2013 9:42 AM, Tom W8JI wrote:
> 31 materials has a Q of about 1 on 80 meters. This means a good part 
> of the impedance is resistive.

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. While there is 
some resistance at HF, it is small compared to the inductive component 
until the inductance is resonated lower by the stray capacitance of 
multiple turns.

To get effective choking impedance from these materials at HF, the 
resonance must be moved down to the HF spectrum by winding multiple 
turns through the core.  Those multi-turn chokes _do_ show a low Q 
resonance (on the order of 0.5 for #31, a bit higher for #43) in the HF 
range.  Both the inductance and the resistance coupled from the core are 
multiplied by the square of the turns, making the choking impedance much 
higher. The stray capacitance of the winding increases linearly with the 
number of turns, also helping move the resonance down.

The only material I know of whose resistive component strongly dominates 
at HF is the #73 material chosen by W2DU for his chokes, but they are 
only made to fit rather small coax. It takes a very large number of 
these beads (at least several hundred) to achieve enough choking 
impedance to handle the high common mode voltages encountered in many 
antenna systems at high power.  Most so-called W2DU "current baluns" are 
made with far fewer beads, and many don't even use the #73 mix.

73, Jim K9YC