[TowerTalk] Common-mode choke

[Steve Hunt]

Gerald,

To a large extent the desirable high resistive component is inherent in 
the choice of core material, but the stray capacitance can also give 
useful benefit.

Here's an example: A single turn on an FT240-31 core will have an 
impedance of about 20+j17 Ohms at 5MHz - the high resistive component 
follows from the complex permeability characteristics. Now increase the 
number of turns to 17 and we benefit from the "N-squared effect"; so the 
impedance becomes 5780+j4910 Ohms, ignoring stray capacitance. Now add 
in stray shunt capacitance of, say, 3pF (a reactance of -j10,600 Ohms at 
5MHz) and the impedance is transformed to 9870-j880 Ohms.

Steve G3TXQ





On 20/05/2014 16:30, TexasRF--- via TowerTalk wrote:
> Jim, trying to understand the source of the high resistance that is
> frequently mentioned for choke design.
>   
> I can see one scenario where a series to parallel conversion might explain
> it. Say 400 ohms xl in series with 20 ohms R would transform to 8020 R in
> parallel with 401 ohms xl. Added shunt C would increase the effective xl up
> to  the point of resonance.
>   
> Is this a possible explanation of the fundamentals?
>   
> Thanks/73,
> Gerald K5GW
>   
>