On 1/23/2026 6:49 PM, Brian Beezley wrote:
Common-mode choke impedance is sensitive to capacitance across the choke.
In the parallel equivalent circuit of effective HF chokes, C is on the
order of 4 to 25pF, depending on the band(s) for which the choke is
designed.
When a cylindrical core is used as a choke, the resonance is between the
inductance of the wire and capacitance from one end of the wire to the
other, through the core, acting as a dielectric. Plots of the complex
impedance of these parts clearly shows the resonance, including loss
coupled from the core. Doing the arithmetic for that parallel equivalent
circuit will show that capacitance to be quite small.
73, Jim K9YC
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