[TowerTalk] Test Fixture for Common Mode Chokes

[Jim Brown]

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