A poster on social media recently wrote:
Remember that a CM choke acts like the cut-off end of a wire antenna. That includes performing the reflection that creates the standing wave for resonance – it IS your ‘end point’ for the frequency it’s built for and any higher frequency.
Let’s explore that proposition with two simple NEC models.
A half wave dipole above average ground
The baseline model is a half wave dipole 10m above ‘average’ ground (σ=0.005, εr=13).
Above is a graphic of the current distribution on the wire. The current distribution is the classic half wave dipole current distribution, approximately sinusoidal in shape and falling to approximately zero at each end.
The feed point impedance is 50.6-j2.28, the dipole is approximately resonant, impedance is quite what is expected of a dipole low to the ground.
A half wave dipole above average ground with common mode choke and eighth wave extension
This is the same model scenario, but with a 10m wire extension and common mode choke (Zcm=1000+j1000) at the connection between the dipole end and the extension.
Above is a graphic of the current distribution on the wires. The common mode choke is the blue square on the diagram.
Above, the model summary shows the feed point impedance is now 193-79.2Ω, it is nowhere near resonant and resistance is almost four times that of the baseline dipole.
A more complicated graphic shows the phase and amplitude of the standing wave current.
Key things to note:
- there is significant current through the choke, and of course in the wires on each side of the choke;
- the choke does not result in the classic half wave dipole current distribution on the left hand half wave, it is not symmetric and current does not fall to zero a half wave from the left hand end; and
- the choke does not “act like the cut-off end of a wire antenna.”
Conclusions
The common mode choke in this example did not behave as the quote claimed. Myth busted!
Read widely, question everything.