# Optimal common mode impedance of a common mode choke

In recent days we see two online experts with diametrically opposite views of the optimal common mode impedance Zcm of a common mode choke…

…the inductance of the CMC is responsible for the CM
attenuation.

and…

A practical choke is RESISTIVE, not INDUCTIVE.

Emphatic statements indeed.

They are very unlikely to both be correct, and it is possible neither applies generally.

This article calculates the common mode current Icm for three Zcm scenarios of a current balun or common mode choke applied to my own antenna system documented at Equivalent circuit of an antenna system at 3.6 and 7.1MHz. The three choke Zcm values are:

• 1500+j1500Ω;
• 1500+j0Ω; and
• 0+j1500Ω.

The measured antenna system common mode impedances Zc are:

3.6MHz: 59+j167Ω; and

7.1MHz: 134-j44Ω.

Be aware that most back-of-the-envelope ohms law analyses are seriously flawed, but in this case we have the common mode impedance of the antenna system measured at the grounded shack patch panel

 F Zc \ Zcm 1500+j1500Ω 1500+j0Ω 0+j1500Ω 3.6MHz 59+j167Ω 438µA 638µA 600µA 7.1MHz 134-j44Ω 457µA 612µA 684µA

Above a summary of the magnitude of common mode currents Icm for 1V of common mode drive at the patch panel.

Keep in mind that the common mode current is usually a standing wave in a three dimensional context and cannot be adequately described by a current at a single point.

The broad conclusions for this scenario are:

• though the antenna common mode impedance Zc is significantly different from frequency to frequency (including inductive vs capacitive), Icm does not vary much from band to band, Icm is dominated by Zcm and Zc has little influence;
• Icm is significantly less for Zcm=1500+j1500Ω case than either of the others on both frequencies;
• comparing Zcm=1500+j0Ω with 0+j1500Ω, one is slightly better than the other on one frequency, and the opposite on the other frequency.

There is no evidence in the analysis of this real antenna scenario that choke resistance or choke reactance dominates Icm, neither of the online experts are correct in this case.