[RFI] coax shield outside impedance?

[Jim Brown]

On 11/25/2020 10:47 AM, Ray LaRue wrote:
> *When attempting to minimze CMI on the outside of 50 ohm coax, the 
> shield is not normally 50 ohms. What shield Z do you use for calculating 
> the needed choke Z and thus the attenuation to expect?

In the common mode circuit, the coax shield is acting as part of the 
antenna, so like any antenna, current and voltage, thus the impedance, 
vary along its length. Impedance is highest at ends, lowest a quarter 
wave from ends. A good choke has a very high impedance (5-10K ohms), so 
forces a high impedance, which results in a low impedance a quarter wave 
away. To minimize common mode current on the feedline coupling to the 
antenna, the optimum point for the choke is at the feedpoint (that is, 
up in the air).

73, Jim K(YC