[RFI] coax shield outside impedance?

Dave Cole dave at nk7z.net
Wed Nov 25 15:52:33 EST 2020

I have an additional question along this line...

If the coax is buried, I have heard, there is no need for a choke.  Is 
this true?

73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 11/25/20 11:38 AM, Jim Brown wrote:
> 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
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