Jim,
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)
https://www.nk7z.net
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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