[RFI] coax shield outside impedance?

[Dave Cole]

Hi Frank,

Thank you Frank!!

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 2:45 PM, donovanf at erols.com wrote:
> Hi Dave,
> 
> 
> Burial prevents common mode radiation onto the the buried portion of
> the feed line. This is an excellent common mode suppression technique
> for very low signal level receiving antennas such as small loops, high
> impedance short verticals (e.g., Hi-Z verticals) and multi-element
> receiving antennas with very close spaced elements (e.g., Waller flag
> arrays).
> 
> 
> Coaxial cables laid directly on the ground provide adequate common
> mode suppression when used with more efficient receiving antennas
> such as Beverages and low impedance short verticals. A fairly
> low impedance ground system at their feed point provides
> additional common mode signal suppression.
> 
> 
> Transmitting antennas (except ground based verticals) are almost always
> elevated. Common mode currents will be radiated onto the outside of
> the elevated portion of the feed line, they then enter the coaxial cable at
> the antenna feed point. Buried cables can't prevent common mode
> signals from entering the elevated feed point.
> 
> 
> 73
> Frank
> W3LPL
> 
> 
> 
> 
> ----- Original Message -----
> 
> From: "Dave Cole" <dave at nk7z.net>
> To: rfi at contesting.com
> Sent: Wednesday, November 25, 2020 8:52:33 PM
> Subject: Re: [RFI] coax shield outside impedance?
> 
> 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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