[RFI] coax shield outside impedance?

Dave Cole dave at nk7z.net
Wed Nov 25 17:32:06 EST 2020

Thanks Jim, I'm off to build a choke...

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

On 11/25/20 2:07 PM, Jim Brown wrote:
> On 11/25/2020 12:52 PM, Dave Cole wrote:
>> 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?
> The answer to this is not simple. First, the skin depth of soil can be a 
> significant fraction of a wavelength, depending on its electrical 
> properties, and varies with moisture content. W8JI has suggested 60 ft 
> on 160M. Second, most soil is a big resistor, so would be unlikely to 
> provide much shielding. Third, a feedline would need to be buried for 
> nearly all of its length for even good soil to do much.
> Remember that the earth is not a sump into which hum, buzz, and RFI is 
> poured. Indeed, we use radials on verticals to shield the antenna's 
> field and return current from seeing the lossy earth in the near field, 
> minimizing loss. Given skin depth and soil characteristics, there's 
> little difference from coax or radials buried or laying on the ground. 
> The principal benefit of either would be if skin depth is small enough 
> and conductivity are good enough that it can serve as a return for 
> induced current, forming a transmission line in the common mode circuit, 
> just as a "ground" layer does with a circuit board trace.
> 73, Jim L9YC
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