[RFI] coax shield outside impedance?

Jim Brown jim at audiosystemsgroup.com
Wed Nov 25 17:07:34 EST 2020

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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