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Re: [RFI] coax shield outside impedance?

To: rfi@contesting.com
Subject: Re: [RFI] coax shield outside impedance?
From: Dave Cole <dave@nk7z.net>
Date: Wed, 25 Nov 2020 14:32:06 -0800
List-post: <mailto:rfi@contesting.com>
Thanks Jim, I'm off to build a choke...

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 2:07 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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