Coax-shielded "Ground" Conductors for Part 15.219?

[rfry]

Unfortunately such coax shields produce little effect, because the inside
and outside surfaces of a coax shield in this application are at r-f ground
potential only where they physically connect to a real r-f ground (conduct-
ors buried in the earth).

Due to skin effect (please Google), the r-f energy traveling on the outer sur-
face of the coax inner conductor between the transmitter r-f ground terminal
and a real r-f ground in this configuration couples to the inside surface of the
coax shield.

Most of that r-f energy crosses to the outside surface of the shield at the top
of the shield, confined again to that outer surface by skin effect, which then
produces radiation from the outer surface of the coax outer conductor.

The physical location where the base of the shield connects to a real
(functional) r-f ground is the only relevant r-f ground for the system.

The greater is the length of such a "shielded" ground lead, the more that conduc-
ting path will radiate, and the greater is the likelihood that a system using it
will be functionally non-compliant with15.219(b).

The link below leads to a NEC analysis of this configuration generated/posted
more than a year ago, for the stated conditions.

http://i62.photobucket.com/albums/h85/rfry-100/3-mMonopoleComparisons.gif

 

[Tom Wells]

If an elevated installation is required, why not bring "ground" to an elevated position with
a twisted pair to cancel radiation then terminate at the top with a center-tapped coil?

Connecting the coax feed outer shield to that center tap would send balanced opposing currents on the tiwsted pair,
while bringing a lightning ground to the elevated position. May take some experimentation,
but I think it would "work".

Meaning it woudl radiate off the intended element and not the ground lead ( the two-wire twisted lead).
I don't think it would work as well as a ground mounted antenna, but if someone really wants to elevate an
antenna and minimize radiation, I think it would minimize radiation where the coax-to ground is
no real help at all, as R Fry points out.

 

[rfry]

The following link has more information on the subject.

http://www.w8ji.com/coaxial currents/coaxial_line_and_shielded_wires.htm

 

[druidhillsradio]

I think many people do not understand that coax has 3 conductive surfaces. The referenced URL in Rich's post bears that out in an easy to understand fashion.