Topband: Impedance of two wires to ground?

[Terry Conboy]

Michael James W0BTU wrote:
> I'm looking for the formula to calculate the surge impedance of two wires parallel to the ground, given the diameter of the wires, the wire spacing, and height above ground.
>   
 From the Johnson & Jasik "Antenna Engineering Handbook, 2nd Edition", 
page 42-9

Differential mode impedance (between conductors):
Zo = 276/sqrt(epsilon) * log10 (4h / (d * sqrt(1 + (2h/S)^2)))

Common mode impedance (parallel conductors relative to ground):
Zo = 69/sqrt(epsilon) * log10 ((4h/d) * sqrt(1 + (2h/S)^2))

where
h = height above ground
S = conductor spacing on centers
d = conductor diameter (assumed small relative to S and h)
epsilon = relative permittivity of dielectric medium (=1 in vacuum)

I entered the equations in an Excel spreadsheet and for #12 wire (0.0808 
inch diam), 6 inch spacing, and 6 feet (72 inches) above ground, I 
obtained Zo differential = 599.3 ohms and Zo common mode = 340.3 ohms.

If anyone is interested in a copy of this simple spreadsheet, contact me 
directly.

73, Terry N6RY