[TowerTalk] 75 or 70 Ohm twinlead or ladderline cable - does it exist?

[Steve Hunt]

Jim,

They do, by virtue of the way the data is derived!

I make Open-Circuit and Short-Circuit S11 impedance measurements of the 
line; from which Zo=SQRT(Zoc.Zsc).
Loss is determined from the input resistance at frequencies where the 
line is multiples of a quarter-wave long.
Vf is determined by comparing the physical length with the 
multiple-quarter-wave electrical lengths.

At that point the loss model is assumed to be of the form:

Total Loss = k1.SQRT(F) + k2.F

k1 is a coefficient representing the copper loss and k2 represents the 
dielectric loss; the two coefficients are determined by best fit to the 
measured total loss. As a check, k1 can be calculated directly from the 
known RF resistance of the conductors.

Some of the underpinning maths here:
http://www.karinya.net/g3txq/wet_ll/tl_formulas.pdf

Hope that helps,
Steve G3TXQ



On 26/03/2014 12:31, Jim Lux wrote:
>
> An interesting test would be to see if the copper losses go as the 
> square root of frequency (skin effect) and dielectric losses go 
> linearly with frequency.
>