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

[Steve Hunt]

On 26/03/2014 14:01, Jim Lux wrote:
>
> OK.. so you didn't actually independently measure the copper and 
> dielectric loss (I haven't had my coffee yet, and I'm not sure how you 
> would do that other than by how you did it); You fit the measured data 
> to the standard square root and linear model, so of course, the data 
> matches..
>
Correct - I also don't know how you would separate them directly!

That said, the directly calculable copper losses usually verify the data 
match; and with the more usual types of feedline the dielectric losses 
in the low frequency region can be assumed to be negligible, and 
therefore copper losses can be directly measured.

> One catch, for others thinking of trying this approach, is when the 
> wire is small enough that the assumption of it being a thin tube is 
> violated.  Two ways I know that can happen are if the skin depth is a 
> significant fraction of the diameter or if the wire is plated (e.g. 
> silver over stainless steel, used for cryogenic coax to minimize heat 
> leakage), particularly if the materials involved are magnetic. [Yes, 
> indeed, I've been caught by the latter...]
>
> At 1 MHz skin depth in copper is  2.5 mil/65 micron.  AWG 24 wire is 
> 20 mil/511 micron diameter, which is 8 times the skin depth. Start 
> going much lower, or using AWG 40 wire, and that thin tube assumption 
> breaks down.
>

The loss model in AC6LA's TLD software allows inclusion of a third 
coefficient - k0 - which takes some account of the skin depth problem. 
This issue hit me when measuring some commercial "450 Ohm" ladderline 
which uses CCS conductors. In the low HF region, because an increasing 
proportion of the current is carried in the steel rather than the copper 
coating, the losses did not decrease with SRT(F) as might be expected. 
Including the k0 coefficient in the loss model provided a much better 
match to the measured data.

You can see the effect of adding k0 in the chart 4th from bottom, here:
http://www.karinya.net/g3txq/wet_ll/

Steve G3TXQ