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
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