On 3/26/14 7:16 AM, Steve Hunt wrote:
On 26/03/2014 14:01, Jim Lux wrote:
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.
that's probably "good enough"..
There's no nice analytical expression for ac resistance when skin depth
is a significant fraction of diameter. There's a series approximation,
but I think when people get to that level of detail, they just do a FEM
model.
Hmm, how thick is the cladding on CCS? Looking up a CommScope 21% CCS
table (claimed to be same AC resistance at >5 MHz) I see that the
cladding on AWG 14 is 0.002 inches/53 microns. That's less than 1 skin
depth at 1 MHz, and bit less than 2 skin depths at 5 MHz. I think
their 5MHz claim only really holds in the bigger sizes..
some RG-8 type coax has copper coated steel center conductor too.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|