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

[Jim Lux]

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.