[TowerTalk] KF4BWG

[jimlux]

On 4/18/19 4:30 PM, David Gilbert wrote:
> 
> I do have a question for the experts about what I posted below. What 
> effect, if any, is there on skin effect for individual wires when you 
> have several in close proximity?  Since skin effect is a field-related 
> phenomenon, does having multiple fields in phase mitigate the effect on 
> the individual wires?
> 
> Just double checking ...
> 

if the wires are "close enough" then the current tends to flow on the 
"outer surface" of the ribbon.  In the limit, a flat strap is just a 
bunch of conductors close together.

For typical ribbon cable with 50 mil spacing and 26 AWG wire (16 mil 
diameter) - the wires are spaced about 3 diameters - that's "close-ish"

Skin Depth at 1 MHz is 65 microns which is 2.5 mil (roughly)
at 10 MHz it's 0.81 mil

I'm going to guess that proximity effects are small at HF frequencies. 
A bigger issue might be the dissipation losses in the insulation.  tan d 
for PVC is around 0.06, which is pretty high. polyethylene is .0003, 
PTFE is .0002

OTOH, lots of people have measured the loss in zip cord, which is also 
PVC insulation and it's not that huge.