[NLRS] looking for an equation

[Barry B]

I'm trying to find the formula for determining the coupling coefficient of
Broadside coupled lines with different materials (e^r) for the line to
ground and between the coupled lines.

For example, I have a 2 layer pcb with a z=50 trace. Taking an identical
pcb and placing the traces to face each other, the traces would be
"broadside" coupled (rather than "edge" coupled in most literature.  e^r
for PCB=~4.  e^r for air (between lines)=1.

My objective is to make a graph of distance 'd' between the lines vs.
coupling (db) with a curve for each band. To simplify this, the line length
is arbitrarily 100mm (not really arbitrary for construction purposes). Some
distance d will result in approx -20db of coupling.

I've been looking at lots of web pages with no luck. Will try my Dubus
archives when I get home FROM SUNNY ORLANDO where the fest today was ONLY
IN THE MID 60s. A far cry from the 93 we had at the BEACH on Wed. (not that
I want to rub that in).

In a couple of weeks I'll have PCB and can experimentally determine the d
vs. coupling, but it would be nice to have a reference which would also
likely tell me the effect of coupling on the line Z.

I expect after this I'll be banned from the list and Tuesday night meetings.

Barry w0iy