[SI-LIST] Re: CPWG

From: Bob Patel <whizplayer@xxxxxxxxx>
> Hi! I had a question regarding CPWG:
> For Edge coupled CPWG in Wadell book it is mentioned
> that to gurantee coplanar propagation:
> s+2.0w+2.0d<h
> where s=spacing between the traces
> w=width
> d=distance from trace to gnd trace
> h=height of dielectric.
> I want to know why I need to meet that equation and
> what if I don't?
> THanks in advance.
> Bob

Bob,

When the ground plane at the bottom of the dielectric gets too close to the 
traces (smaller h), then there is coupling to this plane.  This coupling does 
two things: it changes the impedance of the lines, and it can introduce 
parasitic modes.

On the other hand, if h is too big, parallel-plate waveguide modes will show up 
at certain frequencies.

--
Daniel

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