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Date: Tue, 20 May 2025 14:09:46 -0400
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Subject: Re: Asymmetric Stripline / Microstrip online calculators for impedance and velocity
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On 5/20/2025 1:59 PM, Bill Sloman wrote:
> On 20/05/2025 11:53 pm, Mike Perkins wrote:
>>
>> I can find numerous calculators that provide impedance for the above 
>> structures, but are there any that give propagation velocity too?
> 
> This is sort of nuts. Microstrip is on the surface of a printed circuit 
> board. Half the field is located in the substrate and the other half in 
> the air above the board. It's consequently dispersive - different 
> frequency components propagate at different velocities.
> 
> Strip-line is buried inside a printed circuit board and propagates in 
> what can be a uniform environment. It's non-dispersive. A thicker layer 
> of the insulating substrate above the strip line than below it could 
> make it asymmetric, but I've no idea if this would mess up the 
> propagation velocity. A different insulating substrate above the strip- 
> line than below it presumably could make it dispersive.
> 

Ya as I've been trying to explain, the propagation velocity has to be 
taken as a given to make finding either the symmetric or asymmetric 
stripline capacitance (and therefore Z_0) tractable to closed-form 
analysis. The simple online calculators don't do shit but take it as a 
constant for stripline, based on the relative permeability of the 
substrate, in either the symmetric or asymmetric case.

I didn't think this required a PhD to explain but maybe you or Dr. Hobbs 
or someone can explain it better than I can..