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Date: Tue, 20 May 2025 14:07:28 -0400
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Subject: Re: Asymmetric Stripline / Microstrip online calculators for impedance and velocity
Newsgroups: sci.electronics.design
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From: bitrex <user@example.net>
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On 5/20/2025 11:38 AM, Joe Gwinn wrote:
> On Tue, 20 May 2025 11:20:05 -0400, bitrex <user@example.net> wrote:
> 
>> On 5/20/2025 11:15 AM, bitrex wrote:
>>> On 5/20/2025 11:00 AM, Mike Perkins wrote:
>>>> On 20/05/2025 15:56, bitrex wrote:
>>>>> On 5/20/2025 10:06 AM, Mike Perkins wrote:
>>>>>> On 20/05/2025 14:53, Mike Perkins wrote:
>>>>>>>
>>>>>>> I can find numerous calculators that provide impedance for the
>>>>>>> above structures, but are there any that give propagation velocity
>>>>>>> too?
>>>>>>
>>>>>> After numerous failed searches I found:
>>>>>>    https://www.multekpcb.com/calculators/
>>>>>>
>>>>>
>>>>> I think the stripline would have to be pretty far off-center for the
>>>>> phase velocity to be much different than c/sqrt(Er); in pen-and-paper
>>>>> analysis to derive the relatively simple equations for characteristic
>>>>> impedance, the dominant propagation mode is considered to be TEM.
>>>>>
>>>>> If the geometry is so screwy that it can't be well-approximated by
>>>>> TEM the characteristic impedance equation is wrong, also.
>>>>>
>>>>> I don't know what "asymmetric microstrip" is..?
>>>>
>>>> I would hope the calculators cope with standard Stripline and Microstrip.
>>>>
>>>>
>>>> I just needed details for Asymmetric Stripline and standard Microstrip.
>>>>
>>>>
>>>
>>> In the asymmetric stripline calculator you posted the propagation delay
>>> is calculated from the relative permittivity of the substrate alone,
>>> just so you know it's not returning anything different for that than the
>>> standard one.
>>
>> IOW the phase velocity of the TEM mode is taken as a given to develop
>> the pen-and-paper equations for the capacitance, and and thereby the
>> characteristic impedance of both the symmetric and asymmetric stripline.
> 
> It's not just the relative permittivity of the substrate, as part of
> the EM field is in air.  In some designs this is small enough to
> ignore the air part, but this must be determined, not just assumed.
> 
> Joe

The model of stripline amenable to pen-and-paper calculation has a 
conductor floating between two ground planes on the z axis, and 
dielectric to infinity in the xy plane, there's no field in the air in 
that model.

The microstrip model has field in air and is only quasi-TEM but they're 
not the same thing. What the hell is a "asymmetric microstrip" anyway?!

Not sure what y'all think these online calculators are doing. They're 
just automating the kind of pen-and-paper derived equations you can find 
in the textbooks, which make a number of assumptions to make the problem 
tractable to closed-form analysis.