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Using a hammer instead of pliers

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8 months 1 week ago #875 by Fahmi
Fahmi replied the topic: Using a hammer instead of pliers
What I meant was that increasing the box size will in essence allow a better capture of the field distribution outside of the substrate, the field decays exponentially and one wants to allow the fields to decay enough such that the outer box boundaries do not disturb the field too much, the more wavelengths in air the better, and yes the port boundaries in this case are PEC, so the port size will have to extend enough above the substrate for the fields to decay. I recall Sonnet recommended something like 10 times the substrate thickness.
The safest bet is to see the actual housing where the circuit will be housed in and use those dimensions, as this will mimic the actual operating conditions.
And for the original question from Steve regarding the seemingly erratic calculations, the use of integration lines should take care of that and the results should be consistent.

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8 months 1 week ago #876 by thadrien
thadrien replied the topic: Using a hammer instead of pliers
The field decays exponentially, but wavelength has not so much influence in that process. The frequency dependence here is really low.

In the original question from Steve, I think that the main cause was the box influence. Integration line ambiguity has a much lower effect on microstrip lines sufficiently small.

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8 months 1 week ago #877 by Fahmi
Fahmi replied the topic: Using a hammer instead of pliers
You are right, the box and the port surface just have to be large enough (in absolute terms not in frequency dependent fashion) for sufficient decay of the fields, but then I believe care must be taken when working with really thin substrates, as in absolute terms , the multiplier of substrate thickness for the space needed above the substrate for the fields to decay appreciably, will be different, say if we consider a 25 mil alumina vs. 100 micron GaAs substrates.

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8 months 1 week ago #878 by thadrien
thadrien replied the topic: Using a hammer instead of pliers
The additional spacing changes with thickness, but the multiplying factor does not. What influences the multiplying ratio are the ratio w/h (microstrip line over substrate thickness): larger microstrip lines will need lower multiplying factors, and dielectric constant: substrate with lower dielectric constants will need more spacing.

I typically use 5 for this ratio, but I already had to use at least 10 in some simulations where I needed high accuracy on S11.

Pro tip: always look at the mode patterns. When there is field on the port boundary, it is a sign you need to increase your box. If you want to be really sure, start from 5 and increase the value until impedance does not change anymore. In all cases, be careful not to introduce higher order modes by using too high spacings. I remember a tricky simulation where I had to reduce a lot the spacing to remove this effect.

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