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Substrate

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Fishfood

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I'm working on a project for an upconverter around 27 GHz, and looking to use Isola Itera. Which substrate height is the best to use for this frequency? I have heard rules thumb for 10% of the wavelength and up to quarter wavelength to avoid resonant modes. For er=3.38, this would be 0.054mm or 0.135mm respectively. Does this indicate I should be using 0.127mm substrate height, or is this overkill? The linewidth for a 50Ohm line would be 0.29 mm for the 0.135mm case which feels way too thin.

The chain will include predominantly SMT QFN components and planar designed hair pin filters.
 

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Probably you miscalculated something. Using QucsStudio "Transmission Line Calculator" 50 ohm 360 degree angle line length is 5.19mm, width is 1.23mm (on a 0.5mm er=3.38 substrate)

Using formulas lambda=C/(f*sqrt(er)):
lambda=299792458/((27*10^9)*Sqrt(3.38))*1000 'mm
lambda = 6.039467
lambda*0.1 ' 10% mm
Ans = 0.603947

It looks like around 0.6mm

I've checked some old designs:
FR4 amplifier + antenna X band: 1.0mm substrate, approximately 7% of wavelength in dielectric
F4B antenna K band: 0.5mm substrate, approximately 7% of wavelength in dielectric

If you plan to manufacture it, first make sure required height is available at PCB fab. In my experience for some substrate heights manufacturing price may be around 4 times higher, I try to avoid those.
 

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Ah you're right, I did miscalculate. Thanks so much for the correction, and the example old designs was really helpful. Much appreciate it!
 

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