How to choose the substrate's thickness for a 9.35Ghz application

Hello everyone!

I have to choose a substrate for my design, and i was wondering how do i choose its thickness, i looked around and i found a person recommending the use of a substrate of height equal to the wavelenght in the substrate/10, but at the same time i've used in previous projects substrates of 35μm thickness (0.125mil) for applications at 2.45GHz and had no problem.

i'm working at a frequency of 9.35Ghz.

Thank you in advance.

The thickness of your substrate will determine your linewidths at the impedance you desire. Select a thickness which will give you linewidths that are manufactureable at your impedance.

Your wavelength is about 1.25", so a WL/10 would be 0.125", which is ridiculously high. I've not heard this "rule of thumb" before - it may be more applicable at millimeter wavelengths and higher.

What kind of substrates are you talking about? Or are you confusing copper thickness of regular PCB with substrate height?

There's of course a maximum height to avoid waveguide modes, but it's surely irrelevant for 9 GHz.

No i'm not confusing them, I've used the Arlon AD1000 substrate with that thickness.

Something's wrong. 0.125 mil equals 3.5 μm.

Rogers fabricates AD1000 laminates in a range of 0.020 to 0.127 inch (0.5 to 3.2 mm).

Have also used the wavelength/10 guidance and found it helpful to avoid higher order modes. I am assuming that we are discussing micro strip transmission lines. I would also note that as the dielectric gets thicker the trace width will also increase if one wants to keep the impedance constant. The larger lines will have a lower insertion loss. That is also a consideration. The same thing happens with coax. The bigger the coax the lower the loss, up to the point where higher order modes can propagate.

As in antennas, the same rule applies here: "They are always too big. Make them smaller but reduce the loss (or increase the gain) as well."