Substrate thickness for L-Band

I am trying to design a matching network for a power transistor at 1.25GHz, and I'm having some issues. I'm using Roger's TMM10i that's 75mil thick. Are there any issues at using thick substrates at 1.25GHz? Typically I see thin substrates being used, but I wanted to use the thick material to keep the line widths closer to component sizes at L-Band.

75mil height and Er = 9.8, means a 50 ohms trace with 2.3mm width, which is a reasonable dimension.

Your reason for using thick substrate is not reasonable. especially at L band there will not be a considerable loss if your line width is not same as your component size.
In my opinion use thinner substrates.

The question is, why is a thinner substrate "better" than the thicker substrate? What's the difference? Shouldn't they work the same?

If you use a transmission line calculator as TX Line from AWR you will see that for the same dielectric, at high frequencies, using thicker substrate you get lower loss than using thinner substrate.

AWR | TX-Line | Optional Products - TX-Line

thinner substrate is very cheaper.

although this discussion is old one but if someone read it can u plz tel me how diid you calculate impedance from height and relative permitivty?any formula

Now we use many software to do so, such as TXLine from AWR, ADS, etc.