Low and high impedance of low pass stepped impedance filter

I'm designing a low pass chebyshev filter using stepped impedance.

I'm using the book 'Microstrip Filters for RF/Microwave Applications' as a reference.

I have difficulty understanding how low and high impedances are defined. The book uses $$\Z_{0L} = 93\,\Omega\$$ and $$\Z_{0C} = 24\,\Omega\$$. I could not find an explanation for these values, I searched several articles but all uses an impedance between 20 and 120 Ω but does not explain how to calculate.

How can I get to this definition?

• If you're doing RF, "low" or "high" impedances are relative to 50$\Omega$. If you're doing video, 75$\Omega$. The "why" is partially traditional, and partially because that's what Just Works(TM). If you're doing digital logic, then 120$\Omega$, for the same reasons. Nov 15 '19 at 21:22

I don't know that book, but chances are that $$\Z_0\$$ is simply the impedance of that piece of microstrip; it's pretty standard to use $$\Z_0\$$ as the symbol for reference impedance.