How can I prove the fractional bandwidth of Radar absorber?

Hello dears

I desire to find the original proof of this equation of the fractional bandwidth Radar absorber

I found it in a an open access paper and the reference of it was in " Radar Cross section Handbook, by G. Ruck, Chapter 8 " , but unfortunately, I do not have this book. I hope somebody has this book or knows how to prove this equation.