Hi folks,
Thanks for all your replies, these helped me a lot...
I found from a textbook (Radio Frequency Integrated Circuit Design, 2nd Edition, J. Rogers and C. Plett: Section 6.17 "Layout of Spiral Inductors") very clear explanation about it, the only difference is, as volkver mentioned, I'm using microstrip, but the book refers to on-chip inductors, so, that brought me another question: do I need to place ground shield in microstrip design? since I already have a big ground plane below the circuit. Thank you!