design spiral coil

Can anyone explain or point me towards any papers that have been published to describe coil behaviour beyond the self resonance of a spiral PCB coil. I have created two different length PCB coils, both with similar inductance and self resonant frequecies but viewing the impedance of the coils past their SRF, the smaller coil has secondary series resonant points that have much lower Q (Zo is large). However, the larger coil has several secondary resonant points that have a very high Q (Zo is very small at resonance). I need to dampen these secondary series resonant points while maintaining the SRF and inductance but I am struggling to understand how these resonant points are generated.