Refer to the Coilcraft data sheets for the conical inductors.
https://www.coilcraft.com/getmedia/299d39b5-dce9-4982-b5c7-799e9a3da4d8/bcl.pdfBecause the diameter reduces along the coil length, the distributed Inductance reduces, and the distributed interturn Capacitance reduces.
In a certain GHz range, the coil has approximately a uniform broadband parallel resonance.
Refer to the two graphs for S11 and S21, when a conical coil was connected as shunt element on a 50 Ohm line.
Phase is not given, so assume a resistive loss.
I used qucs to model a pi attenuator in S parameters so I could better understand the losses. ( Note: I had trouble using two of the online Z to S converters).
The attenuation by the coil is roughly 4% in voltage.
Thanks Dejan, Very interesting, I did not know about these inductors until today.