there is really only one mechanism responsible for e" (imaginary part of
the dielectric constant). And that is oscillating dipoles, or what Neven
Orhanovic called a "dielectric hysteresis."
The other mechanism - leaky current in a dielectric suggested by Nirmal
- is not there at our typical 1 Ghz.
Why? In order to create a conductive electron in a dielectric, you need
to excite it into a conduction band, creating an electron-hole pair (a
localized electron would absorb a photon/quant of the field and jump up
over the dielectric gap). How much energy does it require? Several
electron-Volts, which is a million times the energy of the 1 GHz EM
field photon. To create a conductive electron in a dielectric you need
to have optical or even higher energy photons (UV, X-rays).
Signal Integrity Engineer