Look to me the primary, pickup, secondary and chokes are the same type of material.
If it is iron wire it has more resistance than copper wire.
If you look at the resistance measurements of the VIC (Dynodon provided) than you go for copper wire. (you don't want amps in the primary because the wire thickness and current is 'wasted' as heat!!!)
Material Element/Alloy (ohm-cmil/ft) (microohm-cm)
Iron --------- Element -------------- 57.81 --------- 9.61
Copper ------- Element -------------- 10.09 --------- 1.678
What the copper wire is doing is shake (voltage) the inner core material and generate magnetic pulse fields.
"Current (magnetic field) is the opposite side of voltage potential (electrostatic)"
The pulsating field is restricting the amps, what SM explains!!!
The closed loop core is providing an adding magnetic field to all the coils, so restricting more current on the right pulse frequency. That's why the current is going down on resonance.
That's what I think.

Br,
Webmug