>You are correct, air wound pick up coils do not work.  Last night I tried
> using a coil out of a large solenoid.  It is wound with small wire, like 
> #30 awg, thousands of turns on a plastic form.  I passed the main RF 
> ground wire from the secondary through the center.  Running the coil 
> at half power, I could not measure any AC voltage on my meter.

If you wound the wire on the solenoid like you would a regular Tesla 
coil (with the windings going around the solenoid rather than through 
the center), then I don't think it's surprising that you didn't get much 
output.  As someone else mentioned, to get any current out, the 
windings on a transformer have to enclose the magnetic field lines.  
Since the magnetic field around a current carrying wire circles the wire, 
the wires from a solenoidal pickup coil would also be parallel to the 
field.  Thus, no field lines would be enclosed and you would not get any 

And concerning air wound coils, I thought that a ferrite toroid is simply 
used to concentrate the magetic field within the windings, which then 
gives a higher current output from the transformer.  Although an air 
wound coil would not produce as large a current output for a given 
current input as a coil wound on a ferrite core, Physics is Physics and a 
time varying magnetic field enclosed by a conductor will generate a 
time varying electric current  in the conductor.  Also, given the fact that 
ferrite exhibits hysterisis and can saturate, I would think that an air 
wound coil could give results over a much broader range of frequencies 
and currents.

Steven Roys (sroys-at-radiology.ab.umd.edu)