Free space Z is ~377 ohms, based on Helmhotz equations.
We had to derive it from first principles in EM class.

http://en.wikipedia.org/wiki/Impedance_of_free_space



John Gardner wrote:

>>How does it "move" charges up and down the antenna if there isn't a
>>closed circuit?
> 
> 
> If you insist on thinking about it in terms of circuits, free space
> constitutes  ~  75 Ohm path to "ground", in the "RF" spectrum.
> 
> Experts, feel free to substitute your favored impedance.
> 
> best regards,
> 
> Jack
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