Hi Mike:
The characteristic impedance of free space is 120 * PI. So an antenna is just an impedance matching device from whatever impedance you start with (say 50 Ohms) to 120*PI Ohms. There are no mechanisims for reasonance in free space but there is one for the earth called the Shuman resonance. You can Google it.
Something I just learned in the past couple of years is that the impedance of transmission lines is not constant for audio frequencies. For more on that see:
Have Fun,
Brooke Clarke, N6GCE
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