Magne raises a good point. One needs to consult the regulations in one's
home country. Over here, frequencies below 9 kHz are not regulated, but the
ITU requirement is interpreted to mean harmonic energy appearing on
frequencies above 9 kHz must be kept below field strength levels specified in
other parts of our FCC's rules.
This makes the sub-9 kHz region ideal here for earth dipole experiments, and
even conventional antennas, but only if one takes adequate harmonic