LF: Harmonics

[Richard Rogers <vk7ro@netspace.net.au>]

G'day,

When Robert, AX2TAR, was experimenting here on 178KHz, I built a 100W 
class D transmitter for him to try.
He thought the five element low pass filter on the output was redundant 
as the antenna loading coil should provide sufficient selectivity.
So he tried it one night without the filter, feeding the square waves 
via his LF loading coil to the antenna.   I could barely hear his 3rd 
and 5th harmonics, but the  17th, 19th, 21st and 23rd harmonics were 
loud and clear.   The strongest was about S8. Robert lived about 15KM 
away from me.
I think this was because his antenna wire was about a halfwave long near 
these frequencies and thus had a high input impedance which was 
efficiently coupled to the transmitter by the stray capacitance across 
the LF loading coil.
Perhaps some of the current VVLF experimenters would like to try 
listening for higher harmonics than the third.
--
73, Ric   VK7RO