Rik, thanks for the explanations.
I would have never imagined that the gain provided by having a radiation
diagram 'squeezed' in the vertical plane could more than compensate for
the losses caused by the very short dimensions (compared to the wavelength)
of the antenna.  Today I learnt a new one. TNX.

Hi Alberto,

If an short vertical monopole is placed over perfect ground and brought to
resonance with a perfect loadingcoil there are no losses, so its
performance will be independent of its length. It looks a bit strange at
first sight but in the perfect world there are no losses and so every mW
you put into the antenna will be radiated regardless wether the antenna is
1m or 100m high.
Unfortunately there is no perfect ground and no lossless loading coil so in
practice (on 136kHz) a longer antenna will outperform a shorter and the one
who can put a lambda/2 dipole at lambda/4 height will have the
super-antenna (but this antenna just doesn't fit in my back yard).

73, Rik  ON7YD