Bob ZL2CA wrote:

. . . . Once the
LF antenna has been "tuned and matched" using the likes of a (big)
loading coil with taps or L match, the relatively small variations in
reactance with changing conditions could be handled with a motor driven
variometer (say 50 uH range). . . . .

I have been using such a fine-tuner on 136 kHz for several months with
great success.  Although not 'automatic' in operation, the tuner is
remote- controlled, and enables rapid changes in operating frequency to
be made.

Details of the general arrangement can be found at:
http://www.alg.demon.co.uk/radio/136/rem_vario.htm

Regards to all,
Steve GW4ALG