G8DSU has been doing some measurements on height gain at UHF and is also 
interested in active antennas at LF for cave radio purposes.  I will copy this 
to him because I'm not sure whether he is currently reading this list.

John G3PAI

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On 05/02/2006 at 09:14 Walter Blanchard wrote:

The theory behind the "height gain" observed using small LF active
antennas at different heights has to do with compression of the near-earth
LF potential gradient caused by the grounded "mast" holding the antenna.
The essential bit is that there is a grounded connection between the
antenna and receiver, which may be just the outer of the co-ax cable. It
would be an interesting experiment to repeat the "height gain" experiment
without any connection to ground. This could be done by building an active
antenna with a little transmitter to re-radiate the received LF signal (on
2.4 GHz?) and poking it up using a fibreglass mast. If the theory is right
then there wouldn't be any height gain. Might do it myself sometime but
anyone else interested?

Walter G3JKV.