I am very pleased with the discusion about the 'gain' (ahum ... apart from
the many dB groundloss) of a short vertical monopole. It is very
interesting to read the various points of view.

What about following experiment :
1. Someone with good 'ground-connection' (let's say less than 50 Ohm loss)
adjusts the TX power to get a certain antennacurrent. The (relative)
signalstrength is measured accurately (can be done by one or more other
hams at their stations).
2. Now the TX station disconnects the 'ground-connection'. This (hopefully)
will increase the groundloss to a significantly higher value. The TX power
now is adjusted (increased) to get exactly the same antennacurrent as in 1.
The (relative) signalstrength is now measured again and compared to the
signalstrength of 1.

If the signalstrength has changed this indicates that the 'gain' is
dependent of the groundloss (and vice versa).

The distance between TX and RX should be in the 30 to 100km range and maybe
the experiment is best done during daytime. This makes shure that you are
measuring the 'far-field' and surface wave.
The Rx stations should measure the relative signalstrength with an
AF-voltmeter at the audio-output of the RX (or using spectogram with linear
scale) and you might need a very sharp eye to see any differences on a
(more or less logaritmic) s-meter.

In practice it might be better to start with step 2 (at full TX power) and
the reduce the power to get the same antennacurrent is step 1.

73, Rik ON7YD