Neil: Have you considered winding your coil with a larger gauge of wire, say #12?
http://www.mtcradio.com/texas-bugcatcher-6160-hi-q-coil-160-meter-coil/
No, since the skin depth at my operating frequency is very small compared to the diameter of the wire, though a larger wire would give a bigger cross section at this depth for the current. It would be a question of how much is gained for the effort.
I have considered winding a coil with spacing between the turns to reduce the effect of the turn to turn capacitance which distorts the current distribution and tends to pinch the cross section of the current (note the coils in your link are wound this way). This is a bit down on my list.
My next try will be to wind a toroid and compare the loss with the air coil. This is a chance for someone else to beat me to posting the loss data for a toroid. It would be interesting to compare our results.
Neil
The answer may be in this thread, but I missed it: Will a elevated horizontal ground plane give me the same range as, say, running a copper wire from the ground lug on a transmitter down to the cold water pipe of a typical home?
A copper wire from transmitter ground run vertically down to water pipe ground will most likely radiate and give you more range than a properly designed elevated ground plane.