Laurie

I think the critical factor in this discussion is the proposal to make a 'valid' comparison between the existing and additional short mast. To do this, in practice you will have to be using the same ground plane, (I think you already suggested this) and this I believe will prevent you from being sufficiently far away from the 300 footer (regardless of its electrical condition, grounded, resonant or otherwise) to prevent pattern distortion from mutual coupling effects.

Commercial antenna farms do have these problems but as we all know, any antenna system is always a compromise.None are perfect.

I think an experiment along the lines of what you suggest is an excellent idea for just seeing what the results are, but I think any suggestion of a 'valid' comparison would need to be modified in accordance with prevailing experimental conditions.

Using other peoples antennas is always a lot of fun anyway- just my luck that all those I have used so far have been bowled over a few weeks after the event! I have to say that watching a 300 footer being dropped is quite an experience- they always seem a lot bigger when thay are spread out along the ground!
In summary, go for it!

Dave, ZL3FJ