It would not surprise me if the "real world" proved this wrong, however:
Vertical should give the greatest circular area coverage (no end
attentuation like a horiz. dipole).
However, most external receiving fm antennas are horizontal in
polarization--most portables have short pullup whips...
If you do go vertical, a 5/8 (at least in theory) drops the lobes of the
radiation pattern and gives you the most usable low angle of radiation
(direct correlation to max distance)...
The vertical is omnidirectional of course--a horz dipole is more
bi-directional...
If any of this is in error--I am sure it will be corrected...

Regards,
John

"Gavin" wrote in message
...
OK, so here is the situation.

I have a 10W FM transmitter set to 87.7MHz. I also have a folded Dipole
antenna and a 20 meter high mast.

Would it produce and great coverage area with the antenna horizontal or
vertical?

Many Thanks

Gavin.