Gain wrote:
"I have a 10W FM transmitter set to 87.7 MHz. I also have a folded
dipole antenna and a 20 meter high mast."

Range is determined by line of sight.

Your antenna is about 66 feet high. Over smooth earth, your grazing
point, in miles, is the square root of twice the antenna height in feet,
or the sq, rt. of 132 = 11 miles. If your receiving antenna happened to
be at the same elevation and could share a grazing point, the total
distance you could reach is 22 miles.

Antenna polarization has little effect on range unless you can
cross-polarize with an interfering signal which is unlikely. Broadcast
stations in the U.S.A. use both polarizations as many listeners are in
vehicles which have better luck with vertically polarized signals. The
same conditions prevail everywhere.

Have luck and fun. Try to stay out of jail.

Best regards, Richard Harrison, KB5WZI