Howdy... I was playing around with this propagation software on my
friend's laptop. I had a coverage simulation running on 98.1MHz using a
vertically polarized dipole. I decided to pick a location where they
have rolling plains to be as close to an ideal freespace calculation.
Here is my spec:

transmitter location: +41.083N, -98.395W
antenna height: 80 to 120 ft
antenna type: dipole (I decided to use 0dB for my gain just for
reference)
polarization: vertical
terrain data: DTED
propagation model: TIREM
frequency: 98.1MHz
system losses: 5dB
TX power: 50W

So I came up with the following data:

antenna height: 80 ft
distance where power level is -110dBW: 13.3 miles

antenna height: 100 ft
distance where power level is -110dBW: 12.32 miles

antenna height: 120 ft
distance where power level is -110dBW: 13.3 miles

Why did my signal level drop at 100 ft?