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?

I'd have to look at the antenna in my own antenna modeling program,
which I don't have right now to be sure of this, but I'd guess it has
something to do with the antenna being an integer number of wavelengths
above ground. 100 feet is almost exactly 10 wavelengths at 98.1MHz.

Do you have an antenna program that lets you look at the elevation
pattern of the antenna? My guess is that at 10 wavelengths high,
there's more energy in higher angle lobes which I would expect that a
line-of-sight coverage prediction program doesn't report anything
about.

Dan
N3OX

"MRW" wrote
... Why did my signal level drop at 100 ft?
_____________

The number that TIREM calculates will depend among other things on the
vector sum of the direct and ground-reflected or diffracted rays for the
model conditions -- which are a function of ERP, antenna height, path
length, and the terrain profile along that radial. Probably the combination
of these at the 100 ft elevation of the transmit antenna was responsible for
the result you saw.

If you get a chance, check several other radials to see what happens.

RF