Hi there,
I am not a pilot and don't know how you are supposed to calculate this sort of thing, but from what you have written, assuming you meant VHF and with careful application of common sense and general knowledge it should be:
Lets work in meters:
altitude=16000ft=4876.8m
tower height=120ft=36.5m
I attached a picture which should help to comprehend my thinking. My drawing skills are a bit sh*t, but if it was done properly you could see right angle near the tower. That means we get a triangle with a right angle and can use Pythagoras theorem which says c^2 = a^2 + b^2 where c is hypotenuse. So in our case:
(R + altitude)^2 = (R + tower height)^2 + (distance)^2
R here is the radius of our lovely blue planet and it is equals 6378100m
(NOTE: for calculation purpose every number has to be the same unit)
If you rearrange this equation then:
distance = square root(( R + altitude)^2 - (R + tower height)^2)
and insert the numbers you get:
distance = 248548m
from here you can convert it to any units you like
248548m = 248.548km = 154.4m = 134.2nm
This is very rough calculation. We assumed that distance was a straight line, which in reality is not. Electromagnetic waves are bent by gravity. Also, earth radius is not the same everywhere. Moreover, wave propagation depends on weather conditions and obstacles along the way. I heard, that engineers, who deal with vhf, uhf and shf communications use coefficient of 4/3 to get more realistic results. In our case it would equate to 178nm. How much of it is true I don't know.
Sorry for my broken English
Bottle of wine is empty, I guess it's bed time
Goodnight