USCG, on the
west coast, positions antennas on the highest peaks along the coast. From
http://www.tscm.com/rdr-hori.pdf you can see that an
antenna at 5000 ft talking to a ship with
antenna at 40 ft could easily achieve a 100-150 mile range. I was recently delivering my
boat from
Hawaii to SF. At 300 miles from the coast I called the USCG and was shocked to get a response from Humboldt Bay as they were a bit to the North. I could hear SF and LA at least sporadically. This was likely a case of ducting but their antenna is up pretty high. On
AIS range, I live at 1500 feet, 40 mi south of SF, with a clear view of the bay. I have a homemade
AIS set up with a vertical antenna. I can often read signals
offshore up to 100 miles to the north where there are gaps in the coastal range, and of course can read all the ships on the bay. On days when there is a temperature inversion nearly all the bay reports disappear. This is due to the inverted temp layer reflecting the low angle
VHF signals down. At the same time I can easily hear signals that emanate above the inversion layer from peaks up to 200 miles distant (line of sight propagation).