PEHowland commented:
Not true. Radar at microwave frequencies (1GHz+) is line of sight limited, but at HF (3-30MHz) propagation is either via refraction in the ionosphere to ranges between 500 and 3500 km (HF Over the Horizon Radar) or typically 0-500km when vertically polarised and over the sea (HF Surface Wave Radar).
To get an idea of the technology (and impact) of over the horizon radar look up the Russian Woodpecker on Wikipedia. There are some nice pictures and pointers to other nice pictures of these abandoned steel yards. The green you see far far below in the Wikipedia pictures are tall evergreens to give you an idea of the scale involved. (
File:Woodpecker array.jpg - Wikipedia, the free encyclopedia )
You also need to study radio propagation and antennas a little more. The 0-500km use is called NVIS, Near Vertical Incidence Signaling. It's used for emergency communications quite a bit. It's achieved with low horizontal antennas. Vertical antennas or VERY high horizontal antennas, as with the Steel Yards, are the ones with the low angle of radiation needed for long distances.
JD-EE
edit - added URL.