There are numerous radars running in the HF bands.
Some of them run continuously.

Frequencies heard in San Diego.
4407
4814
12180 to 12100
13370 to 13450
13500 to 13570
13920 to 13970
26340 to 26190

We can examine their echos too.
I recorded the one at 26190 (a while back) for 60
seconds and ran an autocorrelation.
A plot of it is posted in alt.binaries.alt.
We can see that it echos over great distances,
on the order of the circumferance of the Earth.
(The horizontal scale is time converted to distance
figuring the signal travels at the speed of light.)

--
rb