Radio propagation depends on the sunspot cycle. Sunspots come and go in about an 11 year cycle. The highs and lows vary greatly. The more sunspots, the higher frequency radio signal the ionosphere is capable of returning to Earth. With fewer sunspots, higher frequencies are lost into space.
Heliographs were used in Arizona and New Mexico during the Indian wars. The Indians would cut the telegraph lines, but couldn't thwart communication using mirrors and the sun. A shutter mechanism or a button to depress the mirror angle was used to send Morse code.