• www.youtube.com/watch?v=stdi6XJX6gU: Ripple tank refraction movie
• http://en.wikipedia.org/wiki/Ionosphere
• http://spaceweather.com/
• http://www.spacew.com/www/realtime.php Realtime critical frequencies.
• http://www.spacew.com/www/fof2.html Realtime maximum usable frequecies.

Chris Myers: Antennas