Here is a good military level primer on HF radio propagation with some links on how to make improvised antennas. See:
http://www.globalsecurity.org/military/library/policy/army/fm/7-93/Appd.htm [1]
For more fun, try Googling on the search phrase: “stealth, apartment, and wire antennas.” There is an amazing science behind these home-built wonders.