An outdoor antenna can be easily constructed by stringing up a wire between two trees or the house and a tree. Each end needs to be insulated from the wire that attaches it to the tree or house. Then at the end nearest your receiver connect an insulated lead-in wire that runs to your receiver's antenna input. Radio Shack provides a simple kit that makes it easy to construct such an antennas. Other outdoor antennas designed specially for shortwave listening are available from Grove, The Antenna Supermarket and Alpha Delta.
Active antennas are especially useful for apartment dwellers although anyone can benefit from one.
These antennas are essentially a whip antenna with an amplifier although some will have more features that allow tuning to specific bands, noise reduction, or selecting from more than one antenna. Even with an active antenna, getting the whip outdoors, such as on a balcony, will improve reception. One disadvantage of an active antenna is that while it is amplifying shortwave signals it also amplifies noise. Still it can be a big improvement over a receiver's built-in antenna. Active antennas are available from MJF, Datong, and McKay Dymek.