While a bit finicky to set up, sloper antennas can be a popular way of covering the low bands if you already have a tower for the upper HF bands.
- The 160-Meter Sloper System at K3LR
QST August 1994, pp. 36-38
A quick review of the theoretical design, the construction, testing and operation of a 160-meter sloper on a 190 foot tower.
- A Reduced-Size Half Sloper For 160 Meters
QST March 1998, pp. 54-56
Here's a limited-space antenna that you can put on your 40 foot tower or even a tree.
- A Two-Band Half-Sloper Antenna
QST June 1980, pp. 32-35
A trapped 40- and 80-meter sloper for your 40 foot tower.