The gain per metal economies of a two element beam are particularly apparent when compared to other antennas. Consider for example the quarter wavelength ground plane that 27 MHz CBers often used at home. That had one vertical element and three drooping radials. A total of one wavelength of element stock. All for 0 dB gain over a dipole. It's a similar deal with other antennas such as folded dipoles and J-poles. Whereas about the same amount of element material made into a two element yagi will give so much more gain.
Excited? So you should be! Follow these links for some two element beam ideas applicable on bands from low HF through to high VHF.
* WB2VUO 2 element direct feed yagi
* 2 element parasitic yagis by DK7ZB
* M0MCX bidirectional 40m wire yagi
* VE7CA's portable two element triband yagi (pdf)
* More VE7CA portable yagis (pdf)
* 2 element wire beam for 24 and 28 MHz by KL7JR
* IW5EDI 2 element yagi for 28 MHz
* K3MT rabbits ears 2 element beam for 144 MHz (pdf)
* Folding beam for 144 MHz portable (video)