The W5GI Mystery Antenna is a phased dipole array (3 elements) that shows great promise as a 20 meter gain antenna and also usable as a multi-band dipole. You can get all the details at John's W5GI's web site.

Some analysis of phased arrays was provided in my Genesis -- Birth of Antennas presentation and I recently provided some additional insight in my paper entitled "Thoughts on the Mystery Antenna."

You can download this paper here.

Thoughts on the Mystery Antenna (40 KByte PDF file)