It is the metal antenna elements phasing relationship. It has to do with wave length and fractions of wave lengths. The commonly used wave length fractions are , 2/3 wavelength , 1/2 wavelength , 1/4 wavelength. It has to do with having the metal receiving elements of the antenna phased together or not phased together. It has to do with the spacing of metal receiving elements. It has to do with the diameter of the metal receiving elements. It has to do with if a metal reflector is placed on one side of the metal receiving elements. It has to do with the spacing of the metal reflector between the metal receiving elements and the metal reflector. There is volumes of antenna design information on the internet and in several hundered books and computer design programs. Here are a places to start , ,
http://www.dxzone.com/catalog/Antennas/Theory ,
http://www.whoburns.com/steve/antenna.htm ,
http://www.kyes.com/antenna/antennadex.html ,
http://www.hdtvprimer.com