Folded dipole

The terminal impedance of a folded dipole is four times that of a simple dipole of about the same dimensions. Other ratios are possible by using two rods of differing diameters, or by adding a third rod. The impedance of a freestanding half-wave folded dipole is 300 ohms, but the presence of nearby elements will usually lower this considerably.
http://www.hdtvprimer.com/ANTENNAS/folded.gif