The drawing here shows the essential elements of a folded dipole and consists of two parallel elements having a constant spacing S with each element having a certain diameter, d1 and d2.
The ends of the parallel elements are connected to form a continuous loop and the feedpoint is at the center of the element having the diameter d1.
Consequently, in this calculation we use the value of single-wire dipole, the feedpoint impedance will be transformed upward by the ratio R according the equation seen below.
In free space the impedance, of a resonant 1/2 wave dipole antenna with a centre feed is aproximately 72 Ohms. Hence, a folded dipole using equal diameters for both elements will have a ratio of 4 and therefore an impedance of about 288 Ohms.