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The drawing below shows the essential elements of a folded dipole which 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 feed-point is at the centre of the element having the diameter d1.

Consequently, in this calculation we use the value of single-wire dipole, the feed-point impedance will be transformed upward by the ratio R according the equation seen below.

To make things a little easier here is a Folded Dipole Calculator...

In free space the impedance, of a resonant 1/2 wave dipole antenna with a centre feed is approximately 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.

**Please note: ** This is only a guide for experimentation, when looking around the Internet you'll find many variations to this calculation due of a number of reasons ranging from rounding down/up or ways to match the impedance of the antenna with the feed line.