The Folded Dipole

A folded dipole has two conductors in parallel, as you can see in Fig-3. This antenna has a high feed impedance (300 ohms). You may use a 4:1 balun transformer (see later chapter on baluns) to convert the 300-ohm balanced line to 75-ohm coaxial cable. A folded dipole may be used in the same manner as the single-wire dipoles in article Building and using Dipole Antennas .

The advantage of a folded dipole is that it provides slightuiy greater bandwidth between the 2:1 SWR points (most modern transmitters have a protective circuit that reduces the power as the SWR increases) than is characteristic of a single-wire dipole. For example, an 80-meter single wire dipole might have an SWR bandwidth (2:1 SWR boundries) of 75 kHz. 

A folded dipole for the same frequency may have an SWR bandwidth of 100 kHz. Apart from this consideration there is nothing to be gained from choosing a folded dipole. I have included it in this chapter for your enlightenment.

Example of a folded-dipole antenna. The characteristic feed impedance is 300 ohms. A 4:1 balun transformer converts the balanced feeder to a 75-ohm unbalanced line (RG-11 cable). This antenna may be fabricated from 300-ohm TV ribbon line or 450-ohm ladder line. The outer ends of the dipole are shorted as shown by the dots near the end insulators.

source : Novice Antenna Notebook

               by DeMaw, Doug