Folded Dipole Antenna 

Folded Dipole Antenna:

Recall that the standard half-wavelength dipole (Hertz antenna) has an input impedance of . Recall also that it becomes very inefficient whenever it is not used at the frequency for which its length equals (i.e., it has a narrow bandwidth). The folded dipole antenna is shown in Figure 1(a) offers the same radiation pattern as the standard Hertz antenna but has an input impedance of (approximately 4 x ) and offers relatively broadband operation.

A standard Hertz antenna can provide the same broadband characteristics as the folded dipole by incorporating a parallel tank circuit, as shown in Figure 1(b). With the tank circuit resonant at the frequency corresponding to the antenna’s length, the tank presents a very high resistance in parallel with the antenna’s and has no effect. However, as the frequency goes down, the antenna becomes capacitive, while the tank circuit becomes inductive. The net result is a resistive overall input impedance over a relatively wide frequency range.

The folded dipole is a useful receiving antenna for broadcast FM and VHF TV. Its input impedance matches well with the input impedance terminals common to these receivers. It can be inexpensively fabricated by using a piece of standard parallel wire transmission line cut to at the mid-band and shorting together the two at each end. Folded dipoles are also invariably used as the driven element in Yagi-Uda antennas. This helps to maintain a reasonably high input impedance since the addition of each director lowers this array’s input impedance. It also gives the antenna a broader band of operation.

In applications where a folded dipole with other than a impedance is desired, a larger-diameter wire for one length of the antenna is used. Impedances up to about are possible in this manner.