What is a Folded Dipole Antenna?

A Folded dipole antenna is a half-wave dipole antenna with an additional parallel wire or rod connecting its two ends and folded to form a cylindrical closed shape. One pole (rod) is continuous with a length of λ/2 and the other is split at the center. During transmission, the antenna is fed at the center terminals of the two rods. Similarly, while receiving, the antenna receives the signal from these two center terminals. Folded dipole antennas usually operate in the frequency range from 3 kHz to 300 GHz and are ideal for use where an optimum power transfer and large input impedance is required.

How does a Folded Dipole Antenna Work?

When a sinusoidal signal at a certain frequency is applied to the folded dipole antenna, it results in the back-and-forth movement of the electrons (oscillating charges). Due to the oscillating charges, time-varying electric and magnetic fields are created between the conductors and propagate away from the antenna. The radiation patterns of half-wave folded dipole antenna and half-wave dipole antenna are almost the same.

Fig. 1: Radiation pattern of Half-wave folded dipole antenna

A conventional dipole antenna is a type of antenna that features two identical wires or rods (Fig. 2). The length of each rod is equal to the quarter wavelength (λ/4) of the operating frequency and the two rods are separated by an insulator at the center. A dipole antenna is also known as a half-wave dipole antenna because the length of the dipole is half of the operating wavelength (λ/2).

The main benefit of a folded dipole antenna is that it provides higher input impedance as compared to the conventional half-wave dipole antenna (also called dipole antenna). Typically, the input impedance of folded dipole antenna (Zf) is four times the input impedance of dipole antenna (Zd ≈ 70 ohms).

At the resonant condition, an input impedance in the range of 300 ohms can be achieved for a folded dipole antenna, which is suitable for connections to “twin-lead” transmission lines. The resonant condition occurs when the antenna is operating at a resonant frequency, which is the frequency at which the input impedance of antenna is entirely real. In the resonant condition, the real input impedance of the antenna can be matched with a real characteristic impedance of the transmission line. Hence, at resonant frequency, impedance matching is possible that results in well-radiated power and antennas are operated at their resonant frequency.

How is Impedance Variation Obtained in Folded Dipole Antenna?

In the folded dipole antenna, greater step-up transformation of input impedance can be obtained by adding more elements (i.e., more parallel rods) with identical diameters and lengths (l ≃ λ∕2). For example, a three-element folded dipole offers input impedance approximately nine times greater than that of the conventional dipole antenna or about 650 ohms, making it a good match for open wire feed cable. Small variations in impedance are also possible by using elements of slightly different diameters and/or lengths, and more accurate impedance variation is possible with smaller spacing-to-diameter (s/d) ratio.

Symmetrical and Asymmetrical Planar Folded Dipoles

Symmetrical and asymmetrical planar folded dipole antennas can be designed and constructed by using microstrips. The microstrip can be fabricated using printed circuit technology, and the input impedance variations can be obtained by adjusting the width of the strips.

Advantage of folded dipole antenna

• Higher input impedance, from 73 ohms to around 300 ohms, compared to conventional half dipole antennas
• Draws lower current from source
• Matches well with characteristics impedance of transmission lines
• Better bandwidth characteristics than a single dipole of the same size
• Offers superior signal/noise ratio performance, good signal reception
• Receives the desired signal from a band of frequency without losing signal quality

Disadvantage of folded dipole antenna

• Displacement and adjustment of the antenna are difficult.
• Outdoor management of antenna can be difficult when antenna size increases

Applications of folded dipole antenna

The folded dipole antenna is ideal for use as a feeder element in Yagi antenna, Parabolic antenna, turnstile antenna, log periodic antenna, phased and reflector arrays, etc. It is also used in radio receivers and is commonly used in TV receiver antennas.