Antenna Theory - Half-Wave Folded Dipole

A folded dipole is an antenna, with two conductors connected on both sides, and folded to form a cylindrical closed shape, to which feed is given at the center. The length of the dipole is half of the wavelength. Hence, it is called as half wave folded dipole antenna.

Frequency range

The range of frequency in which half wave folded dipole operates is around 3KHz to 300GHz. This is mostly used in television receivers.

Construction & Working of Half-wave Folded Dipole

This antenna is commonly used with the array type antennas to increase the feed resistance. The most commonly used one is with Yagi-Uda antenna. The following figure shows a half-wave folded dipole antenna.

Half Wave Dipole

This antenna uses an extra conducting element (a wire or a rod) when compared with previous dipole antenna. This is continued by placing few conducting elements in parallel, with insulation in-between, in array type of antennas.

The following figure explains the working of a half-wave folded dipole antenna, when it is provided with excitation.

Half-Wave Folded Dipole

If the diameter of the main conductor and the folded dipole are same, then there will be four folded (two times of squared one) increase in the feed impedance of the antenna. This increase in feed impedance is the main reason for the popular usage of this folded dipole antenna. Due of the twin-lead, the impedance will be around 300Ω.

Radiation Pattern

The radiation pattern of half-wave folded dipoles is the same as that of the half-wave dipole antennas. The following figure shows the radiation pattern of half-wave folded dipole antenna, which is Omni-directional pattern.

Half-Wave Folded Radiation

Half-wave folded dipole antennas are used where optimum power transfer is needed and where large impedances are needed.

This folded dipole is the main element in Yagi-Uda antenna. The following figure shows a Yagi-Uda antenna, which we will study later. The main element used here is this folded dipole, to which the antenna feed is given. This antenna has been used extensively for television reception over the last few decades.

Yagi Uda Antenna

Advantages

The following are the advantages of half-wave folded dipole antenna −

  • Reception of balanced signals.

  • Receives a particular signal from a band of frequencies without losing the quality.

  • A folded dipole maximizes the signal strength.

Disadvantages

The following are the disadvantages of half-wave folded dipole antenna −

  • Displacement and adjustment of antenna is a hassle.

  • Outdoor management can be difficult when antenna size increases.

Applications

The following are the applications of half-wave folded dipole antenna −

  • Mainly used as a feeder element in Yagi antenna, Parabolic antenna, turnstile antenna, log periodic antenna, phased and reflector arrays, etc.

  • Generally used in radio receivers.

  • Most commonly used in TV receiver antennas.

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