Three Wire Folded Dipole Antenna (Folded Tripole)

If three dipole antennas are connected in parallel to form a thin wire loop, then it is known as a three-wire folded dipole antenna or Folded Tripole antenna.

If three wire folded dipole antennas are used with equal radii, then the equal current flows in all the three conductors. If the total current fed at the terminal is ‘I’ then each dipole will have current ‘I/3’.

Thus, with the same power applied, only one-third of the total radiating current flows in the first dipole, and hence the input impedance increases and it becomes nine times. The folded dipole antenna is shown in the figure below.

Thus, for a three-wire folded dipole with equal radius, the input impedance or radiation resistance is given by

$\begin{align*} \begin{split} Z = R_r = n^2*73 \\ = 3^2 * 73 \\ = 9 * 73 \\ Z = 657 \,\, \Omega \end{split} \end{align*}$

Therefore, three-wire folded dipole or folded tripole can be fed with a conventional 600 Ω two-wire open transmission line without any matching device.

So, the folded dipole antenna or tripole antenna has an important impedance transforming properties. This makes it easy to match with a transmission line that feeds the antenna. It is also possible to change the input impedance by keeping the radius of the two dipoles unequal. In this condition, a larger current flow through a thicker dipole and hence we can maintain any desired input impedance.

Advantages of a Folded Dipole Antenna

Some of the advantages of a folded dipole antenna include:

It has high input impedance; hence it makes it easy to match with the transmission line.
It has wideband frequency i.e., wide bandwidth, hence it is suitable for FM and TV broadcast.
It has high gain and high directivity compared to a simple dipole antenna hence, it can be used in the Yagi-Uda antenna.

FM Dipole Antenna

FM dipole antenna is defined as the vertically polarized half-wave half dipole antenna. The FM dipole antenna is mostly used as it is to build and it is suitable to provide improved reception of VHF FM broadcast. The FM dipole antenna is shown in the figure below.

The radiation pattern of the FM dipole antenna is perpendicular to the axis of the antenna i.e., It is horizontal because the FM dipole antenna is a vertically polarized half-wave dipole antenna. The radiation pattern of the FM dipole antenna is shown in the below figure.

Radiation Pattern of A FM Dipole Antenna — Radiation Pattern of a FM Dipole Antenna

The FM dipole antenna is generally used for FM broadcasting frequency range between 88 MHz to 108 MHZ.

Fan Dipole Antenna

Fan dipole or Multi-band wire antenna is one in which multiple dipoles are connected with a common feed line, and they are spread out like a fan hence it is called a fan dipole antenna.

As the name implies that the form of the fan dipole antenna looks like a fan. It is also known as a Parallel dipole antenna.

In the fan dipole antenna, each dipole of the multi-band is cut from the center of the bands and connected to a common feeder. The dipole should be cut from where we want it to radiate. When the signal is transmitted, we only receive the signal from that band which is radiating, because the other dipoles have a higher impedance compared to that radiating element.

The fan dipole antenna is shown in the above figure. It is 4 Bands fan dipole antenna. Here, we used 80 m, 40 m, 20 m, and 10 m multi-band dipole that are connected in parallel with a common feeder line.

When 80 m dipole is radiating means the current pass through 80 m dipoles, in this condition, we receive signal only from 80 m bands because it has a lower impedance while other 40 m, 20 m, and 10 m band have a higher impedance compared to the 80 m radiating element.

Similarly, when the 40 m dipole is radiating, we only receive the signal from the 40 m bands because it has a lower impedance compared to the other dipole. Note that according to Kirchhoff’s Current Law, other dipoles are also radiating but not efficiently.

Half-Wave Dipole Antenna

The Half-Wave Dipole Antenna is the most widely used type of dipole antenna. As the name suggests that, the total length of the dipole antenna is equal to the half-wavelength ( $\frac{\lambda}{2}$ ) at the frequency of operation.

It is known as a half-wavelength dipole or simply dipole or doublet. It is also known as the Hertz antenna.

The half-wave dipole antenna consists of a two-quarter wavelength conductor with the feed point at the center. It is the symmetrical antenna in which the two ends are at the equal potential to the center point.

The current distribution in the half-wave dipole antenna is approximately sinusoidal along the length of the dipole i.e., a standing wave in nature. The basic half-wave dipole antenna and voltage and current distribution on it are shown in the below figure.

Current and Voltage Distribution In A Half Wave Dipole Antenna — Current and Voltage Distribution in a Half-Wave Dipole Antenna

The radiation pattern of the half-wave dipole antenna is shown in the below figure. It shows that the directions of maximum radiation are perpendicular to the conductor or the axis of the antenna. It is also perpendicular to the direction of the antenna current.

Radiation Pattern of a Half Wave Dipole Antenna In Free Space — Radiation Pattern of a Half-Wave Dipole Antenna

The half-wave dipole antenna can operate around 3 kHz to 300 GHz frequency range; hence It is mostly used in radio receivers.

Short Dipole Antenna

The short dipole antenna is the simplest type of antenna from all the antennas. A short dipole antenna is one in which the length of the wire is less than half of the wavelengthi.e.,( $<\frac{\lambda}{2}$ ).

In a short dipole antenna, the feed impedance starts to increases and its response is less dependent upon the frequency changes. The current distribution in the short dipole antenna is approximately triangular.

The length of the short dipole antenna in between $\frac{\lambda}{50}$ to $\frac{\lambda}{10}$ . i.e., $\frac{\lambda}{50} < l < \frac{\lambda}{10}$ . The current distribution on the short dipole antenna is approximately triangular. The basic short dipole antenna and current distribution on it are shown in the below figure.

Current Distribution in a Short Dipole Antenna

The radiation pattern of the short dipole antenna is simply a circle. It is slightly different compared to the half-wave dipole antenna. The radiation pattern of the short dipole antenna and its comparison with the half-wave dipole antenna is shown in the below figure.

Radiation Pattern of a Short Dipole Antenna

Radiation Pattern of a Short Dipole Antenna And Half Wave Dipole Antenna — Radiation Pattern of a Short Dipole Antenna in Comparison With the Half-Wave Dipole Antenna

A short dipole antenna is used in place of the full half-wave dipole antenna in certain applications where the full half-wave dipole antenna is too large.