# Folded Dipole Antenna

## Folded Dipole Antenna

A folded dipole antenna is a modified λ/2 dipole with an additional wire connecting its two ends. The folded dipole antenna is an extremely practical wire antenna and is also called an ultra closed spaced array. It consists of two parallel, closely spaced λ/2 dipoles joined together at the outer ends forming a narrow wire loop (d << L and d << λ). The antenna is fed at the center of one dipole, i.e., the dipoles have the same voltage at their ends. As well as radiation fields are concerned it is the same as the λ/2 dipole antenna, but input impedance differs and is equal to 300 Ω.

Folded dipole antenna differs from the conventional dipole mainly in two respects: directivity and wider bandwidth. The directivity of folded dipole antenna is bi-directional but because of the distribution of current in the parts of the folded dipole antenna the input impedance becomes higher, however, the radiation patterns of both are equal.

Yagi Uda Antenna

The folded dipole antenna does not accept power at any even harmonics (i.e., 2nd, 4th, …, etc.) of the fundamental frequency, however, it works with a low value of VSWR on odd harmonics (i.e. 3rd, 5th, …). This is because the current distribution of λ/2 and 3 λ/2 antennas are almost similar. That is, if any folded dipole antenna functions at 20 MHz, it will also function at 60, and 100 MHz frequencies. Since a simple λ/2-dipole antenna carries 73 Ω radiation resistances, so it is inconvenient to match it with a feed line of characteristic impedance (Z0) 300 Ω or so. However, a folded dipole offers terminal resistance of nearly 300 Ω and is found suitable for such impedance matching.

## Theoretical Analysis

The folded dipole is basically an unbalanced Tx line with unequal currents which radiates because of its unbalanced condition. A folded dipole antenna operation may be analyzed by considering its current to be composed of two distinct modes, namely Tx line mode and antenna mode. A model composed of these modes has been referred to as a transmission line model. This model accurately calculates the input impedance of folded dipole antenna provided the parallel wires are electrically close so that the usual transmission line equations apply. Using the transmission line model the input impedance of folded dipole antenna can be expressed as:

Z_{in}=\frac{4Z_{T}Z_{D}}{Z_{T}+2Z_{D}}=4Z_{D}\left ( \frac{1}{1+\frac{1}{2}\frac{Z_{D}}{Z_{T}}} \right )

Where, Z_{T}=jZ_{0}tan\left ( \beta \times \frac{L}{2} \right ). So if the length of antenna, i.e., L=λ/2,

Then Z_{T}=jZ_{0}tan\left ( \frac{2\Pi }{\lambda } \times \frac{\lambda }{4} \right )=\infty

Hence, Z_{in}=4Z_{D}

i.e. the half-wave folded dipole antenna offers a four-fold increase in input impedance over its λ/2 dipole version. Since the input impedance of a resonant antenna λ/2-dipole antenna is 73 Ω, therefore, Zinf = 4 × 73 = 292 ≈ 300 Ω (i.e. very close to impedance of common twin lead transmission). Similar to a half-wave dipole, a λ/2-folded dipole antenna also has real input impedance at resonance.

## Input Impedance of Folded Dipole

In general the input impedance of a folded dipole antenna is given by Zin = n2 × 73 provided all the wires carry equal currents. Where n is number of λ/2 dipoles having radiation resistance 73 Ω. However for folded dipole of unequal radii of the two dipoles the input impedance is modified to

Z_{in}=73 \left ( 1+\frac{a_{2}}{a_{1}} \right )^{2}

Since input impedance depends not only on radius of wires, but also on the separation between them, Uda and Mushiake proposed another formula for calculating the input impedance in terms of d as:

Z_{in}=73 \left [ 1+\frac{log\frac{d}{a_{1}}}{log\frac{d}{a_{2}}} \right ]^{2}=73\times Z_{r}

Where Z_{r}=\left [ 1+\frac{log\frac{d}{a_{1}}}{log\frac{d}{a_{2}}} \right ]^{2} is termed impedance transformation ratio/impedance set-up ratio.

The above equation is well suited when matching is done with low impedances, e.g. directive arrays using parasitic elements because the radiation resistance of these arrays is quite low.

## Salient features

1. It is a single antenna but consists of two elements.
2. The first is fed directly and the second is inductively coupled at the ends.
3. Its radiation pattern is the same as that of a straight dipole.
4. If the current fed is I, then the current in each arm is I/2 provided the two arms have the same dimensions. If it is a straight dipole, the total current I flows.
5. When the same power is applied, only half of the current flows in the first arm. Therefore, the input impedance is four times that of the straight dipole. That is, Rr = 4 × 73 = 292 Ω.
6. If the diameters of the two arms of the folded dipole are different, impedance transformation of 1.5 to 25 is achievable.
7. The spacing between the arms is very small and is of the order of λ/100.
8. It is used in the Yagi-Uda antenna as an active element.
9. It has the advantages of high input impedance, greater bandwidth, ease, and low cost of construction with better impedance-matching characteristics.
10. A typical radiation pattern of a folded dipole is shown in the figure below.

## Application of Folded Dipole

The folded dipole antenna is very useful as an FM broadcast band receiving antenna, particularly as an element in the Yagi- Uda, which is mostly used in television. In this Yagi, the driven element is folded dipole and the remaining elements are simple λ/2 dipole antennas.

It is constructed by cutting a piece of 300 Ω twin lead transmission line of length about λ/2. The ends are soldered together maintaining an overall length slightly less than λ/2 at the desired operating frequency.