Folded Dipole Antenna
- notes and summary about the folded dipole antenna, folded dipole impedance, unequal conductor folded dipoles, and multi-wire folded dipoles.
The standard dipole is widely used in its basic form. However under a number of circumstances a modification of the basic dipole, known as a folded dipole antenna provides a number of advantages.
The folded dipole antenna or folded dipole aerial is widely used, not only on its own, but also as the driven element in other antenna formats such as the Yagi antenna.
Folded dipole antenna basics
In its basic form the folded dipole antenna consists of a basic dipole with an added conductor connecting the two ends together to make a complete loop of wire or other conductor. As the ends appear to be folded back, the antenna is called a folded dipole.
The basic format for the folded dipole aerial is shown below. As can be seen from this it is a balanced antenna, like the standard dipole, although it can be fed with unbalanced feeder provided that a balun of some form is used to transform from an unbalanced to balanced feed structure.
Half wave dipole antenna
The folded dipole antenna uses an extra wire connecting both ends of the previous dipole as shown. Often this is achieved by using a wire or rod of the same diameter for all sections of the antenna, but this is not always the case.
Also the wires or rods are typically equi-spaced along the length of the parallel elements. This can be achieved in a number of ways. Often for VHF or UHF antennas the rigidity of the elements is sufficient, but at lower frequencies spacers may need to be employed. To keep the wires apart. Obviously if they are not insulated it is imperative to keep them from shorting. In some instances flat feeder can be used.
Half-wave folded dipole antenna
One of the main reasons for using the folded dipole aerial is the increase in feed impedance that it provides. If the conductors in the main dipole and the second or "fold" conductor are the same diameter, then it is found that there is a fourfold increase (i.e. two squared) in the feed impedance. In free space, this gives an increase in feed impedance from 73Ω to around 300Ω ohms. Additionally the RF antenna has a wider bandwidth.
Folded dipole impedance rationale
In a standard dipole the currents flowing along the conductors are in phase and as a result there is no cancellation of the fields and radiation occurs. When the second conductor is added to make the folded dipole antenna this can be considered as an extension to the standard dipole with the ends folded back to meet each other. As a result the currents in the new section flow in the same direction as those in the original dipole. The currents along both the half-waves are therefore in phase and the antenna will radiate with the same radiation patterns etc. as a simple half-wave dipole.
The impedance increase can be deduced from the fact that the power supplied to a folded dipole antenna is evenly shared between the two sections which make up the antenna. This means that when compared to a standard dipole the current in each conductor is reduced to a half. As the same power is applied, the impedance has to be raised by a factor of four to retain balance in the equation Watts = I^2 x R.
Folded dipole transmission line effect
The folded element of the folded dipole antenna has a transmission line effect attached with it. It can be viewed that the impedance of the dipole appears in parallel with the impedance of the shorted transmission line sections, although the arguments for the impedance given above still hold true - it is just another way of looking at the same issue.
This can help to explain some of the other properties of the antenna.
The length is affected by this effect. Normally the wavelength of a standing wave in a feeder is affected by the velocity factor. If air is used, this will by around 95% of the free space value. However if a flat feeder with a lower velocity factor is used, then this will have the effect of shortening the required length.
The feeder effect also results in the folded dipole antenna having a flatter response, i.e. a wider bandwidth than a non-folded dipole.
It occurs because at a frequency away from resonance, the reactance of the dipole is of the opposite form from that of the sorted transmission line and as a result there is some reactance cancellation at the feed point of the antenna.
Folded dipole advantages
There are a number of advantages or reasons for using a folded dipole antenna:
- Increase in impedance: When higher impedance feeders need to be used, or when the impedance of the dipole is reduced by factors such as parasitic elements, a folded dipole provides a significant increase in impedance level that enables the antenna to be matched more easily to the feeder available.
- Wide bandwidth: The folded dipole antenna has a flatter frequency response - this enables it to be used over a wider bandwidth.
Unequal conductor folded dipoles
It is possible to implement different impedance ratios to the standard 4:1 that are normally implement using a folded dipole antenna. Simply by varying the effective diameter of the two conductors: top and bottom, different ratios can be obtained.
Folded dipole with unequal conductor diameters
In order to determine the impedance step up ratio provided by the folded dipole, the following formula can be used:
d1 is the conductor diameter for the feed arm of the dipole
d2 is the conductor diameter for the non-fed arm of the dipole
S is the distance between the conductors
r is the step up ratio
When determining the length of a folded dipole using thick conductors, it should be remembered that there is a shortening effect associated with their use as opposed to normal wire or thin conductors.
Multiconductor folded dipoles
Although the concept of a folded dipole antenna often implies the use of one extra conductor, the concept can be extended further by adding additional 'folds' or conductors. This has the effect of increasing the overall impedance even more and further widening the bandwidth.
Three conductor folded dipole
Assuming the special case where all the conductors have the same diameter, then the impedance is increased by a factor of three squared, i.e. 9. This means that the nominal value for a folded dipole with three 'elements' is 600Ω
Again this can be useful in antennas such as Yagis, where the close spacing of the reflector and directors can significantly reduce the impedance of the driven dipole element. Using a folded dipole with three 'elements' may bring back the drive impedance to the required level.
Folded dipole applications
Folded dipole antennas are sometimes used on their own, but they must be fed with a high impedance feeder, typically 300 ohms. This on its own can be very useful in certain applications where balanced feeders may be used.
However folded dipoles find more uses when a dipole is incorporated in another RF antenna design with other elements nearby. The issue is that incorporating a dipole into an antenna such as a Yagi where elements are closely coupled reduces the feed impedance. If a simple dipole was used, then the feed impedance levels of less than 20 Ω or less can easily be experienced. Using a folded dipole enables the impedance to be increased by a factor of four or whatever is required by having multiple wires in the folded dipole.
The widespread use of folded dipole antennas can be seen when looking at their use in domestic television and VHF FM broadcast antennas that are used. In addition to this, folded dipoles are used in very many commercial applications as well.
By Ian Poole
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