Folded dipole antenna

- notes and summary about the folded dipole antenna, folded dipole impedance, unequal conductor folded dipoles, and multi-wire folded dipoles.

Dipole antenna tutorial includes:

    •  Dipole antenna
    •  Dipole length calculation
    •  Dipole feed impedance
    •  Folded dipole antenna

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 provides a number of advantages.

The folded dipole 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 basics

In its basic form the folded dipole 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 dipole 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 balan of some form is used to transform from an unbalanced to balanced feed structure.

Simple half-wave folded dipole antenna

One of the main reasons for using the folded dipole 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 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 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 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 advantages

There are a number of advantages or reasons for using a folded dipole:

• 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 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. 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:

Where:
    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.

Folded dipole applications

Folded dipoles are sometimes used on their own, but they must be fed with a high impedance feeder, typically 300 ohms. However they find more uses when a dipole is incorporated in another RF antenna design with other elements nearby. This has the effect of reducing the dipole impedance. To ensure that it can be fed conveniently, a folded dipole may be used to raise the impedance again to a suitable value.

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