Folded dipole antenna

- Overview, summary, tutorial about the folded dipole antenna, a form of RF antenna design that is widely used on its own and as the basis for other RF antenna designs, proving greater bandwidth and higher impedance.

This dipole antenna tutorial is split into several pages each of which addresses a different aspect of dipole antenna technology:

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 that can be used to advantage.

In its basic form a dipole consists of a single wire or conductor cut in the middle to accommodate the feeder. It is found that the feed impedance is altered by the proximity of other objects, especially other parasitic elements that may be used in other forms of RF antenna design. This can cause problems with matching and because resistance losses in the RF antenna system can start to become significant.

Additionally many RF antennas have to be able to operate over large bandwidths and a standard dipole may be unable to fulfil this requirement adequately.

A variation of the dipole, known as a folded dipole provides a solution to these problems, offering a wider bandwidth and a considerable increase in feed impedance. The folded dipole is formed by taking a standard dipole and then taking a second conductor and joining the two ends. In this way a complete loop is made as shown. 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 a feed impedance of around 300 ohms. Additionally the RF antenna has a wider bandwidth.

Impedance increase

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 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.

Further pages from this tutorial
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