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Dipole antenna

FM dipole antenna

Types of RF feeder

Dipole antenna

- Introduction to the dipole antenna or dipole aerial that is widely used on its own or within other antenna designs.

The dipole antenna or dipole aerial is one of the most important and also one of the most widely used types of antenna. It can be used on its own, or there are many other types of antenna that use the dipole as the basic element within the antenna.

The basic construction of a dipole is quite straightforward - a simple dipole antenna can be constructed from a few simple pieces of wire. In this way antennas including FM dipole antennas, or antennas for the short wave bands can easily be made. These antennas, while not having the performance of other more complicated types of antenna can nevertheless prove very effective and quite satisfactory in many applications.

Basic dipole facts

The name dipole means two poles and the antenna does in fact consist of two "poles" or sections. These are normally equal in length, making the antenna what is termed a centre fed antenna. Sometimes a dipole may not be fed in the centre, although this is not normally done in most antenna designs.

The power is applied to the dipole antenna itself through a feeder. Conversely if the dipole antenna is used for receiving, the received signals are taken away to the receiver through a feeder. The feeder serves to transfer the power to or from the antenna with as little loss as possible.

The basic dipole antenna configuration

The most common form of dipole has an electrical length of half a wavelength. As a result this antenna is called a half wave dipole. As before the lengths of the wires are both the same. As the total length of the dipole is a half wavelength, this makes each section or leg of the dipole a quarter wavelength long.

The basic half wave dipole antenna

Current and voltage on a dipole

In order that power flows into or out of an antenna that is transmitting or receiving, there must be associated currents and voltages. The levels of current and voltage vary along the length of the antenna, and it is found that the current distribution along a dipole is roughly sinusoidal. It falls to zero at the end and is at a maximum in the middle. Conversely the voltage is low at the middle and rises to a maximum at the ends. It is generally fed at the centre, at the point where the current is at a maximum and the voltage a minimum. This provides a low impedance feed point which is convenient to handle. High voltage feed points are far less convenient and more difficult to use.

When multiple half wavelength dipoles are used, they are similarly normally fed in the centre. Here again the voltage is at a minimum and the current at a maximum. Theoretically any of the current maximum nodes could be used.

Three half wavelength wave dipole antenna

Dipole feed impedance

All antennas have what is termed a feed impedance. This is the impedance that is seen at the point in the antenna where the feeder is connected. The impedance is measured in ohms, and to ensure that the maximum amount of power is transferred between the feeder and the antenna, it is necessary to ensure that the antenna and feeder impedances are matched, i.e. they have the same value.

The feed impedance of a dipole antenna is dependent upon a variety of factors including the length, the feed position, the environment and the like. A half wave centre fed dipole antenna in free space has an impedance 73.13 ohms making it ideal to feed with 75 ohm feeder.

The feed impedance of a dipole can be changed by a variety of factors, the proximity of other objects having a marked effect. The ground has a major effect. If the dipole antenna forms the radiating element for a more complicated antenna, then elements of the antenna will have an effect. Often the effect is to lower the impedance, and when used in some antennas the feed impedance of the dipole element may fall to ten ohms or less, and methods need to be used to ensure a good match is maintained with the feeder. One method is to use the folded dipole, outlined later on this page.

Polar diagram

Polar diagram of a half wave dipole in free space

If the length of the dipole antenna is changed then the radiation pattern is altered. As the length of the antenna is extended it can be seen that the familiar figure of eight pattern changes to give main lobes and a few side lobes. The main lobes move progressively towards the axis of the antenna as the length increases.

Dipole antenna length

The length of a dipole is the main determining factor for the operating frequency of the dipole antenna. Although the antenna may be an electrical half wavelength, or multiple of half wavelengths, it is not exactly the same length as the wavelength for a signal travelling in free space. There are a number of reasons for this and it means that an antenna will be slightly shorter than the length calculated for a wave travelling in free space.

For a half wave dipole the length for a wave travelling in free space is calculated and this is multiplied by a factor "A". Typically it is between 0.96 and 0.98 and is mainly dependent upon the ratio of the length of the antenna to the thickness of the wire or tube used as the element. Its value can be approximated from the graph:

Multiplication factor "A" used for calculating the length of a dipole

In order to calculate the length of a half wave dipole the simple formulae given below can be used:

Length (metres) = 150 x A / frequency in MHz

Length (inches) = 5905 x A / frequency in MHz

Using these formulae it is possible to calculate the length of a half wave dipole. Even though calculated lengths are normally quite repeatable it is always best to make any prototype antenna slightly longer than the calculations might indicate. This needs to be done because changes in the thickness of wire being used etc may alter the length slightly and it is better to make it slightly too long than too short so that it can be trimmed so that it resonates on the right frequency. It is best to trim the antenna length in small steps because the wire or tube cannot be replaced very easily once it has been removed.

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 that can be used to advantage. This type of antenna is often used in the simple FM dipole antennas that can be bought to use as temporary FM broadcast antennas. They are also used within other larger antennas such as the Yagi.

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 antenna. This can cause problems with matching and because resistance losses in the antenna system can start to become significant.

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

The basic folded half wave dipole

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 antenna has a wider bandwidth.

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.