Dipole antenna

Dipole antenna tutorial includes:

The dipole antenna or dipole aerial is one of the most important and commonly used types of RF antenna. It is widely used on its own, and it is also incorporated into many other RF antenna designs where it forms the radiating or driven element for the antenna.

The dipole is a simple antenna to construct and use, and many of the calculations are quite straightforward. However like all other antennas, the in-depth calculations are considerably more complicated.

Dipole antenna basics

As the name suggests the dipole antenna consists of two terminals or "poles" into which radio frequency current flows. This current and the associated voltage causes and electromagnetic or radio signal to be radiated. Being more specific, a dipole is generally taken to be an antenna that consists of a resonant length of conductor cut to enable it to be connected to the feeder. For resonance the conductor is an odd number of half wavelengths long. In most cases a single half wavelength is used, although three, five, . . . . wavelength antennas are equally valid.

The basic half wave dipole antenna

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 polar diagram

The polar diagram of a half wave dipole antenna that the direction of maximum sensitivity or radiation is at right angles to the axis of the RF antenna. The radiation falls to zero along the axis of the RF antenna as might be expected.

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.

The dipole antenna is a particularly important form of RF antenna which is very widely used for radio transmitting and receiving applications. The dipole is often used on its own as an RF antenna, but it also forms the essential element in many other types of RF antenna. As such it is the possibly the most important form of RF antenna.

Dipole antenna length calculation & formula

notes and details about the dipole antenna length calculation & formula.

 Dipole antenna tutorial includes:

   The length of a dipole is the main determining factor for the operating frequency of the dipole antenna. Typically a dipole is a half wavelength long, or a multiple of half wavelengths.

However the dipole length is not exactly the same as the wavelength in free space - it is slightly shorter.

Dipole length variation from free space length

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

Dipole length formula

It is quite easy to use

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.

Computer simulation programmes are normally able to calculate the length of a dipole very accurately, provided that all the variables and elements that affect the operation of the dipole can be entered accurately so that the simulation is realistic and therefore accurate. The major problem is normally being able to enter the real-life environmental data accurately to enable a realistic simulation to be undertaken.

Dipole antenna feed impedance

Dipole antenna tutorial includes:

 The feed impedance of a dipole antenna is of particular importance. To ensure the optimum transfer of energy from the feeder, or source / load, the feed impedance of the dipole should be the same as that of the source or load.

By matching the feed impedance of the dipole to the source or load, the antenna is able to operate to its maximum efficiency.

Dipole feed impedance basics

The feed impedance of a dipole is determined by the ratio of the voltage and the current at the feed point. A simple Ohms Law calculation will enable the impedance to be determined.

Although a dipole can be fed at any point, it is typically fed at the current maximum and voltage minimum point. This gives a low impedance which is normally more manageable.

Most dipoles tend to be multiples of half wavelengths long. It is therefore possible to feed the dipole at any one of these voltage minimum or current maximum points which occur at a point that is a quarter wavelength from the end, and then at half wavelength intervals.

Three half wavelength wave dipole antenna showing feed point
points λ/4 from either end could also be used

The vast majority of dipole antennas are half wavelengths long. Therefore they are centre fed - the point of the voltage minimum and current maximum.

The basic half wave dipole antenna with centre feed point

The dipole feed impedance is made up from two constituents:

• Loss resistance:   The loss resistance results from the resistive or Ohmic losses within the radiating element, i.e. the dipole. In many cases the dipole loss resistance is ignored as it may be low. To ensure that it is low, sufficiently thick cable or piping should be used, and the metal should have a low resistance. Skin effects may also need to be considered.
• Radiation resistance:   The radiation resistance is the element of the dipole antenna impedance that results from the power being "dissipated" as an electromagnetic wave. The aim of any antenna is to "dissipate" as much power in this way as possible.

As with any RF antenna, 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.

Factors that alter the dipole feed impedance

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 form of RF antenna, then elements of the RF 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.

Folded dipole antenna

  •  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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