Dipole antenna length calculation & formula

- notes and details about the dipole antenna length calculation & formula for a half wave dipole.

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:

Factor A used for calculating the length of a dipole

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.

By Ian Poole


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