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The length of a dipole is the main consideration for determining its operating frequency.
Typically a dipole is an electrical half wavelength long, or a multiple of half wavelengths. It is therefore necessary to determine this length when designing the dipole antenna.
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, this length is not exactly the same as the wavelength for a signal travelling in free space.
There are several reasons for this and it means that an antenna will be slightly shorter than the length calculated for a wave travelling in free space. This results from the fact that the antenna is normally operating surrounded by air, and the signal is travelling in a conductor which is of finite length. The ‘end effects’ are the major reason for the different length.
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:
Dipole length formula
It is quite easy to calculate the length of a half wave dipole using the simple formulae given below. Two versions are given for this length calculation – one using the length in metres and the other for the length in inches.
The calculations from these formulas give a good starting point for determining he length of a dipole antenna. However factors like the proximity of the ground and other nearby objects also have an impact on the length and it is not easy to determine these beforehand.
Accordingly it is always best to make any prototype antenna slightly longer than the calculations might indicate and then shorten the antenna, measuring its performance each time. In this way the optimum performance can be obtained for that antenna.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.
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