However factors like the proximity of the ground and other nearby objects as mentioned above also have a significant impact on the length and it is not easy to determine these beforehand.Īccordingly 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. The calculations from these formulas give a good starting point for determining the length of a dipole antenna. The actual figure derived is 467.48, but this is close enough for most applications, especially as the other factors including nearby objects, etc have a significant effect on the length. This can be derived by taking the figure of 492 seen in the formula above and multiplying it by the typical A or end effect factor of 0.95. Often a formula for the length of a dipole in feet is seen as 468 / frequency. Two versions are given for this length calculation – one using the length in metres and the other for the length in inches. It is quite easy to calculate the length of a half wave dipole using the simple formulae given below. Accordingly, it is usual practice to determine the most likely length for the antenna, cut it a little longer than is expected and then check the characteristics of the antenna experimentally, altering the physical length as necessary. This makes an accurate length calculation difficult if not impossible to make under practical conditions. This may even exceed the variation in length caused by practical variations in conductor diameter. Its value can be approximated from the graph: Multiplication factor "A" used for calculating the length of a dipoleĪs a further complication, the supporting insulators, feed systems, and other surrounding objects, such as the earth, buildings, trees, etc have a noticeable effect upon the electrical length.
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.
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Wire diameter also has a marked influence on this as shown in the diagram.įor a half wave dipole the length for a wave travelling in free space is calculated and this is multiplied by a factor "A". It is found that the antenna end effect increases with frequency and it also varies with different installations. This serves to effectively lengthen the antenna.
More specifically, the antenna end effect results from a decrease in inductance and an increase in capacitance towards the end of the antenna conductor. The end effect 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. 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 as a result of what is lightly termed the "End Effect." Current and voltage waveforms on a half wave dipole showing the need to calculate the antenna length Dipole length variation from free space lengthĪlthough the antenna may be an electrical half wavelength, or multiple of half wavelengths, this physical length is not exactly the same as the wavelength for a signal travelling in free space. However the physical dipole length is not exactly the same as the electrical wavelength in free space - it is slightly shorter, and it is often necessary to calculate the dipole length as best as possible. Most dipole antennas are a half wavelength long, and accordingly it is often necessary to calculate the physical length of a half wavelength dipole antenna. The length of a dipole is the main consideration for determining its operating frequency, and as a result, calculating the length is a key element of designing and installing any dipole antenna whether for HF, VHF, or UHF, etc. Dipole Antenna Length: calculation & formula Notes and details about the dipole antenna length calculation & formula for a half wave dipole.ĭipole antenna basics Current & voltage Half wave dipole Folded dipole Short dipole Doublet Dipole length Dipole feeds Radiation pattern Build HF ham dipole Inverted V dipole HF multiband fan dipole HF multiband trap dipole G5RV antenna FM dipole design
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