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. 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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