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One relatively easy method of creating a multi-band dipole is to have several individual dipoles fed from the same point on one feeder, either with wires running parallel to each other, or as a fan emanating from the centre point. As a result, these dipoles are often called fan dipoles or fan multi-band dipoles.
Each dipole is resonant on its own frequency and will radiate as a resonant dipole for its own frequency, making this an easy way to provide a multi-band capability that enables a number of different bands to be covered using a single feeder.
These fan or parallel dipole antennas provide multi-frequency or multi-band oepration that is used for a variety of commercial radio communications applications as well as for ham radio where they can allow operation on multiple bands using a single feeder.
Operation of HF multiband fan dipole
The way in which the HF multi-band fan dipole operates is that each dipole presents a low impedance at the feed point at its resonant frequency. As the frequency moves away from the resonant frequency of one dipole, its impedance increases and it does not absorb power.
However at the resonant frequency of another dipole, the impedance falls and it will take power from the feeder.
In this way, the multi-band fan dipole will appear to have a number of resonant frequencies, each corresponding to the resonant frequency of the different dipoles. In the case of the fan dipole above, the length L1 is a quarter wavelength at the lowest frequency band, L2 the next one up, and finally L3 is a quarter wavelength at the highest band.
When designing a fan dipole, care should be taken to ensure that the one dipole resonant frequency does not correspond to the third fifth, etc harmonic of another dipole as both will have a low impedance at this point.
Implementations of the fan dipole
Although the theoretical concept of the multi-band fan dipole is quite straightforward, there are a number of ways in which it can be implemented.
If the antenna is set up as a set of parallel dipoles, then the dipole for the lowest frequency will tend to carry the weight of all the other dipoles and this can causes the whole antenna to sag.
To reduce the sag there are several approaches that can be taken. This first is to reduce the number of additional dipoles added to reduce the weight, and another is to implement the parallel dipole antenna as an inverted V as this helps reduce the sag quite considerably.
It is not necessary that al the wires run in parallel making a parallel wire version of the fan dipole. It is also possible to take the different dipole wires away from the feeder in different directions, fanning them out as required. When this approach is taken it is necessary to have a number of different anchor points - one for each end of each dipole .
When this approach is adopted, it is quite common to have the different wires, fanning out almost as a cone, creating a number of different inverted V dipoles, but all fed from the same feeder. It is also possible to have these wires horizontal as well, although finding sufficient anchor points may not be so easy.
However this solution is particularly applicable when a single central pole or mast is available to provide a high centre point which will enable the areas of the antenna that give the main radiation to be as high as possible and hence radiate the best signal.
Determining the lengths of the fan dipole
Each of the halves of the dipoles is an electrical quarter wavelength long - the two halves giving a total of a half wavelength. The feed is in the centre where the current is highest and the voltage lowest and this gives a convenient low feed impedance fort he antenna.
A good starting point for calculating the length of the various sections is the standard dipole length calculation equations. However be aware that the actual lengths for resonance can be longer than the calculated lengths.
The different elements of the antenna can affect each other. In view of this, it is always best to add a little extra length and be prepared to cut it back or 'prune' the lengths to get the required VSWR, etc and thereby enabling successful radio communications performance.
In view of the effect that the different dipoles have on each other, adjusting the length can be difficult, especially if the dipole has many sections for different bands. Often, two or three dipoles connected to the same feeder can give an antenna that can be reasonably adjusted. Adding further dipoles can make the job of accurately adjusting the different dipoles to length exceedingly difficult.
It is found that not only can the length of the dipoles be sometimes longer that expected by a reasonable margin, but also the length adjustment of one affects al of the others, so by the time all have been adjusted, the first one needs readjustment and so on.
The multi-band fan dipole or multi-band parallel dipole provides a very neat solution for an antenna with multiple band capability. These dipoles are used for many radio communications applications and can provide a very neat and easy to implement solution for a multi-band dipole. One of the keys tot he successful implementation of these antennas is not to be too greedy and want to add too many bands. My experience has shown that two or possibly three band versions work very well and can provide excellent performance for some favourite bands.
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