Short Dipole Antenna / Aerial Tutorial
The short dipole antenna is a small version of the popular dipole antenna, typically less than a tenth of a wavelength in its size.
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The short dipole antenna is one that is short when compared to a wavelength at the operating frequency.
Typically a short dipole antenna is taken to be one that is less than a tenth of a wavelength long. However this is very much a 'rule of thumb' and slightly different definitions may appear in various quarters.
The short dipole antenna consists of two co-linear conductors that are placed end to end, but with a small gap between them for the feeder.
Although the half wave dipole antenna is the most popular, the short dipole is also used, especially where space may be an issue, or where a non-resonant antenna is needed.
Short dipole antenna basics
As already mentioned the short dipole is a form of dipole antenna created by feeding a wire, typically in the centre with a signal. The electrical length of the overall radiating element typically has to be less than a tenth of a wavelength to make a short dipole antenna.
In practice, short antennas, and in this case the sort dipole antenna is rarely satisfactory from an efficiency viewpoint because much of the power entering it is dissipated as heat as the resistive losses are normally very high.
This factor reflects into the reception instance where only low reception efficiency is attained and low signal levels pervade. Accordingly poor signal to noise ratios will be achieved in the receiver.
The basic concept of a short dipole antenna is shown in the diagram below.
Current distribution for the short dipole follows the same sinusoidal curve as used for all other forms of dipole. However as only the end section of the sine curve is applicable, this can be equated to a straight line without introducing any major errors.
Short dipole antenna radiation resistance
As with any antenna, one of the key parameters of the antenna is its radiation resistance. This is required to be able to determine the overall feed impedance and hence the required matching. It is possible to calculate the radiation resistance of the short dipole antenna.
Rr = radiation resistance in ohms
L = length of antenna element (both sections together)
λ = wavelength
Both length measurements must be in the same units.
The overall input feed impedance for the short dipole antenna is comprised of a number of different elements: the series inductance, the capacitance, radiation resistance, and the Ohmic resistance. These need to be combined vectorially to obtain the overall feed impedance.
By definition the antenna is shorter than a half wavelength - it operates below the natural half wavelength resonant frequency. As a result, the main reactive element will be the capacitive reactance. Accordingly the antenna impedance is complex, i.e. contains capacitive reactance (in this case) as well as the resistive component. Rather than using a coaxial feeder, that might be the easy option, a balanced feeder should be used between the antenna itself and any impedance matching circuit. This will be able to handle the high standing wacv levels better.
Short dipole radiation pattern
As with other antennas there are two planes of interest for the directional pattern or polar diagram of the short dipole antenna.
When the dipole is vertical, the horizontal radiation pattern is just a circle. Then in any vertical plane through the axis, the field strength varies according to sinθ. In fact the radiation pattern of a short dipole looks like that of a half wave dipole - the familiar figure of '8' shape..
Whilst the short dipole may not be as efficient or effective as a half wave resonant dipole, it nevertheless meets a need where a smaller antenna is required, or where a non-resonant antenna is needed. It will present a high impedance to the feeder as the antenna is fed where the voltage remains high, however it is still possible to feed it and utilise its advantages, especially when low power levels are used. At high power levels the voltages on the short dipole rise and can become an issue if not managed sufficiently well..
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