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The current and voltage waveforms that appear along the length of a dipole antenna are of importance in many instances.
Both the dipole current and voltage waveforms may impact the way that the antenna is used, and therefore an understanding of these is important.
Dipole current & voltage
The current and voltage on any radiating element vary along its length, and this is true for the dipole as well as for any other antenna.
The current variation occurs because standing waves are set up along the length of the radiating element. This result peaks and troughs along the length of the antenna element.
The current falls to zero at the end and then varies sinusoidally reaching a peak a quarter wavelength away from the end.
Conversely, the voltage peaks at the end and then varies as the cosine as the distance away from the end increases. It reaches a minimum a quarter wavelength from the end.
Dependent upon the length of the antenna, there may be several peaks and troughs of current and voltage along the length of the radiating element.
The most popular form of dipole antenna is the half wave and for this, the current is at a minimum at the ends and rises to a maximum in the middle where the feed is applied.
Conversely the voltage is low at the middle and rises to a maximum at the ends. It is generally fed at the centre, at the point where the current is at a maximum and the voltage a minimum. This provides a low impedance feed point which is convenient to handle. High voltage feed points are far less convenient and more difficult to use.
It is easy to remember where the current and voltage minima are. As an aide memoire it can be thought of that the voltage is at a maximum at the ends because it is the point where it is effectively open circuit, and the current is zero there, because there is nowhere for it to flow.
Other forms of dipole
When multiple half wavelength dipoles are used, they are similarly normally fed in the centre. Here again the voltage is at a minimum and the current at a maximum. Theoretically any of the current maximum nodes could be used.
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