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The radiation pattern of a dipole antenna is of particular importance for many reasons it needs to be orientated so that it picks up the maximum level of signal or radiates the maximum amount of signal in the required direction.
The radiation pattern reflects the amount of power radiated from the dipole in any given direction. As the performance of the antenna is the same in transmit and receive, it also reflects the 'sensitivity' of the antenna in different directions.
An understanding of the dipole radiation pattern enables the antenna to be orientated in the optimum direction at any time.
Radiation pattern & polar diagram
As the directional performance of an antenna is the same in transmit and receive, it does not matter which mode is measured to produce a plot of the radiation pattern. As it is easier to undertake measurements with power being transmitted from the antenna with measurements of the signal strength received around it, this is the method that is adopted.
The radiation pattern of any antenna can be plotted. This is plotted onto a polar diagram.
A polar diagram is a plot that indicates the magnitude of the response in any direction.
At the centre of the diagram is a point of referred to as the origin. This is surrounded by a curve whose radius at any given point is proportional to the magnitude of the property measured in the direction of that point.
Polar diagrams are used for plotting the radiation patterns of antennas as well as other applications like measuring the sensitivity of microphones in different directions, etc.
The radiation pattern shown on a polar diagram is taken to be that of the plane in which the diagram plot itself. For a dipole it is possible to look at both the along the axis of the antenna and also at right angles to it. Normally these would be either vertical or horizontal planes.
One fundamental fact about antenna radiation patterns and polar diagrams is that the receiving pattern, i.e. the receiving sensitivity as a function of direction is identical to the far-field radiation pattern of the antenna when used for transmitting. This results from the reciprocity theorem of electromagnetics. Accordingly the radiation patterns the antenna can be viewed as either transmitting or receiving, whichever is more convenient.
Half wave dipole radiation pattern
The radiation pattern polar diagram for a half wave dipole antenna shows that the direction of maximum radiation or sensitivity is at right angles to the axis of the RF antenna. The radiation falls to zero along the axis of the RF antenna as might be expected.
In a three dimensional plot, the radiation pattern envelope for points of equal radiation intensity for a doughnut type shape, with the axis of the antenna passing through the hole in the centre of the doughnut.
Radiation patterns for multiple half wavelength dipoles
A dipole antenna does not necessarily need to be half a wavelength long. Other lengths for dipoles are frequently used.
The radiation pattern changes with the length of the antenna. As the length increases in proportion to a wavelength, the number of major lobes or points of maximum radiation increases, and they move outwards aligning further with the axis of the antenna..
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