Dipole Radiation Pattern : Polar Diagram

- polar diagrams are used to indicate the response of dipole antennas and other antennas in different directions.

The radiation pattern of a dipole antenna is of particular importance.

The radiation pattern reflects the 'sensitivity' of the antenna in different directions and a knowledge of this allows the antenna to be orientated in the optimum direction to ensure the required performance.

Radiation pattern and polar diagram

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.

Diagram of the radiation pattern displayed as a polar diagram.
Antenna polar diagram concept

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 of a half wave dipole antenna that the direction of maximum sensitivity or radiation 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.

Diagram of the radiation pattern of a half wave dipole in free space, showing the maximum radiation at right angles to the axis of the antenna.
Radiation pattern of a half wave dipole antenna in free space

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

If the length of the dipole antenna is changed from a half wavelength then the radiation pattern is altered. As the length of the antenna is extended it can be seen that the familiar figure of eight pattern changes to give main lobes and a few side lobes. The main lobes move progressively towards the axis of the antenna as the length increases.

By Ian Poole

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