Dipole Antennas

Dipole Antennas

Dipole antennas are cheap, easy to manufacture and install. They are used as tv antennas for example. This is a tutorial that uses an interactive java applet to examine the radiation properties of these types of antennas.

Let us consider the dipole antenna of length 2L at the origin of the spherical coordinate system shown in the figure below.

dipole in spherical coordinate system

If we assume that the current flowing in the dipole is along the z axis and is of the form

dipole in spherical coordinate system

then the amplitudes of the electric and magnetic components of the field radiated by the dipole in the far field zone may be approximated by the formulas (in free space the intrinsic impedance may be approximated by 120Pi).

dipole in spherical coordinate system


where

dipole in spherical coordinate system
and L is the half length of the dipole.

The average radiated power density is given by the formula

dipole in spherical coordinate system

We now use an applet to examine in more details the distribution of the power density in terms of angle theta (Θ) as the length 2L of the antenna dipole is changed. Click on the button below to start the applet and change the position of the slider, in the left panel, in order to change the length 2L. For each value of the length, we obtain an intensity pattern on the left panel that also displays the length of the dipole in terms of the wavelength. The half power beam width ( the angle between the two red lines) which indicates the directivity of the antenna is also displayed. The beam width changes with the length 2L of the antenna.

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