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Antenna Fundamentals part. 1

Antenna Fundamentals part. 1

ctual result.

Figure 4. Power flow resulting in radiation.

Figure 5. a Dipole current with ??? wave excitation, b full wave excitation, c dipole impedance.

Figure 6. a Side view of half wave dipole array and b top view with power distribution.

A magnetic field that is time-varying induces a current in the perfect conductor. The current opposes the magnetic field so that no normal component can penetrate the conductor surface. Thus the current image shown in Figure 7c causes the resulting normal component of H to disappear at the surface.

The effect of the image is very important because antennas are often nearby conductive surfaces such as the Earth, or the sheet metal of a car or airplane, or the ground plane of a circuit board. The fields that radiate into space are the sum of those from the antenna and those from the image. If we consider the E-field from a dipole, it is easy to see the effect. In Figure 8a a dipole parallel to conductor is shown with its image. When the dipole is perpendicular to the ground plane, an image of the dipole with inverted charge exists below it -- as shown in Figure 8b. In these two examples, the field at some point in space is the sum of the fields from the dipole and its image. When the field radiating from a dipole hits the conductor, as shown in Figure 8c, the reflection can be interpreted as the wave from the image.


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