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RF and Microwave Engineering: Fundamentals of Wireless Communications by Frank Gustrau

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7.1 Fundamental Parameters

7.1.1 Nearfield and Farfield

If we consider the electromagnetic fields of an antenna it makes a big difference if we look at the rather complex field distribution in the close vicinity of the antenna structure (nearfield) or if we look at the electromagnetic field distribution at greater distances from the antenna (farfield). Usually antennas that are used to establish a wireless link are far away from each other, therefore the important radiation characteristics are derived under farfield conditions.

In Section 7.3 we mathematically derive the electromagnetic fields of a canonical antenna element (Hertzian dipole). We will see that the equations simplify if we progress from the nearfield region to the farfield region. Figure 7.2 shows some commonly used terms to describe field regions around an antenna. The field region closest to the antenna is known as the reactive nearfield. In the nearfield region fields are mainly associated with the storage of electric and magnetic energy. On the other hand, in the farfield region (Fraunhofer zone) the fields are associated with radial power transfer. The product of the Poynting vector images/c07_I0001.gif and distance squared (images/c07_I0002.gif) is independent of the distance r. As we will see in Section 7.1.3 this leads to a radiation pattern with an ...

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