Antennas and Radiation F12 class.pdf

7/27/2019 Antennas and Radiation F12 class.pdf

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The simplest radiation sources are the Hertzian electric dipole and the magnetic

dipole. An understanding of these sources is often used to analyze much more

complicated structures. The Hertzian dipole is an infinitesimally small current

element.

The radiated fields can be found from the magnetic vector potential generated by

the dipole.

When far away from the dipole (in the far-field), the 1/βr term dominates. The

assumption here is that we are at least 6 wavelengths away (i.e. 2 pi wavelengths)

and that the length of the dipole is small compared to a wavelength.

When far away from the dipole (in the far-field), the 1/βr term dominates. The

assumption here is that we are at least 6 wavelengths away (i.e. 2 pi wavelengths)

and that the length of the dipole is small compared to a wavelength.

Fields are proportional to I and f and inversely proportional to r. Electric and

magnetic fields are perpendicular to one another and to r. The ratio of E to H gives

the intrinsic impedance of the medium, which in free space is η0=377 ohms. The

field strength for both E and H go to zero directly above or below the dipole.

Electric and magnetic fields generated by an infinitesimally small loop – a magnetic

dipole – can be found in a similar manner to the Hertzian (electric) dipole.

The far-field generated by a magnetic dipole and a Hertzian dipole are similar. For

a current loop, the fields are proportional to the loop area and the current and are

related to beta squared. For the current dipole, the fields are proportional to the

dipole length and the current and are related to beta.

A wire above a return plane can be dealt with using image theory, so the loop is

essentially twice as large (same current).

A dipole source is not a good radiation source, because the radiation resistance is

very small – it is very difficult to efficiently deliver much energy to the radiation

resistance, which is to say it is very difficult to convert energy into propagating

electromagnetic energy.

The impedance of is inversely proportional to level of current at the input. For

maximum efficiency, the current should be maximum at the source location (i.e. it is

easiest to force radiating current into the antenna).

To find the field strength from the voltage measured at the antenna requires

knowledge of the antenna factor and possibly the cable loss. The field strength can

be obtained simply by adding the measurement, antenna factor, and cable loss in

dB, as shown in this slide. Note that the antenna factor changes as a function of

frequency. Gain describes the directionality of the antenna. It is given by the ratio of

the maximum power radiated in a direction by the antenna divided by the power thatwould be radiated by an isotropic antenna.

Antennas and Radiation F12 class.pdf

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