3. Poynting vector
• The quantity used to describe the power
associated with an electromagnetic wave is
the instantaneous Poynting vector defined
as
3
4. Total Power
• Since the Poynting vector is a power density,
the total power crossing a closed surface can
be obtained by integrating the normal
component of the Poynting vector over the
entire surface
4
5. Average Power
• For applications of time-varying fields, it is
often more desirable to find the average
power density which is obtained by
integrating the instantaneous Poynting vector
over one period and dividing by the period
5
6. Average power density
• For time-harmonic variations of the form
e(jωt) , we define the complex fields E and H
which are related to their instantaneous
counterparts
• Using the identity
6
10. • An isotropic radiator is an ideal source that radiates equally in all
directions. Although it does not exist in practice, it provides a
convenient isotropic reference with which to compare other
antennas. Because of its symmetric radiation, its Poynting vector
will not be a function of the spherical coordinate angles θ and φ. In
addition , it will have only a radial component. Thus the total power
radiated by it is given by
10
11. Radiation intensity
• Radiation intensity in a given direction is defined as
“the power radiated from an antenna per unit solid
angle.” The radiation intensity is a far-field parameter,
and it can be obtained by simply multiplying the
radiation density by the square of the distance. In
mathematical form it is expressed as
11
12. Total radiated power
• The total power is obtained by integrating the
radiation intensity
12
16. Directivity
16
The directivity of an isotropic source is unity since its power is radiated
equally well in all directions. For all other sources, the maximum
directivity will always be greater than unity, and it is a relative “figure of
merit” which gives an indication of the directional properties of the
antenna as compared with those of an isotropic source
17. In case of dual polarization
• the total directivity is the sum of the partial
directivities for any two orthogonal
polarizations
17
