THE
EMISSION OF RADIATION
20.1.Radiation
from an Ocillating Dipole
A simple example of radiation from a
prescribed, time-dependent charge-current distribution is provided by
calculation of the radiation from an oscillating electric dipole. The dipole
will be assumed to consist of spheres located at z = 
l/2
connected by a wire of negligible capacitance.
I = +q
Where I is
positive in the plus z direction.
20.2.Radiation
From A Half-wave Antenna
The restriction to lenghts small
compared with one wavelenght can be removed in some cases by relatively simple
means. In particular, a wire that is just one-half wavelenght in lenght can be
broken into infinitesimal elements, to each of which the method of the
preceding section can be applied. Let the wire lie along the z-axis from 
to 
and carry
current
20.3.Radiation From A Group Moving Charges
The maximum
power is radiated at 900 to P. The total radiated power is obtained
by integrating the Poynting vector over a closed surface surrounding the charge
distribution. A convenient choice for such a surface is a sphere, centered in
the charge distribution, with sufficienly large radius so that all parts of its
surface are in the radiation zone.
20.4.Near
and Intermediate Zone Fields
The radiation field or the static field dominates when kr>>1 or
kr<<1, respectively-that is, when r is large or small compared to the
wavelenght of the emitted radiattion.
20.5.Radiation
Damping: Thomson Cross Section
mv = eE
so that
Is the total power
radiated by one selection. The Thomson scatting cross section 
is defined as P divided by the incident
Poyntingvector (power per unit area)
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