2. Radiation
• For a charge to radiate electromagnetic waves
in free space, it must be either accelerated or
decelerated.
• Near the dipole, the field lines are essentially
those of a static dipole leaving a positive
charge and ending up at a negative charge
• However, at a distance of the order of half
wavelength or greater, the field lines are
completely detached from the dipole
3. • This detachment characterizes radiation fields
which propagate freely (without being attached
to charges) in free space at the speed c.
Electric field lines of static dipole.
4. Field Radiating
The current oscillates in the wires
and the field lines propagate away
from the dipole and form closed
loops.
6. • The line of force created b/w the arms of small center fed dipole in the first quarter
period.
• During this time, the charge has reached its max. value and the lines have travelled
outwardly a radial distance λ/4.
• During the next quarter of the period, the original 3 lines travel an additional λ/4.
• The charge density on the conductors begins to diminish.
• This is achieved by introducing opposite charges which at the end of the 1st half of
the period have neutralized the charges on the conductors.
• The lines of force created by the opposite charges are 3 and travel a distance λ/4
during the 2nd quarter of the 1st half shown by dashed line.
• i.e there are 3 lines of force pointed upward in the 1st λ/4 distance and the same no.
of lines directed downward in the 2nd λ/4.
• There is no net charge on the antenna.
• The lines of force must have been formed to detach themselves from the conductors
and to unite together to for closed loop.
• In the remaining 2nd half of the period the same procedure is followed, but in the
opposite directions.
• The procesds is repeated and continues indefinitely and electric field patterns are
produced.
