2-3
Initially, you can consider that the generator output is zero and that no fields exist about the antenna,
as shown in view A. Now assume that the generator produces a slight potential and has the instantaneous
polarity shown in view B. Because of this slight potential, the antenna capacitance acts as a short,
allowing a large flow of current (I) through the antenna in the direction shown. This current flow, in turn,
produces a large magnetic field about the antenna. Since the flow of current at each end of the antenna is
minimum, the corresponding magnetic fields at each end of the antenna are also minimum. As time
passes, charges, which oppose antenna current and produce an electrostatic field (E field), collect at each
end of the antenna. Eventually, the antenna capacitance becomes fully charged and stops current flow
through the antenna. Under this condition, the electrostatic field is maximum, and the magnetic field (H
field) is fully collapsed, as shown in view C.
As the generator potential decreases back to zero, the potential of the antenna begins to discharge.
During the discharging process, the electrostatic field collapses and the direction of current flow reverses,
as shown in view D. When the current again begins to flow, an associated magnetic field is generated.
Eventually, the electrostatic field completely collapses, the generator potential reverses, and current is
maximum, as shown in view E. As charges collect at each end of the antenna, an electrostatic field is
produced and current flow decreases. This causes the magnetic field to begin collapsing. The collapsing
magnetic field produces more current flow, a greater accumulation of charge, and a greater electrostatic
field. The antenna gradually reaches the condition shown in view F, where current is zero and the
collected charges are maximum.
As the generator potential again decreases toward zero, the antenna begins to discharge and the
electrostatic field begins to collapse. When the generator potential reaches zero, discharge current is
maximum and the associated magnetic field is maximum. A brief time later, generator potential reverses,
and the condition shown in view B recurs.
NOTE: The electric field (E field) and the electrostatic field (E field) are the same. They will be
used interchangeably throughout this text.
The graph shown in figure 2-2 shows the relationship between the magnetic (H) field and the electric
(E) field plotted against time. Note that the two fields are 90 degrees out of phase with each other. If you
compare the graph in figure 2-2 with figure 2-1, you will notice that the two fields around the antenna are
displaced 90 degrees from each other in space. (The H field exists in a plane perpendicular to the antenna.
The E field exists in a plane parallel with the antenna, as shown in figure 2-1.)
Figure 2-2.—Phase relationship of induction field components.
