TC 9-64 _________________________________________________________________________
2-2. The way energy is propagated into free space is a source of great
dispute among people concerned with it. Although many theories have been
proposed, the following theory adequately explains the phenomena and has
been widely accepted. There are two basic fields associated with every
antenna; an induction field and a radiation field. The field associated with
the energy stored in the antenna is the induction field. This field is said to
provide no part in the transmission of electromagnetic energy through free
space. However, without the presence of the induction field, there would be
no energy radiated.
2-3. Figure 2-1, which illustrates a low-frequency generator connected to an
antenna, will help you understand how the induction field is produced. Let us
follow the generator through one cycle of operation.
2-4. Initially, you can consider that the generator output is zero and that no
fields exist about the antenna, as shown in figure 2-1, 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. Because 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.
2-5. 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
figure 2-1, 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.
2-6. 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. The terms are used interchangeably throughout this text.