________________________________________________________________Wave Propagation
1-120. The electric and magnetic fields radiated from an antenna form the
electromagnetic field. This field is responsible for the transmission and
reception of electromagnetic energy through free space. An antenna, however,
is also part of the electrical circuit of a transmitter or a receiver and is
equivalent to a circuit containing inductance, capacitance, and resistance.
Therefore, the antenna can be expected to display definite voltage and
current relationships with respect to a given input. A current through the
antenna produces a magnetic field, and a charge on the antenna produces an
electric field. These two fields combine to form the induction field. To help
you gain a better understanding of antenna theory, the following subsections
review some basic electrical concepts, including voltage and its electric field,
current and its magnetic field, and their relationship to the propagation of
electrical energy.
Electric Field
1-121. Around every electrically charged object is a force field that can be
detected and measured. This force field can cause electric charges to move in
the field. When an object is charged electrically, there is either a greater or a
smaller concentration of electrons than normal. Thus, a difference of
potential exists between a charged object and an uncharged object. An
electric field is, therefore, associated with a difference of potential, or a
voltage.
1-122. Lines that are drawn to show the paths along which the force acts
commonly represent this invisible field of force. The lines representing the
electric field are drawn in the direction that a single positive charge would
normally move under the influence of that field. A large electric force is
shown by a large concentration of lines; a few lines indicate a weak force.
1-123. When a capacitor is connected across a source of voltage, such as a
battery, it is charged by a particular amount, depending on the voltage and
the value of capacitance (see figure 1-25). Because of the EMF (electromotive
force) of the battery, negative charges flow to the lower plate, leaving the
upper plate positively charged. Along with the growth of charge, the electric
field is also building up. The flux lines are directed from the positive to the
negative charges and at right angles to the plates. When the capacitor is fully
charged, the voltage of the capacitor is equal to the voltage of the source and
opposite in polarity. The charged capacitor stores the energy in the form of an
electric field. It can be said, therefore, that an electric field indicates voltage.
1-124. If the two plates of the capacitor are spread farther apart, the electric
field must curve to meet the plates at right angles (figure 1-26). The straight
lines in view A of figure 1-26 become arcs in view B, and approximately
semicircles in view C, where the plates are in a straight line. Instead of flat
metal plates, as in the capacitor, the two elements can take the form of metal
rods or wires and form the basic antenna.
1-35
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