4-7
the antenna must be such that a wave will travel from one end to the other and back again during the
period of 1 cycle of the rf voltage. Remember, the distance a wave travels during the period of 1 cycle is
known as the wavelength and is found by dividing the rate of travel by the frequency.
Look at the current and voltage (charge) distribution on the antenna in figure 4-6. A maximum
movement of electrons is in the center of the antenna at all times; therefore, the center of the antenna is at
a low impedance. This condition is called a STANDING WAVE of current. The points of high current
and high voltage are known as current and voltage LOOPS. The points of minimum current and minimum
voltage are known as current and voltage NODES. View A shows a current loop and current nodes. View
B shows voltage loops and a voltage node. View C shows the resultant voltage and current loops and
nodes. The presence of standing waves describes the condition of resonance in an antenna. At resonance
the waves travel back and forth in the antenna reinforcing each other and the electromagnetic waves are
transmitted into space at maximum radiation. When the antenna is not at resonance, the waves tend to
cancel each other and lose energy in the form of heat.
Figure 4-6.—Standing waves of voltage and current on an antenna.
Q4. If a wave travels exactly the length of an antenna from one end to the other and back during the
period of 1 cycle, what is the length of the antenna?
