2-4
All the energy supplied to the induction field is returned to the antenna by the collapsing E and H
fields. No energy from the induction field is radiated from the antenna. Therefore, the induction field is
considered a local field and plays no part in the transmission of electromagnetic energy. The induction
field represents only the stored energy in the antenna and is responsible only for the resonant effects that
the antenna reflects to the generator.
RADIATION FIELDS
The E and H fields that are set up in the transfer of energy through space are known collectively as
the radiation field. This radiation field is responsible for electromagnetic radiation from the antenna. The
radiation field decreases as the distance from the antenna is increased. Because the decrease is linear, the
radiation field reaches great distances from the antenna.
Let's look at a half-wave antenna to illustrate how this radiation actually takes place. Simply stated, a
half-wave antenna is one that has an electrical length equal to half the wavelength of the signal being
transmitted. Assume, for example, that a transmitter is operating at 30 megahertz. If a half-wave antenna
is used with the transmitter, the antenna's electrical length would have to be at least 16 feet long. (The
formula used to compute the electrical length of an antenna will be explained in chapter 4.) When power
is delivered to the half-wave antenna, both an induction field and a radiation field are set up by the
fluctuating energy. At the antenna, the intensities of these fields are proportional to the amount of power
delivered to the antenna from a source such as a transmitter. At a short distance from the antenna and
beyond, only the radiation field exists. This radiation field is made up of an electric component and a
magnetic component at right angles to each other in space and varying together in intensity.
With a high-frequency generator (a transmitter) connected to the antenna, the induction field is
produced as described in the previous section. However, the generator potential reverses before the
electrostatic field has had time to collapse completely. The reversed generator potential neutralizes the
remaining antenna charges, leaving a resultant E field in space.
Figure 2-3 is a simple picture of an E field detaching itself from an antenna. (The H field will not be
considered, although it is present.) In view A the voltage is maximum and the electric field has maximum
intensity. The lines of force begin at the end of the antenna that is positively charged and extend to the
end of the antenna that is negatively charged. Note that the outer E lines are stretched away from the inner
lines. This is because of the repelling force that takes place between lines of force in the same direction.
As the voltage drops (view B), the separated charges come together, and the ends of the lines move
toward the center of the antenna. But, since lines of force in the same direction repel each other, the
centers of the lines are still being held out.
