_______________________________________________________________________ Antennas
being received increases, the design and installation of the receiving antenna
become more critical. An example of this is a television-receiving antenna. If
you raise it a few more inches from the ground or give it a slight turn in
direction, you can change a snowy blur into a clear picture.
4-15. The conventional antenna is a conductor, or system of conductors, that
radiates or intercepts electromagnetic wave energy. An ideal antenna has a
definite length and a uniform diameter, and is completely isolated in space.
However, this ideal antenna is not realistic. Many factors make the design of
an antenna for a communications system a more complex problem than you
would expect. These factors include the height of the radiator above the
earth, the conductivity of the earth below it, and the shape and dimensions of
the antenna. All of these factors affect the radiated-field pattern of the
antenna in space. Another problem in antenna design is that the radiation
pattern of the antenna must be directed between certain angles in a
horizontal or vertical plane, or both.
4-16. Most practical transmitting antennas are divided into two basic
classifications, hertz (half-wave) antennas and Marconi (quarter-wave)
antennas. Hertz antennas are generally installed some distance above the
ground and are positioned to radiate either vertically or horizontally. Marconi
antennas operate with one end grounded and are mounted perpendicular to
the earth or to a surface acting as a ground. Hertz antennas are generally
used for frequencies above 2 megahertz. Marconi antennas are used for
frequencies below 2 megahertz and may be used at higher frequencies in
certain applications.
4-17. A complete antenna system consists of the following three parts
(figure 4-3):
The coupling device (coupling coil).
The feeder.
The antenna.
Figure 4-3. Typical Antenna System
4-5
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