An antenna is a device for collecting (receiving) or transmitting (radiating)
radio signals, the design of which depends on the wavelength and amplitude of the signals. In essence a transmitting antenna is a combination
of conductors which converts AC (alternating current) electrical energy
into radio waves.
|The signal emitted by a transmitting antenna (A)
can be visualized as a magnetic and an electrical sine wave perpendicular
to one another. The electrical wave, which carries television signals,
is shown to be vertical and this situation is known as vertical polarization.
The most favorable length of antenna to receive this signal is ½L.
An antenna may receive a direct signal as well as a number of reflected
signals. The effect of this is to produce a multiple image called
ghosting (B). This is because a plain antenna is equally receptive
in al directions and the shape representing this is shown in C. This
defect can be partially overcome by the addition of a reflector (D),
or a system of reflectors and directors (E). The effect is to make
the antenna sensitive in the forward direction. The mechanism of the
directors is to concentrate the incoming wave and the reflector behaves
in the same way as a mirror (F).
The simple dipole consists of two straight conductors aligned end on and
energized at the small gap which separates them. The length of the dipole
determines the frequency for which this configuration
is most efficient. It can be made directional by adding electrically isolated
director and reflector conductors in front and behind.
Other configurations include the folded dipole, the highly-directional loop
antenna, and the dish type used for microwave links.
Receiving antenna can consist merely of a short dielectric rod or a length
of wire for low-frequency signals. For VHF and microwave signals, complex
antenna configurations similar to those used for transmission must be used.
See also radio telescope.
ASTRONOMY AND COMMUNICATIONS