How works antenna

An electrical device designed to receive and transmit electromagnetic waves is called an antenna. Antennas are commonly used in radio, television broadcasting, cell phones, radar and other systems involving the use of electromagnetic waves. Since electromagnetic waves can pass through any medium at a certain frequency, this simply means that antennas can work not only in air but also under water and even thru solid ground. An antenna is also defined as an arrangement of conductors that generate electromagnetic field as a reaction to applied alternating electrical current or induce an alternating current between its terminals when placed in an electromagnetic field.

From the time of Hertz and Marconi to the present, one thing has
remained constant in wireless communications: radio waves travel, as if
by magic, from a transmitting antenna to a receiving antenna.

Whether the two antennas are across the garden from each other,
across continents and oceans, or on the Earth and the Moon, if there is
not a transmitting antenna and at least one receiving antenna in the system
then no communications can take place.
At one time, physicists believed that there must be some invisible medium
for carrying the radio signal. But we now know that no such medium exists,
yet radio waves travel even in outer space. Being electromagnetic waves,
radio signals need no medium in order to propagate. If radio signals traveled
only in the Earth’s atmosphere, then we could make some guesses about
a medium for carrying the wave, but space communications demonstrates
that the atmosphere is not necessary (although it does affect radio signal
propagation).

The waves have an amplitude A, which corresponds to the signal
strength. They also have a wavelength (j), which corresponds to the distance
traveled by the wave in one complete up-and-down cycle. In radio work, the
wavelength is measured in meters (m), except in the microwave region where
centimeters (cm) and millimeters (mm) make more sense. Wavelength can be
measured at any pair of points on the wave that are identical: two peaks,
two troughs, two zero crossings, as convenient in any specific case.
The number of cycles that pass a given point every second is the frequency
of the wave. The classic measure of frequency was cycles per second (cps or
c/s), but that was changed in 1960 by international consensus to the hertz
(Hz), in honor of Heinrich Hertz.

There are two basic types of antennas. The omni-directional antenna radiates electromagnetic signal equally in the three dimensional plane.
Its gain is low, but if you are in a city and surrounded by stations, this might be the perfect antenna.
Common omni-directionals include these disk antennas:
Channel Master 3000 SMARTenna, 22” circular VHF/UHF. The 3038 amplifier is optional.
Winegard MS-1000 or MS-2000 MetroStar, circular VHF/UHF. Amplifier optional.
Directional antennas receive the signal more from one direction.
Adding conducting elements such as a rod or a coil and varying their spacing, direction and length can create a specific antenna with specific properties.
By connecting several antennas to a common source, it becomes an antenna array that has a directive radiation pattern.
The conductive means for conveying signal energy between the active antenna and the source is called antenna lead-in while the components between an amplifier and the active antenna is called antenna feed.

Some terms:
Gain:- a measure of how much signal the antenna will collect.
Beam width:- how directional an antenna is.
Bandwidth:- how the gain varies with frequency. A narrowband antenna will receive some channels well, but other channels poorly

There are certain parts of an antenna that perform a distinct function.
A few of these parts are the reflector and director. The reflector is usually a metallic plate or screen that reradiates the electromagnetic radiation coming from or going to the active antenna and adding a component in an opposite direction to the velocity of the interrupting electromagnetic wave.
The director on the other hand, works in a similar manner but adds a component on the same direction to the velocity of the interrupting electromagnetic wave.