Unguided media or wireless communication transport electromagnetic waves without using a physical conductor (cables, wires, etc.).
send communications signals through air or space.
And thus signals are available to anyone who has a device capable of receiving them.
Antenna:
Antenna is defined as the region of the transmission between a guided wave and a free space wave or between a free space wave and a
guided waves.
In other words a transmitting antenna connected to a transmitter by a transmission line, forces electromagnetic waves into free space which travel in space
with velocity of light.
Similarly a receiving antenna connected to a radio receiver receives or intercepts a portion of electromagnetic waves travelling through space.
Structures of Antenna:
- Antenna size
- Antenna supports
- Antenna feeders
- Antenna conductors
- Antenna insulators
- Antenna weather protection
Antenna Size:
These vary from micro Size (much smaller) to huge.
Although there is a general proportionality the antenna size and the wavelength at the frequency of operation but this relationship
is not hard and fast. Large antennas are sometime used at high frequencies to obtain highly directional radiation pattern.
Antenna Supports:
An antenna should be situated in the open space for better results.
It should be away from by conducting or absorbing object.
Antenna are usually supported by device like, masts (a tall upright post), pedestals and towers.
Towers are needed when the height requirement is more.
Masts also may be high but they are generally as short as a few maters only.
Pedestals are the base stations of antenna, for which height is not as important as the strength.
Antenna feeders:
Feeders are used to connect transmitters and receivers to the antennas.
The feeders are connected between antenna input terminals and transmitter output.
Antenna Conductors:
Antennas wires of rods are made up of conducting materials like copper and aluminium and its alloys.
The size of the antenna design mainly depends upon the permissible ohmic losses and heating effects.
Antenna Insulators:
The conducting portions of an antenna carry r.f. current and r.f. voltage.
In order to avoid short circuiting these voltages, insulators must sometimes be employed between antenna and its supports or between different parts of the antenna.
The materials used for constructing such antenna insulators
Antenna Weather Protection:
Antennas are usually for "out doors".
Guy wires are employed with tell structures like tower to avoid overturning in high winds.
At very low frequency in which long spans of wires have been used.
Protection against lighting induced currents and static charge build up is necessary for some types of antennas like broadcasting towers which stands high above its surroundings if conducting path to ground is not heavy and direct.
Insulators may be protected by horn or ball gaps.
