26-12-2012, 04:28 PM
Antennas
[attachment=45324]
Introduction
The antenna is the interface between the transmission line and space
Antennas are passive devices; the power radiated cannot be greater than the power entering from the transmitter
When speaking of gain in an antenna, gain refers to the idea that certain directions are radiated better than others
Antennas are reciprocal - the same design works for receiving systems as for transmitting systems
Simple Antennas
The Isotropic Radiator would radiate all the power delivered to it and equally in all directions
The isotropic radiator would also be a point source
The Half-Wave Dipole
A more practical antenna is the half-wave dipole
Dipole simply means it is in two parts
A dipole does not have to be one-half wavelength, but that length is handy for impedance matching
A half-wave dipole is sometimes referred to as a Hertz antenna
Radiation Resistance
The half-wave dipole does not dissipate power, assuming lossless material
It will radiate power into space
The effect on the feedpoint resistance is the same as if a loss had taken place
The half-wave dipole looks like a resistance of 70 ohms at its feedpoint
The portion of an antenna’s input impedance that is due to power radiated into space is known as radiation resistance
Antenna Characteristics
It should be apparent that antennas radiate in various directions
The terms applied to isotropic and half-wave dipole antennas are also applied to other antenna designs
Radiation Patterns
Antenna coordinates are shown in three-dimensional diagrams
The angle f is measured from the x axis in the direction of the y axis
The z axis is vertical, and angle q is usually measured from the horizontal plane to the zenith
Beamwidth
A directional antenna can be said to direct a beam of radiation in one or more directions
The width of this bean is defined as the angle between its half-power points
A half-wave dipole has a beamwidth of about 79º in one plane and 360º in the other
Many antennas are far more directional than this
Front-to-Back Ratio
The direction of maximum radiation is in the horizontal plane is considered to be the front of the antenna, and the back is the direction 180º from the front
For a dipole, the front and back have the same radiation, but this is not always the case
The Monopole Antenna
For low- and medium-frequency transmissions, it is necessary to use vertical polarization to take advantage of ground-wave propagation
A vertical dipole would be possible, but similar results are available from a quarter-wavelength monopole antenna
Fed at one end with an unbalanced feedline, with the ground conductor of the feedline taken to earth ground
Cell-Site Antenna
For cellular radio systems, there is a need for omnidirectional antennas and for antennas with beamwidths of 120º, and less for sectorized cells
Cellular and PCS base-station receiving antennas are usually mounted in such a way as to obtain space diversity
For an omnidirectional pattern, typically three antennas are mounted on a tower with a triangular cross section and the antennas are mounted at 120º intervals
[attachment=45324]
Introduction
The antenna is the interface between the transmission line and space
Antennas are passive devices; the power radiated cannot be greater than the power entering from the transmitter
When speaking of gain in an antenna, gain refers to the idea that certain directions are radiated better than others
Antennas are reciprocal - the same design works for receiving systems as for transmitting systems
Simple Antennas
The Isotropic Radiator would radiate all the power delivered to it and equally in all directions
The isotropic radiator would also be a point source
The Half-Wave Dipole
A more practical antenna is the half-wave dipole
Dipole simply means it is in two parts
A dipole does not have to be one-half wavelength, but that length is handy for impedance matching
A half-wave dipole is sometimes referred to as a Hertz antenna
Radiation Resistance
The half-wave dipole does not dissipate power, assuming lossless material
It will radiate power into space
The effect on the feedpoint resistance is the same as if a loss had taken place
The half-wave dipole looks like a resistance of 70 ohms at its feedpoint
The portion of an antenna’s input impedance that is due to power radiated into space is known as radiation resistance
Antenna Characteristics
It should be apparent that antennas radiate in various directions
The terms applied to isotropic and half-wave dipole antennas are also applied to other antenna designs
Radiation Patterns
Antenna coordinates are shown in three-dimensional diagrams
The angle f is measured from the x axis in the direction of the y axis
The z axis is vertical, and angle q is usually measured from the horizontal plane to the zenith
Beamwidth
A directional antenna can be said to direct a beam of radiation in one or more directions
The width of this bean is defined as the angle between its half-power points
A half-wave dipole has a beamwidth of about 79º in one plane and 360º in the other
Many antennas are far more directional than this
Front-to-Back Ratio
The direction of maximum radiation is in the horizontal plane is considered to be the front of the antenna, and the back is the direction 180º from the front
For a dipole, the front and back have the same radiation, but this is not always the case
The Monopole Antenna
For low- and medium-frequency transmissions, it is necessary to use vertical polarization to take advantage of ground-wave propagation
A vertical dipole would be possible, but similar results are available from a quarter-wavelength monopole antenna
Fed at one end with an unbalanced feedline, with the ground conductor of the feedline taken to earth ground
Cell-Site Antenna
For cellular radio systems, there is a need for omnidirectional antennas and for antennas with beamwidths of 120º, and less for sectorized cells
Cellular and PCS base-station receiving antennas are usually mounted in such a way as to obtain space diversity
For an omnidirectional pattern, typically three antennas are mounted on a tower with a triangular cross section and the antennas are mounted at 120º intervals