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Antenna

VIEWS: 30 PAGES: 15

									Antenna
Antenna Radiation, Antenna Gain, Resistance, Bandwidth, Beam width.


Antenna: It is a structure that is generally a metallic object, often a wire or group of
wires, used to convert high frequency current into electromagnetic waves or vice
versa.
Transmitting and receiving antennas have similar characteristics, which means that
their behaviour is reciprocal.
The spacing, length and shape of the device is related to the wavelength of the
desired transmitter frequency. i.e mechanical length is inversely proportional to the
numerical value of the frequency.
- Physically, an antenna is simply an arrangement of one or more conductors, usually
called elements.
- In transmission, an alternating current is created in the elements by applying a
voltage at the antenna terminals, causing the elements to radiate an electromagnetic
field.
- In reception, the inverse occurs: an electromagnetic field from another source
induces an alternating current in the elements and a corresponding voltage at the
antenna's terminals.
Electromagnetic Radiation
• When RF energy is fed into a mismatched transmission line, standing waves occur.
• Here energy is lost or radiated into the space surrounding the line. This process is
considered unwanted in the transfer of energy to the radiation device.
• when the two ends of the transmission line are separated, more surface area of the
wire is exposed to the atmosphere. This enhances the radiation process. (fig 9-2a)
• When the two wires are bent at 90 degree to each other, the radiation efficiency
improves. (fig 9-2c)
• The electric and magnetic fields are fully coupled to the surrounding space instead
of being confined between the two wires.. Here the maximum radiation exists. This
type of radiator is called dipole.
• When the total length of the two wires is a half wavelength, the antenna is
called a half-wave dipole.
• This configuration has similar characteristics to its equivalent length transmission
line(1/4 λ)
•. It results in high impedance (Hi Z) at the far ends reflected as low impedance (Lo Z)
at the end connected to the transmission line. (fig 9-2c)
• This causes the antenna to have large current node at the centre and large
voltage nodes at the ends, resulting in maximum radiation.
A Hertzian antenna is a set of terminals that does not require the presence of a ground
for its operation.
Basic dipole facts :

The name dipole means two poles and the antenna does in fact consist of two "poles" or
sections.
These are normally equal in length, making the antenna what is termed a centre fed antenna.

The power is applied to the dipole antenna itself through a feeder. Conversely if the dipole
antenna is used for receiving, the received signals are taken away to the receiver through a
feeder.

The feeder serves to transfer the power to or from the antenna with as little loss as possible.




                     The basic dipole antenna configuration
The most common form of dipole has an electrical length of half a wavelength. As a
result this antenna is called a half wave dipole.

As the total length of the dipole is a half wavelength, this makes each section or leg of
the dipole a quarter wavelength long.




                           The basic half wave dipole antenna
    Antenna Gain and effected radiation pattern
•   Certain antennas can be Omni directional (isotropic) which radiates in all
    directions, and certain types focus their radiation pattern in a specific direction.
•   Antennas have gain which is measured in decibels.

    Directive Gain
•   Directive gain is defined as the ratio of power density in a particular direction of
    one antenna to the power density that would be radiated by an omni directional
    antenna.
•   The power density of both types of antenna is measured at a specified distance
    and a comparative ratio Is established.




    Half wave dipole
    Gain = 2.15 dB – resonant antennas in free space
    Gain = 5.05 dB – non resonant antennas n free space
Two important point here are:




Resonance is the tendency of a system to oscillate at larger amplitude at some frequencies
than at others. These are known as the system's resonant frequencies (or resonance
frequencies).

Power Gain

Power gain is a comparison of the output power of an antenna in a certain
direction to that of an isotropic antenna.

Gain of an antenna is a power ratio comparison between an omni directional and
unidirectional radiator.

This ratio can be expressed as
•   Effective radiated power (ERP) applies to the field gain of the antenna and
    the efficiency of the transmitter.


Radiation measurement and field intensity

•   The voltage induced in a receiving antenna are very small, generally in the
    microvolt range.
•   Field strength measurements are given in micro volts per meter.

•   Field intensity:
•   Field strength of an antenna’s radiation, at a given point in space, is equal
    to the amount of voltage induced in a wire antenna 1 m long , located at
    that given point.
•   Field strength or induced voltage is affected by a number of conditions
    such as the time of the day, atmospheric conditions and distance.

Antenna Resistance
• Radiation Resistance is the ratio of the power radiated by the antenna to
  the square of the current at the feed point.
Antenna losses and efficiency:
• In addition to the energy radiated by an antenna, the power losses must be
   considered.
• Antenna losses can be caused by ground resistance, corona effects, imperfect
   dielectric near the antenna, energy loss due to eddy currents induced into
   nearby metallic objects, and I*I*R losses in the antenna itself.
• The losses can be combined and represented as:
•    Low and medium frequency antennas are least efficient because of difficulty in
     achieving the proper physical (resonant) length. These antennas can approach
     efficiencies of only 75 to 95 percent.
•    Antennas at higher frequencies can easily achieve values approaching 100
     percent.
•    Radiation resistance values may vary from a few ohms to several hundred ohms
     depending upon the choice of feed point and physical and electrical
     characteristics.


    Bandwidth, beam width and polarization
•    These are important terms dealing respectively with the operating frequency
     range, the degree of concentration of the radiation pattern and the space
     orientation of the radiated waves.


    Bandwidth:
•    Bandwidth refers to the range of frequencies the antenna will radiate effectively
     i.e. the antenna will perform satisfactorily throughout this range of frequencies.
•    When the antenna power drops to ½ (3 dB), upper and lower extremities of these
     frequencies have been reached and the antenna no longer performs
     satisfactorily.
• Beam width
•   Beam width of an antenna is described as the angles created by comparing half
    power points (3 dB) on the main radiation lobe to its maximum power point.
•   In fig 9-9, the beam angle is 30 degree, which is the sum of two angles created at
    the points where the field strength drops to 0.707 ( field strength is measured in
    microvolts/m) of the maximum voltage at the centre of the lobe (these points are
    knows as half-power points)
• Polarization
•   Polarization of an antenna refers to the direction on space of the E field (electric
    vector) portion of the electromagnetic wave being radiated (fig 9-10) by the
    transmitting system.
•   Low frequency antennas are usually vertically polarized because of ground
    effects (reflected waves etc) and physical construction methods.
•   High frequency antennas are generally horizontally polarized.
•   High frequency antennas are more desired because of its rejection to noise made
    by people which is mostly vertically polarized.



• Corona Effects
The presence of corona can reduce the reliability of a system by degrading insulation.
While corona is a low energy process, over long periods of time, it can substantially
degrade insulators, causing a system to fail due to dielectric breakdown. The effects of
corona are cumulative and permanent, and failure can occur without warning.

An eddy current (also known as Foucault current) is an electrical phenomenon caused
   when a conductor is exposed to a changing magnetic field due to relative motion of
   the field source and conductor; or due to variations of the field with time. This can
   cause a circulating flow of electrons, or a current, within the body of the conductor.
   The dipole antenna is simply two wires pointed in opposite directions arranged
  either horizontally or vertically, with one end of each wire connected to the radio
     and the other end hanging free in space. Since this is the simplest practical
 antenna, it is also used as a reference model for other antennas; gain with respect
       to a dipole is labeled as dBd. Generally, the dipole is considered to be
   omnidirectional in the plane perpendicular to the axis of the antenna, but it has
  deep nulls in the directions of the axis. Variations of the dipole include the folded
 dipole, the half wave antenna, the ground plane antenna, the whip, and the J-pole.
 The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements
 added which are functionality similar to adding a reflector and lenses (directors) to
                               focus a filament light bulb.
http://www.electronics-
radio.com/articles/radio/antennas/dipol
e/dipole-antenna.php                  Antennas are used in systems such as radio and
                                      television broadcasting, point-to-point radio
                                      communication, wireless LAN, radar, and space
                                      exploration.

                                       Antennas are most commonly employed in air or
                                      outer space, but can also be operated under water
                                      or even through soil and rock at certain
                                      frequencies for short distances.

								
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