Transcript Antenna Primer

Antenna Primer
Wang Ng
References
• Balanis; Antenna Theory
• Collin; Antennas and Radiowave Propagation
What is an antenna?
• The American Heritage Dictionary: A
metallic apparatus for sending and receiving
electromagnetic waves.
• Webster’s Dictionary: A usually metallic
device (as a rod or wire) for radiating or
receiving radio waves.
• Balanis; Antenna Theory: An antenna is a
transitional structure between free-space
and a guiding structure.
• Assumption: Lossless antennas
Types of Antennas
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Wire antennas
Aperture antennas
Array antennas
Reflector antennas
Lens antennas
Patch antennas
Radiation Mechanism
Basic Antenna Parameters
• Radiation pattern: The relative distribution
of radiated power as a function of direction
in space – an (hypothetical) isotropic
antenna radiates equally in all directions.
• Gain G : The ratio of the radiated power in
the maximum direction to the radiated
power of an isotropic antenna. The gain of
an antenna represents the ability to focus its
beam in a particular direction – an isotropic
antenna has a gain of 0 dB.
Basic Antenna Parameters
• Radiation Resistance RA: The equivalent
resistance which would dissipate the same
amount of power as the antenna radiates.
• Input impedance ZA : The ratio of the voltage
to the current at the antenna terminals
(Thevenin equivalent circuit).
• Polarization: The direction of the E-field.
Basic Antenna Parameters
• Beamwidth: The angle between the two
directions in which the radiated power is half of
the maximum value of the beam.
• Bandwidth: Half-power frequency band.
• Antenna efficiency: e = Prad / Pin
• Effective aperture: It describes the effectiveness
of an antenna in the receiving mode. It is defined
as the ratio of the power delivered to the load to
the incident power density.
Friis Transmission Equation
• For polarization matched antennas aligned for
maximum directional radiation and reception,
the ratio of the transmitted and received
power is:
2
Pr   
 et Gt er Gr
 
Pt  4 R 
Space Loss Factor
Radiation Pattern of an
Infinitesimal Dipole
Radiation Pattern of a Thin Dipole
Radiation Pattern of a Circular Loop