ece 3323 antennas
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Radiation regions - the volume surrounding the antenna can generally
be subdivided into three regions based on the characteristics of
the radiated fields in these regions.
Reactive Near Field- the region immediately surrounding theantenna where the reactive field (stored energy - standing
waves) is dominant.
Near-Field(Fresnel region) - the region between the reactive
near-field and the far-field where the radiation field
(propagating waves) is dominant and the field distribution
is dependent on the distance from the antenna.
Far-Field(Fraunhofer region) - the region farthest away from
the antenna where the field distribution is essentially
independent of the distance from the antenna (propagating
waves).
Antenna-Related Concepts
Electromagnetic Interference (EMI) - radiation from external sources
that couples to a device, and can negatively impact
performance. EMI can be generated by devices not designed to
act as antennas.
Electromagnetic Compatibility (EMC) - designing systems to
minimize the problems with EMI.
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Fundamentals of Antenna Radiation
Antenna as the termination of a transmission line
The open-circuited transmission line does not radiate effectively because
the transmission line currents are equal and opposite (and electrically close
together). The radiated fields of these currents tend to cancel one another.
The current on the arms of the dipole antenna are aligned in the same
direction so that these radiated fields tend to add together making the dipole
an efficient radiator.
Antenna as the termination of a waveguide
The open-ended waveguide radiates, but not as effectively as thewaveguide terminated by the horn antenna. The wave impedance inside the
waveguide does not match that of the surrounding medium creating a
mismatch at the open end of the waveguide (causing reflections). The horn
antenna acts as a matching network by providing a gradual transition in the
wave impedance from the waveguide to the surrounding medium (reducing
reflections).
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~ ~~
~
The electric scalar potential and magnetic vector potential are related
by the Lorentz gauge:
For time-harmonic fields, the Lorentz gauge can be written in terms of
phasors as
Solving for the electric scalar potential in terms of the magnetic vector
potential gives
This equation defines the electric scalar potential in terms of the magnetic
vector potential and allows for the fields radiated by an antenna to be
defined in terms of the magnetic vector potential only.
Fields in terms ofV and A
Fields in terms ofA only
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