Ece 3323 Antennas

7/29/2019 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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~ ~~

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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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Documents

Ece 3323 Antennas