Antennas-9 - Travelling Wave Antennas

8/13/2019 Antennas-9 - Travelling Wave Antennas

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Dr.-Ing. Getahun Mekuria

Department of Electrical and Computer Engineering (ECE)

Addis Ababa Institute of Technology (AAiT)

Addis Ababa University (AAU)

ECEG 6308

Analysis and Design of Antenna

Travelling Wave Antennas


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• Antennas with open-ended wires where the currentmust go to zero (dipoles, monopoles, etc.) can be

characterized as standing wave antennas or resonant

antennas.

• The current on these antennas can be written as a sum

of waves traveling in opposite directions (waves which

travel toward the end of the wire and are reflected in the

opposite direction).

• For example, the current on a dipole of length l is

given by


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• For example, the current on a dipole of length l isgiven by


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• The current on the upper arm of the dipole can bewritten as

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• Traveling wave antennas are characterized by matchedterminations (not open circuits) so that the current is

defined in terms of waves traveling in only one direction

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Example of a Travelling Wave Antenna


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• Consider a segment of a traveling wave antenna (anelectrically long wire of length l lying along the z-axis).

• A traveling wave current flows in the z- direction.

Where:

the attenuation constant

the phase constant


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• If the losses for the antenna are negligible (ohmic lossin the conductors, loss due to imperfect ground, etc.),

then the current can be written as:

• The Magnetic Vector potential is given by:


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• And the Far Field Electric Field Strength is given by:

•

And •

Where


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• The if we assume that the phase constant of theantenna is the same as an unbounded medium ( = k),

then


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• Then the Travelling Power Density is given by:

• And the Radiated Power can be given as:


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• And the Radiation Resistance is then:

•

Where C i(x) is the cosine Integral given by:

• The radiation resistance of the ideal traveling wave

antenna (VSWR = 1) is purely real just as the inputimpedance of a matched transmission line is purely

real.

• A plot of the radiation resistance of the traveling wave

segment as a function of segment length.


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A plot of the radiation resistance of the traveling wavesegment as a function of segment length.


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• The Radiation Intensity is also given by:

• The normalized pattern function can be written as


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• The polar plots for different values of length:


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• The polar plots for different values of length:


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•The approximate angle of the main lobe for the travelingwave segment is found by determining the first peak of

the function in the normalized pattern function.


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•Directivity

• The Maximum Directivity, is then


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Travelling Wave Antennas - Examples

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•V-Travelling Wave Antenna

• V-Antenna Radiation Pattern


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Travelling Wave Antennas - Examples

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•Rhombic Travelling Wave Antenna

• Rhombic Antenna Radiation Pattern

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Antennas-9 - Travelling Wave Antennas