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Page 1: BroadBand Antennas and Traveling Wave Antennas

BROADBAND ANTENNASANDTRAVELING WAVE ANTENNAS

Veli YILDIRIMMithat Sacit ATAR

Cukurova University/ADANA/TURKEYElectrical-Electronics Engineering

Page 2: BroadBand Antennas and Traveling Wave Antennas

CONTENT: -Helical Antenna -Normal mode -axial (end-fire) Mode -Design Procedure -feed design

-Yagi-uda antenna -History -effects of elements -optimization

-input impedance and matching techniques -Traveling wave antennas -Calculation of radiation resistance -Pattern function of traveling wave segments -Vee traveling wave antenna -Rhombic antenna

Page 3: BroadBand Antennas and Traveling Wave Antennas

Basic, simple and practical configuration of an electromagnetic radiator is that of an conducting wire wound in the form of a screw thread forming a helix. In most cases the helix is used with a ground plane.

Helical Antenna

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Helical Antenna Normal Mode:

In the normal mode of operation the field radiated by the antenna is maximum in a plane normal to the helix axis and minimum along its axis.

To achive the normal mode of operation, the dimensions of the helix are usually small compared to the wavelength (i.e. NL₀<<λ).

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Helical Antenna Normal Mode:Near Field-Electric field component for short dipole:

Far Field-Electric field component for short dipole:

Near Field-Electric field component for small loop:

Far Field-Electric field component for small loop:

Eθ and Eφ indicates that the two components are in time-phase quadrature, a necessary but not sufficient condition for circular or elliptical polarization.

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Helical Antenna Normal Mode:

• The ratio of the magnitudes of the Eθ and Eφ components is defined here as the axial ratio (AR)

AR = 0, Eθ=0 AR=∞, EΦ=0

when AR is unity (AR = 1);

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Helical Antenna End-Fire Mode (axial):

A more practical mode of operation, which can be generated with great ease, is the axial or end-fire mode. In this mod of operation, there is only one major lobe and its maximum radiation intensity is along the the axis of the helix.

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Helical AntennaDesign Procedure:

“ Empirical expressions ”

The input impedance (purely resistive)

which is accurate to about ±20%

The half-power beamwidth by

The beamwidth between nulls by

The directivity by

The axial ratio (for the condition of increased directivity) by

The half power beamwidth by

(All these relations are approximately valid provided 12◦ < α < 14◦, 3/4 < C/λ0 < 4/3 and N >3.)

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Helical AntennaDesign Procedure:

“ Empirical expressions ”

Ordinary End-Fire Hansen-Woodyard end-fire

(All these relations are approximately valid provided 12◦ < α < 14◦, 3/4 < C/λ0 < 4/3 and N >3.)

Page 10: BroadBand Antennas and Traveling Wave Antennas

For ordinary end-fire the relative phase ψ among the various turns of the helix.

Helical AntennaDesign Procedure:

Wave velocity along the helix wire between turns; v = ρv0 (ρ < 1 where v0 is the wave velocity in free space).

θ

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In a similar manner, it can be shown that for Hansen-Woodyard end-fire radiation.

which when solved for ρ leads to

Helical AntennaDesign Procedure:

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Helical AntennaFeed Design

At the beginning of the formation of the helix, in the form of a strip of width w by flattening it and nearly touching the ground plane which is covered with a dielectric slab of height:

Where;ω = width of strip conductor of the helix starting at the feedεr = dielectric constant of the dielectric slab covering the ground planeZ0 = characteristic impedance of the input transmission line

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Typical radiation patterns of a 5- to 35-turn helix antenna; α =12.8◦ and D =107.4 mm.

725MHz 925MHz 1067MHz

725MHz 925MHz 1067MHz

725MHz 925MHz 1030MHz

779MHz 925MHz 1000MHz

Measured field patterns of monofilar axial-mode helical antenna of 6 turns and 14◦ pitch angle. Patterns are characteristic of axial-mode of radiation over a range of circumferences from about 0.73λ to 1.22λ

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Helical Antenna example:

Current Distribution (3.5 GHz)

3D Far Field Pattern (7.2 GHz)

Simulation ResultsDirectivity of Antenna Reflection Coefficient (S11)

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Yagi-Uda

• Invented by Shintaro Uda and described in english by Hidetsugu Yagi in 1926

• Relatively unknown until World War 2• Now used for amateur radio, TV, satellite communications…

Dr. Yagi and his Yagi antenna(Photo: Hitachi Kokusai Electric, Inc. Yagi Antenna

Division))

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Yagi-Uda Array of Linear Elements

Another very practical radiator in the HF (3–30 MHz), VHF (30–300 MHz), and UHF (300–3,000 MHz) ranges is the Yagi-Uda antenna.

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Only Driver Element

Driver and Reflector Elements

Driver, Reflector and Director Elements

Elevation Azimuth

Yagi-UdaEffects of Elements

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Yagi-Uda Optimization

Perturbation of All Element Spacings

Perturbation of Directors Spacings

Directivity Optimization for Six-Element Yagi-Uda Array

The lengths of the elements are constant and vary spacings between them.

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Yagi-Uda Optimization

The spacings between all the elements constant and vary the lengths of the elements.

Directivity Optimization for Six-Element Yagi-Uda Array

Perturbation of All Element Lengths

Page 20: BroadBand Antennas and Traveling Wave Antennas

Yagi-Uda Input Impedance and Matching Techniques

The input impedance of a Yagi-Uda array, measured at the center of the driven element, is usually small and it is strongly influenced by the spacing between the reflector and feed element.

Input Impedance of a 15-Element Yagi-Uda Array (Reflector Length = 0.5λ; Director Spacing = 0.34λ; Director Length = 0.406λ)

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Yagi-Uda Input Impedance and Matching Techniques

Maximum energy of rf transfer at the design frequency occurs when the impedance of the feed point is equal to the impedance of the feedline.

Impedance matching devices are highly recommended for getting the best antenna performance.

Gamma Match

Yagi-Uda with Folded Dipole

Folded dipole as the driven element which acts as a step-up impedance transformer.

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Yagi-Uda Design Procedure

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Yagi

• The animation above shows a Yagi antenna driven by a sinusoidal signal at 454MHz. The animation shows the magnetic field perpendicular to the plot using a dB scale.

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TRAVELING WAVE ANTENNA

The current on these antennas can be written as a sum of waves traveling in opposite directions. Forexample, the current on a dipole of length l is given by

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The current on the upper arm of the dipole can be written as

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CALCULATION OF RADIATION RESISTANCE

T this antennas commonly called beverage antennas.

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CALCULATION OF RADIATION RESISTANCE

T this antennas commonly called beverage antennas.

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If the losses for the antenna are negligible then the current can be written as

And the far field vector potential is

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If we assume that the phase constant of the traveling wave antenna is the same as an unbounded medium (B = k), then

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Given the far field of the traveling wave segment, we may determine thetime- average radiated power density according to the definition of thePoynting vector such that

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The normalized pattern function of the traveling wave segment is shown below for segment lengths of 5, 10, 15 and 20.

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Vee Traveling Wave Antenna

A vee traveling wave antenna is formed by connecting two matched traveling wave segments to the end of a transmission line feed at an angle of 2o relative to each other.

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Rhombic AntennaA rhombic antenna is formed by connecting two vee traveling wave

antennas at their open ends.

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References:

-ANTENNA THEORY ANALYSIS AND DESIGN (Constantine A. Balanis)3th Edition-ANTENNAS FOR ALL APPLICATIONS (John D. Kraus)3th Edition-MODERN ANTENNA BOOK(Constantine A. Balanis)-http://www3.dogus.edu.tr/lsevgi/ (Prof. Dr. Levent Sevgi)-http://antenna-theory.com-http://yagi-uda.com/