BroadBand Antennas and Traveling Wave Antennas

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BROADBAND ANTENNAS AND TRAVELING WAVE ANTENNAS Veli YILDIRIM Mithat Sacit ATAR Cukurova University/ADANA/TURKEY Electrical-Electronics Engineering

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BROADBAND ANTENNAS AND TRAVELING WAVE ANTENNAS

Veli YILDIRIM Mithat Sacit ATARCukurova University/ADANA/TURKEY Electrical-Electronics Engineering

CONTENT:

-Helical Antenna

-Normal mode -axial (end-fire) Mode-Design Procedure -feed design

-Yagi-uda antenna

-Traveling wave antennas

-History -effects of elements -optimization -input impedance and matching techniques

-Calculation of radiation resistance -Pattern function of traveling wave segments -Vee traveling wave antenna -Rhombic antenna

Helical Antenna

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 AntennaNormal Mode:

I th e n o rm a l m o d e o f o p e ra ti n n o th e fi l ra d i te d b y th e a n te n n a i ed a s m a xi u m i a p l n e n o rm a l to th e h e l x m n a i a xi a n d m i i u m a l n g i a xi . s n m o ts s To a ch i ve th e n o rm a l m o d e o f o p e ra ti n , th e d i e n si n s o f th e h e l x a re o m o i u su a l y sm a l co m p a re d to th e w a ve l n g th l l e ( ie . NL < < . ).

Helical AntennaNormal Mode:Near Field - Electric field component for short dipole : Near Field - Electric field component for small loop :

Far Field - Electric field component for short dipole :

Far Field - Electric field component for small loop :

E and E indicates that the two components are in timephase quadrature, a necessary but not sufficient condition for circular or elliptical polarization.

Helical AntennaNormal Mode: The ratio of the magnitudes of the E and E components is dened here as the axial ratio (AR)

A R = 0, E=0

AR=, E=0

when AR is unity (AR = 1);

Helical AntennaEnd-Fire Mode (axial):

A m o re p ra cti l m o d e o f o p e ra ti n , w h i ca o ch ca n b e g e n e ra te d w i g re a t e a se , i th e axial or th s e n d -fi m o d e . I th i m o d o f o p e ra ti n , th e re i re n s o s o n l o n e m a j r l b e a n d i m a xi u m ra d i ti n y o o ts m a o i te n si i a l n g th e th e a xi o f th e h e l x. n ty s o s i

Helical AntennaDesign Procedure: Empirical expressions The input impedance (purely resistive)which is accurate to about 20%

The directivity by

The half power beamwidth The half-power beamwidth by by

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

The beamwidth between nulls by

(All these relations are approximately valid provided 12 < < 14, 3/4 < C/0 < 4/3 and N >3.)

Helical AntennaDesign Procedure: Empirical expressions

Ordinary EndFire

Hansen-Woodyard endfire

(All these relations are approximately valid provided 12 < < 14, 3/4 < C/0 < 4/3 and N >3.)

H e lica l A n te n n a

Design Procedure :

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

Wave velocity along the helix wire between turns; v = v 0 ( < 1 where v0 is the wave velocity in free space).

Helical Antenna

Design Procedure :

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

which when solved for leads to

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; = width of strip conductor of the helix starting at the feed r = dielectric constant of the dielectric slab covering the ground plane Z 0 = characteristic impedance of the input transmission line

725MHz

925MHz

1067MHz

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

725MHz

925MHz

1067MHz

725MHz

925MHz

1030MHz

779MHz

925MHz

1000MHz

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

Helical Antenna example:Current Distribution ( 3 . 5 GHz )

Directivity of Antenna Results

SimulationReflection Coefficient ( S11 )

3D Far Field Pattern ( 7 . 2 GHz )

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 communicationsDr. Yagi and his Yagi antenna (Photo: Hitachi Kokusai Electric, Inc. Yagi Antenna Division))

Yagi-Uda Array of Linear Elements

Another very practical radiator in the HF (330 MHz), VHF (30300 MHz), and UHF (3003,000 MHz) ranges is the Yagi-Uda antenna.

Yagi-UdaEffects of ElementsOnly Driver Element

ElevationDriver and Reflector Elements

Azimuth

Driver, Reflector and Director Elements

Yagi-UdaOptimization

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

Directivity Optimization for Six ement Yagi - Uda Array Perturbation of Directors Spacings

Perturbation of All Element Spacings

Yagi-UdaOptimizationThe spacings between all the elements constant and vary the lengths of the elements.

Directivity Optimization for Six - Element Yagi - Uda ArrayPerturbation of All Element Lengths

Yagi-UdaInput Impedance and Matching TechniquesThe 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 .5 ; Director Spacing = 0 . 34 ; Director Length = 0 . 406 )

Yagi-UdaInput Impedance and Matching TechniquesMaximum 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.Yagi - Uda with Folded DipoleFolded dipole as the driven element which acts as a step-up impedance transformer.

Gamma Match

Yagi-UdaDesign Procedure

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.

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

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

CALCULATION OF RADIATION RESISTANCET this antennas commonly called beverage antennas.

CALCULATION OF RADIATION RESISTANCET this antennas commonly called beverage antennas.

If the losses for the antenna are negligible then the current can be written as And the far field vector potential is

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

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

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

Vee Traveling Wave AntennaA 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.

Rhombic AntennaA rhombic antenna is formed by connecting two vee traveling wave antennas at their open ends.

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/