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Broadband AntennasYagi-Uda Antennas
Set X
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 1
Yagi-Uda Antenna- StructureThe Yagi-Uda antenna is one of the mostbrilliant antenna designs.
• It consists of a fed element and at least twoparasitic (non-excited) elements.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 2
Yagi-Uda Antenna- Structure• The shorter
elements in
the front are
directors.
• The longer
element in
the back is a reflector.
• The conventional design has only onereflector, but may have up to 10~15 directors.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 3
Yagi-Uda Antenna-Advantages
Advantages:
• It is simple to construct.
• It has a high gain, typically greater than 10 dB.
These antennas typically operate in the HF toUHF bands (about 3 MHz to 3 GHz), frequentlyemployed as TV/FM antennas.
However:
• It has a typically small bandwidth (on the orderof a few percent of the center frequency).
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 4
Yagi-Uda Antenna- Geometry
• The basic geometry of a Yagi-Uda antenna isshown in the following figure:
• The single 'feed' or 'driven' element (F) istypically a dipole or a folded dipole antenna.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 5
Yagi-Uda Antenna- Geometry• The rest of the elements are parasitic:
They reflect (Reflector R) or help to transmitthe energy in a particular direction (Directors-D1, D2, D3, … DN).
• This feed antenna is often altered in size tomake it resonant in the presence of theparasitic elements (typically, 0.45-0.48wavelengths long for a dipole antenna).
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 6
Yagi-Uda Antenna- Geometry• The reflector length is given as R and the
separation between the feed and the reflectoris SR.
• The reflector element is typically slightlylonger than the feed element (~5%).
• Adding more reflectors improves performancebut very slightly.
• This element is important in determining thefront-to-back ratio of the antenna.
So, what is the front-to-back ratioDr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 7
Yagi-Uda Antenna- Geometry• The front-to-back ratio: is the ratio of the
maximum directivity in the forward directionto that in the back
direction:
• Having the reflector slightly longer than itsresonant length, makes the impedance of thereflector capacitive.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 8
Yagi-Uda Antenna- Geometry
Hence, the current on the reflector leads thevoltage induced on the reflector Out ofphase with the backward wave and in phasewith the forward wave.
• The director elements (those to the right ofthe feed) will be shorter (~4%) than itsresonant length, making them inductive, sothat the current lags the voltage.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 9
Yagi-Uda Antenna- Geometry
• This will cause a phase distribution tooccur across the elements, so to be inphase with the driven element forwardwave.
• This leads to the array being a travellingwave antenna end-fire array wherethe radiation is along the +y axis asshown above.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 10
Yagi-Uda Antenna- Geometry
• Each director is of length Di , andseparated from the adjacent director bya length SDi , so the chosen lengthsstrengthen the wave propagation in thedirection of the directors.
• Separations are mostly optimized bysimulations or measurements to achievemaximum gain.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 11
Yagi-Uda Antenna- Geometry• Example measurement of two-element Yagi
antenna (1 reflector, 1 feed dipole element, 0directors). The gain as a function of theseparation is
shown right
• G increases by
about 2.5 dB if
SR is:
0.15 to 0.3Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 12
Yagi-Uda Antenna- Geometry• Similarly, you can plot the gain as a function of
director spacings or:
• As a function of the number of directors.
Typical Data:
• The first director will add approximately 3 dB ofoverall gain (if designed well).
• The second will add about 2 dB.
• The third about 1.5 dB. So larger number than10 ~ 15 is not always a good idea.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 13
Yagi-Uda Antenna- Design• The design of a Yagi-Uda antenna is actually
simple. Because Yagi antennas have beenextensively analyzed and experimentallytested. The design steps basically follow thisoutline:
1. Look up a table of design parameters for Yagiantennas.
2. Build it (or model it numerically), andoptimize it until the performance isacceptable.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 14
Yagi-Uda Antenna- Design• Example: Consider the table published for
"Yagi Antenna Design“ (available in Balanis):
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 15
Yagi-Uda Antenna- Design
• Note that the "boom" is the long element thatthe directors, reflectors and feed elementsare physically attached to, and dictates thelength of the antenna.
• The spacing between the directors is uniformand given in the second row from bottom ofthe table.
• The diameter of the elements is given by:
d = 0.0085
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 16
Yagi-Uda Antenna- Design
• The above table helps to estimate the requiredlength of the antenna (the boom length), anda set of lengths and spacings that achieves thespecified gain Just a starting design.
• There are numerous other tables that furthergive results, such as how the diameter of theboom affects the results, and the optimaldiameters of the elements.
• This is a good starting point which can befurther improved by simulations.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 17
Yagi-Uda Antenna- Design• As an example results: The radiation patterns
for a 6- element Yagi antenna (with axis alongthe +x axis) is simulated by FEKO software (seelink below).
• (1 reflector, 1 driven dipole, 4 directors).The resulting antenna has a 12.1 dBi gain, andthe plots are given next slide
• You can download a light version of FEKOsoftware from the link:http://www.feko.info/feko-product-info/feko-lite/feko-lite/
Or http://www.feko.info/Download_CentreDr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 18
Yagi-Uda Antenna- Design
• Yagi-Uda Antenna Radiation Pattern: E-plane
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 19
Yagi-Uda Antenna- Design
• Yagi-Uda Antenna Radiation Pattern: H-plane
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 20
Yagi-Uda Antenna- Design• Yagi-Uda Antenna 3-D Radiation Pattern
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 21
Yagi-Uda Antenna- DesignComments on Results:
• The above plots are just an example to give anidea of how the radiation pattern of a Yagi-antenna looks like.
• The gain can be increased (and the patternmade more directional) by adding moredirectors or optimizing spacing (or rarely, addinganother reflector).
• The front-to-back ratio is approximately 19 dBfor this antenna. It can also be optimized further.
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 22
The Example in BalanisInput Data for the program YAGI_UDA.m • NUMBER OF MODES PER ELEMENT (A POSITIVE INTEGER) = 8
• NUMBER OF ELEMENTS N= 15
• DO ALL DIRECTORS HAVE THE SAME LENGTH?
• ANSWER: (Y OR N) ...... y
• THE UNIFORM LENGTH (in WAVELENGTHS) OF THE DIRECTOR
= 0.406
• LENGTH (in WAVELENGTHS) OF THE REFLECTOR = 0.5
• LENGTH (in WAVELENGTHS) OF THE DRIVEN ELEMENT = 0.47
• SEPARATION (in WAVELENGTHS) BETWEEN DRIVEN ELEMENT & 1st DIRECTOR = 0.34
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 23
The Example in Balanis• IS THE SEPARATION BETWEEN DIRECTORS UNIFORM?
• ANSWER: (Y OR N) ...... Y
• THE UNIFORM SEPARATION (in WAVELENGTHS) BETWEEN DIRECTORS = 0.34
• SEPARATION (in WAVELENGTHS) BETWEEN REFLECTOR & DRIVEN ELEMENT = 0.25
• RADIUS (in WAVELENGTHS) FOR ALL ELEMENTS USED
= 0.003
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 24
Computed ResultsResults:
*****************************************
PROGRAM OUTPUT FOR THE YAGI UDA ARRAY
*****************************************
• 3-dB BEAMWIDTH IN THE E-PLANE PATTERN
= 26.98 DEGREES
• 3-dB BEAMWIDTH IN THE H-PLANE PATTERN
= 27.96 DEGREES
• FRONT-TO-BACK RATIO IN THE E-PLANE
= 36.6416 dB
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 25
Computed Results• FRONT-TO-BACK RATIO IN THE H-PLANE
= 36.7225 dB
• DIRECTIVITY = 14.637 dB
• The plots follow in next pages
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 26
The Field Pattern E & H plane
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 27
Current Amplitudes on Elements
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 28
Radiation Patterns in E & H planes
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 29
End
of Set X
Thank You for Your Attention
Dr. Zuhair M. Hejaz Set X-Antennas and Wave propagation 30