yagi-uda antennas - zuhair m. hejaz

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.


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.


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).


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.


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).


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.



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.



• 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.



• 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.


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.


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.


Yagi-Uda Antenna- Design• Example: Consider the table published for

"Yagi Antenna Design“ (available in Balanis):


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



• 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.


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

http://www.feko.info/feko-product-info/feko-lite/feko-lite/









http://www.feko.info/Download_Centre


• Yagi-Uda Antenna Radiation Pattern: E-plane



• Yagi-Uda Antenna Radiation Pattern: H-plane


Yagi-Uda Antenna- Design• Yagi-Uda Antenna 3-D Radiation Pattern


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.


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


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


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


Computed Results• FRONT-TO-BACK RATIO IN THE H-PLANE

= 36.7225 dB

• DIRECTIVITY = 14.637 dB

• The plots follow in next pages


The Field Pattern E & H plane


Current Amplitudes on Elements


Radiation Patterns in E & H planes


End

of Set X

Thank You for Your Attention


yagi-uda antennas - zuhair m. hejaz

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