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EXPERIMENT NO.3

Analysis ofYagi-Uda antenna

Time

( min)

Content

Learning Aid / Methodology

Faculty Approach

Typical Student Activity

Skill / Competency Developed

10

Relevance and significance of experiment

Chalk & Talk , Presentation

Introduces, Facilitates, Monitors

Listens, Participates, Discusses

Knowledge, Communication, intrapersonal

20

Explanation of experiment

Chalk & Talk , Presentation

Introduces, Facilitates, Explains

Listens

Knowledge, Communication, intrapersonal, Application

60

Reading

Role play/ demonstration

Explains,

Monitors

Participates, Discusses

Debate, comprehension, Hands on experiment

20

Calculations

N/A

Explains,

Monitors

Participates, Discusses

Knowledge, Communication, Intrapersonal, Application

10

Results and conclusions

Keywords

Lists, Facilitates

Listens, Participates, Discusses

Knowledge, Communication, intrapersonal, Comprehension

Practical Session Plan


Title: Analysis of Yagi-Uda antenna

Objectives: Mount and set up the connections of Yagi-Uda antenna to analyze the performance parameters

Aim:

1) To sketch the radiation pattern of Yagi-Uda antenna

2) To compute gain, beam width, return loss & impedance of Yagi-Uda antenna

Apparatus: Antenna, transmitter, receiver-stepper motor controller, pair of dipole antenna, stepper pod, connecting cables, direction coupler, slotted line, 20 db attenuator etc

Theory:

1) Basic type of antenna: Antenna array

2) Theoretical Information:

An antenna array is a group of radiators whose currents are of different amplitudes and phases. They use electromagnetic wave interference phenomena to enhance the radiative signal in the desired direction and diminish it in the non-desired direction. Antenna arrays are the solution to the problem defined as the limitations of operating a single antenna.

A Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element (typically a dipole or folded dipole) and additional parasitic elements (usually a so-called directors and one or more reflector). The name stems from its inventors, as the Yagi-Uda array was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan, with a lesser role played by his colleague Hidetsugu Yagi. The reflector element is slightly longer (typically 5% longer) than the driven dipole, whereas the so-called directors are a little shorter. This design achieves a very substantial increase in the antenna's directionality and gain compared to a simple dipole.

Highly directional antennas such as the Yagi-Uda are commonly referred to as "beam antennas" due to their high gain. However the Yagi-Uda design only achieves this high gain over a rather narrow bandwidth, making it more useful for various communications bands (including amateur radio).

Fig. 1: Three elements Yagi-Uda Antenna

Yagi-Uda antennas are routinely made with rather high gains (over 10dB) making them a common choice for directional antennas especially in VHF and UHF communications systems where a narrowband antenna is acceptable

Procedure:A) To plot the radiation pattern of Yagi-Uda antenna in azimuth plane on polar plots.

1) Connect the dipole antenna to the tripod & set the transmitter frequency to 600 MHz

2) The length of antenna is c/2f=(3*10^8)\(2*600*10^6)=25cm end to end.

3) Connect Yagi-Uda antenna to stepper pod & set the receiver to 600 MHz +.Connect 20dB

attenuator at receiver side to avoid saturation & set the length of receiver dipole antenna same

as 25cm at both ends.

4) Rotate the Yagi-Uda antenna of receiver end around its axis in steps of using stepper motor controller.

5) The reading rotating receiver antenna is automatically saving the power received at each

rotation of antenna to the corresponding memory location .

6) Connect RS232 from receiver to computer & transfer the reading by enabling serial mode at receiver.

Hence radiation pattern is plotted on polar plot from which gain & beam width is obtained.

7) Beam width is obtained on log plot at 3dB level.

B) Calculation of return loss using direction coupler:

1) Connect the direction coupler to the transmitter tripod to RF out connector .

2) Connect the antenna to the direction coupler to RF in connector.

3) Connect the receiver to the direction coupler at RF sample connector and insert attenuator.

4) Bring the transmitter & receiver to 600MHz and take the reading this will give forward power .

5) Now interchange the connection of transmitter tripod & antenna is connected at RF sample.

6) Now the transmitter tripod is at RFin and antenna is connected at RFout .

7) Note down the reading at receiving at same frequency (600MHz) this will give reverse power.

8) The return loss is hence obtained by Return loss=forward power-reverse.

C) Calculation of Impedance:-

1. Connect generator at 0 cm pointof slotted line . Set the frequency of source as 600 MHz. Hence free space wavelength lambda =20cm.

2. Connect short load at 20cm end of slotted line.

3. Move the probe from 0cm to 20cm that is generator to load end of line.

4. Observing Rx reading that goes Max to Min again Min to Max. Hence with short connected 1st minima

is at & 2nd minima is at .

5 . Now connect unknown load ( antenna) at 20cm end & measuring shift in minima. Observing

whether minima has shifted towards load end or generator end.

6 . Thus with antenna connected at load end, 1st minima occurs at cm & 2nd occurs at cm.

7. Hence w.r.t short on slotted line the minima has shifted by cm towards load end when antenna is

replaced with short.

8. Hence shift in minima /lambda =

9. For calculating VSWR, using directional coupler measuring Return loss of antenna.

10. Now using formula for calculating Reflection coefficient which is

R.L. = -20log

11. After calculating , calculating VSWR using formula

VSWR = 1+ / 1-

This gives VSWR which is

12 . Draw a circle on Smith chart with radius taking center of smith chart. This circle is called constant

VSWR circle.

13 With ratio of shift in minima /lambda = , rotating anticlockwise from left & locate on outer circle.

14 . Draw a line connecting to center of circle such that it crosses VSWR circle.This point is read as

as real part & as imaginary part.

15 . Hence normalized impedance is

16. Actual impedance is ( ) 50 = ohms.

Observations:

Sample Calculations:

Results:

1) Max.Gain=

2) Beamwidth=

3) Forward power=

4) Reverse power=

5) Return loss=-

Radiation Pattern:

Conclusion: The yagi-uda antenna was analyzed on the antenna trainer kit. Field Strength of various angle were obtained and radiation pattern were plotted for same.

Oral Question Bank

Theory Question Bank

Q. No

Description

1

Define antenna

2

Define Radiation pattern

3

What are the two types of radiation pattern

4

What is the relationship between effective aperture and directivity?

5

Write the principle of pattern multiplication?

6

Define Beam solid angle or beam area?

7

Define beam efficiency?

8

Define directivity?

9

Define antenna gain?

10

Define effective aperture?

11

What is collecting aperture?

12

Define HPBW?

13

Define FBR?

14

Define BWFN?

15

What is Yagi-uda antenna?

16

What are factors affecting yagi-uda antenna gain?

17

What are factors affecting yagi-uda antenna beam width?

18

What are factors affecting yagi-uda antenna impedance and return loss?

List the formulae for

a) Reflection coefficient

b) VSWR

c) Return loss

Of Yagi-uda Antenna

19

Yagi antenna is generally used for which frequency range?

20

What are the parameters to be considered for the design of a Yagi-uda antenna?

21

Explain the special features of Yagi-uda antenna?

22

What is the main advantage of Yagi-uda antenna?

23

Draw the radiation pattern for Yagi-Uda antenna?

24

List application of Yagi-uda antenna

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