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Basic RF Technic and Laboratory Manual

Dr. Haim Matzner&Shimshon Levy

April 2002

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CONTENTS

I Experiment-8 Antenna Radiation Pattern 5

1 Introduction 71.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2 Prelab Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.3 Background Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3.1 RF and Microwave Antennas . . . . . . . . . . . . . . . . . . 81.3.2 Antenna Radiation Pattern . . . . . . . . . . . . . . . . . . . 81.3.3 Antenna Radiation Pattern Lobes and Nulls . . . . . . . . . . 81.3.4 Antenna Beamwidth. . . . . . . . . . . . . . . . . . . . . . . . 81.3.5 Antenna Directivity . . . . . . . . . . . . . . . . . . . . . . . . 81.3.6 Antenna Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.3.7 Antenna polarization . . . . . . . . . . . . . . . . . . . . . . . 101.3.8 Antenna Bandwidth . . . . . . . . . . . . . . . . . . . . . . . 10

2 Experiment Procedure 112.1 Required Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2 Measuring input impedance of an antenna using Network Analyzer . 112.3 Applying the Flam & Rusell software for measuring and analyzing

antennas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3.1 Measuring E and H Fields of Antenna . . . . . . . . . . . . . 13

2.4 Final Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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4 CONTENTS

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Part I

Experiment-8 Antenna RadiationPattern

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Chapter 1

INTRODUCTION

1.1 Objectives

Upon completion of the study, the student will become familiar with the followingtopics:

1. Antenna Pattern.(amplitude and phase).

2. E-plane and H-plane diagrams.

3. Omni-directional pattern.

4. Beamwidth and beam efficiency.

5. Main lobe, sidelobes and radiation nulls.

6. Far field and near field.

7. Antenna impedance (SWR and Reflection Coefficient) in various formats:linear, log and polar.

8. Bandwidth.

1.2 Prelab Exercise

1. Define a radiation pattern.

2. Explain the following antenna parameters:

- E-plane and H-plane diagrams

- Beamwidth

- Main and lateral antenna lobes

- Beam efficiency

- Far-fi

eld region, near-fi

eld region and reactive near-fi

eld region- Bandwidth

- Omni-directional

3. What is the mimimal distance between the transmitting and receivingantennas to obtain far field condiitons ?

4. Describe and explain how to measure E-plane and H-plane patterns of adipole antenna.

5. Describe the experiments you intend to perform.

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8 Introduction

1.3 Background Theory

1.3.1 RF and Microwave Antennas

In modern communication, the transmission and reception electromagnetic power toor from space, is primary necessity. The process is effected by a transformer betweenthe space and the source, known as Antenna. The function of an antenna is twofold.At the transmitter end, the antenna broadcasts the electromagnetic signal in thedirection of the receiver. At the receiver end, the antenna picks up as much power aspossible from the transmitter. Definition of primary specification discussed.

1.3.2 Antenna Radiation Pattern

The basic source of radiation is a short dipole and theoretically it radiates equalenergy in all direction and so it is called IsotropicAntenna. A practical antenna,however does not radiated isotropically. Radiation patterns generally defined as the

farfi

eld power orfi

eld strengh produced by the antenna as a function of the direction(Azimuth and elevation) measured from the antenna position. The behavior of thefields is changed with the distance from the antenna, and generally three regions aredefined:

Reactive near-field region - The region in the space immediately surround-ing the antenna in which the reactive field dominated the radiating field (d < /(2)).

Radiating near-field region - Beyond the former region and for which d 1, except in the case of an isotropic antennafor which D = 1. An antenna with directivity D >> 1 is called a directive antenna.

1.3.6 Antenna Gain

The definition of antenna gain is close to that of antenna directivity. The differencebetween these two parameters is that the gain takes into account the efficiency of theantenna and its directivity together, that is, the gain of a lossless antenna equals toits directivity. Hence the gain is given by

G = etD (1.1)

where et is the total efficiency of the antenna and D is its directivity.

1.3.7 Antenna polarization

Polarization describes the movement of the tip of the electric field vector going outfrom the transmitting antenna. The antenna is said to be linearly polarized whenthe electric field remains parallel to a constant direction. In this case the minorperpendicular field is called the crosspolarized field. If the tip of the electric fieldtraced a circle the wave is said to be circularly polarized. In the case of circular or

elliptical polarizations, the sense of the rotation is defined as CW or CCW as seenby the observer.

1.3.8 Antenna Bandwidth

The bandwidth of an antenna is defined as The range of frequencies within whichthe performance of the antenna, with respect to some characteristic, conforms to aspecified standard.

The reason for this qualitative definition is that all the antenna parametersare changed with frequency and the importance of the different parameters as gain,return loss, beamwidth, side-lobe level etc. much depends on the application.

For example, the bandwidth of an antenna for gain (-1dB from the maximum)

is defined as

Bandwidth(%) =fU fL

fC 100

where fU is the upper frequency, fL is the lower frequency, and fC is the centerfrequency. Another example is the bandwidth related to the mismatch loss definedby the SWR.

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Chapter 2

EXPERIMENT PROCEDURE

2.1 Required Equipment

The following equipment needed to the pattern experiment:

Network analyzer Hp - 8714B

Transmitting dipole antenna - Cushcraft S8060BReceiving dipole antenna - Cushcraft S8060B

2.2 Measuring input impedance of an antenna using Network Analyzer

In this part of the experiment you measure the impedance of the antenna in variousformat, the impedance is defined for open site area, therefore the measuremend willbe affected by the enviroment.

1. Connect the corner reflector to the reflection port of the network analyzer,as indicated in Fig.-3.

2. Stabilize the antenna in free space far from moving person (as far as posible).3. Set the frequency of the network analyzer, accordind to the frequency range

of the antenna.4. Measure || absolute value of Reflection coefficient of the antenna , by

pressing Format menu and lin Mag. Save the Data on magnetic media.

RF INRF OUT

Antenna impedance measurement

Figure 1 Setup for input impedance of antenna

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12 Experiment Procedure

The return loss of the antenna is displayed. Save the Data on magneticmedia.

4. Measure Re|| real part of Reflection Coefficient of the antenna , by pressing

Format menu and lin Mag.and More Format and Real Save the Data on magneticmedia.

5. Measure Im|| imaginary part of Reflection Coefficient of the antenna , bypressing Format menu and lin Mag.and More Format and Imaginary Save theData on magnetic media.

6. Press Polar to get amplitude and phase of the Reflection Coefficient inpolar notation. Use the central knob to move across the frequnecy range in order toget relevant values. Save the Data on magnetic media.

6. Measure SWR of the antenna and, Save the Data on magnetic media.

9. Press Format and Smith Chart for getting a display of the real and

imaginary values of the impedance as function of frequency.

2.3 Applying the Flam & Rusell software for measuring and analyzingantennas.

Define a measurement

1. Switch on the PC and prepare the system for E-plane measurement.

2. Double click the FR959 icon.

3. Press Define Test in the pull down menu and choose Primary Axis inorder to control the azimuth range of the measurement.

4. Choose CW/CCW and set the start azimuth to -90 and stop azimuthto 90. Press Replace and OK for a final setting. Check that Incr=2,Data span=1Motion=continuous, and enable primary axis=X.

5. Press Define Test in the pull down menu and choose Frequencies inorder to control the frequnecy of the measurement. Change the frequnecy with thecontinuous frequency control icon, press Add and OK.

6. Press test in the FR959 Acquire window and choose a filename for thedata of the measurement.

7. Press Measuere Antenna and OK to start the measurement.

The measuerement is started and a radiation cut is displayed.

8. Press File, Save and Exit for saving your measuerement results.9. Press Data Pro for analyzing the measurement results.

A phase graph of the radiation cut is displayed. In order to go back to thepower cut use the key at the left - down corner of the screen.

10. Press Analysys in the pull down menu, and then use Beampeak,Beamwidth, Gain, Null-Depth and Sidelobes in order to find the relevant values.Be sure that Enable icon is on.

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Final Report 13

Network analyzer

Positioner controler AL4901-3A

Azimuth positioner

AL360-1

Transmitting antenna

HPIB

HPIB

Receiving antenna

RF INRFOUT

123.45123

Figure 2 Measuring E and H field of antenna

2.3.1 Measuring E and H Fields of Antenna

Follow the instructions below:

1. Connect the system as indicated in the figure.2. Switch on the network analyzer, computer, antenna controller antenna

positioner and printer.3. Prepare the antennas for E-Plane measurements.4. Prepare FR959 software to azimuth between -90o and 90o and get power

and phase patterns, including pattern analysis: beamwidth, sidelobe level, max. gain,angle for max. gain.

5. Repeat the measurements for 3 different frequencies fc, fc +f, fc f (f/fc 6 10%).

6. Prepare the antennas for H-Plane measurements.7. Repeat paragraphs 3 and 4.8. Print all your results.

2.4 Final Report

Please preform the following requirements

1. Using your Data Draw the following graphes.a. Absolute value of Reflection Coefficient as a function of frequency.

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14 Experiment Procedure

b. Real part of Reflection Coefficient as a function of frequency.

c. Imaginary part of Reflection Coefficient as a function of frequency.

d. SWR as a function of frequency.

e. Impedance as a function of frequency.

2. Using the measured Data of Reflection Coefficient, calculate and drawgraphes of SWR and Impedance as a function of frequency, compare the calculatedand measured graphes.

3. Describe and explain each graph of your measurement results. All your

printed patterns have to be included in yourfi

nal report.

2. Plot the maximum of the main lobe as a function of frequency. Find thebandwidth for -1dB and -3dB gain, and the frequency bandwidth of the antenna.Explain your considerations.

3. Draw theoretical E-Plane and H-Plane patterns from the geometrical pa-rameters of the antenna and calculate also the beamwidth for the E-Plane patternand compare your theoretical calculations to the measurement results.

4. Prepare a 3D theoretical pattern of the antenna (using MATLAB or anequivalent software. See Appendix A). Add a diskett containing your source code tothe Report.Antenna Radiation Pattern.