KULLIYYAH OF ENGINEERING

DEPARTMENT OF ELECTRICAL & COMPUTER

ENGINEERING

ANTENNA AND WAVE PROPAGATION

LABORATORY

(ECOM 4101)

EXPERIMENT NO 3

“RADIATION PATTERN AND GAIN CHARACTERISTICS

OF THE DISH (PARABOLIC) ANTENNA”

INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA

KULIYYAH OF ENGINEERING

ECOM 4101 ANTENNA LAB

EXPERIMENT NO: __

NAME OF EXPERIMENT: _____________________________ Student Name : __________________________ Matric Number : __________________________ Submission date : __________________________ Mark obtained : __________________________

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EXP NO: 3

RADIATION PATTERN AND GAIN CHARACTERISTICS

OF THE DISH (PARABOLIC) ANTENNA

OBJECTIVE

To become familiar with the dish or parabolic antenna.

To investigate the radiation pattern and gain characteristics of parabolic dish antenna.

To compare the effect of changing the distance of parabolic reflector.

MATERIAL

1 Rotating antenna platform 737 400

1 Gunn power supply with SWR meter 737 021

1 Gunn oscillator 737 01

1 Isolator 737 06

1 Pin Modulator 737 05

1 Large Horn Antenna 737 21

2 RF cable, L = 1 m 501 02

2 Supports for waveguide components 737 15

2 Stand base MF 301 21

1 Set of microwave absorbers 737 390

1 Set of 10 thumb screws M4 737 399

1 Remote control for rotating antenna platform 737 401

1 Dipole antenna kit 737 410

1 Dish Antenna Kit 737450

BRIEF THEORY

A parabolic antenna is used for microwave radio communications. It is often referred to as a dish

antenna. It consists of a parabolic reflector which collects and concentrates an incoming parallel beam

of radio waves and focuses them onto the actual antenna placed at its focal point or focus. The actual

antenna at the focus is sometimes referred to as the antenna feed. The main advantage of a

parabolic antenna is that it is highly directive; it functions analogously to a

searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves

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from one particular direction only. Parabolic antennas have some of the highest gains, that they can

produce the narrowest beam width angles, of any antenna type.

They are used as high-gain antennas for point-to- point radio, television and data

communications, and also for radiolocation (radar ), on the UHF and microwave (SHF) parts of

the radio spectrum. The relatively short wavelength of electromagnetic radiation at these frequencies

allows reasonably sized reflectors to exhibit the desired highly directional response. The directive

qualities of an antenna are measured by a dimensionless parameter called its gain, which is the ratio of

the power received by the antenna from a source along its beam axis to the power received by a

hypothetical isotropic antenna.

Gain of a parabolic reflector

Using the formula for the area of a circle, the area of the aperture of a parabolic reflector is:

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A D

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This area is used in calculating the gain of a parabolic reflector. The gain G of a parabolic reflector is

proportional to the ratio of the area of the aperture to the square of the wavelength l of the incoming

radio waves

G = 10 log10( 24

A) ; where ;

is the efficiency of the parabolic reflector and has a practical value of 50%. In electrical

engineering, it is common practice to express gain ratios such as G in terms of decibels which is 10

times the common logarithm of the gain ratio. The units of G are in dBi.

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The equation of a parabola with focal length F can be written in the (x,z) plane as:

This is plotted in Figure 4.

Figure 4: Illustration of parabola with defining parameters.

The parabola is completely described by two parameters, the diameter D and the focal length F. We

also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle

between the focal point and the edge of the dish ( ). These parameters are related to each other by

the following equations:

To analyze the reflector, we will use approximations from geometric optics. Since the reflector is

large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will

analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave.

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Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in

Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely

reflected.

Figure 5: Two rays leaving the focal point and reflected from the parabolic reflector.

There are two observations that can be made from Figure 5. The first is that both rays end up

travelling in the downward direction (which can be determined because the incident and reflected

angles relative to the normal of the surface must be equal).

The rays are said to be collimated. The second important observation is that the path lengths ADE and

ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These

facts can be proved for any set of angles chosen. Hence, it follows that:

All rays emanating from the focal point (the source or feed antenna) will be reflected towards

the same direction.

The distance each ray travels from the focal point to the reflector and then to the focal plane is

constant.

PROCEDURES

Initial Setting (Based on Experiment 1)

1. The antenna measurement system is set-up according to Figure 2.

2. Switch on the rotating antenna platform.

3. Switch on the computer and run the antenna measurement software.

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4. Rotate the transmitting horn antenna to produce the required wave polarization for E-

plane and H-plane pattern measurements.

Figure 2

Parabolic Dish Antenna Configurations

A. Default Distance:

5. Carefully insert the dish antenna kit (please see the Appendix) into the antenna rod as shown in

Figure 3.

6. Set the range switch v/dB of the SWR meter to 20 dB.

7. Switch on the rotating antenna platform.

8. Set the bias current to setting 3 using the remote control. An incoming signal should now appear

on the scale of the SWR meter.

9. Now bring the rotating antenna platform slowly (“SPEED” on setting 2 or 3) into motion by

activating the toggle lever “─ ← → + “ on the remote control.

10. Observe the scale of the SWR meter. Stop the rotating base when the maximum incoming signal

in reached. Calibrate the ‘GAIN ZERO” display of the SWR meter to “0 dB”.

11. 13. Now try to turn the rotating base of the platform in the desired direction by activating to

toggle lever “─ ← → + “ on the remote control. (“SPEED” set to setting 1). The angular position

of the antenna fastened to the rotating platform is indicated on the display of the remote control.

Observe the power scale of the SWR meter for a possible correction of the gain setting.

12. Now carry out an additional test to see whether the bias current setting at setting 3 (7.5 μA)

provides us with the highest sensitivity of the antenna detector.

13. Try to find a more optimal setting in order to measure with. It may be necessary to calibrate the

SWR meter display to “0 dB” again.

r=170cm

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14. Now position the test antenna in a desired angular position, e.g. + 10º by activating the toggle

lever “─ ← → + “. Measure the incoming signal of the test antenna. The magnitude of the

incoming signal can be read directly on the SWR meter. There is a logarithmic scale in dB and

also a linear scale in % available.

15. Use the polar graph to draw a line through the measured points in order to obtain a complete

directional diagram for the test antenn17.

16. Determine the 3 dB width of the two lobes of the parabolic dish.

Figure 3

B. Other Distance between Focal Feed to Reflector:

1. Repeat part A by changing the distance between focal feed and the reflector.

2. Plot by manual and computer, and compare both. Discuss the difference that can be observed

when the distance is changed.

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RESULTS

1. Manual Procedures for Plotting Radiation Pattern

(A) Distance 1: Default Distance (_Distance between parabolic reflector and feed_)

Table 1: Directional Diagram

Types of Test Antenna: Polarization:

Type of Source Antenna: Polarization:

Distance Between Source &

Test Antenna:

cm

Detector Bias Current: µA

WR Meter Range: dB Frequency:

Angle [º] SWR Meter Level [dB] Angle [º] SWR Meter Level [dB]

0 0

-10 10

-20 20

-30 30

-40 40

-50 50

-60 60

-70 70

-80 80

-90 90

-100 100

-110 110

-120 120

-130 130

-140 140

-150 150

-160 160

-170 170

-180 180

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(B) Distance 2 : ___________________________

Table 1: Directional Diagram

Types of Test Antenna: Polarization:

Type of Source Antenna: Polarization:

Distance Between Source &

Test Antenna:

cm

Detector Bias Current: µA

WR Meter Range: dB Frequency:

Angle [º] SWR Meter Level [dB] Angle [º] SWR Meter Level [dB]

0 0

-10 10

-20 20

-30 30

-40 40

-50 50

-60 60

-70 70

-80 80

-90 90

-100 100

-110 110

-120 120

-130 130

-140 140

-150 150

-160 160

-170 170

-180 180

2. After plot manually, then change the device in order plot by computer. Attach the output from

the computer generated result(s).

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a ( ) – Diagram

a / dB

Directional Diagram in Polar Coordinates: R-Axis – Relative Amplitude (Log)

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a ( ) – Diagram

a / dB

Directional Diagram in Polar Coordinates: R-Axis – Relative Amplitude (Log)

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QUESTIONS

1. How is the directivity of a dish antenna determined? Determine the 3 dB-width of the experiment antenna.

Technical specifications on the main reflector:

Focal distance f ≈ 160 mm, D = 400 mm,

Free-space wavelength for 9.40 GHz: λ0 ≈ 32 mm

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2. Approximately determine the gain of the dish antenna to be expected based on the following assumptions :

a. Operating frequency f = 9.40 GHz, corresponding to a free-space wavelength of λ0 ≈ 32 mm

b. Reflector diameter D = 400 mm

c. Aperture efficiency q = 0.5

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3. How large in the error in dB caused by the estimation of the aperture efficiency?

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4. How would a doubling of the reflector diameter affect the antenna gain in dB?

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5. What is value of default distance? Briefly compare the effect between distance 1(default

distance) and distance 2. Explain why it happens?

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DISCUSSION & CONCLUSION

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APPENDIX / GLOSSARY

ATTENTION!!

Microwave Radiation

The power of the microwave generated here is only slight (≈ 20 mW). But in view of normal

professional working conditions with sources of higher power, we recommend that the student be

trained certain points of safety when dealing with this material. When carrying out changes in the

experiment set-up. Switch the modulation of the PIN modulator to “EXT”. This reduces the power of

the radiated microwaves by approx. 10 dB. Nevertheless, avoid looking into the radiating aperture. If

this cannot be avoided, then there is no other alternative but to briefly switch the Gunn oscillator off.

This, however, results in corresponding temperature effects (TC approx. 0.3 MHz/K).

The Parabolic Reflector Antenna

Parabolic reflectors typically have a very high gain (30-40 dB is common) and low cross

polarization. They also have a reasonable bandwidth, with the fractional bandwidth being at least 5%

on commercially available models, and can be very wideband in the case of huge dishes (like the

Stanford "big dish" above, which can operate from 150 MHz to 1.5 GHz).

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The smaller dish antennas typically operate somewhere between 2 and 28 GHz. The large dishes can

operate in the VHF region (30-300 MHz), but typically need to be extremely large at this operating

band.

The basic structure of a parabolic dish antenna is shown in Figure 3. It consists of a feed

antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a circular

aperture.

Figure 4: Components of a dish antenna.

The fields across the aperture of the parabolic reflector are responsible for this antenna's radiation.

The maximum possible gain of the antenna can be expressed in terms of the physical area of the

aperture:

The actual gain is in terms of the effective aperture, which is related to the physical area by the

efficiency term ( ). This efficiency term will often be on the order of 0.6-0.7 for a well designed

dish antenna:

Radiation Efficiency

The radiation efficiency is the usual efficiency that deals with Ohmic losses, as discussed on the

efficiency page. Since horn antennas are often used as feeds, and these have very little loss, and

because the parabolic reflector is typically metallic with a very high conductivity, this efficiency is

typically close to 1 and can be neglected.