KULLIYYAH OF ENGINEERING

Transcription

1 KULLIYYAH OF ENGINEERING DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING ANTENNA AND WAVE PROPAGATION LABORATORY (ECE 4103) EXPERIMENT NO 3 RADIATION PATTERN AND GAIN CHARACTERISTICS OF THE DISH (PARABOLIC) ANTENNA

2 INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA KULIYYAH OF ENGINEERING ECEE 4103 ANTENNA LAB EXPERIMENT NO: NAME OF EXPERIMENT: Student Name : Matric Number : Submission date : Mark obtained :

3 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 Gunn power supply with SWR meter Gunn oscillator Isolator Pin Modulator Large Horn Antenna RF cable, L = 1 m Supports for waveguide components Stand base MF Set of microwave absorbers Set of 10 thumb screws M Remote control for rotating antenna platform Dipole antenna kit Dish Antenna Kit 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 1

4 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: Π A = D 2 4 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 log 10 ( 4η Π A ) ; where ; 2 λ η 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.

5 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 ( the following equations: ). These parameters are related to each other by 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.

6 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 Switch on the rotating antenna platform. 3. Switch on the computer and run the antenna measurement software.

7 4. Rotate the transmitting horn antenna to produce the required wave polarization for E- plane and H-plane pattern measurements. r=170cm 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 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 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. 5

8 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 antenn 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.

9 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: Type of Source Antenna: Polarization: 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]

10 (B) Distance 2 : Table 1: Directional Diagram Types of Test Antenna: Type of Source Antenna: Polarization: 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] After plot manually, then change the device in order plot by computer. Attach the output from the computer generated result(s). 8

11 a (θ ) Diagram a / db Directional Diagram in Polar Coordinates: R-Axis Relative Amplitude (Log)

12 a (θ ) Diagram a / db Directional Diagram in Polar Coordinates: R-Axis Relative Amplitude (Log)

13 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 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 λ 0 32 mm b. Reflector diameter D = 400 mm c. Aperture efficiency q = How large in the error in db caused by the estimation of the aperture efficiency? 4. How would a doubling of the reflector diameter affect the antenna gain in db? 11

14 5. What is value of default distance? Briefly compare the effect between distance 1(default distance) and distance 2. Explain why it happens? DISCUSSION & CONCLUSION 12

15 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). 13

16 The smaller dish antennas typically operate somewhere between 2 and 28 GHz. The large dishes can operate in the VHF region ( 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 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.