Radiation characteristics of a dipole antenna in free space

Transcription

1 Department of Electrical and Electronic Engineering (EEE), Bangladesh University of Engineering and Technology (BUET). EEE 434: Microwave Engineering Laboratory Experiment No.: A1 Radiation characteristics of a dipole antenna in free space Objectives: 1. Observe the radiation pattern of a dipole antenna in horizontal plane and vertical plane. 2. Observe the received power vs. frequency characteristics of a dipole antenna. 3. Observe the radiation pattern of a dipole antenna at different frequencies. Introduction: One of the simplest forms of antenna is a dipole antenna. A dipole is a wire type antenna. It is basically a conductor that is divided in the middle and is connected to a feeder (feed line). The feeder connects the antenna to the receiver or transmitter. The most commonly used feeder is coaxial cable. This type of feeder is used in the Antenna trainer which will be used in this experiment. Radiation pattern is the mathematical or graphical representation of the radiation properties of an antenna as a function of space coordinates. In most cases, radiation pattern is determined in the far-field region. If we imagine an antenna as a transmitter, then, the radiation pattern (field pattern/power pattern) of the antenna is a trace of received electric field/magnetic field/power at different points in space at a constant distance from the antenna. In far-field region, the radiation pattern is usually independent of the distance between the antenna and the observation region. So, the radiation pattern is a function of the angular coordinates (azimuth angle and zenith angle/elevation angle). Radiation pattern can be mathematically formulated from the expression of electric field/magnetic field in the far-field region. It can also be formulated from the expression of gain. The gain of a small dipole antenna with uniform current distribution (Hertzian dipole) can be approximated as: G 1.5sin

2 As this is the function of zenith angle only, the radiation pattern is expected to be independent of azimuth angle. To visualize the radiation pattern, it is important to understand the co-ordinate system in relation to the geometry of the dipole antenna. Note that the arms of the dipole are assumed to be parallel to the z-axis. In this experiment, the radiation pattern of the half-wave dipole will be studied. A halfwave dipole is a dipole antenna whose length is equal to half of the operating wavelength. Although the expression of gain of a half-wave dipole is different from the expression of gain of Hertzian dipole, the basic radiation characteristics are similar. The Feedback Antenna Lab USB hardware will be used for the experiment. The instrument can perform radiation characteristics analysis in the frequency range 1200 MHz to 1800 MHz. Initial Hardware Setup: 1. Connect the interface unit to the computer using the USB port. The interface unit should be connected to the generator tower using the 25-way ribbon cable. 2. Connect the generator tower and receiver tower together using the 15-way socket. 3. Connect the power supply to the generator tower unit through a voltage stabilizer. Do not turn on the power. 4. Make sure that the receiving tower is connected with four 5 element log periodic antenna fed to a four-way microstrip combiner. This array of antennas will be used as the reference antenna. The antenna connected to the generator tower is the test antenna whose radiation characteristics will be measured. 5. Make sure that the RF coaxial cable of the receiving tower is connected to the receiving antenna array. 6. Turn on the computer. 7. Make sure that the generator and the receiver tower are aligned with each other and no obstacles are located between them. 8. The distance between the generator tower and the receiver tower should be about 1 (one) meter.

3 z-axis diretion l l = Dipole length = λ/2 for a halfwave dipole Dipole arm Microwave source Coaxial feed line Figure 1: A dipole antenna. Experimental Procedure: 1. The Feedback Antenna Lab USB contains multiple types of antennas and antenna elements. Identify a dipole antenna element. The dipole can be identified by its shape and the coaxial feed line that is attached to it. Elements that look like a dipoles but does not contain a feed line are passive antenna elements which can be used as directors or reflectors in an antenna array. 2. Identify a Yagi boom assembly rod. The Yagi boom assembly is a long rod with that has screw holes in it. The assembly also has a distance scale printed on it. Connect the Yagi boom assembly to the generator tower with screws. Note that assembly can be connected to the generator tower both horizontally and vertically. The assembly should be connected horizontally at this time. 3. Connect a single dipole antenna to a Yagi boom assembly. The dipole element has a groove in it. It can be connected to the Yagi boom assembly by aligning the groove to the rod and applying slight pressure. Make sure that the dipole is located near the center of the generator tower shaft and that the dipole arms are aligned horizontally. 4. Note that the ends of the dipole antenna are extendable. So, the length of the dipole can be adjusted. Adjust the length of the dipole so that it is equal to half wavelength. For a frequency of 1500 MHz, the required length is 10 cm.

4 5. Make sure that the test antenna and the reference antenna are polarized in the same direction. This can be done by checking if they both are in similar physical orientation. For example, if the arms of the dipole are horizontally oriented, the arms of the log-periodic elements should also be horizontally oriented. 6. Turn on the Discovery IMS (IMS = Instructional Management System) software. Using the navigation panel at the left side of the application window, navigate to Signal strength vs. Angle application page. Select 2D polar graph window. A window with a blank polar graph should appear. 7. Make sure that the feed line of the dipole is NOT connected to the generator tower feed line. Turn on the power of the generator tower (it is a green switch that is located at the back of tower). A green light should turn on. After that, turn on the motor enable switch. At this time, the generator tower shaft may start to rotate. The coaxial feed line is not connected because this rotation may cause the coaxial cable to tangle up. 8. From the polar graph window, select file and then new plot. A dialogue box asking for operating frequency will appear. At the same time, if the generator tower shaft was rotating before, it should stop. Connect the feed line of the dipole element to the generator tower. Set the frequency to 1500 MHz and click OK. The test antenna should rotate in one direction and after completing 360 degree rotation, it should rotate the opposite direction to return to initial position. This ensures that the connecting wires do not get tangled. 9. After the rotation, a 2D polar graph of the radiation pattern of the dipole should appear. This is the radiation pattern of the dipole in the horizontal plane. 10. Navigate to signal strength vs. frequency graph from application window. It should take a few seconds for the graph to appear. Note the frequency range where the signal strength is high. 11. Turn off the motor enable switch first. Then turn off the power switch. Remove the coaxial feed connection from the generator tower to the dipole antenna. Remove the dipole antenna from the Yagi boom assembly. Now connect the dipole antenna to the Yagi boom assembly so that the dipole arms are aligned vertically. In this position, we can get the radiation pattern of the dipole in the vertical plane. 12. Change the orientation of the receiving tower antenna so that the log-periodic antenna element arms are oriented vertically. The orientation can be changed by loosening the round screw located at the center of the receiving antenna array, then rotating the antenna to a desired direction and tightening the screw. This will ensure that the test antenna and the reference antenna are in the same polarization.

5 13. Navigate to signal strength vs. angle 2D polar graph in the software window. Select new plot. Connect the coaxial feed connector to the generator tower. Set frequency at 1500 MHz. The resulting graph gives radiation pattern of the dipole antenna in the vertical plane. 14. Observe the radiation pattern of the dipole antenna (both horizontal and vertical plane) at different frequencies (keeping dipole length fixed) by following similar procedure. Note the change in radiation pattern of the dipole with frequency. Report: 1. Comment on the radiation pattern of the dipole antenna. 2. Compare the observed data with theoretical values. Prepared by: Mohammad Asif Zaman Lecturer, S. M. Shafiul Alam Lecturer, Supervised by: Dr. Md. Shah Alam Associate Professor, Dr. Md. Abdul Matin Professor, Date created: 18 th April, Date last modified: 22 nd April, 2011.