Department of Electrical and Electronic Engineering (EEE), Bangladesh University of Engineering and Technology (BUET). EEE 434: Microwave Engineering Laboratory Experiment No.: A2 Radiation characteristics of an array of two dipole antennas Objectives: 1. Observe the radiation pattern of an array of two dipole antennas in horizontal plane and vertical plane. 2. Observe the received power vs. frequency characteristics of the antenna array. 3. Observe the change in radiation characteristics when the spacing between the two dipole elements is changed. 4. Observe the change in radiation characteristics when a phase difference exists between the excitation of the two dipole elements. Introduction: In many practical applications, an antenna system is required to have specific radiation characteristics. For example, an antenna may be required to have a fixed beamwidth, sidelobe levels below a threshold value, and the main-lobe directed towards a specific direction. It is very hard to find a single antenna element that can satisfy all these requirements. In such cases, arrays of multiple antenna elements are used. In an antenna array, multiple antennas operate together and act like a single antenna. Each individual antenna in an array is called an antenna element, array element, radiating element or just element. The radiation pattern of an antenna array depends on: i. The geometry at which the antenna elements are positioned. ii. The distances between the antenna elements. iii. The amplitude of excitation current of the antenna elements. iv. The phase difference among the excitation currents of the antenna elements. By adjusting these parameters, an antenna array can produce radiation pattern of almost any arbitrary shape.
Antenna arrays can be one dimensional (elements located along a line), two dimensional (elements located on a plane) or three dimensional. The number of elements in an antenna array can vary depending on the application. In this experiment, an array of two dipole elements will be studied. The Feedback Antenna Lab 57-200-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. Experimental Procedure: 1. The Feedback Antenna Lab 57-200-USB contains multiple types of antennas and antenna elements. Identify two dipole antenna elements. 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 the Yagi stack base assembly. It is the thinner of the two grey strips with screw holes in them. 3. Identify Yagi boom assembly rods. The Yagi boom assembly is a long rod with that has screw holes in it. Connect two Yagi boom assembly rods on to the Yagi stack base assembly. One assembly rod should be connected 4 holes above the center of the stack base and the other should be connected 4 holes below the center of the stack base. Measure the distance between the rods. 4. Connect the Yagi stack base assembly vertically to the side of the generator tower. 5. Connect two dipole elements to each of the Yagi boom assembly rod. The elements should be connected near the center of the rod (close to the shaft of the generator tower). If everything is assembled correctly, the arms of the dipoles should be horizontally aligned at this moment. Measure the distance between the two dipole antennas. 6. Adjust the length of the dipole elements so that each dipole is 10 cm long (Making two half wave dipole for operating frequency of 1500 MHz). 7. Identify the 2-way combiner. It is the small green printed circuit with 3 coaxial sockets mounted on it. The sockets are mounted in the shape of a triangle. Identify the sockets located close to each other. When input is supplied to these 2 terminals, the third terminal gives the combined (added) signal. And, if input is supplied to the third terminal, the total power is equally divided in the first two terminals (the signal is split). 8. Identify two 183mm lengths of coaxial cable. Make sure that the two cables have equal length. Equal length of the cable ensures that there is no phase difference between the excitation of the two elements. Connect the cables to the two terminals located close to each other of the 2-way combiner. The other ends of the cable should be connected to the dipole elements. 9. 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. 10. 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. 11. Make sure that the generator tower feed line is not connected to anything. 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 feed line is kept disconnected so that the cables do not get tangled up during this initial rotation. 12. 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 generator tower to the third terminal of the 2-way combiner. 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. The rotation in the opposite direction ensures that the cables do not get tangled up. 13. After the rotation, a 2D polar graph of the radiation pattern of the dipole should appear. This is the radiation pattern of the array in the horizontal plane. 14. 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. 15. 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 Yagi stack base assembly. Now connect the Yagi stack base assembly horizontally on the generator tower. If the assembly is done correctly, the dipole arms should align vertically. In this position, we can get the radiation pattern of the array in the vertical plane. 16. 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. 17. 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 to 1500 MHz. The resulting graph gives radiation pattern of the dipole antenna in the vertical plane. 18. Change the distance between the two dipole antennas by changing the position of the Yagi boom assembly on the Yagi stack base. Measure the distance between the dipoles. At this condition, find the radiation pattern at both planes by following similar procedure as described in previous steps. 19. To create a phase difference between the excitation current of the antenna elements, two coaxial cables of unequal length should be used to connect the
dipole elements to the 2-way combiner. Measure the length of each cable. The amount of phase shift can be calculated from the difference in length of the wires. 20. Find the radiation pattern of the array (in both planes) when a phase difference exists between excitation current of the array elements. Report: 1. Comment on the radiation pattern of the two element array when no phase difference exists between the excitation currents of the dipole elements. 2. Comment on the radiation pattern of the two element array when phase difference exists between the excitation currents of the dipole elements. 3. 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: 22 nd April, 2011. Date last modified: 22 nd April, 2011.