KULLIYYAH OF ENGINEERING DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING ANTENNA AND WAVE PROPAGATION LABORATORY (ECE 4103) EXPERIMENT NO 1 “RADIATION PATTERN AND GAIN CHARACTERISTIC OF DIPOLE ANTENNA” INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA KULIYYAH OF ENGINEERING ECE 4103 ANTENNA LAB EXPERIMENT NO: __ NAME OF EXPERIMENT: _____________________________ Student Name : __________________________ Matric Number : __________________________ Submission date : __________________________ Mark obtained : __________________________ EXP NO: 1 RADIATION PATTERN AND GAIN CHARACTERISTIC OF DIPOLE ANTENNA OBJECTIVE • To become familiar with dipole antennas. • To investigate radiation patterns and gain of the dipole antenna with different length of dipole antenna. • To plot manually the radiation pattern of dipole antenna and compare with computer generated radiation pattern. 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 BRIEF THEORY Antennas are a fundamental component of modern communications systems. By definition, an antenna acts as a transducer between a guided wave in a transmission line and an electromagnetic wave in free space. Antennas demonstrate a property known as reciprocity that is an antenna will maintain the same characteristics regardless if it is transmitting or receiving. When a signal is fed into an antenna, the antenna will emit radiation distributed in space a certain way. A graphical representation of the relative distribution of the radiated power in space is called a radiation pattern. 1 The radiation pattern of the antenna is of principle concern when engineering a communications system. Let’s assume that a signal needs to be sent from an antenna on the ground to a satellite in orbit. This would require a radiation pattern with the majority of its radiated power focused into orbit. If the antenna is not engineered to do so, contact cannot be established between the signal source and its target. There are many different ways to manipulate a radiation pattern to meet the demands of a specific task. These concepts are the principle focus of this lab assignment. Implementing this lab assignment, students will examine the radiation patterns of several antennas by hands on field testing. Only the most fundamental antennas were chosen for this lab assignment. This allows us to see visually how the most common types of real-world antenna designs function. The dipole is one of the oldest and simplest forms of antenna. It is used in all of the microwave frequency ranges and on up to the long-wave range. Its radiation properties are dependent on a ratio1/ (dipole length/wavelength). In actual practice, the antenna length is normally between 1/3 and 5/ and only rarely exceeds 2 . Since our antenna experiments are carried out in the X- band, in the frequency range f = 9.40 ±0.05 GHz dimensions suitable for work in the laboratory. Consequently, they can be investigated without entailing difficulties. Dipole antenna basics The dipole antenna or dipole aerial is one of the most important and commonly used types of RF antenna. It is widely used on its own, and it is also incorporated into many other RF antenna designs where it forms the radiating or driven element for the antenna. The dipole is a simple antenna to construct and use, and many of the calculations are quite straightforward. However like all other antennas, the in-depth calculations are considerably more complicated. The basic half wave dipole antenna Dipole antenna is an antenna that can be made by a simple wire. Typically a dipole antenna is formed by two quarter wavelength conductors or elements placed back to back for a total length of . This antenna consists of two linear wires with same length and distance between the wires (2 λ ) is assumed to be infinitely small. The center of the antenna is located at the origin of the coordinate system and the dipole wire run along the z-axis. Here some figure about the elementary dipole and field vectors of an outgoing wave which the wire length and diameter can be almost any value. Dipoles that are much smaller than the wavelength of the signal are called Hertzian, short, or infinitesimal dipoles. These have a very low radiation resistance and a high reactance, making them inefficient, but they are often the only available antennas at very long wavelengths. Dipoles whose length is half the wavelength of the signal are called half-wave dipoles, and are more efficient. In general radio engineering, the term dipole usually means a half-wave dipole (center-fed). Radiation Patterns for Dipole Antennas The far-fields from a dipole antenna of length L are given by: The normalized radiation patterns for dipole antennas of various lengths are shown in Figure 3. Figure 3: Current characteristics and vertical directional diagrams for Dipole Antenna From the Figure 3, it shows the current characteristics and vertical directional diagrams of linear dipoles under the assumptions of sinusoidal current distributions: a) Half-dipole Antenna b) Full-dipole Antenna c) 2cDipole Antenna d) 6 λ Dipole Antenna Dipole polar diagram Polar diagram of a half wave dipole in free space The polar diagram of a half wave dipole antenna that the direction of maximum sensitivity or radiation is at right angles to the axis of the RF antenna. The radiation falls to zero along the axis of the RF antenna as might be expected. If the length of the dipole antenna is changed then the radiation pattern is altered. As the length of the antenna is extended it can be seen that the familiar figure of eight pattern changes to give main lobes and a few side lobes. The main lobes move progressively towards the axis of the antenna as the length increases. PROCEDURES 1. Assemble the experiment set-up as specified in Fig.1 and setting distance between source antenna and test antenna, r = 170 cm. r=170cm Figure 1 2. The dipole antenna (Cat. No. 737 410) generally serves as the object under test without any restrictions. Connect the antenna rod, with the holder provided. Set the holder into the central mounting bore for the stand rods in the rotating antenna platform so that the axis runs parallel and perpendicular to the marked reference lines on the rotating base in accordance with Fig.2. Figure 2 Note: The following generally applies: the axes of the test antenna and the rotating base must align. This is fulfilled in antennas, which are inserted into the central mounting bore of the rotating base. However, there is also the possibility of mounting test antennas with the aid of stand base. If this is selected, the system must be aligned very carefully. When the antenna is rotating, it may not carry out any eccentric movements. Otherwise asymmetries can arise in the directional diagrams. If necessary, turn the experiment set-up manually to test the accuracy of the assembly. The built-in slip clutch prevents any damage from occurring to the electro-mechanical drive. 3. Connect the plug of the antenna output cable to the BNC input socket on the rotating base. Set the antenna to 0º position, as shown in Fig 2. 4. Switch on the Gunn power supply with SWR meter. Select a Gunn supply voltage of 9.5 V. 5. Set the PIN-modulator switch to INTern and turn the rotary knob for the modulation amplitude to the right limit (maximum modulation amplitude). 6. Set the range switch v/dB of the SWR meter to 25 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. No bring the rotating antenna platform slowly (“SPEED” on setting 2 or 3) into motion by activating the toggle lever “─ ← → + “ on the remote control. 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”. Here you can expect a voltage of approx. 7 V at the amplifier output “AMP.OUT”. Note: When the maximum incoming signal is reached, we find ourselves in the main radiation direction of the major lobe of an antenna, or in the case of several desired radiating receiving directions in the maximum of the antenna. 10. 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. 11. Now carry out an additional test to see whether the bias current setting at setting 3 provides us with the highest sensitivity of the antenna detector. 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. 6 RESULTS 1. Manual Procedures for Plotting Radiation Pattern (A) Length of Dipole Antenna : Default Length (______) 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 Angle [º] SWR Meter Level [dB] Frequency: Angle [º] 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 SWR Meter Level [dB] 7 (B) Length of Dipole Antenna : ___________________________ 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 Angle [º] SWR Meter Level [dB] Frequency: Angle [º] 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 SWR Meter Level [dB] 8 (C) Length of Dipole Antenna : ___________________________ 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 Angle [º] SWR Meter Level [dB] Frequency: Angle [º] 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 SWR Meter Level [dB] 2. After plot manually, then change the device in order plot by computer. Attach the output from the computer generated result(s). 9 a ( θ ) – Diagram a / dB Directional Diagram in Polar Coordinates: R-Axis – Relative Amplitude (Log) a ( θ ) – Diagram a / dB Directional Diagram in Polar Coordinates: R-Axis – Relative Amplitude (Log) a ( θ ) – Diagram a / dB Directional Diagram in Polar Coordinates: R-Axis – Relative Amplitude (Log) QUESTIONS: 1. What are the effects of the wavelength to the directivity and radiation pattern for dipole antennas? …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… 2. Does the dipole antenna have the same response in all directions in the azimuth (horizontal) plane? …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… 3. What is the default length of the dipole antenna that in our lab? Brief about its radiation pattern. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… 4. What happen when the default length of dipole antenna is doubled? Brief in term of radiation patterns and gain. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… 5. What happen when the default length of dipole antenna is reduced to half or less? Brief in term of radiation patterns and gain. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… 13 DISCUSSION & CONCLUSIONS --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 14 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 following is a glossary of basic antenna concepts. i. Antenna An antenna is a device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise reception and/or transmission will be impaired. ii. Radiation Patterns The radiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna, at a fixed or constant distance. The radiation pattern is a "reception pattern" as well, since it also describes the receiving properties of the antenna. The radiation pattern is three-dimensional, but it is difficult to display the three dimensional radiation patterns in a meaningful manner, it is also time consuming to measure a three-dimensional radiation pattern. These pattern measurements are presented in either a rectangular or a polar format. iii. Near-Field and Far-Field Patterns The radiation pattern in the region close to the antenna is not exactly the same as the pattern at large distances. The term near-field refers to the field pattern that exists close to the antenna; the term far- field refers to the field pattern at large distances. The far-field is also called the radiation field, and is what is most commonly of interest. The near-field is called the induction field (although it also has a radiation component). Ordinarily, it is the radiated power that is of interest, and so antenna patterns are usually measured in the far-field region. For pattern measurement it is important to choose a distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength. 15