Lab Exercise: Antenna Basics

Transcription

1 Experiment Objectives: Lab Exercise: Antenna Basics In this lab you will analyze received radar signals from the P400 to gain an understanding of the basics of electromagnetic antennas. This lab will demonstrate the importance of electromagnetic wave polarization, antenna radiation patterns, and the law of reciprocity. Part 1: Overview 1.1 Introduction Understanding antennas is a key component to understanding electromagnetics and their numerous applications. There are many types of antennas, but all antenna performance can be described by two aspects: its impedance and radiation properties. These properties are dictated by the shape, size, and material of the antenna. The impedance of the antennas determines the amount of energy that is coupled to the antenna from the generator when the antenna is transmitting or, vice-versa, when the antenna is receiving. The simplest case is when the antenna is properly matched, thereby resulting in full transmission. The radiation properties, which are of greater interest for this lab, include the radiation pattern and the polarization of the radiated field. Polarization is a critical aspect of antenna design and usage. In general, electromagnetic waves are time varying electric and magnetic fields that are a function of spatial coordinates and are coupled by Maxwell s equations. A uniform plane wave is characterized by electric and magnetic fields which do not have any components along the direction of propagation and have uniform properties across a plane perpendicular to the direction of propagation. The polarization of a uniform plane wave describes the shape and locus of the tip of the E vector in the plane orthogonal to the direction of propagation, and depends on the relative phase between the field components, Figure 1. In the most general case, a wave is elliptically polarized, meaning that the vector of the E field traces an ellipse as it propagates. The rotation can be either clockwise or counterclockwise depending on the properties of the wave, and dictated by the direction of propagation. In certain cases, the field can simplify to circular or linear polarization. Figure 1: Example of circular polarization In Figure 1, the blue and green lines depict the E x and E y components which are propagating in the z direction. The red line is the combined effect of the E field components, since E x and E y are 90 out-of-phase, the result is circular Lab Instructions: Experiment #6 Page 1

2 polarization. In this lab we will focus on linear polarization. If a wave is traveling in the z direction, the wave is said to be linearly polarized if it contains only one field component (either E x or E y ), or if the E x and E y components are either in-phase or out-of-phase by 180. In these particular cases, at any specified value of z, the tip of the E field will trace a straight line in the x y plane. For instance, considering Figure 1, if the E y field component (green) was zero, then for any z value, the E x field component would appear to be a line along the x-axis changing from positive to negative. Depending on an antenna s physical properties, the radiated field can be elliptical, circular, or linear in nature. Aside from radiating a particular polarization type, the antenna will also only be able to receive that particular polarization type. For example, if the transmitting antenna only radiates an E x component, then given a receiving antenna which cannot receive E x components no power will be received. If the transmitting antenna is circularly polarized, and the receiving antenna is linearly polarized (e.g. in E x ), then since the transmitting antenna radiates both an E x and E y component, only part of the transmitted power will be received. The antenna pattern, also referred to as the radiation pattern, describes the strength of the radiated wave in the threedimensional space around the antenna. It is the far-field directional properties of the antenna measured at a fixed distance from the antenna. An isotropic antenna is one that radiates equal power in all directions; it is not physically realizable but it is used as a reference to calculate antenna properties such as antenna gain. Measuring an antenna s radiation pattern can be difficult, as is graphically displaying the three-dimensional results. Most often, radiation patterns are discussed in terms of the spherical coordinates, and typically they are shown using either a threedimensional plot, or two separate plots for orthogonal planes. The two planes most commonly used are the elevation and azimuth planes. The elevation plane, also called the plane, corresponds to a constant; whereas the azimuth plane, also called the plane, corresponds to the x y plane where = 90. The antenna radiation patterns can vary greatly depending on the application; however two general classes are omnidirectional antennas or directional antennas, Figure 2. Omnidirectional antennas radiate power uniformly in all directions in a single plane, usually defined as the azimuth plane, and then have decreasing power in the elevation plane. Conversely, directional antennas, also called beam antennas, radiate greater power in one or more directions. Directional antennas are often described in terms of their lobes, where the maximum power is confined to the main lobe, and additional power results in the undesirable side and back lobes. Radiation patterns not only define the directions along which an antenna will transmit power, they also define the directions in which an antenna can receive power. If an antenna is directional, and does not transmit energy in a certain direction, it also will not be able to receive energy coming from that direction. This is the definition of reciprocity, and is a fundamental property of antennas. Reciprocity means that the transmitting and receiving antennas can be switched, and the resulting signal will be exactly the same. Figure 2: 3D representation of an omnidirectional radiation pattern (left) and a directional radiation pattern (right). Lab Instructions: Experiment #6 Page 2

3 1.2 Background The P400 emits an electromagnetic pulse that has a center frequency of 4.3GHz and a bandwidth of 2GHz, Figure 3. The antennas are planar elliptical dipole antennas, which produce a linearly polarized electromagnetic field. The radiation patterns can be found in the Broadspec UWB Antenna data sheet which was included in your Time Domain information packet. This specification sheet will be used for comparison to the experimental data collected. Note that the antennas are omnidirectional in the azimuthal plane, and have greater variability in the elevation plane. Also, note that the radiation patterns are shown for individual frequencies. In the following experiments, the peak received signal will be used to analyze the radiation pattern. Since we are considering the results in the time domain, these measurements will include the entire bandwidth of the P400 excitation. Figure 3: P400 excitation in the time domain (left) and frequency domain (right). Images from Time Domain. Part II: Antenna Experiment 2.1 Required Equipment Hardware: Hardware: Laptop, P400 radar, coax cables, assortment of fixed attenuators, tape measure, string (1 meter), protractor Software: MATLAB, MATLAB scripts (DataAcquistion.m, Initialize.m, getscan.m) Note: In this lab you will be rotating and moving the antennas often, and you should be sure to check that the coaxial connections are still tight after each move. 2.2 Setup: Step 1: Mount the antennas some distance apart, with the flat sides facing each other. The antennas should be level, at the same angle, and orthogonal to the ground. This setup (see Figure 4a) will be referred to as Position 1 and will be the starting point for various data sets. Recall our lab about multipath reflections, and assure that the first received signal will be the direct signal between the two antennas and will not constructively interfere with any reflections. (This can be accomplished by mounting the antennas, for example on tripods or cardboard boxes, such that there are no RF reflectors close to either antenna. In this case a reflector is too close if it closer to either antenna by a distance less than the distance between the two antennas.) Step 2: In Matlab open DataAcquisition.m Lab Instructions: Experiment #6 Page 3

4 Step 3: Using the provided coaxial cables, attenuators, and transmitgain setting in Initialize.m, determine a combination where the received signal is strong but not clipped. 2.3 Test Procedure: Step 4: Use DataAcquisition.m to record a scan with the current setup at Position 1. Step 5: Next, rotate one of the antennas 90 o, and record a scan. Repeat for 180 o, and 270 o. Save the current data set. How does the signal change? Does this agree with the antenna pattern given on the Broadspec UWB Antenna specifications? Step 6: Start recording a new data set. With the antennas back at Position 1, record a new scan. Step 7: Tie a string around the center of the transmitter, and tie the other end around the receiver. Next, move one of the antennas around the other antenna, and keep the string taunt so that the antenna separation is the same, Figure 4. Both antennas should still be orthogonal to the ground at all positions. Record scans at different positions. How do the results change? Is this expected? (a) (b) (c) Figure 4: Diagrams for Procedure Part 7; (a) Tie a string connecting the antennas at Position 1; (b) Keeping the string taunt, raise the receiver some angle above the transmitter; (c) position the receiver directly above the transmitter Lab Instructions: Experiment #6 Page 4

5 Step 8: Again, start recording a new data set with the antennas back at Position 1. Using a protractor, change the angle of the receiver so that is no longer orthogonal with the ground. Move the antenna in increments of 10 degrees from 0 degrees (orthogonal to the ground) to 90 degrees (parallel with the ground), Figure 5. What changes? Why? Do you need to repeat this experiment for the transmitter, why or why not? 2.4 Lab Report Prepare a Lab report which summarizes your experiment and specifically addresses the questions asked in the experimental procedure. 2.5 References 1. Balanis, Constantine A. Advanced Engineering Electromagnetics. New York: Wiley, Ulaby, Fawwaz. Fundamentals of Applied Electromagnetics. Prentice Hall, Lab Instructions: Experiment #6 Page 5