ENGR1 Antenna Pattern Measurements November 29, 2006 Instructor: Dr. Milica Marković Office: Riverside Hall 3028 Email: milica@csus.edu Abstract In this lab we will calculate and measure antenna parameters. 1 What is an antenna? IEEE Standard 145-1983 defines antenna as usually metallic device that radiates or transmits radio waves 1. More recent version of the same standard defines the antenna as that part of a transmitting or receiving system that is designed to radiate or to receive electromagnetic waves 2. Antenna helps the signal escape the cable and continue moving forward in air. A simplified transmitter block diagram is shown below. It consists of a signal source, cables that connect various parts (we call cables transmission lines) and an antenna. The receiver consists of an antenna, signal sensor and a consumer (user). Figure 1: Simplified block diagram of a communication system. 2 Types of antennas Some of the most common antennas are shown in Figures 2-7. 2.1 Antenna Jargon 1. Far-Field. This is the minimum distance between two antennas. In todays experiment you will first have to calculate this minimum distance as given by FF =2 d2 λ (1) 1 IEEE Standard Definitions of Terms for Antennas IEEE Std 145-1983 2 IEEE Standard Definitions of Terms for Antennas IEEE Std 145-1993 1
Figure 2: Horn Antenna. Figure 3: Dipole Antenna. Figure 4: Yagi-Uda Antenna. Figure 5: Helix Antenna. Figure 6: Loop Antenna. 2
Figure 7: Dish Antenna. Where d is the maximum size of the antenna. For a horn antenna this is the length between two opposite corners of the largest rectangular opening. λ is the wavelength of the signal, and is calculated as λ = c/f. c is the speed of light 3 10 8 m/s and f is frequency of signal in Hz. 2. Antenna Radiation Pattern. Antenna radiates most power in a certain direction in space. The exact distribution of this power in space is called antenna radiation pattern. We will measure this antenna property in the lab today. This property of an antenna is usually drawn on x-y plane as power (on y-axis) as a function of degrees away from the antenna axis. An example of an antenna pattern is shown in Figure 8. Figure 8: Example of an Antenna Pattern. 3. Antenna Beam. The shape of the radiated power in space looks like a beam and it is called antenna beam. 4. Beam-width. Beam width is the measure of antenna beam in degrees. We define the point at which the beam-width is calculated as the point where the antenna power falls of to 0.5 of the maximum power (or if you measure voltage on the oscilloscope, then find 0.707 point). 3
5. Directivity. Directivity is the measure of the narrowness of the antenna beams. The higher the directivity the narrower the beam is. The formula to estimate directivity is D = 4π (2) β xyβ yz β xy is the beamwidth in the xy plane and β yz in the beamwidth in the perpendicular plane. Both βs need to be converted to radians first. Directivity is usually quoted in decibels. To find directivity in decibels, use the formula below: D db =10log(D) (3) 3 Laboratory setup In the laboratory we have the two measurement setups shown in Figures 9 and 10. Note that the only difference between Figure 1 and Figures 9 and 10 is the terminology. Figure 9: Measurment Setup 1. Figure 10: Measurement Setup 2. 3.1 Antenna Pattern Measurements 3.1.1 Calculating Far Field of an antenna To measure antenna pattern of the transmitting antenna, we first set the two antennas in the far field. Now we will set up the experiment. 1. Make the grid on the paper showing degrees from -90 to 0 to +90 degrees. 2. Then set the transmitting antenna on the gridded paper showing degrees. 3. Orient the transmitting and receiving antenna to point to each other. 4. First we will measure the radiation pattern in x-y plane as shown in Figure 11. (a) Turn on the oscilloscope (or power meter) first. (b) Then turn on the HP oscillator. (c) You should see a square wave on the oscilloscope (or the power measurement on the power meter). (d) Then move the transmitting antenna clockwise for a few degrees at a time. (e) Record the number of degrees and the voltage on the scope (or the power reading on the power meter). You should stop recording when you get to 45 degrees or you cant see the square wave (or the reading on the power meter is zero). 5. Flip the antennas. 6. Repeat for counterclockwise direction. 7. Then move both antenna sideways on the holders, as shown in Figure 12. 8. Repeat the measurements. 4
Figure 11: X-Y plane antenna pattern measurement. Figure 12: How to flip the antenna. 5
Figure 13: Y-Z plane antenna pattern measurement. 3.2 Data Post-Processing Type all the data in Exell. In the first column write the number of degrees, in the second column write oscilloscope or power-meter readout. Select Graph x-y plot. Plot a pattern. From the plots calculate the beam-widths in both planes. Calculate antenna directivity. 6