Project Description and Guidelines

Transcription

1 EE 351 Project Due Friday, Apr. 30, 2010 Project Description and Guidelines For this project your team is required to build and characterize an antenna (half-wavelength, waveguide, etc.) that will operate in the band GHz which corresponds to the operation range of b and g WiFi networking equipment. You will write a report and give a brief oral presentation on Friday of dead-week, April 30th. (If you are ready to present your work on Wednesday April 28, you may do so.) Your design should aim for an impedance of 50 ohms at 2.4 GHz. Material, such as connectors (SMA), flexible coax cable, semi-rigid coax, brass tubing, etc., will be provided to aid in the construction of your antenna but this is subject to availability. Measured Characteristics: You will measure the antenna s reflections coefficient (S11) and the vertical and horizontal radiation patterns. To do this you will use a network analyzer and a spectrum analyzer that are available in the wireless lab (EME 240). Details concerning these measurements are provided below. Report: You are required to submit a four to six page double-spaced report on your project that should include the following sections: 1. Introduction/objective. 2. Theory of how the antenna operates. This should include the NEC modeling that you did. 3. Experiment: Details of building the antenna and measured characteristics. 4. Discussion and Conclusion. Approximately one page out of the minimum four will be devoted to the main plots (these are described in more detail below). You can use more space for the display of plots but this is to say that you should have no less than three pages of text. Oral Presentation: Each team will make a brief oral presentation on your project. You should aim for approximately four minutes. (You may go slightly over four minutes, but you should not go over five.) Teams: You should work in teams of two or three. Teams are self-selected. Division of labor within the team is up to the team. All the team members will receive the same grade. (If you feel a particular team member is not making an appropriate contribution to the effort, you may inform me of that

2 fact.) Please send me by Friday, April 2nd letting me know who your team members are. If you have trouble forming a team I will appoint you team a team (the appointments will be made essentially randomly). Resources: This project is rather open-ended in that your team must come up with the design for your antenna. Please feel free to use whatever resources that may be appropriate including magazines and Web sites. Some URL s that may be of interest are: (Spider omni antenna) Numerical Simulation: Simulate your antenna design numerically using nec2 software. You are free to use any software you can obtain to accomplish this. Free software that provides both a front-end for nec2 as well as the underlying nec2 code itself is 4nec2 which can be obtained from arivoors Provide simulated values for all of the characteristics that you will measure for your actual antenna. Plots Your report and presentation must include three plots as follows: Vertical and horizontal field patterns, in units of db. Normalize these to a maximum of 0 db. Note that db is a logarithmic scale, so normalization is done using subtraction instead of division. For these plots, you should have data points for every 15 degrees (24 total measurement for each pattern). You may take the field patterns at either the resonant frequency or the design frequency, but include this information in the figure captions. The reflection coefficient magnitude which is also known as S11. Label the resonant frequency. Also label the values of S11 at the resonant frequency and the design frequency. Discuss features of the plots in the report, including: What is your antenna s impedance at the resonant frequency? If your antenna has a built-in transmission line, try to compensate for the rotation of the reflection coefficient to get the impedance at the driven element (this will require that you know the length of the line and the speed of light in the line).

3 Note that resonant frequency is where S11 is minimum. After compensating for any built-in transmission line, the antenna input impedance should be close to real at the resonant frequency. Find the input impedance in rectangular form (X +jy ) for both your design frequency and the resonant frequency. Comment on the field pattern plots. Do they look like you would expect given the modeling? What limitations are in the test setup? Can you note specific features of the field patterns that might be caused by the test setup?

4 Measurement Details The following are some specifics concerning the measurements you will make. First, we will measure S11 (essentially the reflection coefficient) of your antenna using the HP 8719D Network Analyzer. We start by setting the range over which to measure S11. We want to measurements from 0 to 4 GHz (actually 50 MHz to 4 GHz). In the STIMULUS group of buttons hit the Start button. If you see 50 MHz on the screen, then you are all set. If you see any other value, on the keypad type 0 and then hit the G/n button. The display should now say 50 MHz. In the STIMULUS group of buttons hit the Stop button. You will probably see GHz, which we don t want. On the keypad type 4 and then hit the G/n button. Next, we must must calibrate the machine. Press Cal button in the RESPONSE group of buttons. From the programmable buttons select CALIBRATE MENU and then S11 1-PORT. Make sure a line with an SMA terminated with a SMA male connector is attached to PORT 1. Once that is in place, with nothing at the end of the line, hit the OPEN button. Once you do that you should hear a beep and OPEN will be underlined. Attach the load to the end of the line and hit the LOADS button. Hit the BROADBAND button. You will hear a beep and BROADBAND will be underlined. Hit the DONE: LOADS button. At this point LOADS should be underlined. Attach the short to the end of the line. Hit the SHORT button. You should hear a beep. Hit the DONE 1-PORT CAL button. We are now ready to measure S11 for your antenna. Attach your antenna to the end of the line. Hit the Meas button in the RESPONSE group of buttons. Hit the programmable button Refl: FWD S11 (A/R).

5 You may want to change the scale (you are probably seeing 10 db per division). To do this, hit the Scale Ref button in the RESPONSE group of buttons. Then, in the ENTRY group of buttons hit the down arrow (or up arrow) as appropriate. Hitting the down arrow twice should give 2 db per division. Hit the Marker button in the RESPONSE group. Using the marker, which is positioned using the thumb wheel, locate the deepest null (which is hopefully in the vicinity of 2.4 GHz). We now want to record your measurement to the floppy disk (which should be inserted in the drive on the left). In the INSTRUMENT STATE group of buttons, hit the Save/Recall button. Hit the DEFINE DISK-SAVE programmable button. Use the DATA ONLY button to ensure that ON is underlined. Use the SAVE USING button to ensure ASCII is underlined. Hit the RETURN button. Hit SELECT DISK Hit INTERNAL DISK From the INSTRUMENT STATE group of buttons, hit Save/Recall Hit SAVE STATE. At this point a file should be created called DATAnn.D1 where nn is an integer. This is your S11 data. I (John) will get this off the floppy and a copy to your team. See the class Web site for the Matlab dot-m file read hp8719d.m that can be used to conveniently read this file. That s it for measuring S11! Next we will measure the radiation pattern for your antenna, but really we will measure its reception. The source is an HP 8664A Synthesized Signal Generator. Turn on the machine. Hopefully the frequency will be set to 2.4 GHz. If not hit the FREQ button then type 2.4 on the keypad and hit the GHz button. (If the display shows the word oven, the frequency may not be very stable. This should go away after everything is warmed up.) The amplitude should be V. If this is not the case, hit the AMPTD button then type 1 on the keypad and hit the MHz/V button.

6 Make sure a spider antenna is attached to the output port labeled RF. Node of the lights in the UTILITY, SWEEP, or MODULATION sections should be on. You can toggle the RF signal via the RF ON/OFF button. If you hit this button once, the display of the amplitude should show RF OFF. If you hit it again, it should return to V. That is all you have to do with the source. You measure your antenna s radiation/reception pattern using a Tektronix RSA 3308 Real-Time Spectrum Analyzer. Turn on the machine. It takes a minute or two to boot. Attach your antenna via a cable to the input. Hit the MEASURE button in the MEASUREMENT group. Hit Channel Power is that is not already underlined. Hit the FREQUENCY/CHANNEL button in the MEASUREMENT group. If the Center Freq isn t 2.4G, enter this frequency via the keypad: type 2.4 and then hit the GHz button. Hit the SPAN button in the MEASUREMENT group. This should be 2 khz. If it isn t, enter this value via the keypad. (You may notice that the peak signal doesn t line up with the center of the display although, in theory, it should if the source and receiver agreed perfectly on what 2.4 GHz is. Don t worry about this as we will be measure things over a sufficiently broad band of frequencies to account for this offset.) If, at the bottom right side of the display, the Channel Bandwidth doesn t indicate 2 khz, do the following: Hit the MEAS SETUP button in the MEASUREMENT group. Use the keypad to enter 2 khz for the Channel Bandwidth. You are now set to take your measurements. You will have to record this by hand. We are interested in the Channel Power. This is given in dbm at the bottom center of the screen. Try to take measurements every 15 degrees in both the horizontal and vertical directions. Try to keep bodies away from the path between the antennas.