Using the Remote Educational Antenna Laboratory
The URL for the project is easy to remember: research.ece.ncsu.edu/preal, where "preal" is short for "project remote educational antenna laboratory." From the project home page you can find more technical details about the facility, and observe any measurements that may be underway.
First have a look at the chamber by clicking the Webcam item on the menu bar at the upper left. The picture shows the view of the chamber left by the previous user. It likely shows the antenna mount and any antenna that happens to be mounted at the moment. To get a better view of the chamber, click the "zoom out" button (-) on the image. Also note that you can move the camera to look at different parts of the chamber using the arrow keys.
There are two camera pre-sets for your convenience. The first re-centers and zooms the camera for a good view of the antenna. The second takes you to a view of the chamber horn. In this view, we are looking at the upper face of the horn. You can verify the polarization of the horn by observing large red letters on the upper face: V for vertical, and H for horizontal.
If you have a high bandwidth connection, you may want to switch to a higher-performance Java viewer. Click "Switch to the java viewer" just below the picture. Next click the "Start Control" button on the lower right, and you can move and zoom the camera using the scroll bars. With the Java viewer, the camera presets are available from the drop down menu just below the image.
When measurements are in progress, there will usually be an administrator on duty. This is indicated by the color of the bullet by the "Chat with a Project REAL Administrator" item at the bottom. A green bullet indicates that an administrator is on duty, while a yellow bullet indicates that an administrator is not currently available.
Next, click About in the menu bar. Here you can find more information about the goals of the project, and the technical specifications of the chamber. Diagrams are also available showing the chamber dimensions and absorber layout. Finally, links to presentations and publications by the PREAL developers related to remote laboratories are given, as well as some links to other remote laboratory sites.
To actually make measurements with the chamber, you must first log in, so let's return to the project home page. Click Create Account near the bottom of the page to take you to the registration page. The only information that is required is a valid email address. Providing a valid email address helps to ensure the security of the facility, and will only be used in connection with chamber assistance. This email address will not be shared with any other organization.
Also, under Resources, a short MATLAB script is available to assist you in processing measurements you have downloaded, along with an extensive set of tutorials and experiments developed by Prof Satish Sharma at San Diego State University. These include an introduction to Antenna Fundamental Measurements, experiments describing the design and construction of simple antennas, and tutorials to popular tools such as Ansoft Designer and HFSS.
Now, click Schedule in the top menu bar. Here the times where an administrator will be available are listed. To make a measurement on an antenna that you have sent to the facility, choose a time that is far enough in advance to ensure that the antenna has arrived, and click Make next to the desired time slot. A short message can be entered for the administrator, such as identification of the specific antenna you would like to have mounted prior to the scheduled time slot.
At the scheduled time, log in and check the webcam to ensure that the correct antenna is mounted. If everything looks ok, click Testing from the top menu bar.
Note that you do not have to schedule time to use the PREAL facility, if the chamber is not scheduled and an antenna is mounted. You can proceed to make measurements on whatever antenna is mounted by clicking Testing from the top menu bar.
Note that the frequency range is fixed. This is determined by the last calibration process performed on the system. During the test, measurements will be made at all frequencies in the range for each angular position of the antenna.
The first parameter to be selected is the rotation increment. Smaller increments give smoother patterns, but take longer. For the purposes of this tutorial, we will choose 10 degrees.
Next, select the polarization of the test and chamber antennas. To measure the E-plane pattern of a linearly-polarized antenna, we will select Horizontal for both. Click Set Parameters and confirm that the measurement is set up as you intended. If everything looks ok, click Run Test.
The page then switches to the webcam view so that you can watch the measurement. After a few seconds, the test antenna will rotate into the horizontal position (if it is not already in this position), and the rotation will begin. After the rotation stops, there will be a pause of a minute or less while the data is processed. The words Test Completed will appear at the top of the screen when the processing is over. Before we view the test results, let's check the chamber antenna webcam pre-set. We see that the chamber antenna is also in the Horizontal polarization orientation as we specified.
Now click View your Results at the top left of the screen. The first plot that appears is the return loss for the antenna. The resonant frequency of the antenna is the frequency corresponding to the lowest value of S11. To obtain pattern and gain plots at this frequency, enter it into the box labeled Frequency for Pattern Plots and click Apply.
Next, we can view the polar pattern by clicking the second icon on the left. The polar pattern plots are normalized to zero dB, so to view the gain, click on the Rectangular Gain icon on the left. The peak value of this curve gives the realized gain. It is also possible to view the phase of the pattern by clicking the fourth icon from the top on the left. Any curvature of the phase plot indicates that the rotation axis did not coincide with the phase center of the antenna.
Finally, the complex impedance can be displayed on a Smith Chart by clicking the icon on the bottom left. The portion of the curve that is closest to the center of the chart corresponds to the resonant frequency.
After you are satisfied that you have the measurement you need, the data can be downloaded in MATLAB format by clicking the link at the bottom of the screen. Also available is a short MATLAB script showing you how to read and plot the data for subsequent analysis. Just extract the zip file into the same local directory where the data is stored. Next open MATLAB and change the working directory to the one where the data and scripts are located. Then open the file
antennaPattern.mand change the line,
dataFile = 'data_XX.mat'to match the name of your data file (i.e., change XX to match the number of your file). You may also want to change the values of
freqGain. Save the modified file, and type
antennaPatternat the command line. The code will then generate several example plots.
Returning to the web interface, you can add notes and/or change the name of the data set using the Metadata - Edit link. You may either delete the data set, or leave it on the server after you are finished.
Log off, and your measurement is complete!