Microwave Holography - Monitoring the antennas the monitor the galaxy
ATSC has been tracking spacecraft since the creation of NASA. The large reflector antennas that are used for tracking are subject to deterioration due to changes in the reflector panels and backstructure caused by weather, the earth's gravity, and age. These changes in the surface tolerance of the reflector and changes in antenna feed positioning cause a reduction in the performance of the antenna which may be significant.
Deviations in antenna and feed positioning as small as .25 mm can be measured using a holography system. For the past 10 years, ATSC has been able to test the performance of reflector type antennas using Microwave Holography.
"Microwave holography testing can be used to test antennas ranging from a few feet to 100 meters in diameter," said Bill Hitzeman, ATSC project engineer. "The measurements can be performed in all kids of weather, any time of day or night. Testing of smaller antennas is normally performed at the antenna test range at the Columbia, MD headquarters. In order to test larger antennas, the holography equipment is packed up and transported to the antenna site."
An additional feature of this type of testing is that it requires very little down time and can usually be performed during periods of minimum activity.
The holography test itself is performed using a satellite which is in the frequency range of the antenna under test. A small reference antenna, the antenna under test, and the holography system, including its control computer, are used to perform the holography measurement. The satellite signals collected by the antennas are fed to the holography system where the data is collected and transformed into the performance characteristics of the antenna under test. Examples of the characteristic data provided are surface tolerance, panel alignments, efficiency, and gain in addition to other features.
The information provided can be displayed and printed out as a color map of the antenna surface, and charts and tables of recommended correction data. If the calculated improvement based on correction information is significant, adjustments will be made to the antenna which will improve the performance of the overall system. The tests are then repeated to verify the expected improvement has been realized.
In May, Hitzeman will be traveling to Matera, Italy to perform this procedure on a 20-meters VLBI antenna. "I'll be going with Al Wu to perform the tests, and we'll be there for three weeks," said Hitzeman. "We'll be performing tests on the antenna, but the site is providing a crew of people to perform the panel and feed adjustment in addition to some additional needed maintenance. This will be the first time ATSC has performed this type of work at Matera."
Holography testing should be performed every 10-15 years. It is important to check for deterioration of the antenna's shape since a shift of a few millimeters in the panel surface can reduce the amplitude of the received signal. Performance of this kind of maintenance ensures that the antenna system is performing as well or better than when it was new.
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