Computer Science – Performance
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsa43a1872s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SA43A-1872
Computer Science
Performance
[2403] Ionosphere / Active Experiments
Scientific paper
The imminent completion of the major tasks in the construction of the New Arecibo HF facility means that we must verify that the components are working as intended. The antenna system and the transmitters must be separately commissioned before they can be connected together so that we an be sure it will provide 83 dbW at 8.175 MHz, and 80 dbW at 5.1 MHz. The antenna system will be ready for initial testing in September. It Illuminates the 305 meter dish using dipoles near the surface of the dish transmitting upward to a wire mesh sub-reflector. There are three crossed dipoles for each of the two frequencies. The dipoles are supported on towers mounted on concrete pads underneath the dish. Each dipole element is connected to a transmitter through a three inch coax line. The tower foundations are nearly complete, and the towers will be erected in early September, and we expect to have at least one crossed dipole in place for initial testing by the end of September. We will need to make some measurements on the antenna system to ensure that it meets our requirements. One requirement is to match the antenna impedance to the transmission line and the transmitter closely enough to meet the transmitter specifications, the closer, the better. We have additional requirements; for example, it is necessary for efficient use of the facility that the polarization be close to circular. In most experiments, we want O mode excitation. If we were to transmit linear only half the power would reach the reflection height. The symmetry of the system assures that most of the conditions for achieving accurate circular polarization are met, but one condition is not: that exciting the transmitters driving the orthogonal elements at 90 degrees assures 90 degree separation in the corresponding antenna currents. One of the dipoles of each pair points toward the center of the array. If we think of the three crossed dipoles as consisting of a reference and two that are excited relative to it, one is excited 120 degrees ahead, and the other 120 degrees behind. The two elements in each crossed pair must have currents shifted by 90 degrees. In most systems all the elements excited 90 degrees apart are also spatially orthogonal. That is not the case in this array; modeling results showed better array performance with this geometry. Recent analysis shows that it might be necessary to excite the transmitters at 90 degrees plus or minus some offset in order to achieve 90 degrees in the antenna currents. We must make the measurements to determine the size of this offset. The electrical substation that provides power to the transmitters and the heat exchangers providing cooling is complete. Wiring of the power to the transmitters is nearly complete; we are beginning the commissioning of the transmitters. Four one MW generators have been installed; we have tested the power system with the 430 MHz transmitter and the site together. This system provides a flexible complete way to power the planetary radar, the HF facility, and site backup,as well as the 430 transmitter if required. The commissioning tasks that we complete in the next few months will affect the scientific usefulness of the facility, especially in the first campaign. It is important that we bring up the system in an efficient and orderly fashion.
González Sixto A.
Sulzer Michael P.
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