Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsa11b..03s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SA11B-03
Astronomy and Astrophysics
Astronomy
[2403] Ionosphere / Active Experiments, [2494] Ionosphere / Instruments And Techniques
Scientific paper
The problem of illuminating the Arecibo 300 m dish was solved in the 1970s by hanging first a dipole antenna and later a log periodic antenna from the central area of the azimuth arm. This approach somewhat blocked the area, added undesirable weight to the structure, and did not provide optimum illumination. The new HF facility, now under construction and to be completed next year, uses a very different approach. For each of two frequency bands, there is a set of three crossed dipoles located relatively near the surface of the dish. The dish acts as a reflector for each of these dipoles. They transmit to a reflecting screen that is supported from the three main towers and hangs below the astronomy and radar feeds supported from the platform above. Modeling results give an expected antenna gain of about 25.5 db at 8.175 MHz. A building has been constructed for six transmitters, which were originally used in the Maine over the horizon radar facility. Each of these transmitters has a maximum power of 100 KW CW, and it will be possible to transmit various modulated waveforms. The effective radiate power will thus be greater than 150 MW. The power at 5.1 MHz will be about 80 MW. The transmitters have been installed in the building. There are three main tasks underway which must be completed before the transmitters can be used. These are electrical power input, the rf transmission line system, and the cooling systems, air and water. The electrical power will be supplied by generators which will replace the observatory's aging turbine. These are expected to be available at the end of February. The dipole antennas and the reflecting screen have been designed and are in the process of bidding for construction. Also a control system will be built to allow the certain transmitter parameters to be set from the observatory control room to allow coordinated experiments with the radar. The dipole antennas are to be constructed in a manner that will allow their length to be modified so that the frequency band can be shifted somewhat by constructing different length sections. The matching is designed so that loading networks can be added to give additional degrees of freedom to match the new impedance at the altered frequency. We plan for the facility to be ready for use in the coming year.
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