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The radioastronomical bands protection
Radio telescopes are large antennas that are used to study naturally occurring radio emission from stars, galaxies, quasars, and other astronomical objects, between wavelengths of about 10 meters (30 megahertz [MHz]) and 1 millimeter (300 gigahertz [GHz]). By international agreement and Italian regulations, a number of frequency bands in this range are allocated to radio astronomical research. The radio signal (its spectral power flux density) arriving on Earth from astronomical objects is in fact extremely weak – typically a million billion of times (0.0000000000001%) weaker than the signal by an FM radio station.
RF spectrum up to 2.5GHz
Since radio telescopes are extremely sensitive receiving devices, transmitting is generally prohibited in the radio astronomy bands. However, transmitters using frequencies near those assigned to radio astronomy can cause radio interference (RFI) to radio telescopes. This problem mainly occurs at low frequency (VHF and UHF bands), where strong emissions due to FM radio broadcasts, television, cellular telephone, radar, radio link, etc. may be found. In some case the transmitter's output is unduly "broad," spilling over into the radio astronomy frequencies, or the transmitter emits frequencies outside its intended range. Other interference arises because radio transmitters often unintentionally emit spurious signals or at multiples of their intended frequency.
At the Radioastronomical Station of Medicina the presence of RFI over the main radiastronomical bands is kept constantly under control by the Center for RFI monitoring. The center consists of the following facilities:
1) a 25-m high tower equipped with a senstive receiving system
This system is composed of 11 antennas (Log-periodic, Yagi, parabolic dish, in both single and double polarization) connected to a device wich performs the selection, filtering and pre-amplification of the received radio signal. It is fully steerable on the horizontal plane (0-360 °) and can be remotely controlled. The frequency coverage of the receiving system currently allows RFI monitoring within a range of 300 MHz - 12 GHz.
Tower with antennas
2) a fixed based laboratory for RFI control
Radio signals coming from the receivers on the top of the tower reach a control room by means of special low-loss coaxial cables. Herein a spectrum analyzer and a multimode receiver for telecommunications allow the identification of the main features of the interfering signal (type, bandwidth, modulation, duty cycle, etc..), the source direction and, in some cases, its identity. All this informations are reported to the competent Territorial Inspectorate of the Department of Communications.
3) a mobile Laboratory for locating RFI sources.
A mobile Laboratory for locating RFI sources is also available at the Medicina Radiostronomical Station. It is set up in a FIAT Ducato 14 2500 TDI van, and it is equipped with an autonomous power supply for the onboard intruments (auxiliary alternator – batteries – inverter 24Vdc/220Vac). A telescopic pole extensible up to a height of 11 mt, manually rotable, to support antennas and the box with filters and radio signals amplifiers. The receiving antenna -a log-periodic or Horn type according to the monitored frequency band - is usually used in linear polarization at 45° to pick up simultaneously Vertical and Horizontal signals, albeit with an attenuation of 3 dB. The frequency coverage of the receiving system can perform measurements in radioastronomical bands between 300MHz and 18GHz and between 18 and 40 GHz. The mobile laboratory equipment allows the analisys of the RFI emission and the determination the source direction, as well as the triangulation and localization of the signal source through a GPS mapping system.
Mobil unit used by the Interference Monitoring Group and internal instrumentation
4) software tools
In order to perform a daily search for interferences it is necessary to utilize both commercial and in house developed software aimed at simplifying various operations. Amongst the others the pathprofile program performs simulations of a generic radio path attenuation in function of its vertical profile, deduced from a 3D high resolution map. Further software tools, as some programs allowing the remote control of the Spectrum analyzer for real time monitoring of interference permanence, are developed in LabView environment.