Radio telescopes are large and high-tech antennas used to receive natural radio emissions from many astronomical objects in the range 10 m (30 MHz) – 1 mm (300 GHz). Thanks to both international and Italian laws, small sub-sets of frequencies are dedicated to the radio astronomical research: these bands must be kept free from other radio signals, in order to avoid "pollution" in the astronomical data acquisition.
Radio Frequency Interferences (RFI) are widely found in the lower radio bands (VHF and UHF) where many strong emissions are present, coming from radio stations (FM), TV broadcast signals, cell phones, radars, and long distance communication systems. These signals are transmitted by means of high-power devices. The spurious signals generated by these systems often fall into the assigned radio astronomical bandwidths, making their use difficult.
RFI can be produced by faulty devices or by unapproved devices (used for unauthorized services). It is important to notice that the sum of all the sub-bands dedicated to radio astronomy amounts only to about 2% of the entire available radio spectrum! For this reason, radio astronomical bands cannot be considered as an obstacle for other telecommunication services.
The Medicina Radio Astronomical Station monitors 24/7 the RFI presence in the radio astronomical bands, exploiting a proper Monitoring Center equipped with:
- Receiving and Monitoring System placed on top of a tower (25 m of height). On the tower summit 11 different antennas (i.e. Log-periodic, Yagi, parabolic dish) are installed. Some of them receive in single polarization, others in double polarization. In this way it is possible to keep under control a frequency range of 300 MHz - 12 GHz.
- RFI Monitoring laboratory where the signals received by the above mentioned antennas are controlled. The instrumentation inside this lab is able to detect the main characteristics of a RFI signal (type, bandwidth modulation, duty-cycle, etc..), the direction it is coming from and, in some cases, its “identity”. Collecting all this information makes it possible to submit proper reports to the authorities.
- Mobile Unit Lab to localize the RFI sources, equipped with an extendible pole (up to 11 m high). Inside the unit are placed all the necessary instruments for the analysis of the RFI emission in the ranges 300 MHz – 18 GHz and 18 GHz – 40 GHz. Using a cartographic GPS system, for the localization and triangulation of the signals, the on-board equipment can exactly determine the RFI direction.
- Software: Both commercial and custom software is available in the RFI monitoring lab, in order to assist every operation. A particular "path-profile" software allows the simulations of a generic radio path attenuation vs its vertical profile, derived from 3D high resolution maps. This special software is useful to analyse "doubtful cases", when requested by international stations and notified to our office by the Ministry of Economic Development.