RFI shielding and mitigation techniques for a sensitive search for the 327 MHz line of Deuterium
Alan E.E. Rogers (Haystack) aeer@franklin.haystack.mit.edu
Joseph C. Carter (Haystack) jcc@franklin.haystack.mit.edu
Preethi Pratap (Haystack) preethi@franklin.haystack.mit.edu
Marcos A. Diaz (Boston U.) mardiaz@franklin.haystack.mit.edu

An array of 25 stations each with 24 crossed-dipoles is being built at the Haystack Observatory in Westford MA. This array has been designed to make a sensitive search for the 327 MHz spectral line of Deuterium. Since the deuterium line is expected to be about 50 dB weaker than the the 1420 MHz hydrogen line the amelioration of RFI is the major challenge for the "Deuterium array". Locally generated RFI both from the array and from nearby sites has been reduced by extensive shielding and in some cases the removal of consumer electronics, like certain digital answering machines, which emit strong signals in the 327.3 to 327.5 MHz band of prime importance for the search. Since almost all the RFI comes from the horizon the station array have parasitic directors added to the dipoles to reduce the response at the horizon. A RFI monitor with 12 active yagi antennas pointed every 30 degrees in azimuth provides a means of determining the direction of the RFI and information of frequencies and time spans with need to be excised from the array data. We present details of the array design, the RFI excision algorithms, levels of spectral and continuum RFI measured at the observatory and the performance of the array from initial long integrations from a subset of the full array.