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The results of an extensive study on analog PAL signals interfered with by digital DVB-T signals are presented. The research work covers field measurements, laboratory measurements, a theoretical evaluation of the interference problems and a national survey among TV installation companies. The main objective is the identification and explanation of the interference problems reported since the start of DVB-T broadcasting. The report is organized in two parts. This paper, part I, describes the field measurements, whereas the laboratory measurements and the poll results are described in part II (see Acuna, M.A. et al., ibid., p.116-22). It has been found that the frequency plan is part of the source of the interference problems, particularly in the case of cochannel interference. Another source of problems is use of different radiation elements at the same site for broadcasting DVB-T and analog TV signals. As stated in ETSI documents (see ETSI TR-101-190, 1997), the use of a different antennas for DVB-T may require ERP restrictions to protect existing analog services. The power ratios between digital and analog TV signals at the broadcasting station should be adjusted to meet the protection ratios everywhere within the coverage area. The required power ratio depends on the specific shape of the radiation pattern of each antenna. Finally, it has been found that an important part of the interference problems is due to the amplifiers of the master antenna television (MATV) systems in buildings. The results of the laboratory tests and the theoretical analysis conducted to determine the increment of the protection ratio required by the presence of these amplifiers are presented in part II.