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Dependence of Radar Emission Spectra on Measurement Bandwidth and Implications for Compliance with Emission Mask Criteria

Radar transmitter emission criteria normally include the specification of frequency-dependent emission masks. These masks specify the amount by which unwanted radar emissions (both out-of-band and spurious) must be suppressed relative to the power levels emitted at the radars' fundamental frequencies. Compliance with emission masks is determined through measurements of emission spectra. The measured levels of radar unwanted emissions and fundamental-frequency emissions both vary as a function of measurement system bandwidth, Bm. But the variation with Bm differs between the unwanted emissions and the fundamental-frequency emissions. Moreover, the variation of unwanted emission levels varies as a function of frequency as well as Bm. This creates a problem for radar emission mask-compliance measurements. The National Telecommunications and Information Administration (NTIA) Institute for Telecommunication Sciences (ITS) has explored this problem by performing emission spectrum measurements on a maritime surface search (navigation) radar. In the spectrum data that are presented, the radar unwanted emission levels are found to vary between 12 log(Bm) and 20 log(Bm), depending upon frequency. But the measured power of the radar fundamental frequency is found to vary as 20 log(Bm) for all bandwidths that are less than or equal to 1/(radar pulse width). The result is that the level offset between the unwanted emissions and the fundamental-frequency emission level depends upon the measurement bandwidth and the frequency of the unwanted emissions. This result implies, at a minimum, that measurement personnel must take the effect of Bm into account when performing radar emission spectrum measurements for the purpose of determining emission mask compliance. Based upon the results of these maritime radar spectrum data, some technical strategies for measuring radar emission spectra for emission mask compliance are proposed. Possible technical implications for future development of radar emission masks are also discussed.