Technical Information -- Microwave Products --

Efforts to suppress the unwanted emissions of marine radar systems

3.Actual strategy to suppress unwanted emissions
Although the physical structure of the magnetron makes it difficult to suppress all its unwanted emissions, the unwanted emission of the radar system can be considerably reduced by providing the system with appropriate filters.
In installing a filter in the radar system, great care must be taken of where the filter should be located.For the designed characteristic of the filter may not be obtained due to the influence associated with its relative position with respect to other peripheral circuit components.The optimal positioning of the filter, therefore, is critical in obtaining its rated performance. Moreover, since the reflection frequency band of the installed filter affects the multi-cavity resonance mode of the magnetron, the installation of a filter in the radar system may cause the magnetron to generate unexpected unwanted emissions.In order to avoid this sort of undesired phenomenon, an isolator must be placed between magnetron and filter.It is also important to select such a filter as can handle signals of sufficient power.
The following diagrams show the measured spectra before and after the elimination of the unwanted emission of pai -1 mode by the use of the combination of magnetron, isolator and filter.
filter graph explain filter graph
4.Method of measuring unwanted emissions
ITU-R M-1177 provides as guidance the method of measuring unwanted emissions.
Method of measuring unwanted emissions
5.Efforts to cope with the enhanced restriction on unwanted emissions
International Telecommunication Union (ITU) is now aggressively arguing that the roll-off of the mask defined in its SM1541 ANNEX 6 should be made steeper, from the current value of -20 dB/decade to -40 dB/decade and that the level of the emission in the spurious domain should be further suppressed, from the current value of -60 dBc to -80 dBc. Thus, the restrictions on unwanted emissions will tend to be shifted to the stricter side in future.
filter graph
The example of X band marine radar system
In order to follow such a trend of the industry as described above, we, New Japan Radio Co., Ltd., will continue to make researches and development to improve the emission spectrum of magnetron in the out-of-band domain and the filter characteristics.