Technical Information -- Microwave Products --

Efforts to suppress the unwanted emissions of marine radar systems

1.Restrictions on unwanted emissions
The new limiting value for the power level of unwanted emission was adopted as defined in the Appendix S3 of the Radio Regulations agreed upon in the ITU Radiocommunication Assembly, and the levels of the unwanted emissions of the marine radar systems to be mounted on marine vessels from January 1, 2003 onward are to be subject to the new limiting value.
International Telecommunication Union
The frequency range where unwanted emission occurs covers the out-of-band (abbreviated as Oob) domain and the spurious domain extending outside the Oob domain. The Appendix S3 of the Radio Regulations provides that the level limit to be applied to unwanted emissions in the spurious domain should be the smaller of 43+log(PEP) or 60dBc, while ITU-R SM1541 ANNEX 8 defines the boundary between the Oob domain and the spurious domain.
unwanted emissions of X band marine radar system
The example of X band marine radar system
2.Difficulty in suppressing the level of unwanted emissions
 
An electron tube called magnetron is installed as an oscillator in the transmitter section of a marine radar system. Magnetrons are most widely used in the marine radar systems because they are of low cost, although they have some unfavorable characteristics in relation to their unwanted emissions.
Features of oscillators installed in the transmitter sections of radar systems
  unwanted emission characteristic output cost
Magnetron Poor Good Good
Klystron Fair Good Poor
Solid-state oscillator Good Poor Poor
The unwanted emission of the solid-state oscillator mainly comprises higher harmonics while that of the magnetron includes higher harmonics (originating from the deformation of the oscillating waveforms), emissions having frequencies associated with the internal resonance mode of the magnetron itself (due to multi-cavity resonance mode), and relatively low-frequency emissions of several tens to several hundreds of MHz, whose generation is triggered by noise (due to the drift of oscillating frequency).
Among the multi-cavity resonance modes, the resonance mode having a desired frequency is called "paimode", while the resonance mode wherein the number of the pairs of high-frequency electric fields is less by unity than that of the paimode is called "pai -1 mode". The pai -1 mode generates the unwanted emission, among others, that has the maximum power emission level. (See the diagrams shown below)
π-1 mode
(a)pai mode(normal oscillation mode) (b)pai-1 mode
high-frequency electric fields in magnetron(shown in blue broken curves)
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