An electron tube is a type of vacuum tube that can perform useful work by moving around and skillfully controlling electrons in a vacuum using electric and magnetic fields. Prior to the development of semiconductor devices such as transistors, vacuum tubes were widely used in electrical products. Senior citizens will no doubt remember that there were many such glass vacuum tubes in old televisions and radios.
The microwave electron tube
A microwave electron tube is an electron tube that can handle high-frequency radio waves called microwaves. Many electron tubes have been replaced with semiconductor devices, but electron tubes are still widely used as components for handling microwaves. And for handling high-power microwaves, the electron tube has no rivals.
Microwave electron tube types
There are actually various types of microwave electron tube: magnetrons, klystrons, traveling-wave tubes, and gyrotrons. These differ structurally and in the way they operate.
Marine Radar Equipment
What is Marine Radar?
Marine radar is a radio device that emits radio waves (microwaves). These are reflected by objects such other vessels at sea, allowing the equipment to determine their direction and distance. The use of radio waves means that radar can operate not only in rain and fog but also at night, enabling it to play a vital role in ensuring safe ship navigation.
Attracting a great deal of attention recently is the issue of unwanted emissions. These are radio waves emitted outside the bands required for the transmission of information by wireless devices. They result in interference that affects other wireless devices.
Radio resources are finite and to ensure their effective utilization, unwanted emission levels are regulated worldwide. Current debate about these regulations suggests that unwanted emissions will be even more strictly controlled in the future.
We manufacture and market microwave electron tubes (magnetrons) used in the transmitter units of marine radar equipment, and we are conducting research into unwanted emission suppression technologies from the perspective of the component manufacturer.
Suppressing unwanted emissions from marine radar equipment
Unwanted emission regulations
New regulations governing the level of unwanted emissions are to be found in the Radio Regulations Appendix S3, which was published by the International Telecommunication Union’s World Radio Communication Congress. It has become necessary to ensure compliance with these unwanted emission power levels for marine radar equipment installed after January 1, 2003
The unwanted emission domain is divided into an out-of-band emission domain and a spurious emission domain. Radio Regulations Appendix 3 states that the limit value of the primary radar’s spurious emission domain is “43 + 10 log (PEP), or 60 dBc, whichever is less stringent.”
The boundary between the out-of-band emission domain and the spurious emission domain is specified in Recommendation ITU-R SM.1541 Annex 8.
Difficulty of suppressing unwanted emission levels
An electron tube known as a magnetron is used as the oscillator in the transmitter unit of marine radar equipment. Their use is problematic as regards unwanted emission characteristics, but low cost is the main reason for their use.
The unwanted emissions of a solid-state oscillator are mainly harmonics, but magnetron unwanted emissions are caused by harmonics (due to distortion of the oscillation waveform), by harmonic radiation resulting from the resonator structure inside the magnetron (multi-cavity resonance mode), and by low-frequency radiation (due to oscillation frequency drift) ranging from approximately 30 to 300 MHz resulting from the oscillator structure in response to noise.
In multi-cavity resonance mode, the resonance mode of the required frequency is called π mode; π-1 mode is the resonance mode in which there is only one pair of internal high-frequency electric fields, unlike π mode. In this π-1 mode unwanted emissions are at their highest level.
Practical measures for suppressing unwanted emissions
The structural design of the magnetron makes it difficult to suppress all unwanted emissions, but unwanted emissions can be suppressed by inserting a filter in the radar device.
When inserting a filter, because filter characteristics will vary depending on the positional (phase) relationship between it and peripheral circuit elements, it may be that theoretical filter performance cannot be achieved. Thus, it is necessary to optimize the insertion position. Also, the reflected frequency bandwidth of the filter affects the multi-cavity resonance mode of the magnetron, so unexpected unwanted emissions from the magnetron may be encountered. It is necessary to place an isolator between the magnetron and filter to avoid this happening. It is also important to select a filter with sufficient power handling capability.
Shown below is the effect on the spectrum display when π-1 mode unwanted emissions are eliminated with a combination of magnetron + isolator + filter.
Measurement of unwanted emissions
Guidance regarding measurement methods is provided in Recommendation ITU-R M.1177.
Efforts to strengthen unwanted emission regulations
Within the International Telecommunication Union, it is being discussed whether to change the mask slope stipulated in Recommendation ITU-R SM.1541 Annex 8 from -20 dB / decade to -40 dB / decade and whether to change the spurious domain level from -60 dBc to -80 dBc. It is therefore expected that regulations on unwanted emissions will grow increasingly strict in the future.