MICROWAVE TUBES HELIXTWT MTYPE TUBES February 22 Prof
MICROWAVE TUBES, HELIX-TWT & M-TYPE TUBES
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February 22 Prof KCBRao-UCEVJNTUK_VZM MICROWAVE TUBES 4
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February 22 LIMITATIONS OF CONVENTIONAL TUBES Prof KCBRao-UCEVJNTUK_VZM 6
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February 22 Prof KCBRao-UCEVJNTUK_VZM HELIX-TRAVELLING WAVE TUBE(TWT) 34
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SLOW WAVE STRUCTRES Slow Wave Structures are special circuits that are used in microwave tubes to reduce the wave velocity in a certain direction so that the electron beam and the signal wave can interact. February 22 Prof KCBRao-UCEVJNTUK_VZM 36
February 22 BASIC STRUCTURE OF ATRAVELING WAVE TUBE (TWT) Prof KCBRao-UCEVJNTUK_VZM 37
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Ø Ø Prof KCBRao-UCEVJNTUK_VZM Ø The basic structure of a TWT consists of a cathode and filament heater plus an anode that is biased positively to accelerate the electron beam forward and to focus it into a narrow beam. The electrons are attracted by a positive plate called the collector, which has given a high dc voltage. The length of the tube is usually many wavelengths at the operating frequency. Surrounding the tube are either permanent magnets or electromagnets that keep the electrons tightly focused into a narrow beam. February 22 Ø 41
Ø Ø Ø Prof KCBRao-UCEVJNTUK_VZM Ø The unique feature of the TWT is a helix or coil that surrounds the length of the tube and the electron beam passes through the centre or axis of the helix. The microwave signal to be amplified is applied to the end of the helix near the cathode and the output is taken from the end of the helix near the collector. The purpose of the helix is to provide path for RF signal. The propagation of the RF signal along the helix is made approximately equal to the velocity of the electron beam from the cathode to the collector February 22 FEATURES 42
February 22 FUNCTIONING Prof KCBRao-UCEVJNTUK_VZM The passage of the microwave signal down the helix produces electric and magnetic fields that will interact with the electron beam. Ø The electromagnetic field produced by the helix causes the electrons to be speeded up and slowed down, this produces velocity modulation of the beam which produces density modulation. Ø Density modulation causes bunches of electrons to group together one wavelength apart and. these bunch of electrons travel down the length of the tube toward the collector. Ø 43
February 22 FUNCTIONING Prof KCBRao-UCEVJNTUK_VZM The electron bunches induce voltages into the helix which reinforce the voltage already present there. Due to that the strength of the electromagnetic field on the helix increases as the wave travels down the tube towards the collector. Ø At the end of the helix, the signal is considerably amplified. Coaxial cable or waveguide structures are used to extract the energy from the helix. Ø 44
Ø Ø TWT has extremely wide bandwidth. Hence, it can be made to amplify signals from UHF to hundreds of gigahertz. Most of the TWT’s have a frequency range of approximately 2: 1 in the desired segment of the microwave region to be amplified. The TWT’s can be used in both continuous and pulsed modes of operation with power levels up to several thousands watts. Prof KCBRao-UCEVJNTUK_VZM Ø February 22 ADVANTAGES 45
Ø Ø Prof KCBRao-UCEVJNTUK_VZM Ø Frequency of operation : 0. 5 GHz – 95 GHz Power outputs: 5 m. W (10 – 40 GHz – low power TWT) 250 k. W (CW) at 3 GHz (high power TWT) 10 MW (pulsed) at 3 GHz Efficiency : 5 – 20 % ( 30 % with depressed collector) February 22 PERFORMANCE CHARACTERISTICS 46
Ø Ø Low noise RF amplifier in broad band microwave receivers. Repeater amplifier in wide band communication links and long distance telephony. Due to long tube life (50, 000 hours against ¼th for other types), TWT is power output tube in communication satellite. Continuous wave high power TWT’s are used in troposcatter links (due to larger power and larger bandwidths). Used in Air borne and ship borne pulsed high power radars. Prof KCBRao-UCEVJNTUK_VZM Ø February 22 APPLICATIONS OF TWT 47
February 22 Prof KCBRao-UCEVJNTUK_VZM M-TYPE TUBES 48
Prof KCBRao-UCEVJNTUK_VZM Microwave crossed field tubes(M-Type tubes): � Microwave crossed field tubes derive their name from the fact that the dc electrical field and the dc magnetic fields are perpendicular to each other. � They are also called M-type tube because these are the “tubes for propagation of waves in a magnetic field”. � In the crossed field tube, the electrons emitted by the cathode are accelerated by the electric field and gain velocity, but the greater the velocity, the more their path is bent by the magnetic field. February 22 INTRODUCTION 49
February 22 CROSSED FIELD ELECTRON TUBES Prof KCBRao-UCEVJNTUK_VZM 50
February 22 Prof KCBRao-UCEVJNTUK_VZM MAGNETRON 51
February 22 MAGNETRON OSCILLATORS Prof KCBRao-UCEVJNTUK_VZM Hull invented the magnetron in 1921. Magnetron consists of some form of anode and cathode operated in a dc magnetic field normal to a dc electric field between the cathode and anode. Because of the crossed field between the cathode and anode, the electrons emitted from the cathode are influenced by the crossed field to move in curved paths. If the dc magnetic field is strong enough, the electrons will 52 not arrive in the anode but return to the cathode.
Ø Ø 2) Cyclotron-frequency magnetron: operates under the influence of synchronism between an alternating component of electric field and a periodic oscillation of electrons in a direction parallel to the field. Ø 3) low efficiency Useful only at low frequency(<500 MHz) Prof KCBRao-UCEVJNTUK_VZM Three types: 1) Split-anode magnetron(Negative resistance magnetron): uses a static negative resistance between two anode segments. February 22 CLASSIFICATION Useful only for frequencies greater than 100 MHz. Travelling wave magnetrons: this type depends on the interaction of electrons with a traveling electromagnetic field of linear velocity. They are referred to simply as “MAGNETRONS” Ø Ø High power Also called cavity magnetron. 53
The magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. Ø High-power oscillator Common in radar and microwave ovens Cathode in center, anode around outside Strong dc magnetic field around tube causes electrons from cathode to spiral as they move toward anode Current of electrons generates microwaves in cavities around outside Ø Ø Ø Prof KCBRao-UCEVJNTUK_VZM Ø February 22 Ø 54
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Cylindrical Magnetron February 22 Prof KCBRao-UCEVJNTUK_VZM 57
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CONSTRUCTION February 22 Prof KCBRao-UCEVJNTUK_VZM Magnetron consists of tick cylindrical cathode at the centre and a coaxial cylindrical block of copper as anode. Anode consists of number of cavities around the cathode. The space between the anode and cathode is the interaction space and to one of the cavity is connected a coaxial line for extracting the output. It is a cross field device as the electric field between anode and cathode is radial whereas the magnetic field produced by a permanent magnet is axial. The magnetic lines are parallel to the vertical cathode and perpendicular to the 59 electric field between cathode and anode.
OPERATION Prof KCBRao-UCEVJNTUK_VZM The cavity magnetron shown in the figure has 8 cavities, that are tightly coupled to each other. N-cavity tightly coupled system will have N-modes of operation. These modes must be self consistent so that the total phase shift around the ring of cavity resonator is 2 nπ where n is an integer. Therefore if φ represents the relative phase change of the ac electric field across adjacent cavities, then February 22 60
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