Microwave Vaccum Tube po, < Microwave Engineering (RGTU) punches in the g sed onic admittance a reg the The admittance presented to the external circuit by G, becomes negative at the time of i , i cl equivalent circuit in Fig. 4.15. Here Ye = Ge + JB thee equivale resonator cavilY- oscillation. B, Capacitive Increasing n Increasing (-ve) Vp |, No oscillation Oscillation 1, Fig. 4.16 is shown in Fig, 4.16, where Y, is plotted in the complex admittance (Ge ——_:. in this plane) corresponds t0 infinite repeller voltage. When the repeller voltage gets reduced one moves utwards along the spiral when G, > Ge, (conductance of extemal sccuny) exile iB, is negative, oscillations take place at a frequency higher than the resonance frequency of the cavity, where as positive values of jB, corresponds to a frequency lower than the cavity resonant frequency. This leads to the frequency variations with the repeller voltage as shown in Fig. 4.13. Afte er to he electrons are collected by the body of the resonator which for this reason, is kept at the most e, Maximum at available from two cavity Klystron: Performance characteristics (1) __Frequency range : 4 t @ OmW 25 W (3) Theoretical efficiency: 22.78% — (4) Practical efficiency : 10% to 20% _ 4. a\ traveling Wave Tube (TWT): ‘As seen in previous section, the mai ‘of the klystron amplifier is it (due to use of resonant cavities). So there if need of new type of tube which will work as Scanned with CamScannerineering (RGTU) eS Ent 421 Microwave Vaccum Tube Devices F « tube was proposed by Pi name’ x ieree and others i C 1 evince its in folowing ways, saving wane Te pp use 0 resonators to a \ree devices as compare to TI jfhe‘cecton bea I cai ‘ A sud bund device) in which there are no cavity ‘ rmeraction space (bel ne era She electron beam and a travelling wave) in a TWT is extended andl the RF wave over the full length of the tube. 9, {0 distributed interaction between ai both trav , both travel rection with nearly wtravelling wa ' come dil «RF field rem = &S compared to klystron in which the electron beam -d to prevent the beam used 10 Anode Cathode WLM, Magnetic focusing ‘structures Fig. 4.17 : Simplified circuit of TWT (ue wave on the helix. “S : ilable at the TWT Thus due to this energy transfer t ‘uput The axial 1Tadius of the helix, which is constant over @ range hain at zero field of frequencies. The entering the those electrons entering the helix at the \d are As the electrons ‘or end. The bunching shifts the phase by 7/2. the helix, they Exh electron in thelbuneli experience & \d. Then the microwave energy of the ‘wel further alon; Scanned with CamScanneras. fl Microwave Engineering (RGTU) Microwave Vaccum Tug Le lectrons is delivered by the-electrons bunch to — ‘Se RF signal. Mathematical analysis : Consider a helix, with pitch P, radius r, ‘and velocity of the wave propogating through, the Vg. Helix is acting as a slow wave structure, ie. it will suppor a slow wave with an axial phage in x V, = V_ x sin y where y is the helix ange given by tan"! = =) Such agihelixfean Pea, considering the follow (1) The ty of a helix in the direction of helix is specified to testes (2) ~The of the helix in theairetion perpendicular to the helix wire sero, » Electron beam x ‘The applic signal which is! as a longitudinal component g¢ electric field, whic! the the electron beam. This velocity modulation causes. + Accelerting trons at regular intervals of He ‘one wavelength as shown in Fig. 4.19. i ‘When the id thefeleetrons?are moving with . a continuous interaction between the wave and the beam takes place resulting in (Bunches and these Electron | a « the beam moves further. >" is cumulative interaction can be discussed in terms of space charge waves on the beam. Fig. 4.19 Siler IS TERS an team wl ato essere aio i ce density and current density known as the ‘on the electron beam, and theit haracteristics may be studied a a small sis based on the law! 6f eleétron motion. th juation and Let Vo. Po- Jun Fepresents static RF velocity, charge density and current density, while ¥. Ps Ju corresponds to time varying equivalents of the same. So under small signal approximations various time | varying quantities can be “pressed as v ° wey eh 468) potp,e-™ 2 Tr) Scanned with CamScanner™Microwave Vaccum 7, ' CUM Tug Microwave Engineering (RGTU) i ee « exponentially a8 it travels. This «that grow Oe ee a ar eresponsble for amplification in a Tyr © es Zam res| dominates the other two for large values o! ’ associ va electro magnetic enerRY associated + enero in ecto magnet Tee ith he ‘The real source responsible cee owns i rowth of waves the Kinetic nergy (KE) of the cestons ae Le NT YAN a expense of the the tube, and the helix wave gains energy atthe expense @ on aTWr aancntaanye yan the wave on the helix The gain of the TWT is piv, Ina TWT the bunches travel faster than 7 G = 9544473 CN (1B) (day @ C= gain parameter = Zz No = Helix length => = constant v, = Axial phase velocity J, = de beam current Vo. = de beam voltage The value of the gain will be maximum when the beam velocity is same as that of axial waye velocity V,,. “There is a possibility of feedback from output to input, alongwith generation of spurious sign thin the TWT. These may cause parasitic oscillations within the TWT, which can affect the performance of TWT. So to avoid this, inside wall of the TWT is coated with aquadog material, which Acts as attenuator. This attenuator will absorb parasitic oscillation, alongwith desired signal, so reduces amplification. To avoid this attenuator is placed at the input end of tube in such a way that it wit attenuate reverse waves heavily as compare to forward waves. The high frequency limit of TWT can be increased by decreasing the helix diameter i.e. bandwidth limitations are due to the size limitations in a TWT, The gain is limited at lower frequencies due to the helix length performance characterist (1) Frequency range; 3 GHz and higher (2) “Bandwidth: about 0.8 GHz (3) Efficiency : 20 to 40% (4) Power output : upto 10 KW average (5) Lower gain : upto 60 dB Thus TWT amplifiers are known for their wide bandwidth and low noise characteristics. They find extensive applications in microwave communication systems 4.5 _Magnetorns: ‘The tubes (klystron, reflex klystron, TWT) studi x . icd{GPENROW" are called! aS Tinea Bea tubes of H) type tubes. Alongwith these tubes another type of tubes are present called sae tube in this in which the electric and magnetic fields are cul rw; Dek ‘ae elds are perpendicular to each other, The principal Scanned with CamScannerEngineering (RGTU) —~ Microwave Vaccum Tube Devices ult invented the magneton jn yy jp and Boot around 1039, 4 Vand an : improve ayn YEH high power BNCLIONS ayy Used 'th power magnetron wi 0 generate ms cy is achieved j and RF field, TWT ANd Manenyy jy (iibszigninerion tween aio bea BO mere are three Lypes Of magnetrons, \ negative resistance type Wy cyclotron frequency type YY Fravelling wave oF cavit 1 ‘i © ie aes Op mam use the property o we, ctu TREMNRGUCHEIRS. ( < S00 Mitz), ‘These Wen (0 ase sey ments, (@yeloton egueiey : eater than nan alternating component of electric fil field. ‘ass developed by ind their operation depend on synelonism Periodic oscillation of electrons in a dite nati scHHTatior electrons in a direction By a is based on ‘ng\inFAtion between 1 and We anat Selocity. These provide of very high peak ean in detail, _ most common RF structure of in an annular space called fete + inca ERD. i he cavities are de in the plane of paper one of the rity is connected a coaxial line-or waveguide for extracting the output. It is across field as the between and cathode is as c Pane suget, which i 1 pel to ihevertical ula fo the elf eld between cathode and anode. Coaxial line output ' ——p RF output (a) Cavity magnetron (b) Magnetic Mus Hnes in magnetron a . Fig. 4.22 Operation : ies. Generally any system A shown in Fig, 4.22, Cavity magneton bi ich of which is characterised by a bavi tly coupled cavities have fon relative to adjacent cavity con ey ne ar Scanned with CamScannerMicrowave Engineering (RGTU) » of ly sonal et | ere ‘Thala pase AifParound the FINE OF nagnetron indicates that the Sr ’ ines, passa "been Bc) the electron experiences a greattt rotational force and: I as shown in d. All such electrons may cause eet in after stating of oscillation. This is done to avoid fallin the emitting efficiency of the cat ‘The behaviour of electrons studied above corresponds to static magnetic field (created by permanent magnet, ie, thé@bseneB Ot REG inthe cavity of magnetron IFIRE osciations are inti Within the magnetron, the oscillations will (afro ane pis aie will occur ifthe phase difference baw emai ° ls lof operation as shown in Fig. 424 ‘where anode poles are radians apart in phase. ‘BHOYS of operation as shows ‘The Solid paths corresponds to the electron motion in the presence of ie dled PoP corresponds to electron motion in aiields RE MHeld whi Scanned with CamScannerY} Mh ye Engineering (RGTU) Microwave Vaccum Tube Devices lll 4 retarded by the a ‘ode. Values of static E and Hi are adjusted such that by the again and SE es, The electron 7 y to the RF Field. hhich participate in uc oe Ao on responsible f the Vir and instead of delivering the energy to the weet Seite lation. So this electron will more quickly back towards Such unfavoured electrons 0 back a Anode . Similarly an electron C, which is vw er than a, will moves faster and tries to catch A ¥ like a, ¢ forms a Btinehl’ind electron clouds. process of bunching the favoured electrons 3 ference electron ‘a’ is called as The spokes in the % mode rotate wil tte angular velocity corresponding to two poles per mo ole, Fig. 4.25 : Phase focussing effect nalysis : t commonly used magnetron is the cylindrical ‘magnetron, So We will do the analysis of this magnetron. Let a, and b represents the radius of the cathode and ame, and represents the angular displacement of the eeetron beam, As the electric and magnetic fields are teasverse to each other, the path of the electrons 1m the sence of cross field is parabolic. Fig. 4.26 : Cylindrical magnetron Scanned with CamScanner