Step Tapered Waveguide with Cylindrical Waveguide

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					                                                         (IJCSIS) International Journal of Computer Science and Information Security,
                                                         Vol. 10, No. 3, March 2012

                Step Tapered waveguide with cylindrical
                       Harshukumar Khare,                                                  Prof. R.D. Patane
                  M.E (EXTC) Final year student                                         Asst. Proffessor (EXTC)
                   TEC, Nerul, Navi-Mumbai                                             TEC, Nerul, Navi-Mumbai

Abstract: Tapered Waveguide is a waveguide in which a                   the other standard values associated with circular waveguide
physical or electrical characteristic changes continuously with         can be done relatively easily.
distance along the axis of the waveguide. Tapered waveguide                 A waveguide taper can always be built to have as low a
offer an excellent means of converting microwave mode sizes to          mode conversion as is wanted in a certain frequency band
connect Microwave devices of different cross-sectional                  merely by making it long enough. However, an optimally
dimensions. This paper discusses the waveguide component for            designed taper has the smallest possible length for a given
interconnecting rectangular and circular waveguide using step           difference in diameters at its two ends for a specified
tapering. Model is designed for the frequency range from 2 to 4         unwanted mode level in a given frequency band. Tapered
GHz. Dominant Mode conversions ie from TE10 to TM11 is                  waveguide for matching impedance is nothing but a tapered
considered for tapering techniques. Step tapering is studied at         waveguide in which only one mode is propagating. Power
different step sizes 4mm to 10 mm and analysis is done. All             can only be converted into reflected waves, and it is this
simulations done with CST Microwave studio and S                        reflected power which is kept small in a properly designed
                                                                        transmission line taper. If more than one mode is
parameters and E field parameters are analyzed. Simulation
                                                                        propagating, power will be scattered not only into the
result shows that wave is properly propagated with no power
                                                                        reflected wave but also into the other propagating modes. In
reflection and low power loss.                                          fact , the power scattered into backward traveling waves is
Key words: Single & Double Step Tapering, Cylindrical                   quite small compared to the power scattered into forward
waveguide, CST, S parameter, E Field                                    traveling waves, and only the latter need be considered in a
                                                                        multimode waveguide taper. Therefore, the mode
                                                                        conversion in the waveguide transition corresponds to the
                   I.       INTRODUCTION                                reflection in transmission line taper.
    A rectangular waveguide supports TM and TE modes                        A waveguide mode is a unique arrangement of the
but not TEM waves. A rectangular waveguide cannot                       electric and magnetic fields propagating in the z-direction
propagate below some certain frequency. This frequency is               that satisfies all Maxwell equations and boundary conditions
called the cut-off frequency.                                           imposed by the geometry of the conductors of the
    Circular waveguides offer implementation advantages                 transmission system. Various waveguide modes are TEM,
over rectangular waveguide in that installation is much                 TE, TM and Hybrid modes. Dominant mode in Rectangular
simpler when forming runs for turns and offsets -                       waveguide is TE10 and in circular waveguide TE11. To
particularly when large radii are involved and the wind                 convert dominant mode in rectangular waveguide to
loading is less on a round cross-section, meaning towers do             dominant mode in circular waveguide tapered waveguide is
not need to be as robust. Manufacturing is generally simpler,           used. There are different types of tapering such as step
too, since only one dimension the radius needs to be                    tapering, conical tapering elliptical tapering, etc. Analysis
maintained. Applications where differential rotation is                 has been done using Step tapering with CST Microwave
required, like a rotary joint for a radar antenna, absolutely           Studio.
require a circular cross-section, so even if rectangular
waveguide is used for the primary routing, a transition to                                   II.       DESIGN ASPECT
circular and then possibly back to rectangular is needed.               The simulation was done by Transient solver of CST
Calculations for circular waveguide require the application             Microwave Studio. The Cartesian coordinate system (x, y,
of Bessel functions, so working equations with a cheap                  and z) is used to model the 3D structure. Design & analysis
calculator is not going to happen. However, even
spreadsheets have Bessel function capability nowadays, so               has been done with tapering and without tapering.
determining cutoff frequencies, field strengths, and any of

                                                                                                    ISSN 1947-5500
                                                         (IJCSIS) International Journal of Computer Science and Information Security,
                                                         Vol. 10, No. 3, March 2012


    A. Design without tapering
   The rectangular brick is directly connected to circular
waveguide. Port of rectangular brick (Port 2) is excited and
the S parameters are obtained by Transient Solver. 3D
model of cylindrical waveguide and Rectangular Brick
Without tapering is shown in fig 1.

                                                                                  Fig 4 - 3D model of cylindrical waveguide and
                                                                                     Rectangular Brick with Step tapering
                                                                           Simulated results for S parameters & E field are
                                                                        calculated for different step sizes for sizes of 4 mm & 10mm
                                                                        their S11 and S21 are shown in Fig 5.
                                                                            It is seen from Fig 5 that for step size 4 mm plots the
                                                                        sufficient amount of power is reflected back which shows
                                                                        that taper is not properly coupled and for step size 10 mm it
                                                                        is seen that no power is reflected back hence source is safe
         Fig 1 - 3D model of cylindrical waveguide and
                                                                        in this case.
             Rectangular Brick Without tapering
   S11 plot gives that around 2.1 GHz S11 goes upto 19                          Step size 4mm                      Step Size 10mm
dBas shown in Fig 2 which is not a desirable case as huge                             S11                                  S11
amount of power is reflected back to the source damaging
the network analyzer.

                                                                                      S21                                  S21

                          Fig 2 - S11 plot
   S21 plot is shown in fig. 3 which indicates that no
sufficient output is coupled to the output port as it is
approaching to 0dB i.e. no power is coupled from port 1 to
port 2 and vice versa.
                                                                                  Fig-5– S11 & S21 for Step size 4mm & 10 mm
                                                                          E Field distribution in single step tapered and cylindrical
                                                                        waveguide is shown in Fig. 6.

                          Fig 3 - S21 plot
    B. Design with tapering
i) Single Step tapering
    The 3D model of single step tapering is shown in Fig 4.

                                                                                 Fig-6 E Field distribution in single step tapering

                                                                                                    ISSN 1947-5500
                                                       (IJCSIS) International Journal of Computer Science and Information Security,
                                                       Vol. 10, No. 3, March 2012

   ii) Double Ste Tapering                                                             III.                       ION
                                                                                                  RESULT & CONCLUSI
   The schematic of double ste tapering is s
                c            ep            shown in Fig 7                T
                                                                         The total sim mulation proce              ne
                                                                                                       ess was don by CST
   which two step of 10 mm are used.
in w            ps            e                                       Micr                             hat
                                                                          rowave Studio. It is infer th Tapering b  between two
                                                                      waveeguides is go ood possible solution to c  connect two
                                                                      wave               ed                        lent
                                                                          eguides. Tapere waveguide offer an excell means of
                                                                         verting microw
                                                                      conv                            zes
                                                                                        wave mode siz to connect microwave
                                                                      devic of different cross-sectio              ons. Properly
                                                                                                       onal dimensio
                                                                          e             single stepping with 10 mm s size with
                                                                      wave is guided in s             g            step
                                                                      no r              waveguide. Im
                                                                          reflection in w             mpedance matching is the
                                                                         or              h
                                                                      mino problem with double step.

                                                                         The          e            Dr.
                                                                         T authors are grateful to D R.C Sethi, HOD,EXTC
                                                                      TEC and Prof. Mrs. Jyothi Digge, PG coordinator,
                             e             g
                Fig 7 - Double Step Tapering                          EXT             eir          ort         ble
                                                                         TC,TEC for the great suppo and valuab guidance.
                                                                      They would also like to ackno            p
                                                                                                   owledged help & support
                 21                          tion are shown
     Their S11, S2 plots and E field distribut            n               ived from Dr. Abhay Deshpande, Scientist,
   Figs           .                          m
in F 8, 9 &10. In double step tapering from Fig 8 & 9 it  t           SAMMEER,IIT-B.
sho that imped                g              urs.
                 dance matching problem occu Due to this  s
   ve             ed          om             l
wav is not guide properly fro cylindrical waveguide to    o
   tangular waveg
rect              guide.
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           Fig 9 - S21 plot for double step tap



     Fig 10 - E F              ion         step
                Field Distributi in Double s tapering

                                                                                                    ISSN 1947-5500

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