Waveguides
1
2   Parallel wire transmission
    lines
    Cannot effectively propagate
     EM energy above
     approximately 20 GHz
    Cannot be used to propagate
     signals with high powers
    Impractical for many UHF and
     microwave applications
3   Waveguide
    Is a hollow conductive tube,
     usually rectangular in cross
     section but sometimes
     circular or elliptical
    Generally used to
     frequencies above 1 GHz
4      Signal Injection and
       Extraction
     A microwave signal to be carried by a
      waveguide is introduced into one end of the
      waveguide with an antennalike probe that
      creates an electromagnetic wave that
      propagates through the waveguide. The
      electric and magnetic fields associated with
      the signal bounce off the inside walls back
      and forth as the signal progresses down the
      waveguide. The waveguide totally contains
      the signal so that none escapes by radiation.
    Signal Injection and
5   Extraction
                    • The probe shown in the figure is a
                      one-quarter wavelength vertical
                      antenna at the signal frequency
                      that is inserted in the waveguide
                      one-quarter wavelength from the
                      end, which is closed.
                    • Because the probe is located one-
                      quarter wavelength from the closed
                      end of the waveguide, the signal
                      from the probe is reflected from the
                      closed end of the line back toward
                      the open end. Over a one-quarter
                      wavelength distance, the reflected
                      signal appears back at the probe in
                      phase to aid the signal going in the
                      opposite direction.
6   Signal Injection and
    Extraction
                • A loop can also be used to
                  introduce a magnetic field into a
                  waveguide.
                • Microwave energy applied
                  through a short piece of coaxial
                  cable causes a magnetic field to
                  be set up in the loop.
       Rectangular waveguide
  7
 Most common form of
  waveguide
Must satisfy Maxwell’s
equations through the guide
The wave must propagate
down the guide in a zigzag
manner, with the electric field
maximum in the center of the
guide and zero at the
surface of the walls.
8   Example
    A rectangular waveguide has a
     width of 0.65 in and a height of
     0.38 in.
    (a)What is the cutoff frequency?
    (b)What is a typical operating
       frequency for this waveguide?
    Circular waveguide
9
                    Used in radar
                     and microwave
                     applications
                     when it is
                     necessary or
                     advantageous
                     to propagate
                     both vertically
                     and horizontally
                     polarized waves
                     in the same
                     waveguide
     Circular waveguide
10
 Easier to manufacture than
  rectangular waveguides
 Easier to join together
 Have a much larger area that a
  corresponding rectangular waveguide
  used to carry the same signal
 Plane of polarization may rotate while
  the wave is propagating down it
11   Ridged Waveguide
12   Ridged Waveguide
     Advantages             Disadvantages
      Used for              Will have less
       impedance              power handling
       matching purpose.
       This is because it     capabilities
       helps in               compare to
       decreasing the         rectangular
       characteristic         waveguide of the
       impedance of the       same dimension
       guide
      Increases the
       bandwidth if
       operation
     Flexible waveguide
13
                    Consists of spiral-wound
                     ribbons of brass or copper.
                    Short pieces of this are used in
                     microwave systems when
                     several transmitters and
                     receivers are interconnected to
                     a complex combining or
                     separating unit
                    Also used extensively in
                     microwave test equipment
                    Limitations
                        Increased loss
                        Possible introduction
                         of passive
                         intermodulation
                         products
                        Minimum bend radius
14
     Group velocity
       Velocity at
         which a
         wave
         propagates
     Phase velocity
       Velocity at
         which the
         wave
         changes
         phase
 15
 Where: 𝜆g- guide wavelength (meters/cycle)
      𝜆 o- free-space wavelength (meters/cycle)
      Vph- phase velocity (m/s)
      c – free space velocity of light
NOTE: Phase velocity may exceed the velocity of light
 16     Cutoff frequency
Is an absolute limiting frequency
Frequencies above this will be propagated by
 the waveguide
    Smallest free-space wavelength that is just
     unable to propagate in a waveguide
    Only frequencies less than the cutoff wavelength
     can propagate down the waveguide
17
      Where: 𝜆g – guide wavelength
          f- frequency of operation
          fc – cutoff frequency
           c – free-space propagation
          velocity
         Where: 𝜆g – guide wavelength
               𝜆 o – free-space
          wavelength
              f- frequency of operation
              fc – cutoff frequency
18
     Where: fc – cutoff frequency
            a – cross-sectional length
19   Example
     For a rectangular waveguide with a
     wall separation of 3cm and a
     desired frequency of operation of
     6GHz, determine
       Cutoff frequency
       Cutoff wavelength
       Group velocity
       Phase velocity
20   Modes of Propagation
     In 1955, the Institute of Radio
       Engineers published a set of
       standards.
      TEm,n – transverse-electric
      waves
      TMm,n – transverse-magnetic
      waves
21           Modes of Propagation
 In Figure (a), a vertical probe is generating a vertically polarized
  wave with a vertical electric field and a magnetic field at a right angle
  to the electric field. The electric field is at a right angle to the direction
  of wave propagation, so it is called a transverse electric (TE) field.
 Figure (b) shows how a loop would set up the signal. In this case, the
  magnetic field is transverse to the direction of propagation, so it is
  called a transverse magnetic (TM) field.
       Modes of Propagation
22
      Wave paths in a waveguide at various frequencies. (a) High frequency. (b) Medium
       frequency. (c) Low frequency. (d) Cutoff frequency.
23
 Whenever a
 microwave signal
 is launched into a
 waveguide by a
 probe or loop,
 electric and
 magnetic fields are   Electric and
 created in various
 patterns
                        magnetic
 depending upon         fields in a
 the method             rectangular
 of energy coupling,    waveguide:
 frequency of           (a) top view
 operation, and size
 of waveguide.
                        (b) end view
    Other
24
     waveguide
     operating
     modes
 SUBSCRIPTS:
 1st number - indicates the
 number of half-wavelength
 patterns of transverse lines
 that exist along the short
 dimension
 of the guide through the
 center of the cross section.
 2nd number - indicates the
 number of transverse half
 wavelength patterns that
 exist along the long
 dimension of the guide
 through the center of
 the cross section.
      Characteristic impedance
 25
Waveguides have a characteristic impedance
 that is analogous to the characteristic
 impedance of parallel-wire transmission lines
 and closely related to the characteristic
 impedance of free-space.
26   Example
     Calculate the characteristic impedance
     of a waveguide with a cut-off frequency
     of 3.75 GHz, at a frequency of 5 GHz?
     Impedance Matching
27
           Waveguide Irises
28
     Conducting posts and screws
     Waveguide Hardware and
29
     Accessories
                     Connection Joints
                        consists of two
                         flanges connected
                         to the waveguide at
                         the center. The
                         right-hand flange is
                         flat, and the one at
                         the left is slotted
                         one- quarter
                         wavelength deep at
                         a distance of one-
                         quarter wavelength
                         from the point at
                         which the walls of
                         the guide are
                         joined.
     Waveguide Hardware and
30   Accessories
                    Curved Sections
                       Special curved
                        waveguide sections
                        are available for
                        making 90° bends.
                       Curved sections
                        introduce reflections
                        and power loss, but
                        these are kept small
                        by proper design.
                        When the radius of
                        the curved section is
                        greater than 2λat the
                        signal frequency,
                        losses are minimized.
     Waveguide Hardware and
31
     Accessories   T Sections
                       It is occasionally
                        necessary to split or
                        combine two or more
                        sources of microwave
                        power. This is done with
                        T sections or T junctions
                       The T can be formed on
                        the short or long side of
                        the waveguide. If the
                        junction is formed on the
                        short side, it is called a
                        shunt T. If the junction is
                        formed on the long side, it
                        is called a series T, Each
                        T section has three ports,
                        which can be used as
                        inputs or outputs.
     Waveguide Hardware and
32
     Accessories
                     Hybrid T’s
                        can be formed by
                         combining the
                         series and shunt T
                         sections
                        Sometimes referred
                         to as a magic T
                        used as a duplexer
                         to permit
                         simultaneous use of
                         a single antenna by
                         both a transmitter
                         and a receiver
     Waveguide Hardware and
33
     Accessories    Directional Couplers
                             used to facilitate
                              the measurement
                              of microwave
                              power in a
                              waveguide and
                              the SWR.
                             They can also be
                              used to tap off a
                              small portion of a
                              high-power
                              microwave signal
                              to be sent to
                              another circuit or
                              piece of
                              equipment.