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ENE 429 Antenna and Transmission lines Theory

DATE: 28/08/06 01/09/06. ENE 429 Antenna and Transmission lines Theory. Lecture 8 Rectangular waveguides and cavity resonator. TE waves in rectangular waveguides (1). E z = 0. From Expanding for z-propagating field gets where. TE waves in rectangular waveguides (2).

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ENE 429 Antenna and Transmission lines Theory

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  1. DATE: 28/08/06 01/09/06 ENE 429Antenna and Transmission lines Theory Lecture 8 Rectangular waveguides and cavity resonator

  2. TE waves in rectangular waveguides (1) • Ez = 0 From Expanding for z-propagating field gets where

  3. TE waves in rectangular waveguides (2) • In the x-direction Since Ey = 0, then from we have at x = a and x = b

  4. TE waves in rectangular waveguides (3) • In the y-direction Since Ex = 0, then from we have at y = a and y = b

  5. Method of separation of variables (1) Assume then we have

  6. Properties of TE wave in x-direction of rectangular WGs (1) • in the x-direction • at x = 0, • at x = a,

  7. Properties of TE wave in x-direction of rectangular WGs (2)

  8. Properties of TE wave in y-direction of rectangular WGs (1) 2. in the y-direction at y = 0, at y = b,

  9. Properties of TE wave in y-direction of rectangular WGs (2) For lossless TE rectangular waveguides,

  10. Cutoff frequency and wavelength of TE mode

  11. A dominant mode for TE waves • For TE mode, either m or n can be zero, if a > b, is a smallest eigne value and fc is lowest when m = 1 and n = 0 (dominant mode for a > b)

  12. A dominant mode for TM waves • For TM mode, neither m nor n can be zero, if a > b, fc is lowest when m = 1 and n = 1

  13. Ex1 a) What is the dominant mode of an axb rectangular WG if a < b and what is its cutoff frequency? b) What are the cutoff frequencies in a square WG (a = b) for TM11, TE20, and TE01 modes?

  14. Ex2 Which TM and TE modes can propagate in the polyethylene-filled rectangular WG (r = 2.25, r = 1) if the operating frequency is 19 GHz given a = 1.5 cm and b = 0.6 cm?

  15. Rectangular cavity resonators (1) • At microwave frequencies, circuits with the dimension comparable to the operating wavelength become efficient radiators • An enclose cavity is preferred to confine EM field, provide large areas for current flow. • These enclosures are called ‘cavity resonators’. There are both TE and TM modes but not unique. b d a

  16. Rectangular cavity resonators (2) • z-axis is chosen as the reference. • “mnp” subscript is needed to designate a TM or TE standing wave pattern in a cavity resonator.

  17. Electric field representation in TMmnp modes (1) • The presence of the reflection at z = d results in a standing wave with sinz or cozzterms. Consider transverse components Ey(x,y,z), from B.C. Ey = 0 at z = 0 and z = d • 1) its z dependence must be the sinz type 2) similar to Ex(x,y,z).

  18. Electric field representation in TMmnp modes (2) From Hz vanishes for TM mode, therefore

  19. Electric field representation in TMmnp modes (3) If Exand Ey depend on sinz then Ezmust vary according to cosz, therefore

  20. Magnetic field representation in TEmnp modes (1) • Apply similar approaches, namely • transverse components of E vanish at z = 0 and z = d • - require a factor in Ex and Eyas well as Hz. • factor  indicates a negative partial derivative with z. • - require a factor for Hx and Hy • fmnp is similar to TMmnp.

  21. Dominant mode • The mode with a lowest resonant frequency is called ‘dominant mode’. • Different modes having the same fmnp are called degenerate modes.

  22. Resonator excitation (1) • For a particular mode, we need to • place an inner conductor of the coaxial cable where the • electric field is maximum. • introduce a small loop at a location where the flux of the • desired mode linking the loop is maximum. • source frequency = resonant frequency

  23. Resonator excitation (2) • For example, TE101 mode, only 3 non-zero components are • Ey, Hx, and Hz. • insert a probe in the center region of the top or bottom • face where Eyis maximum or place a loop to couple • Hxmaximum inside a front or back face. • Best location is affected by impedance matching requirements of the microwave circuit of which the resonator is a part.

  24. Coupling energy method • place a hole or iris at the appropriate location • field in the waveguide at the hole must have a component that is favorable in exciting the desired mode in the resonator.

  25. Ex3 Determine the dominant modes and their frequencies in an air-filled rectangular cavity resonator for • a > b > d • a > d > b • a = b = d

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