127. For a rectangular waveguide (a x b, a> b) to suport only the TE10 mode at wave length which one of the following pairs of inequalities is to be satisfied?

A. b < X< 2b; X > 2a

B. b < X < 2b; X < 2a

C. a < X <2b; X >2a

D. a < X <2b; X <2a

128. In a hollow rectangular waveguide, the phase velocity

A. increases with increasing frequency

B. decrease with increasing frequency

C. is independent of frequency

D. will vary with frequency depending upon the frequency range

129. A circuilar air filled waveguide of radius 5 cm is operated in the THoi mode. The waveguide is then completely filled with a dielectric of relative permittivity 2.25 and relative permeability 1.0. The cut-off wavelength for the THoi mode is 0.082 m when the waveguide is air-filled.When the waveguide is filled with the dielectric, the cut-off frequency will become nearly

A. 1.06 GHz

B. 2.44 GHz

C. 3.66 GHz

D. 4.00 GHz

130. In a rectangular waveguide there is one half-wave variation of a electric field across the narrow dimension and two half-wave variations of electric field across the wider dimension. What is the dominant mode?

A. TM12

B. TE12

C. TE21

D. TM21

131. Which one of the following statements is true for electro-magnetic wave propagation through a rectangular waveguide of cross section 2a x a?

A. Critical wavelength for TE10 mode is the same as that for TEO, mode

B. Critical wavelength for TE10 mode is half that for TM?, mode

C. Critical wavelength for TE10 mode is the same as that for IMO, mode

D. Critical wavelength for TE10 mode is double that for TE01 mode

132. The phase velocity of an electromagnetic wave propagating in a hollow metallic rectangular waveguide in the TE10 mode is

A. equal to its group velocity

B. less than the velocity of light in free space

C. equal to the velocity of light in free space

D. greater than the velocity of light in free space

133. The longest wavelength that can be absorbed by silicon, which has the bandgap of 1.12 eV, is 1.1 pm. If the longest wavelength that can be absorbed by another material is 0.87 pm, then the bandgap of this material is

A. 1.416 eV

B. 0.886 eV

C. 0.854 eV

D. 0.706 eV

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