20. Assume the electron density is 10⁶ cm^-3 in a certain layer of ionsphere. Which of the following frequency of electromagnetic waves that could pass through that layer?
(A) 12 MHz
(B) 8 MHz
(C) 4 MHz
(D) 1 MHz

21.If V/m. Find (z，t). (A)mA/m (B)mA/m (C)mA/m (D)m/Am

22. In an air-filled parallel-plate waveguide with dimension a = 4 cm, according to the expression of electric field distribution, E= E0(sin25πx+3sin50πx)sin(8x109πt) What is the value of phase constantβ2 of propagation wave? (A) 7.5 (B) 23.5 (C) 29.2 (D) 74 radm

23. Consider a parallel-plate waveguide (plates at y=0and y=b planes), with wave propagating in the +z direction. For the lowest-order TM mode, if the instantaneous field expression for the z-component of the clectric field at (x, y, z) = (x, b/4. 0) is Ez(t) = E0cos wt, what is the corresponding expression for Ez(t) at (x, y, z) = (x. b/2, d)? (A)2E0cos(wt-βd)(B)(C)2E0sin(wt-βd)(D) sin(wt-βd)

24. Find the spacing d for a parallel-plate waveguide having a dielectric of ε =4ε0, and u= u0 such that 5 GHz is the cutotr frequency of the dominant mode. (A) 1(B)1.5 (C)2 (D) 2.5 cm

25. A wave in air is incident upon a soil surface at θ1 =40°. If soil has εr = 2.2 and μr=1, determine Γ⊥and τ⊥. (A)Γ⊥=-0.48,τ⊥=0.52(B)Γ⊥=-0.27,τ⊥=0.52(C)Γ⊥=0.27,τ⊥=0.73(D)Γ⊥.=-0.48,τ⊥=0.73

26. The magnetic field in a given dielectric medium is given by = y6cos(2z)sin(2x107t -0.1x) (A/m), where x and z are in meters. Determine the phase velocity of the electromagnetic wave. (A) 2x108 (m/s) (B) 3x108 (m/s) (C) 5x108(m/s) (D) 1.5x108 (m/s)

27. The magnetic field associated with a uniform plane wave propagating in the +z-direction in free space is given by  = H0 cos(6πx107t -0.2πz)y (A/m) . Find the instantaneous power fow across a surface of area Im2 in the z=0 plane at t=(1/8)μs(A)0 W (B)1 W (C)1.5 W (D)3 W

28. The electric field of a plane wave is given by(zt) = x3 cos(ωt-kz) + y4 cos(ωt-kz) (V/m). Determine the polarization state of the corresponding electromagnetic wave. (A)Lincar Polarization (B) Right-Hand Circular Polarization (C) Left-Hand Circular Polarization (D)Elliptical Polarization

29. The general expression of the voltage on a lossless transmission line in the sinusoidal steady state is Which one of the following statements is wrong?(A)The current can be expressed as，where Z0 is the characteristic impedance of the transmission line. (B) If B is zero, there is no standing wave on the transmission line. (C) The first term and the second term represent the waves going in the +z and -z directions, respectively. (D) ω /vp is the propagation constant.

30. A 50-Ω transmission line is terminated by a load. Which one of the following statements is wrong? (A)Ifthe load impedance is 0Ω and the electrical length of the transmission line is 90 degree, the input impedance is ∞. (B) If the load impedance is 100 Ω and the electrical length of the transmission line is 90 degree, the input impedance is 20Ω. (C) Ifthe load impedance is 200Ω and the electrical length of the transmission line is 180 degree, the input impedance is 200Ω. (D) If the load impedance is 30 Ω and the electrical length of the transmission line is 360 degree, the input impedance is 30Ω.

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