Nội dung

655 Problems --- 11= cl T, - - 12 = C2T 2 - Section 8-3 16. A transmission line is excited by a voltage source Vo cos wt at z = -1. The transmission line is loaded with a purely reactive load with impedance jX at z = 0. + Vo cosw, 0 g - (a) Find the voltage and current distribution along the line. (b) Find an expression for the resonant frequencies of the system if the load is capacitive or inductive. What is the solution if IXI = Zo? (c) Repeat (a) and (b) if the transmission line is excited by a current source Io cos wt at z = -1. 17. (a) Find the resistance and conductance per unit lengths for a coaxial cable whose dielectric has a small Ohmic conductivity o- and walls have a large conductivity o,, (Hint: The skin depth 8 is much smaller than the radii or thickness of either conductor.) (b) What is the decay rate of the fields due to the losses? (c) If the dielectric is lossless (o = 0) with a fixed value of 656 Guided Electromagnetic Waves outer radius b, what value of inner radius a will minimize the decay rate? (Hint: 1+1/3.6·-ln 3.6.) 18. A transmission line of length I is loaded by a resistor RL. VOC 0 Wa RL 1 - 0 (a) Find the voltage and current distributions along the line. (b) Reduce the solutions of (a) when the line is much shorter than a wavelength. (c) Find the approximate equivalent circuits in the long wavelength limit (kl < 1) when RL is very small (RL << Zo) and when it is very large (RL >> Zo) Section 8-4 19. For the transmission line shown: ~y\n JY jB · VOCos • Zo = 50 Z• = 100(1 -j) =4 4 (a) Find the values of lumped reactive admittance Y = jB and non-zero source resistance R, that maximizes the power delivered by the source. (Hint: Do not use the Smith chart.) (b) What is the time-average power dissipated in the load? 20. (a) Find the time-average power delivered by the source for the transmission line system shown when the switch is open or closed. (Hint: Do not use the Smith chart.) 400 4 S=100 Vo cos wr (b) For each switch position, what is the time average power dissipated in the load resistor RL? Probl•m 657 (c) For each switch position what is the VSWR on each line? 21. (a) Using the Smith chart find the source current delivered (magnitude and phase) for the transmission line system shown, for I= A/8, A/4, 3A/8, and A/2. Vocosw• Zo = 50 L = 50(1 - 2j) (b) For each value of 1,what are the time-average powers delivered by the source and dissipated in the load impedance ZL? (c) What is the VSWR? 22. (a) Without using the Smith chart find the voltage and current distributions for the transmission line system shown. S4 V o coswot ZL = 100 (b) What is the VSWR? (c) At what positions are the voltages a maximum or a minimum? What is the voltage magnitude at these positions? 23. The VSWR on a 100-Ohm transmission line is 3. The distance between successive voltage minima is 50 cm while the distance from the load to the first minima is 20 cm. What are the reflection coefficient and load impedance? Section 8-5 24. For each of the following load impedances in the singlestub tuning transmission line system shown, find all values of the length of the line 11 and stub length 12 necessary to match the load to the line. (a) ZL = 100(1-j) (b) ZL = 50(1 + 2j) (c) ZL=25(2-j) (d) ZL = 2 5(l + 2j) 658 Guided Electromagnetic Waves 25. For each of the following load impedances in the doublestub tuning transmission line system shown, find stub lengths 1 and 12 to match the load to the line. -"----8 8x (a) ZL = 100(1-j) (b) ZL = 50(l+2j) (c) ZL =25(2-j) (d) ZL=25(1+2j) 26. (a) Without using the Smith chart, find the input impedance Zi, at z = -1= -A/4 for each of the loads shown. (b) What is the input current i(z = -1, t) for each of the loads? Problems 4 RL=- Zo + L =- Vo 0 cosw o I --I 659 1 0 V0 coiwof Zo .o - C= 1 (c) The frequency of the source is doubled to 2wo. The line length I and loads L and C remain unchanged. Repeat (a) and (b). (d) The frequency of the source is halved to fao. Repeat (a) and (b). Section 8-6 27. A rectangular metal waveguide is filled with a plasma with constitutive law -'i= ow 1 E (a) Find the TE and TM solutions that satisfy the boundary conditions. (b) What is the wavenumber k along the axis? What is the cut-off frequency? (c) What are the phase and group velocities of the waves? (d) What is the total electromagnetic power flowing down the waveguide for each of the modes? (e) If the walls have a large but finite conductivity, what is the spatial decay rate for TE1 o propagating waves? 28. (a) Find the power dissipated in the walls of a waveguide with large but finite conductivity o, for the TM,,, modes (Hint: Use Equation (25).) (b) What is the spatial decay rate for propagating waves? 29. (a) Find the equations of the electric and magnetic field lines in the xy plane for the TE and TM modes. (b) Find the surface current field lines on each of the 660 Guided Electromagnetic Waves waveguide surfaces for the TEr,modes. Hint: J tan xdx = -In J cos x cot xdx = In sin x (c) For all modes verify the conservation of charge relation on the x = 0 surface: V - K+Lt= 0 30. (a) Find the first ten lowest cut-off frequencies if a = b= 1 cm in a free space waveguide. (b) What are the necessary dimensions for a square free space waveguide to have a lowest cut-off frequency of 10'0, 108, 106, 10', or 102 Hz? 31. A rectangular waveguide of height b and width a is short circuited by perfectly conducting planes at z = 0 and z = 1. (a) Find the general form of the TE and TM electric and magnetic fields. (Hint: Remember to consider waves traveling in the +z directions.) (b) What are the natural frequencies of this resonator? (c) If the walls have a large conductivity a, find the total time-average power dissipated in the TE 1 01 mode. (d) What is the total time-average electromagnetic energy < W> stored in the resonator? (e) Find the Q of the resonator, defined as Q= Qo< W> where wo is the resonant frequency. Section 8.7 32. (a) Find the critical frequency where the spatial decay rate a is zero for all the dielectric modes considered. (b) Find approximate values of a, k,, and k, for a very thin dielectric, where kd << 1. (c) For each of the solutions find the time-average power per unit length in each region. (d) If the dielectric has a small Ohmic conductivity o, what is the approximate attenuation rate of the fields. 33. A dielectric waveguide of thickness d is placed upon a perfect conductor. (a) Which modes can propagate along the dielectric? (b) For each of these modes, what are the surface current and charges on the conductor? Problems 661 E0 ,p 0 (c) Verify the conservation of charge relation: V, - K+ ° at = -0 (d) If the conductor has a large but noninfinite Ohmic conductivity oar, what is the approximate power per unit area dissipated? (e) What is the approximate attenuation rate of the fields? chapter 9 radiation 664 Radiation In low-frequency electric circuits and along transmission lines, power is guided from a source to a load along highly conducting wires with the fields predominantly confined to the region around the wires. At very high frequencies these wires become antennas as this power can radiate away into space without the need of any guiding structure. 9-1 9-1-1 THE RETARDED POTENTIALS Nonhomogeneous Wave Equations Maxwell's equations in complete generality are 0B at VxE= (1) aD VxH=J +- (2) at V B=0 (3) V-D=pf (4) In our development we will use the following vector identities Vx (V V) = O (5) (6) V - (VxA) = 0 2 Vx (Vx A) = V(V - A) - V A (7) where A and V can be any functions but in particular will be the magnetic vector potential and electric scalar potential, respectively. Because in (3) the magnetic field has no divergence, the identity in (6) allows us to again define the vector potential A as we had for quasi-statics in Section 5-4: (8) B=VxA so that Faraday's law in (1) can be rewritten as / Vx(E+- A\ at =0O "/\Ot