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A rectangular waveguide is filled with a polyethylene ( εr = 2.25 ) and operates at 24 GHz. The cutoff frequency of a certain mode is 16 GHz. The intrinsic impedance of this mode is
  • a)
    2248 Ω
  • b)
    337.2 Ω
  • c)
    421.4 Ω
  • d)
    632.2 Ω
Correct answer is option 'B'. Can you explain this answer?
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A rectangular waveguide is filled with a polyethylene (εr= 2.25 )...
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A rectangular waveguide is filled with a polyethylene (εr= 2.25 )...
Problem: Find the intrinsic impedance of a certain mode in a rectangular waveguide filled with polyethylene (r=2.25) operating at 24 GHz, given that the cutoff frequency of the mode is 16 GHz.



Solution:


Step 1: Determine the dimensions of the waveguide:


The cutoff frequency of a rectangular waveguide is given by:


$$f_c = \frac{c}{2\sqrt{(a/\pi)^2 + (b/\pi)^2}}$$


where, c is the speed of light, a and b are the dimensions of the waveguide.


Let's assume that a > b. Then we can rearrange the above equation to get:


$$\frac{a}{b} = \frac{\pi}{2}\sqrt{\left(\frac{c}{f_c}\right)^2 - 1}$$


Substituting the given values, we get:


$$\frac{a}{b} = \frac{\pi}{2}\sqrt{\left(\frac{3 \times 10^8}{16 \times 10^9}\right)^2 - 1} \approx 2.376$$


We can choose any arbitrary value for a or b. Let's assume that a = 2.376b. Then we can use the relation:


$$f_c = \frac{c}{2\sqrt{(a/\pi)^2 + (b/\pi)^2}}$$


to find both a and b. Substituting the given values, we get:


$$b \approx 1.879 \text{ cm}$$


$$a \approx 4.469 \text{ cm}$$


Step 2: Determine the wavelength of the mode:


The wavelength of the mode is given by:


$$\lambda = \frac{c}{f}$$


where, f is the operating frequency of the waveguide.


Substituting the given values, we get:


$$\lambda \approx 1.25 \text{ cm}$$


Step 3: Determine the wave impedance of the mode:


The wave impedance of a mode in a waveguide is given by:


$$Z_{wg} = \frac{377}{\sqrt{\epsilon_r}}\frac{K}{\sqrt{K^2 - 1}}$$


where, K is the ratio of the waveguide dimensions to the wavelength of the mode.


Substituting the given values, we get:


$$K = \frac{a}{\lambda} \approx 3.577$$


$$Z_{wg} = \frac{377}{\sqrt{2.25}}\frac{3.577}{\sqrt{3.577^2 - 1}} \approx 337.2 \text{ }\Omega$$


Therefore, the intrinsic impedance of the mode is 337.2 Ω.
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A rectangular waveguide is filled with a polyethylene (εr= 2.25 ) and operates at 24 GHz. The cutoff frequency of a certain mode is 16 GHz. The intrinsic impedance of this mode isa)2248 Ωb)337.2 Ωc)421.4 Ωd)632.2 ΩCorrect answer is option 'B'. Can you explain this answer?
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