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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