- 2069 Chaitra · 2+2 marks
Define cutoff frequency, dominant mode
Short answer
Cutoff frequency f_c is the minimum frequency for mode propagation; below it, the mode is evanescent. In a rectangular guide, f_c = (c/2)√[(m/a)²+(n/b)²]. The dominant mode has the lowest f_c, TE10 with f_c = c/(2a) when a>b.
Define cutoff frequency and dominant mode for a rectangular waveguide.
Cut-off frequency
The cut-off frequency f_c of a mode is the lowest frequency at which that mode can propagate in the waveguide. For a rectangular guide of width a and height b,
f_c = (c/2)√[(m/a)² + (n/b)²]
For f > f_c, β is real and the mode propagates; for f < f_c, β becomes imaginary (an attenuation constant) and the field dies out quickly (evanescent). At f = f_c the guide wavelength is infinite.
Dominant mode
The dominant mode is the mode with the lowest cut-off frequency. For a rectangular guide with a > b it is TE₁₀:
f_c(TE₁₀) = c/(2a), λ_c = 2a
It is preferred in practice because it gives single-mode operation over the band c/2a < f < c/a (for a = 2b), has simple field patterns, and needs the smallest guide for a given frequency.
✓ Answer checked by Claude Opus 5.5 (high effort)
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