Dominant Mode Property in Waveguides: Which characteristic defines the dominant mode in a waveguide?
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ANo phase shift
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BNo attenuation
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CHighest cut-off frequency
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DLowest cut-off frequency
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EZero group delay
Answer
Correct Answer: Lowest cut-off frequency
Explanation
Introduction / Context:In waveguides, multiple modes can propagate above their respective cut-off frequencies. The 'dominant mode' is the one that first propagates as frequency increases from zero, a key concept for design and analysis.
Given Data / Assumptions:
- Hollow metallic waveguide (e.g., rectangular), standard mode nomenclature (TE10 is dominant in rectangular waveguides).
Concept / Approach:
The dominant mode is defined as the mode with the lowest cut-off frequency. It requires the smallest operating frequency to propagate and is therefore the most readily excited mode in a given guide geometry.
Step-by-Step Reasoning:
Identify the definition: dominant mode ↔ minimum f_c.Rectangular guides: TE10 has f_c = c/(2a), which is lower than other modes.Hence the characteristic property is the lowest cut-off frequency among all modes of that guide.Verification / Alternative check:
Textbook mode tables list TE10 as dominant; corresponding cut-off is lowest for rectangular geometry, validating the definition.
Why Other Options Are Wrong:
- Highest cut-off frequency: opposite of the definition.
- No attenuation / No phase shift / Zero group delay: not defining properties and, in general, not true in realistic guides.
Common Pitfalls:
- Confusing dominant mode property with loss/dispersion characteristics.
Final Answer:
Lowest cut-off frequency