Waveguide/cavity modes – Identify the incorrect statement about TE/TM transmission Which of the following statements is NOT correct regarding modes used to carry electromagnetic energy in hollow guides and resonant structures?
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AIn TE mode the direction of electric field is always and everywhere transverse to the direction of propagation
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BIn TM mode the magnetic field pattern is always and everywhere transverse to the direction of propagation
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CThe subscripts in TE and TM designations denote the numbers of half-sine variations of field components in the x and y directions, respectively
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DTM11 is called the dominant mode
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ETE10 is typically the dominant mode in a standard rectangular waveguide
Answer
Correct Answer: TM11 is called the dominant mode
Explanation
Introduction:Electromagnetic energy in hollow metal guides is carried in discrete modes, commonly labeled TE_mn or TM_mn for rectangular guides. The dominant mode is the lowest-cutoff mode that can propagate and is central to practical design.
Given Data / Assumptions:
- Standard empty (air-filled) rectangular waveguide.
- Mode labels follow conventional definitions.
- Focus on conceptual correctness rather than numerical cutoff values.
Concept / Approach:
In TE modes, E_z = 0, so the electric field is everywhere transverse to the propagation direction. In TM modes, H_z = 0, so the magnetic field is everywhere transverse. The subscripts m, n indicate the number of half-wave field variations along the guide's two transverse dimensions. The dominant mode of a rectangular waveguide is TE10, not TM11.
Step-by-Step Solution:
1) Recall TE definition: E has no longitudinal component → transverse everywhere.2) Recall TM definition: H has no longitudinal component → transverse everywhere.3) Identify dominant mode in rectangular WG: TE10 (lowest cutoff frequency).4) Therefore, the statement naming TM11 as dominant is incorrect.Verification / Alternative check:
Cutoff tables list TE10 with the smallest fc for rectangular guides of given a × b cross-section.
Why Other Options Are Wrong:
Options A, B, C, and E correctly describe TE/TM definitions and the dominant mode for rectangular waveguide.
Common Pitfalls:
Confusing TE vs TM definitions; assuming symmetry implies the same dominant mode across shapes (it does not).
Final Answer:
TM11 is called the dominant mode.