Understanding Planewaves Bouncing Parallel Plates (Waveguide)

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SUMMARY

This discussion focuses on the propagation of modes in waveguides, specifically regarding the behavior of standing waves above and below the cutoff frequency. It is established that for a guided mode to propagate, reflections off the waveguide surfaces must be in phase; otherwise, the waves will attenuate. The confusion arises from understanding what 'in phase' means in this context, particularly concerning the reflections at the bottom surface of the waveguide. The discussion clarifies that without proper phase alignment, destructive interference occurs, preventing effective wave propagation.

PREREQUISITES
  • Understanding of waveguide theory and mode propagation
  • Familiarity with the concept of cutoff frequency in waveguides
  • Knowledge of wave interference and phase relationships
  • Basic principles of reflection in optics or electromagnetism
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  • Study the principles of waveguide design and cutoff frequency calculations
  • Learn about phase relationships in wave interference and their impact on wave propagation
  • Explore the effects of boundary conditions on wave behavior in waveguides
  • Investigate the mathematical modeling of standing waves in waveguides
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Students and professionals in physics, electrical engineering, and telecommunications who are interested in waveguide theory and the behavior of electromagnetic waves in confined structures.

fred3142
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Hi,

I'm rather confused as to how this diagram in my book. I know that in order for a mode to propagate, a standing wave must be formed in the waveguide, and this happens above some cutoff frequency that one designs for. What I don't understand is, below cutoff, what happens? I don't understand what is meant by the waves must be 'in phase'. In phase with what? The picture I've attached is meant to show how out of phase waves are created if the angle is not correct; however, I don't at all see what created these out of phase waves. I also don't understand why there isn't a wavefront traveling in the direction of the ray pointing downwards.

My guess is that when the waves reflect (and go in the direction of the downward ray), they need to be in-phase with the waves moving in the direction of the upward point ray so that they don't destructively interfere.

If anyone could shed some light on this, that would be great.

Thank you.
 

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Your guess is close. Note there are two reflections off the bottom surface - for a guided mode to travel, then these reflections must be in phase. If they are not, then they will attenuate.

The reflection off the top surface has been removed, presumably for brevity.

Claude.
 

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