Why do small waveguides support less modes than larger ones?

In summary, through courses on Electromagnetics and Waveguide, it is common knowledge that smaller waveguides support fewer modes than larger ones. The graphical method can also be used to calculate the number of modes in a 3-layer slab waveguide. However, the reason for this phenomenon is unclear, and an analogy may provide better understanding. It is suggested that in an ideal waveguide, the modes are the same but their frequencies are scaled according to the dimensions. It is possible that the smaller waveguides have higher modes that are attenuated faster due to practical properties of the interior surface.
  • #1
Jin Huang
1
0
I've taken several courses on Electromagnetics and Waveguide. It has become common sense to me that small waveguides support less modes than larger ones. I've also learned the graphical method to calculate the number of modes in a 3-layer slab waveguide.

What I don't get is why small waveguides support less modes than larger ones. Is there a reasonable analogy to help get insight on this fact?

Thanks very much!
 
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  • #2
In an ideal waveguide the modes are the same, but their frequencies scale with dimension.
 
  • #3
marcusl said:
In an ideal waveguide the modes are the same, but their frequencies scale with dimension.
Yes. The modes are all the same - just scaled according to wavelength / waveguide dimensions.
It's possible that the very highest modes for the smaller waveguides may be attenuated faster due to the practical properties of the interior surface of the waveguide - the geometry has to yield to surface impedance.
 

1. Why do small waveguides support fewer modes than larger ones?

Small waveguides have smaller dimensions, which result in a smaller cross-sectional area. This means that there is less space for electromagnetic waves to propagate, leading to fewer possible modes.

2. How does the size of a waveguide affect the number of supported modes?

The size of a waveguide directly affects the number of supported modes. As the dimensions of the waveguide decrease, the number of possible modes also decreases.

3. Can a small waveguide support the same number of modes as a larger one?

No, a small waveguide cannot support the same number of modes as a larger one. This is due to the limited space for electromagnetic waves to propagate in a smaller waveguide.

4. What is the relationship between the size of a waveguide and the number of modes it supports?

The relationship between the size of a waveguide and the number of modes it supports is inverse. As the size of the waveguide decreases, the number of modes also decreases.

5. Are there any advantages to using a small waveguide with fewer supported modes?

Yes, there are potential advantages to using a small waveguide with fewer supported modes. For example, a smaller waveguide may have a lower loss and higher bandwidth compared to a larger waveguide. It may also be easier and more cost-effective to fabricate smaller waveguides.

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