A clarification regarding the definition of cutoff frequency for Dielectric waveguides. Quoting Yeh and Shimabukuro:
"The cutoff frequency does not have the conventional definition as that for the metal waveguides. Here it is defined that at the cutoff frequency the open dielectric waveguide structure ceases to act as a binding medium for the guided surface wave, and the wave can no longer be guided by the structure."
Does this mean that it's more like an upper limit to the frequency of operation rather than the lower limit as in metallic waveguides? Note that .in my situation I am considering a solid cylinder of dielectric material (HDPE)
beyond cutoff , its like a loaded wave guide, so mu and epsilon changes become inevitable and participate in the propagation process resulting in losses
Frankly, speaking, I found exactly same characteristics (high pass filter) for dielectric waveguide slab as found for metallic waveguide.
Can you please provide the title of the paper by 'Quoting Yeh and Shimabukuro'. We want to read that paper before making any comments.
Dr. Maity, the line if from the book "The Essence of Dielectric Waveguides" by Yeh and Shimabukuro. There was a footnote stating the same in the book.
The different modes propagating in such waveguides are analyzed and just to add here it said that there's one mode (HE11 mode) which has no cutoff and propagates at all frequencies while other modes have a cutoff.
Dr. Surducan, here dielectric waveguide refers to waveguide made up of dielectric material. In my case for example a solid cylindrical structure of High Density Polyethylene is being used (in simple words lets say a solid plastic rod)
According to Prof. C Yeh's work (The Essence of DW_C Yeh_2008, pp. 146), "Only one mode, the HE11 mode, has zero cutoff frequency since the first root of (5.46) with m = n = 1 is p1a = 0. The HE11 mode is called the dominant mode." Yes, this is quite possible phenomenon. In rectangular dielectric slab, TM0 and TE0 modes have ZERO cut-off frequency (The Essence of DW_C Yeh_2008, pp. 110).
Now, coming to your original question on cut-off frequency, It is correct that there is no conventional definition to define the cut-off frequency. In metallic waveguide, beta is set to zero to define cut-off frequency. In case case dielectric waveguide, beta = ko, not zero. The answer is simple. We have to find a limiting condition by which at least one mode can be found to have minimum frequency. This is basic definition. If we follow it, we can easily find a relation between metallic waveguide and dielectric waveguide. This condition is satisfied by setting beta = 0 (no wave propagation) for metallic waveguide. In case of dielectric waveguide, decaying factor (in air) is set to zero ( not, beta is set to zero). After setting decaying factor = 0, beta becomes equal to ko in dielectric waveguide. In case of multilayered dielectric waveguide (i.e. having several decaying factors) set one decaying factor to zero such that we can get minimum frequency, beyond that we can not get lower frequency.
But, metallic and dielectric waveguide both are HIGH pass filter, i.e. after cutoff frequency, one mode will propagate
I would like to welcome the colleagues and Dr. Surducan!
Sudipta is right. Metallic and dielectric wave guides behave as a high pass filters in the sense that the waves will be confined in the wave guide either by total reflection on the metallic walls or by the total reflection on the boundary of the dielectric and the surrounding air. Accordingly, only integral multiples of half wave lengths must fit in the width of the wave guide. However the higher frequency modes will suffer from attenuation either in the Metallica walls in case of metallic waveguide or in the polarization of the dielectric material. At the end effect the waveguides are band pass filters at the low frequency because of the mode cut off and at high frequency because of the losses in the material.
Best wishes
Dear Mr. Halder,
yes, we have a cutt off frequency at wave guides. See please to a IEC standard
of waveguides.
All the best
F. Gräbner
Thank you all for your replies.
My query is clarified now. They were referring to the technique of cutoff rather than the High Pass behaviour.
In a metallic one its attenuated below the cutoff while in DWGs the mode is not bound inside the guiding medium with enough radiation.
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