unknown reflection from discontinuity in circular waveguide

[Kaku Yichikun]

Hello, I encountered such a problem: TM01 mode was excited to two circular waveguides. One has a full circular discontinuity and the other has a partial circular discontinuity shown in the figure 1. Results are shown in figure 2. The peak value correspond to the reflection from the discontinuity but I am confused about the reflection component behind the peak when using a waveguide with partial circular waveguide. It is just like there is a source continuously generating the reflection signal. I observed this in both simulations and experiments. I will appreciate your comments.

 

[volker@muehlhaus]

To me, this looks like multiple reflections travelling between two discontinuities.

Does your TM01 port terminate all other modes as well? Is it possibly another mode excited by your partial discontinuity that is NOT terminated by the port?

 

[Kaku Yichikun]

To me, this looks like multiple reflections travelling between two discontinuities.

Does your TM01 port terminate all other modes as well? Is it possibly another mode excited by your partial discontinuity that is NOT terminated by the port?

Thanks for your helpful reply. I agree with your point. There exist TM01 and TE11 mode in my sweeping frequency range as shown in fig 3. But I do no know why the reflection becomes large at some specific frequencies. It seems mode conversion is intensified at these points. Looking forward to your comments.

 

[PlanarMetamaterials]

I would guess at these peaks, the TM01 mode is coupling to some resonant mode supported by the discontinuity, although it's hard to say without more data. It would be interesting to see the fields at the S11 peaks; these could provide some more insight.

 

[Kaku Yichikun]

I would guess at these peaks, the TM01 mode is coupling to some resonant mode supported by the discontinuity, although it's hard to say without more data. It would be interesting to see the fields at the S11 peaks; these could provide some more insight.

Thanks for your reply. I checked the electric field at the cross section of the cavity when frequency is set to the left specific frequency. It looks like TM21 mode in the discontinuity region. This specific frequency is under the cutoff frequency of TM21 in the circular waveguide. I am not sure whether it is called the "trapped mode".

 

[PlanarMetamaterials]

Aha, that sounds about right. Normally, these modes wouldn't couple, but when perturbations are introduced they most certainly could. Is this true of both resonance frequencies?