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Dimitrios V. Peponis

National and Kapodistrian University of Athens

Thank you Dr Alimenti. I have to admit that the solution I have seen before it's about using a transparent screen, which requires the impulse response of the grid to be known

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If your "stimulus" acts by imposing a given value to the E (or H) field in FDTD method, the stimulus region becomes reflector. A simple solution to avoid this is to use the current density J (as in th Maxwell equations) to launch the wave (to create your source or stimulus).

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Ajay Kashi

You might want to experiment with a quartz glass tube.....they are transparent to microwaves. Another option would be to use alumina which is transparent to microwaves.

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Halim Boutayeb

Université du Québec en Outaouais

Hi Dimitris,

I suggest you to take a look to the FDTD powerpoint files in the following website:

http://www.creer.polymtl.ca/Halim_Boutayeb/

I hope it can help you.

Best regards,

Halim

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Dimitrios V. Peponis

National and Kapodistrian University of Athens

Honestly, I would like to thank everyone who answered my question. As I can see, I have a lot work to read :D

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Luca Roselli

University of Perugia

Dear Dimitris,

I'was about to give the same answer of Prof. Alimenti, try it, it is very efficient;

regards, Luca

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Luca Roselli

University of Perugia

I wish to underline that the transparent source concept is exactly the one we have implemented in the paper referred to by professor Alimenti. Looking in the literature for "modal absorption" authored by Alimenti Roselli et al. you will find additional refernces to understand deeply in details how that concept works.

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Dimitrios V. Peponis

National and Kapodistrian University of Athens

Thank you, It's true that the whole concept is about modal absorbion. Again, your work is very useful in mine.

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Federico Alimenti

University of Perugia

The modal absorption can be applied to all kind of homogeneously filled waveguides (not only to rectangular ones) with closed external (metallic) boundaries. Useful examples are: the circular waveguide, the coaxial cable, etc.

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Dimitrios V. Peponis

National and Kapodistrian University of Athens

Till now I use a PML layer with absorbion characteristics that mathc the excited mode. Can It combined with a PML layer, especially UPML

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First of all I've managed to build from scratch a 3D FDTD code for modelling waves travelling in cylindrical waveguides. The problem comes when I have to calculate nr, nf and nz where ni; i=r,f,z are the spatial steps in the computational grid. I use a modified version of an earlier adoption of the same issue, regarding cartesian coordinates. So, that's my assumptions:

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In a similar way, dz is calculated.

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So, df is the angle whose arc (at a radial point equal to the radius) is equal or smaller to that basic step referred before. With this method we are sure that we've taken in account the largest possible distance within our grid in the azimuthial direction.

With this logic, comparing a rectangular waveguide with similar characteristics as cutoff frequency of TE10 mode and dimensions we can make an assignment like that one given below

big dimension of the waveguide assigns to half the perimeter. That's because in the big dimension of the rectangular waveguide is corresponded half wavelength and in the perimeter two wavelengths

small dimension of the waveguide to the radius

length to the length

With this logic, we obtain similar cell number for the propagation direction and the radius, but a larger enough number for the azimuthial direction (nf). In other words, nf is considerably larger than the rectangular equivalent, when using similar dimensions and approach.

And here comes my question. What is wrong? This as you can assume makes timestep very small leading my simulations to last longer. How can azimuthial step (nf) number can be calculated instead? This number I think is very large and we don't need such a big number. One obvious answer is to reduce resolution in the azimuthial direction. But I want a more systematical approach.

Thanks in advance!!!