Learning Forum

[ixg90]

I'm attempting to model the transmission and reflection of a dielectric slab from a plane wave source. The slab is 0.5mm thick polyester. The simulation region is 5mm (x), 5mm (y), 3mm (z) with a plane wave excitation radiating in the positive z-direction from the bottom face of the model. The top/bottom boundary conditions are PML, and the four sides are radiation boundaries (I'm trying to minimuze reflections from the model's boundary). The model is shown in the following images: 

 

The model has a 2-dimensional surface above and below the dielectric slab. I use the field calculator to integrate the real part of the normal component of the Poynting vector to determine the total power flux. This is how I determine the reflection and transmission of the dielectric slab. The solution frequency is set to 300 GHz and a frequency sweep of 1 GHz - 600 GHz, with maximum delta energy set to 0.01 using the direct solver. 

I've calculated what I expect the normalized transmission and reflection of the slab to be based on its dielectric constant. That plot is below: 

 

 

The results of the simulation however show very poor agreement. The raw output of the simulations are shown below. There are three plots in total, obtained by changing the 'edit sources' field of the excitation. Each plot has two traces, one for the top integration surface, and one for the bottom: 

 

 

From this, I deduce that the transmission is represented by top integration surface with the 'edit sources' option set to 'total', and the reflection is represented by the bottom integration surface with the 'edit sources' option set to 'scattered'. Here are those traces overlayed with the previous calculation: 

 

As a sanity check, I've tried to use the conservation of energy to verify that the transmitted vs reflected power is equal to 1. The plot below shows the sum of the transmitted and reflected power vs the incident power (as reported by HFSS): 

 

 

My questions are: 

1. Why don't the simulations of transmission and reflection match the calculations? I assume the low frequency noise is a result of the finite size of the simulation region (where the model is on the order of the size of the wavelength) but that doesn't account for the higher frequency issues. 

2. Why does the reflected + transmitted power not sum to 1, and why is the incident power (as reported by HFSS) not constant? I've set a plane wave excitation to 1 V/m, yet I see a steady rise in incident power as a function of frequency. 

Any help in understanding these issues would be greatly appreciated. I've tried many different boundary conditions, and simulation settings, but haven't been able to track down the problem. 

[ixg90]

Can anyone tell me if the simulation setup is correct for calculating the reflection and tramsmission of a plane wave through a material? For example, is integrating the Poynting vector over some surface above and below the material the proper way to tell the reflected and transmitted power? 

It seems like this should be a very simple setup, yet I can't understand where the descrepancy is coming from. 

[DMARATHE]

Hi,

Thanks for sharing the post and queries over ‘Ansys Learning Forum’.

I would suggest you use lattice boundary assignment to the box and Floquet port assignment for the excitation. This setup and settings can be found from below mentioned examples.

There is an open example available under HFSS example folder (File -> Open Examples -> HFSS -> Metamaterial -> Split_Ring_Resonator.aedt)

There is a step-by-step document to analyse FSS structure (Help > HFSS PDFs > HFSS Getting Started guides > HFSS Floquet ports).

This setup is used very often to study the reflection and transmission characteristic of the metamaterial, frequency selective surfaces (FSS), dielectric slabs etc. The setup models a plane wave excitation with 377 Ohm of wave impedance. The another more simplified setup is to use PEC and PMC boundary instead of lattice and apply wave port excitation. This setup would model like a parallel plane waveguide structure.

The floquete port and wave ports has de-embedding feature, which allows you to shift the measurement plane and measure the S-parameters close to the surface of dielectric slab. The s-parameters can be used further to derive material properties such as permittivity and permeability for parameter extraction.

Hope this help.