# Scattered field larger than incident field!!!! (HFSS problem, Reflectarray cell)

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#### Ben17

##### Newbie level 6
Hi guys,

I am simulating a single unit cell of a reflectarray antenna using HFSS. I place the unit cell inside an imaginary waveguide (a waveguide with periodic boundary conditions, PEC & PMC) in order to study the behavior of the unit cell. I excite the waveguide with an incident planewave and since the waveguide has periodic boundaries it supports the planewave.

I am interested in the pattern of the electric field. I am ploting the magnitude of the complex electric field on a surface 0.2 mm above the unit cell. I have the option of which field to plot (HFSS> Fields> Edit Sources.... see the images attached). In the first image attached, I have plotted the magnitude of the incident field on the surface, it has a constant magnitude of 1 volts/meter which makes perfect sense. The problem is the scattered field or the total field. As you can see (second image attached) the magnitude of the scattered field is ~20 times larger than the incident field!!! This is definitely wrong! Or I am missing something!!

I would like to find the reflection coefficient, \Gamma, on that surface. \Gamma is the scattered field divided by the incident field. Notice that max(mag(\Gamma))=1, using these results the reflection coefficient would be larger than 1!!! Any suggestion? What am I doing wrong?

Thanks,
Ben

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• incident.jpg
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#### dopplerjeff5000

##### Newbie level 5
It is common to see confinement like this in the near-field leading to local "hot spots" with much greater field strength than the incident wave. Much of your scattered field is plotted in blue ... approximately zero strength.

I can't tell what your structure is, but it appears that it may be a square loop or patch. If so, is the excitation at an off-normal angle of incidence? If this is the case, you cannot use true PEC and PMC boundaries as they will not correctly account for the phase gradient from one side to the other. I would suggest using Floquet ports for a simulation such as this. With this method, you can plot the reflection coefficient simply in terms of the s-parameters.

Ben17

### Ben17

Points: 2
V
Points: 2

#### Ben17

##### Newbie level 6
Dopplerjeff,

You are right.

I was thinking that the magnitude of the scattered field should always be smaller or equal to the magnitude of the incident field (because the magnitude of reflection coefficient can not be larger than one). But this is not correct!!!!
In principle the magnitude of reflection coefficient can be greater than one and is not bounded. In a lot of scattering problems such as scattering by a finite metal plate or a metal wedge, the scattered field at the edges blows up (it is equal to infinity). These are called scattering singularities.
I think everything in my model makes sense now and it is correct.

About the model, the incident wave is normal to the structure in my model..... That is true, if it's not normal, floquet port should be used.

V
Points: 2