Hi CST Studio users,
I am trying to simulate the diffraction orders of a very simple wire-grid grating in CST studio. The wires are made of PEC and are free standing in vacuum. The unit cell includes only a single period of the wire and gap with duty cycle of 50%. The width of the wire is 2.5mm and the width of air gap is 2.5mm as well. I use the far-field plot in order to plot the diffraction orders. According to the grating equations sin(theta)=m*lambda/d where m is the diffraction order, lambda is the simulation wavelength (300GHz = 1mm) and d is the grating period (5mm) I expect to get the first order diffraction at 11.5 degrees. However, the far-field plot shows a null at 11.5 degrees and a peak at 16.5 degrees. The far-field plot is attached. Then I did another simulation of the same grating but now without a unit cell. for this simulation I took 20 periods of the grating. This time I got the first order peak at 11.5 degrees as expected (far-field plot attached). Can anyone explain why the unit cell simulation gives me wrong results?
Thanks in advance
Kaynağa bağlı olarak dalganın yoğunluğu değiştiği için teta, m, lamda, d buna bağlı olarak değişir.
the structure is numerically analyzed by means of a full-wave. Floquet-mode simulation of a unit cell using the software.
Akeel M. Kadim , Thank you for your reply.
For the unit cell simulation I indeed used the periodic Floquet boundary conditions. I expected the unit cell simulation to agree with the full structure results (that agree with theory).
The unit cell is just a simple slit and the grating equation cannot be used for it. The first minimum should fall at an angle where sin(angle) = wavelength/width of the slit, i.e. the first minimum should be at an angle ~ 23 deg. It is not clear why the first minimum is at 11.5 deg.
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