Defining a Unit Cell

Define a finite layered structure comprising layers of substrate and metal to solve with periodic boundary conditions. Alternately, imprint the structure onto a surface to construct frequency selective surfaces (FSS).

Some applications for the unit cell are as follows:

Use as periodic structure for solving transmission and reflection coefficients.
Auto-generate the geometry from a unit cell definition and attach the geometry to a flat or curved surface an use as a frequency selective surface (FSS).

On the Periodic Structures tab, in the Structure group, select Unit Cell.

Figure 1. An example of defining a unit cell.
From the Reference Vector drop-down list, select one of the following:
- To specify the skew angle relative to the U axis, select U vector.
- To specify the skew angle relative to the V axis, select V vector.
Under Dimensions, specify dimensions and orientation of the unit cell:
1. In the Skew angle ( $θ$ ) field, specify the unit cell skew angle in degrees.
  
  Tip: A skew angle of 0° yields a rectangular unit cell.
2. In the Distance (U) field, specify the length of the unit cell in the U axis direction.
3. In the Distance (V) field, specify the length of the unit cell in the V axis direction.
For each layer, from the Method drop-down list select one of the following:
- To specify a dielectric substrate or free space layer, select Substrate.
- To specify a metal layer, select Metal.
- To specify a metallic layer minus the shape, select Aperture.
Figure 2. An example of a single layer with a Cross shape. Left: Metal cross with no rotation or U and V ofsets; Centre: Metal cross with 45° rotation and U and V offsets of 1.5; Right: Aperture cross with 45° rotation and U and V offsets of 1.5.
1. Specify the relevant options for each layer:
  - For Substrate:
    - In the Medium field, specify the dielectric medium or free space for vacuum.
    - In the Thickness field, specify the thickness of the layer.
  - For Metal and Aperture:
    - In the Shape field, specify the shape to be used, for example: cross, ellipse, trifilar.
    - In the Rotation field, specify the rotation of the shape in degrees.
    - In the Offset (U) field, specify the offset in the U direction from the corners of the unit cell.
    - In the Offset (V) field, specify the offset in the V direction from the corners of the unit cell.
    - [Optional] To specify a thickness, select the Define thickness check box and specify the metal thickness.
In the Z-value at the top of layer 1 field, specify where the top of layer 1 is located.
Click OK to define the unit cell and to close the dialog.

After a unit cell is defined, it available in the model tree (Construction tab) under Definitions > Periodic Structures > Unit Cells.

Note: Next step is to auto-generate the geometry from the unit cell definition.