Creating the Model

Create the model in CADFEKO. Define any ports and sources required for the model. Specify the operating frequency or frequency range for the model.

1. Set the model unit to millimetres.
2. Define the following variables.
   • d1 = 2.22 (Distance between apertures.)
   • d2 = 12.51 (Distance between apertures.)
   • epsr = 2.2 (The relative permittivity of the substrate.)
   • s = 10 (Length of the aperture.)
   • w = 4.6 (Width of the microstrip.)
   • strip_feed_arc_radius = 2*s (The radius of curved microstrip line.)
   • strip_length = 2* s + d2 + d1 (The straight section length of the microstrip line.)
   • substrate_depth = 50 (The substrate depth.)
   • substrate_height = 1.58 (The substrate height.)
   • substrate_width = 140 (The substrate width.)
   • f_max = 5e9 (The maximum frequency.)
   • f_min = 2.5e9 (The minimum frequency.)
3. Create a dielectric medium.
   • Dielectric loss tangent: 0
   • Relative permittivity: epsr
   • Label: substrate
4. Create the straight section of the microstrip line.
   1. Create a rectangle.
      • Definition method: Base corner, width, depth
      • Base corner (C): (0, -w/2, substrate_height)
      • Width (W): strip_length
      • Depth (D): w
5. Create the feed section of the microstrip.
   1. Create a rectangle.
      • Definition method: Base corner, width, depth
      • Base corner (C): (0, 0, 0)
      • Width (W): w
      • Depth (D): substrate_height
      • On the Workplane tab set the Origin: (strip_length+strip_feed_arc_radius-w/2, strip_feed_arc_radius, 0)
   2. Rotate the workplane of the rectangle 90° around the U axis to align the rectangle in the XZ plane.
      Tip: Right-click on the Origin box of the workplane and click Rotate workplane.
6. Create the arc section of the microstrip.
   1. Create an elliptic arc.
      • Centre point (C): (strip_length, strip_feed_arc_radius, substrate_height)
      • Radius (U): strip_feed_arc_radius + w/2
      • Radius (V): strip_feed_arc_radius + w/2
      • Start angle: -90
      • Stop angle: 0
      • Label: outer_circle
7. Create a line.
   • From: (0, -strip_feed_arc_radius-w/2, 0)
   • To: (0, -strip_feed_arc_radius+w/2, 0)
   • On the Workplane tab set the Origin: (strip_length, strip_feed_arc_radius, substrate_height
8. Pathsweep Line1 on outer_circle.
9. Union all parts.
   The resulting geometry represents half of the top microstrip section.

   
   

   

   
   Figure 1. Geometry after Union operation.

10. Copy and rotate Union1 by 180° around the U axis.
    Note: The new part represents half of the bottom microstrip.
11. Create the ground plate.
    1. Create a rectangle.
       • Base Corner (C): (0, -substrate_depth/2, 0)
       • Width (W): substrate_width/2
       • Depth (D): substrate_depth
       • Label: ground_plate
12. Create an aperture.
    1. Create a rectangle.
       • Base corner (C): (d2/2, -s/2, 0)
       • Width (W):s
       • Depth (D):s
       • Label: aperture_1
13. Create a second aperture.
    1. Create a rectangle.
       • Base corner (C): (d2/2+s+d1, -s/2, 0)
       • Width (W):s
       • Depth (D):s
       • Label: aperture_2
14. Subtract aperture_1 and aperture_2 from ground_plate.
    The resulting geometry is a ground plane between two microstrip lines with two square holes.
15. Copy and mirror all geometry around the VN plane.

    
    

    

    
    Figure 2. Geometry after the copy and mirror operation.

16. Union all the parts.
17. Set all faces to perfect electric conductor (PEC).
    Tip: Faces set to PEC remain PEC when becoming faces of a dielectric region.
18. Add edge ports.

    Port1

    Define an edge port between the bottom microstrip feed on the negative X side and the ground plate.

    Port2

    Define an edge port between the bottom microstrip feed on the positive X side and the ground plate.

    Port3

    Define an edge port between the top microstrip feed on the negative X side and the ground plate.

    Port4

    Define an edge port between the top microstrip feed on the positive X side and the ground plate.

19. Create two substrate layers.
    1. Create a cuboid to construct the top layer.
       • Definition method: Base centre, width, depth, height
       • Base centre (C): (0, 0, 0)
       • Width (W): substrate_width
       • Depth (D): substrate_depth
       • Height (H): substrate_height
       • Label: top_layer
    2. Create a second cuboid to construct the bottom layer.
       • Definition method: Base centre, width, depth, height
       • Base centre (C): (0, 0, -substrate_height)
       • Width (W): substrate_width
       • Depth (D): substrate_depth
       • Height (H): substrate_height
       • Label: bottom_layer
    3. Union top_layer and bottom_layer.
    4. Set both regions for this Union to the dielectric, substrate.
20. Union all the parts in the model.
21. Activate the FDTD solver.
    Tip: Open the Solver settings dialog and click the FDTD tab. Select the Activate the finite difference time domain (FDTD) solver check box.
22. Set the frequency.
    • Linearly spaced discrete points
    • Start frequency (Hz): f_min
    • End frequency (Hz): f_max
    • Number of frequencies: 101