# Constructing the Spiral Antenna

Create the complementary slotted two-arm spiral antenna in CADFEKO.

1. Set the model unit to centimetres.
2. Create the following variables:
• a = 1.1459 (The spiral growth ratio.)
• r1 = 1 (The base radius of the spiral.)
• r2 = 45/2 (The end radius of the spiral.)
• w = 1.8 (The width of the slot.)
• width = 70 (The width and depth of the conducting plate.)
• n1 = r1/(2 * pi * a)
• n2 = r2/(2 * pi * a)
• n = n2 - n1 (The number of turns.)
• fmin = 150e6 (The minimum frequency.)
• fmax = 900e6 (The maximum frequency.)
• lambda0 = 100 * c0/fmax (A scale factor of 100 to compensate for working in centimetres.)
• meshing = min(lambda0/12, 1.3 * w) (The triangle length to be minimum of $\frac{\lambda }{12}$ or .)
3. Create a helix.
• Definition method: Base centre, base radius, end radius, height, turns
• Origin of the helix (C): (0, 0, 0)
• Base radius (Rb): r1
• End radius (Rt): r2
• Height: 0
• Turns (N): n
• Label: Helix1
4. To add width to the spiral, create a copy (duplicate) of Helix1.
1. Rename the copy to Helix2.
2. Modify the start radius and end radius:
• Base radius (Rb): r1 + w
• End radius (Rt): r2 + w
5. Connect the two helices to create a surface (loft).
1. Select Helix1 and Helix2 and loft the two curves to create a surface.
Note: Do not select the Reverse orientation check box.
6. Create half of the feed.
1. Create a polygon.
• Corner 1: (r1 + w/2, -w/2, 0)
• Corner 2: (0, -w/2, 0)
• Corner 3: (0, w/2, 0)
• Corner 4: (r1 + w/2, w/2, 0)
• Corner5: (r1 + w, 0, 0)
• Label: Polygon1
7. To ensure mesh connectivity between the feed and helix, union the lofted surface (Loft1) and the feed (Polygon1). The default label for the new union is Union1.
8. Simplify Union1 to remove non-essential edges and faces.
9. Create the second arm of the spiral antenna.
1. Copy and rotate Union1 by 180° around the N axis.
10. Union all parts of the model. Rename the union to Union2.
11. Create the conductive plate which will contain the complementary slotted spiral antenna.
1. Create a rectangle.
• Definition method: Base centre, width, depth
• Base centre (C): (0, 0, 0)
• Width (W): width
• Depth (D): width
• Label:: Rectangle1
12. Create the spiral slots in the conductive plate.
1. Subtract Union2 from Rectangle1.
13. Create the feed wire.
1. Create a line.
• Start point: (0, -w/2, 0)
• End point: (0, w/2, 0)
14. Add a wire port to the feed wire.
A complementary slotted two-arm spiral antenna with feed wire and wire port were constructed. Add requests, mesh the model and view the results in POSTFEKO.