The antenna design and dimensions are taken from Example 6-2 on page 239 of “Antenna Theory and Design” by Stutzman and Thiele . The center frequency is 8 GHz giving a helix circumference of 3.45 cm. The helix is constructed with a right hand (clockwise) twist to produce a right hand circularly polarized far-field pattern. The spacing between turns is 0.796 cm. The theoretical half-power beam width of 44 degrees and 39 degrees is provided in Figure 6-14a and 6-14c . These two beam widths represent a value taken from measurements and computed from simple theory.
The antenna is constructed in XFdtd using the helix script. The helix will expand along the Z-axis and will have a radius of 0.549 cm. In addition to the helix, a small 2D ground plane is centered on the base of the helix with dimensions of 4 cm x 4 cm. The resulting CAD model geometry is shown in Figure 1. It was meshed with a cubical cell size of 0.3 mm.
The feed is applied to the cell edge that connects the helix to the base plate. A sinusoidal signal at 8 GHz is used as the waveform, and automatic convergence criteria is set to -30 dB. Following the simulation, the far zone patterns are computed in a 2D cutplane and as a 3D pattern. The right hand circularly polarized pattern is shown in Figure 2. Since the antenna is right polarized, the left hand polarization pattern, shown in Figure 3, is much lower.
The gain of the main beam is plotted in a two-dimensional line plot to determine the half-power beam width. The plot in Figure 4 indicates a half power beam width of approximately 42.5 degrees which falls between the two values provided by Stutzman and Thiele. Figure 5 is the full 2D slice at phi = 0 degrees.
- Stutzman and Thiele. Antenna Theory and Design. 2nd ed. New York: John Wiley and Sons, 1998.