A Small, Efficient, Linear-polarized Omni-directional Antenna

CST MICROWAVE STUDIO® (CST MWS) simulates this small dipole antenna in freespace finding the resonant frequency, drive-point impedance, radiation efficiency and total Q. Small antennas are defined by ka<0.5 which means the entire structure must fit inside a sphere with a radius less than 0.0796 wavelengths. In the case presented here, the entire antenna fits inside a sphere of radius 0.0415 wavelengths, ignoring wire diameter. (Model courtesy of Steven R. Best, Air Force Research Laboratory, Hanscom Air Force Base, USA)

The entire structure inside the reactive-energy sphere of 0.0564 wavelengths radius. Inside this sphere non-radiating energy predominates. The feed point is at Ymax at the convergence of 2 windings.

Figure 1: The entire structure inside the reactive-energy sphere of 0.0564 wavelengths radius. Inside this sphere non-radiating energy predominates. The feed point is at Ymax at the convergence of 2 windings.

Sub-gridding allows a reasonable mesh to capture complicated shapes with ease. E-plane symmetry (Electric-plane in the XY-plane) cut the solve time in half.

Figure 2: Sub-gridding allows a reasonable mesh to capture complicated shapes with ease. E-plane symmetry (Electric-plane in the XY-plane) cut the solve time in half.

Zero-reactance occurs at 295.5 [MHz] in simulation compared with the 299.6 [MHz] published value for about 0.01 error. Driving-point impedance simulates as 9 [Ohms] for PEC compared with 10 [Ohms] for copper.

Figure 3: Zero-reactance occurs at 295.5 [MHz] in simulation compared with the 299.6 [MHz] published value for about 0.01 error. Driving-point impedance simulates as 9 [Ohms] for PEC compared with 10 [Ohms] for copper.

Figure 4: Total simulated Q equals 116 for PEC versus 101 for copper. See the attached file for the computations.

Farfield pattern. Although only 1/6th as long in the Z-direction (E-plane normal), the small antenna’s directivity of 1.63 [dBi] compares favorably with the directivity of a full-scale halfwave dipole of 2.16 [dBi].

Figure 5: Farfield pattern. Although only 1/6th as long in the Z-direction (E-plane normal), the small antenna’s directivity of 1.63 [dBi] compares favorably with the directivity of a full-scale halfwave dipole of 2.16 [dBi].

Summary:

The Perfect Boundary Approximation(PBA)® allows even curved surfaces to be simulated using a Cartesian grid. Sub-gridding enhances this feature by minimizing the number of mesh-cells needed for a structure that is made up entirely of 3-dimensional curves mirrored about the E-plane.

Literature:

S. R. Best, "Low Q Electrically Small Linear and Elliptical Polarized Spherical Dipole Antennas," IEEE Transactions on Antennas and Propagation, Vol. 53, No. 3, pp. 1047-1053, March 2005.

S. R. Best, "The Radiation Properties of Electrically Small Folded Spherical Helix Antennas," IEEE Transactions on Antennas and Propagation, Vol. 52, No. 4, pp. 953-960, April 2004.

CST Article "A Small, Efficient, Linear-polarized Omni-directional Antenna"
last modified 5. May 2006 3:53
printed 4. Jul 2008 3:35, Article ID 105
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Article ID: 105 Last modified: 5. May 2006 3:53

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