CST – Computer Simulation Technology

A Small Efficient Linear Polarized Omnidirectional 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)



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.

 

Figure 2: 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: Total simulated Q equals 116 for PEC versus 101 for copper; See the attached file for the computations


Figure 4: 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.

References

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.

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