Steven R. Best

Low Q electrically small linear and elliptical polarized spherical dipole antennas

Electrically small antennas are generally presumed to exhibit high impedance mismatch (high VSWR), low efficiency, high quality factor (Q); and, therefore, narrow operating bandwidth. For an electric or magnetic dipole antenna, there is a fundamental lower bound for the quality factor that is determined as a function of the antennas occupied physical volume. In this paper, the quality factor of a resonant, electrically small electric dipole is minimized by allowing the antenna geometry to utilize the occupied spherical volume to the greatest extent possible. A self-resonant, electrically small electric dipole antenna is presented that exhibits an impedance near 50 Ohms, an efficiency in excess of 95% and a quality factor that is within 1.5 times the fundamental lower bound at a value of ka less than 0.27. Through an arrangement of the antennas wire geometry, the electrically small dipoles polarization is converted from linear to elliptical (with an axial ratio of 3 dB), resulting in a further reduction in the quality factor. The elliptically polarized, electrically small antenna exhibits an impedance near 50 Ohms, an efficiency in excess of 95% and it has an omnidirectional, figure-eight radiation pattern.

A study of the quadrifilar helix antenna for Global Positioning System (GPS) applications

An analytic model for computing the radiation properties of the quadrifilar helix volute antenna is discussed and various design considerations for GPS applications are presented. The effects of modifying the antenna length and diameter on the antenna amplitude and phase performance are presented, and using the antenna for dual-frequency operation is discussed. The effects of phase imbalances are presented and compared with measured pattern anomalies. >

On the performance properties of the Koch fractal and other bent wire monopoles

Koch fractal monopole antennas are known to exhibit lower resonant frequencies than Euclidean monopoles of the same height. It has been concluded that there exists a unique relationship between the antennas fractal geometry and its electromagnetic behavior. Here, the performance properties of the Koch fractal monopole are examined and compared with the performance properties of other bent wire geometry monopoles having the same total wire length and overall height. It is demonstrated that monopoles with less complex shapes exhibit lower resonant frequencies because they are more effective at increasing the electrical volume of the antenna. When these antennas are made to be resonant at the same frequency, they exhibit virtually identical performance properties independent of differences in their geometric shape and total wire length. It is also demonstrated that the effective height of these monopoles converge to that of an electrically small Euclidean monopole near the small antenna limit and they exhibit virtually identical radiation resistance properties at low frequencies. Finally, it is shown that the fractal limit in lowering of resonant frequency is related to the limit in the increase in the antennas effective volume.

Electrically small supergain end‐fire arrays

The theory, computer simulations, and experimental measurements are presented for electrically small two-element supergain arrays with near optimal endfire gains of 7 dB. We show how the difficulties of narrow tolerances, large mismatches, low radiation efficiencies, and reduced scattering of electrically small parasitic elements are overcome by using electrically small resonant antennas as the elements in both separately driven and singly driven (parasitic) two-element electrically small supergain endfire arrays. Although rapidly increasing narrow tolerances prevent the practical realization of the maximum theoretically possible endfire gain of electrically small arrays with many elements, the theory and preliminary numerical simulations indicate that near maximum supergains are also achievable in practice for electrically small arrays with three (and possibly more) resonant elements if the decreasing bandwidth with increasing number of elements can be tolerated.

A monopole superdirective array

In principle, the end-fire directivity of a linear periodic array of N isotropic radiators can approach N/sup 2/ as the spacing between elements decreases, provided the magnitude and phase of the input excitations are properly chosen. Thus, the directivity of a two-element array of isotropic radiators would approach a value of four, that is, 6 dB higher than that of a single isotropic radiator. We have conducted a theoretical, computational, and experimental study for a two-element superdirective array of resonant monopoles. In agreement with the theoretical and computational curves, the measured gain of the monopole array does indeed continually increase with decreasing spacing of the monopoles, provided the relative magnitudes and phases are maintained. However, for very small separation, maximum achievable gain is not reached due to the presence of ohmic loss.

The effectiveness of space-filling fractal geometry in lowering resonant frequency

The space-filling fractal geometry of the Hilbert curve is examined in terms of its effectiveness in lowering resonant frequency. It is demonstrated that the complex geometry associated with the space-filling Hilbert fractal curve is inherently ineffective in lowering resonant frequency compared to other less complex geometries of the same size and total wire length. The effectiveness of the geometry in lowering resonant frequency is a direct function of the current vector alignment established by the wire layout.

On the resonant properties of the Koch fractal and other wire monopole antennas

The Koch fractal monopole antenna has been shown to exhibit a lower resonant frequency than a simple Euclidean monopole having the same overall height. The performance properties of the Koch fractal monopole have primarily been attributed to its fractal geometry. Here, the performance properties of the Koch fractal monopole, normal mode helix, and two meander line configurations are considered. While the resonant frequency of these antennas is a function of both the antennas geometry and total wire length, it is demonstrated that when these antennas are made to be resonant at the same frequency, they essentially exhibit the same performance characteristics independent of the differences in their geometry and total wire length. It is demonstrated that the electromagnetic behavior of the Koch fractal monopole is not uniquely defined by its geometry alone.

Design of a broadband dipole in close proximity to an EBG ground plane

One of the known advantages of an electromagnetic bandgap (EBG) ground plane is the fact that a straight-wire dipole can be located parallel and in very close proximity to the ground plane. For such a structure and geometry, a reasonable impedance match to 50 n) can be achieved with more bandwidth than when the dipole is located in close proximity to a PEC ground plane. However, in addition to impedance, radiation-pattern properties must be considered when evaluating the antennas overall performance and usable bandwidth. Here, we consider the performance properties of a folded bowtie dipole element in close proximity to an EBG ground plane, with emphasis on examining the bandwidth and pattern characteristics. We demonstrate that a dipole element can be designed to exhibit a matched input impedance over approximately a 1.4:1 bandwidth with respect to a 50 n characteristic impedance, when located in close proximity to the EBG ground plane. While the shape of the radiation pattern of the antenna remains relatively unchanged over much of this operating bandwidth, we show that the usable bandwidth of the antenna-EBG combination is limited to approximately 1.5:1, due to pattern degradation at both the lower and upper frequencies. Details of the design approach and feed structure used to match the impedance of the dipole to 50 fl are discussed. Finally, we show that with this element design, the reflection phase bandwidth of the EBG ground plan-e does not limit the matched impedance bandwidth of the antenna. A -portion of ,t ,he work detailed here was presented at the 2005 Antenna Applications Symposium.

The performance properties of electrically small resonant multiple-arm folded wire antennas

In this tutorial article, the resonant radiation properties of electrically small, multiple-arm, folded wire antennas are considered and compared as a function of the antennas height, the cylindrical diameter occupied, the geometry, and the number of folded arms within the antenna structure. The radiation properties considered include resonant resistance, efficiency, radiation patterns, and the operating bandwidth, which is characterized using the antennas quality factor (Q). It is shown that electrically small, multiple-arm, folded wire antenna designs offer significant performance improvements relative to simple open-ended wire antennas, in terms of increased resonant resistance, bandwidth, and efficiency. However, when multiple-arm, folded wire antennas of the same height and cylindrical diameter, but having significantly different geometries, are made to be self-resonant at the same frequency, they exhibit similar resonant performance properties. This illustrates that the resonant performance properties of these antennas are primarily established by their height and the physical volume occupied relative to the resonant wavelength. Various design parameters are considered and described for achieving self-resonance and a reasonable impedance match with an electrically small, multiple-arm folded wire antenna. Finally, the performance properties of the multiple-arm folded wire configurations are compared with those of an impedance-matched, non-folded, open-ended wire configuration. It is shown that the multiple-arm folded wire configurations exhibit a lower Q than the impedance-matched, non-folded wire configuration.

A discussion on the quality factor of impedance matched electrically small wire antennas

The quality factors of several impedance matched, electrically small wire antennas are compared as a function of matching technique and antenna geometry. The antennas considered have the same height and wire diameter, and they are designed to be self-resonant at approximately the same frequency. The antennas are impedance matched to a nominal 50 Ohm characteristic impedance using either a parallel stub, lossless reactive network or lossless transformer. It is shown that the quality factors of the antennas are essentially the same, and for the most part, independent of the matching technique and differences in the antenna geometry. The quality factor of the impedance matched, electrically small antenna is primarily established by the antennas height and effective volume.