Aaron Kerkhoff

Design of a Band-Notched Planar Monopole Antenna Using Genetic Algorithm Optimization

Genetic algorithm (GA) optimization is applied to the design of planar monopole antennas, which exhibit both ultrawideband (UWB) operation and a narrow-band frequency notch. Such an antenna is useful in applications involving wideband communications where it is desired to mitigate interference with other radio systems colocated with the operating band. It is demonstrated in this paper that traditional band-notched planar monopole antennas exhibit asymmetry in radiation patterns within the notch band such that the attenuation provided by the antenna varies as a function of azimuth angle, which lowers the effective bandwidth of the notch. A GA optimizer, which uses of a weighted sum cost function related to impedance matching and radiation patterns at frequencies within both the wide operating band and narrow notch band, is used to improve the performance of the band-notch planar monopole antenna. A two-dimensional (2-D) matrix chromosome is used in the GA to represent a wide-range on planar element shapes. It is shown that the GA generates antenna designs that exhibit equal wideband performance as traditional band-notched designs, but have improved azimuth plane radiation pattern symmetry, which widens the effective notch bandwidth. The GA-generated antenna design is measured and compared with simulation

Experimental verification of three-dimensional plasmonic cloaking in free-space

We report the experimental verification of metamaterial cloaking for a 3D object in free space. We apply the plasmonic cloaking technique, based on scattering cancellation, to suppress microwave scattering from a finite-length dielectric cylinder. We verify that scattering suppression is obtained all around the object in the near- and far-field and for different incidence angles, validating our measurements with analytical results and full-wave simulations. Our near- field and far-field measurements confirm that realistic and robust plasmonic metamaterial cloaks may be realized for elongated 3D objects with moderate transverse cross-section at microwave frequencies.

Design of a planar monopole antenna for use with ultra-wideband (UWB) having a band-notched characteristic

The design of planar monopole antennas for use with UWB and having band-notched characteristics is demonstrated. A GA code is used to develop a reduced size planar monopole which exhibits good impedance matching and stable radiation towards the horizon across the entire UWB band. Part of the metal is then removed from this design to create a band-notched characteristic. While this feature offers significant attenuation in the notched band, it has little effect on the antenna operation outside of the band. An analysis demonstrates the resemblance of this feature to a quarter-wave, open-ended transmission line, and that the notched band can be varied by changing the size of the feature.

Demonstration of an ultralow profile cloak for scattering suppression of a finite-length rod in free space

We present the first experimental realization and verification of a three-dimensional stand-alone mantle cloak designed to suppress the total scattering of a finite-length dielectric rod of moderate cross-section. Mantle cloaking has been proposed to realize ultralow-profile conformal covers that may achieve substantial camouflage, transparency and high-performance non-invasive near-field sensing. Here, we realize and verify a mantle cloak for radio-waves. We report an extensive campaign of far- and near-field free-space measurements demonstrating that conformal cloaks can indeed produce strong scattering suppression in all directions and over a relatively broad bandwidth of operation.

Design and analysis of planar monopole antennas using a genetic algorithm approach

The application of genetic algorithm (GA) optimization to the design and analysis of planar monopole antennas is presented. GA is first used to optimize the impedance matching bandwidth of two particular planar element shapes, the bow-tie (BT) and reverse bow-tie (RBT). The results of this study indicate that the RBT can achieve a significantly wider bandwidth with a much smaller size than the traditional BT. In a follow-on study, GA is used to generate arbitrarily shaped planar monopole designs, which exhibit improved broadband performance and/or reduced size compared with the RBT. The designs generated by the GA demonstrate a better tradeoff between matching bandwidth and electrical size compared with planar monopole designs previously characterized in the literature. Analysis of results from simulation and measurement are presented, which provide insight into the operation of these antennas as well as the key parameters that lead to improved performance. Finally, a performance bound is generated to relate the bandwidth limitation of planar monopoles to size.

Plasmonic cloaking of cylinders: finite length, oblique illumination and cross-polarization coupling

Metamaterial cloaking has been proposed and studied in recent years by following several interesting approaches. One of them, the scattering-cancelation technique or plasmonic cloaking, exploits the plasmonic effects of suitably designed thin homogeneous metamaterial covers to drastically suppress the scattering of moderately sized objects within specific frequency ranges of interest. In addition to its inherent simplicity, this technique also holds the promise of an isotropic response and weak polarization dependence. Its theory has been applied extensively to symmetrical geometries and canonical three-dimensional (3D) shapes, but the application of it to elongated objects has not been explored with the same level of detail. We derive here closed-form theoretical formulae for infinitely long cylinders under arbitrary wave incidence, and validate their performance with full-wave numerical simulations, also considering the effects of finite lengths and truncation effects in cylindrical objects. In particular, we find that a single isotropic (idealized) cloaking layer may successfully suppress the dominant scattering coefficients of moderately thin elongated objects, even for finite lengths comparable with the incident wavelength, providing weak dependence on the incidence angle. These results may pave the way for application of plasmonic cloaking in a variety of practical scenarios of interest.

Scattering suppression and wideband tunability of a flexible mantle cloak for finite-length conducting rods

A simple, thin, flexible mantle cloak for conducting rods based on scattering cancellation is analyzed, designed and experimentally realized. We show strong scattering suppression at all angles of incidence, for both far-field plane-wave and near-field Gaussian excitations. The required effective shunt surface impedance is realized by a subwavelength patch array, targeting the suppression of the dominant omnidirectional scattering contribution of a conductive rod. Full-wave simulations predict a total radar cross-section reduction better than 14 dB in the lossless case and nearly 8 dB when considering a lossy substrate in the cover. Measurements of the realized cloak are consistent and validate these numerical predictions. The proposed geometry is also shown to be an ideal platform for monolithic integration of varactor diodes, allowing real-time tuning of the effective surface capacitance of the cloak. We show with numerical simulations the possibility of tunable scattering suppression over 1 GHz of bandwidth by seamlessly integrating varactor diodes in our mantle cloak design.

A Simplified Method for Reducing Mutual Coupling Effects in Low Frequency Radio Telescope Phased Arrays

This paper presents a new approach to reduce mutual coupling effects in large phased array antennas currently under development for use in low frequency radio astronomy. A receive mode definition of coupling between two antennas is shown to serve as a good indicator of the severity of mutual coupling interactions in a phased array. Thus, an efficient way to reduce mutual coupling in the array is to design antenna elements for low receive coupling. It is demonstrated that wire inverted-V dipole antennas can be designed with significantly reduced receive coupling over a wide range of frequencies and wave directions by proper selection of the antenna element geometry and feed impedance. Corresponding improvements in the performance of a large aperiodic phased array are achieved by using the reduced coupling wire inverted-V dipole design. Both antenna coupling and phased array analyses are performed using numerical simulation. Full-scale, in situ receive coupling measurements are performed to validate the simulation results.

Application of pareto genetic algorithm optimization to the design of broadband dipole elements for use in the Long Wavelength Array (LWA)

The application of Pareto GA multi-objective optimization to the design of planar dipole antennas for use in the LWA has been presented. The Pareto GA was used to optimize antenna performance in terms of sky noise frequency response and radiation pattern quality over the 20 MHz to 80 MHz band assuming the antenna is operated directly over an average lossy earth. It was shown that it is possible to design an antenna with radiation patterns that conform well to a sin(thetas) profile over the entire operating band with minimal pattern ripple at higher frequencies. However, high sky noise dominance cannot be achieved over the entire band with such a design. Antenna designs exist, which exhibit 6 dB dominance over the entire band, though this comes at the expense of reduced pattern beamwidth and higher pattern ripple. For the antenna geometry considered, improved design trade-offs can only be achieved by placing a metal reflector beneath the antenna; this is a topic for further investigation using Pareto GA optimization.

Experimental demonstration of a conformal mantle cloak for radio-waves

We present the first experimental realization and verification of a stand-alone 3D mantle cloak designed to suppress the total scattering of a finite-length dielectric rod. Mantle cloaking has been theoretically proposed as a convenient way to realize ultralow-profile, conformal covers to achieve complete camouflaging, transparency and non-invasive near-field sensing. After theoretically analyzing, designing and realizing a mantle cloak working for all polarizations and incidence angles, we have conducted an extensive campaign of near-field and far-field free-space measurements verifying strong suppression of scattering in all directions and over a relative broad bandwidth. The measured results agree well with analytical theory and full-wave simulations.