Terry H. O'Donnell

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.

An Electrically Small Multi-Frequency Genetic Antenna Immersed in a Dielectric Powder

Emerging military and commercial applications require more compact and light weight systems. Some of the potential applications for small antennas are for satellites, unmanned airborne vehicles, manpacks, radio frequency identification and mobile communications. Since the antenna is often the largest system component, electrically small antennas are desirable. It has been shown that resonant antennas, that will fit in a cubic volume as small as 1/20th of a wavelength on a side, can be designed using a genetic algorithm [1, 2]. Placing an electrically small antenna in a dielectric further reduces its operational frequency, where that frequency is defined as one for which the antenna has an acceptable performance. However, as the antenna becomes electrically smaller, its Quality Factor (Q) becomes larger and the bandwidth becomes smaller, thus limiting it to very narrow bandwidth systems. However, it has been found that an antenna immersed in a dielectric, also has a set of higher frequencies at which it is operational. Thus, although an antenna in a dielectric cannot operate over a wide continuous band of frequencies, it can be used for multi-frequency applications.

An electrically-small multi-frequency genetic antenna immersed in a dielectric powder

In this paper, we investigate the performance of an electrically small genetic antenna, immersed in several different powders having relative dielectric constants as large as 12. Placing the antenna in a dielectric reduces its electrical size from about 1/20th of a wavelength in free space to less than 1/50th of a wavelength, in a powder having a dielectric constant of 12, even for a dielectric volume just slightly larger than the antenna. Although the bandwidth decreases as the electrical size of the antenna becomes smaller, it is shown that for each dielectric powder, there is a set of frequencies for which the antenna is operational. It is thus possible to efficiently operate the antenna at specific frequencies within a wide band.

Behavior of a parasitic supergain two-element array in a dielectric 1

Electrically small driven and parasitic supergain two-element arrays have previously been presented [1]-[7], including the effects of achieving higher gain in parasitic arrays by varying the element spacing and frequency [1]-[3]. It has also been shown that shifting the parasitic arrays frequency, for a given element spacing, can create a current phase difference closer to that which produces the maximum supergain with the fully-driven supergain array at the same element spacing [2]. This technique thereby increases the parasitic arrays supergain, provided that the elements themselves have a sufficiently low Q to accommodate the frequency shift. It has also been shown, both computationally and experimentally, that immersion of electrically small antennas in dielectrics decreases their resonant frequency, making them “electrically smaller” [8,9].

The measurement of antenna gain with transmitting and receiving antennas on a finite ground plane

In this paper the edge effects from a finite ground plane on the gain of an antenna for a wide band of frequencies at UHF and above was examined. Hewlett-Packard 8510 network analyzer was used for the measurements. Simulation is done using Ansofts HFSS code for frequencies from 400 to 1200 MHz. In addition, the simulation is also done for an infinite ground plane so that the edge effects can be more easily understood.

Genetic design and optimization of military antennas

Genetic and evolutionary optimization techniques have been used in military antenna research and design at many levels, ranging from electrically-small antenna element design to broadband applications and array-pattern control. In this paper, we summarize in-house work in these areas, conducted at the Antenna Technology Branch of the Air Force Research Laboratory Sensors Directorate. In particular, we highlight areas where differences in modeling and simulation techniques have proven crucial in avoiding premature convergence and obtaining a valid optimal solution.

Initial Data Sampling in Design Optimization

Evolutionary computation (EC) techniques in design optimization such as genetic algorithms (GA) or efficient global optimization (EGO) require an initial set of data samples (design points) to start the algorithm. They are obtained by evaluating the cost function at selected sites in the input space. A two-dimensional input space can be sampled using a Latin square, a statistical sampling technique which samples a square grid such that there is a single sample in any given row and column. The Latin hypercube is a generalization to any number of dimensions. However, a standard random Latin hypercube can result in initial data sets which may be highly correlated and may not have good space-filling properties. There are techniques which address these issues. We describe and use one technique in this paper.

Genetic optimization of a hybrid DISS Tx antenna

Genetic algorithms have been shown to be effective in the design and optimization of many types of antenna. In this paper, we show how genetic algorithm antenna research at the Air Force Research Laboratory (AFRL) has been applied to a real-world problem - the re-design of the DISS transmit antenna. We present a brief background of the problem, describe our optimization goals and constraints, and compare the existing antenna performance to an LPDA-augmented hybrid antenna and a genetically-optimized hybrid antenna.