Hans Steyskal

Simple method for pattern nulling by phase perturbation

A method of sidelobe hulling, which involves perturbing the array illumination phase only, is presented. The general nonlinear problem is linearized by assuming the perturbations to be small, and an analytic solution is derived. Illustrative examples of sinc and Chebyshev patterns with imposed nulls are given.

Broad-band fragmented aperture phased array element design using genetic algorithms

In this paper, a synthesis procedure to design thin broad-band fragmented aperture array elements is described. The arrays are assumed to be infinite periodic and the elements consist of a conducting pattern etched on a dielectric backed by a groundplane. A genetic algorithm (GA) is used to design the conducting pattern, relative permittivity, and thickness of the dielectric substrate with respect to array scan and bandwidth performance. The fitness function in the GA is evaluated using a finite-difference time-domain code with periodic boundary conditions. For a substrate thicker than about 0.1 /spl lambda//sub L/ (/spl lambda//sub L/= wavelength at the lowest frequency in the frequency band investigated), it was found that a bandwidth of at least one octave can be obtained for arrays scanned within 45/spl deg/ from broadside.

Design considerations and results for an overlapped subarray radar antenna

Overlapped subarray networks produce flat-topped sector patterns with low sidelobes that suppress grating lobes outside of the main beam of the subarray pattern. They are typically used in limited scan applications, where it is desired to minimize the number of controls required to steer the beam. However, the architecture of an overlapped subarray antenna includes many signal crossovers and a wide variation in splitting/combining ratios, which make it difficult to maintain required error tolerances. This paper presents the design considerations and results for an overlapped subarray radar antenna, including a custom subarray weighting function and the corresponding circuit design and fabrication. Measured pattern results will be shown for a prototype design compared with desired patterns.

Pattern synthesis for TechSat21 - a distributed space-based radar system

The TechSat21 space-based radar employs a cluster of free-floating satellites, each of which transmits its own orthogonal signal and receives all reflected signals. The satellites operate coherently at the X band. The cluster forms essentially a multielement interferometer, with a concomitantly large number of grating lobes and significant ground clutter. A novel technique for pattern synthesis in angle-frequency space is proposed, which exploits the double periodicities of the grating lobes in the angular domain and of the radar pulses in the frequency domain, and allows substantial gains in clutter suppression. Gains from 7 to 17 dB relative to the normal random, sparse array appear feasible.

Pattern synthesis for a conformal wing array

Future aircraft may utilize the large aerodynamic areas of the wings also for electrodynamics by structurally embedding conformal phased array antennas. We explore this concept with a computer model for a line array wrapped around a wing. The model uses a realistic wing profile and array element patterns which include the effects of mutual coupling and the local radius of curvature. The study has two objectives: 1) develop a pattern synthesis method which is effective for this non-conventional array shape, and 2) determine whether low sidelobe patterns can be realized. We find that pattern synthesis based on alternating projections is a flexible and highly efficient synthesis method. No convergence problems due to local minima occurred. High quality patterns with uniform low sidelobes were achieved for most beam directions, except in a narrow sector about the difficult forward direction, where there appears to be a sidelobe floor of roughly -23 dB.

On the Power Absorbed and Scattered by an Antenna

We discuss the powers absorbed and scattered by a receiving antenna. Most antennas scatter more than they absorb. Antennas that scatter less power than they absorb are rare, and the few examples that have been presented are all based on numerical optimization. This note presents a simple conceptual model - a partially transparent aperture antenna - which provides physical insight into the case where the scattering is less than the absorption. Finally, we comment on the far field in the forward-scattering direction, noting that the total field actually is increased, although the scatterer blocks the incident field.

Spiral elements for broad-band phased arrays

In this paper, we present a numerical analysis of an infinite periodic array of planar spiral elements with octave bandwidth. For off-broadside scan the array is found to exhibit very narrow resonances, which are independent of scan angle. They occur when the spiral arms are multiples of half a wavelength, in which case the current forms a high amplitude standing wave along the spiral arms. The resonances are conveniently suppressed by making the arms unequally long. We also discuss the equivalent 3-port for this nonsymmetrical array element and evaluate the element polarization performance.

Pattern synthesis for TechSat21-a distributed spacebased radar system

The TechSat21 space-based radar employs a cluster of free-floating satellites, each of which transmits its own orthogonal signal and receives all reflected signals. The satellites operate coherently at X-band. The cluster forms essentially a multi-element interferometer with a concomitant large number of grating lobes and significant ground clutter. A novel technique for pattern synthesis in angle-frequency space is proposed, which exploits the double periodicities of the grating lobes in the angular domain and of the radar pulses in the frequency domain, and allows substantial gains in clutter suppression. Gains from 7 to 17 dB relative to the normal random, sparse array appear feasible.

Digital Beamforming Aspects of Wideband Circular Arrays

Circular array antennas are attractive in many applications since they offer uniform beam coverage over 360 degrees in azimuth, and moreover, they can generate radiation patterns with a main beam and sidelobes that are essentially independent of frequency. In this paper we consider a wide band circular array beamforming concept based on phase modes generated by a spatial Fourier transform of the array excitation. In particular, we evaluate the computational load associated with the digital implementation of this beamforming scheme, and compare it to the load of conventional element level beamforming. Based on preliminary numerical analyses, conventional beamforming is the more economical alternative from a computational point of view.

Pattern synthesis for moving target detection with TechSat21-a distributed space-based radar system

We have explored a novel approach for pattern synthesis in angle-frequency space, which is applicable for highly thinned arrays. It exploits the double periodicities of the grating lobes in the angular domain and the radar pulses in the frequency domain and allows substantial clutter suppression. Gains from 7 to 17 dB relative to a randomly, thinned array appear feasible. Further improvements in signal/clutter ratio can be achieved by joint array and filter weight optimization and by narrow-band Doppler filtering.