H.M. Aumann

Phased array antenna calibration and pattern prediction using mutual coupling measurements

A technique which utilized the inherent mutual coupling in an array to both calibrate and predict the radiation patterns of a phased-array antenna is investigated. The only restriction of the technique is that the ability to transmit and receive with pairs of the array elements is required. The theory associated with array mutual coupling and its relationship to both array calibration and array patterns is discussed. The design of a test bed phased-array antenna is covered. The mutual coupling technique (MCT) is used experimentally to calibrate the test array as well as to predict the array radiation patterns. It is shown that the results obtained by MCT are in good agreement with conventional far-field measurements. >

An experimental adaptive nulling receiver utilizing the sample matrix inversion algorithm with channel equalization

The authors explore two ideas for effectively achieving channel equalization within the framework of a narrowband sidelobe canceler. The first approach relies primarily on digital equalization to enhance conventional sample matrix inversion (SMI) performance, whereas the second utilizes feedback to extend the SMI method into the realm of analog adaptive cancellation. An experimental four-channel receiver that supports open-loop and closed-loop operation is described. As implemented, three different canceler modes are possible: feedforward, feedback, and a tandem feedback/feedforward combination. All three modes have been successfully demonstrated in bench experiments with a broadband noise source using the SMI algorithm. Cancellation ratios in excess of 50 dB have been achieved in a four-channel testbed system. >

Phased array calibrations using measured element patterns

A technique to compensate for differences in phased array element patterns is presented. Each measured element pattern is approximated by a virtual array whose excitation function is determined by the Woodward-Lawson synthesis technique. By extending the virtual array beyond the physical array dimensions, mutual coupling and edge diffraction effects can be separated. An example is given where calibration by coupling matrix inversion resulted in significantly reduced array pattern sidelobes.

Focused near-field adaptive nulling: experimental investigation

Focused near-field adaptive nulling has been demonstrated using an experimental linear phased array and a four-channel receiver. The measured adaptive cancellation of one near field interference source was in good agreement with theoretical predictions. The theory assumes that a dipole array without ground plane is the approximate dual of a rectangular waveguide array with ground plane. The results indicate that adaptive antenna performance testing using focused near-field adaptive nulling at approximately one aperture diameter range distance is a viable alternative to conventional far-field testing.<<ETX>>

Multipath height finding in the presence of interference

A new method is proposed for multipath target height finding with a non-coherent, narrowband radar in the presence of interference. The technique is based on an ultra-wideband fusion technique for sparse-band frequency processing originally developed for wideband imaging. It is illustrated with data collected during the mountaintop program.

Linear array characteristics with one-dimensional reactive-region near-field scanning: simulations and measurements

A model for simulating the performance of a phased-array antenna with one-dimensional near-field scanning is addressed. Measurements and simulations in the reactive near-field region for a linear array with monopole elements are described. The array and near-field probe characteristics are simulated by using the method of moments. Measurements and simulations of the array near-zone received voltage, plane-wave spectrum, and far-field data, using centerline scanning at less than one wavelength distance from the antenna, are shown to be in good agreement. The presence of evanescent grating lobes in the plane-wave spectrum is observed. It is shown that a V-dipole theoretical probe antenna can accurately model a practical near-field measurement probe. >

Receiver channel equalization for adaptive antennas: experimental results

The authors describe two complementary approaches for attacking the channel equalization problem within the framework of a primarily digital sidelobe canceling system. The first relies heavily on digital filtering techniques, while the second extends the SMI (sample matrix inversion) method into the realm of analog adaptive cancellation. The latter approach eases some of the digital signal processing requirements imposed by the former, although not without increasing the complexity of the receiver. In demanding applications, further improvements may be realizable by combining the two basic approaches in a tandem architecture. The experimental system was designed and built with the capability to test all three methods. Its modes of operation are depicted in simplified block diagram form and are explained. Empirical results from bench tests conducted with the experimental receiver demonstrate that the proposed methods can substantially improve cancellation performance.<<ETX>>

Polarization ratio improvement in a spiral element array

A technique is presented for generating high quality circularly polarized radiation from an array of spiral elements by applying a mechanical rotation and a compensating electric phase shift to each element so as to steer the cross-polarized pattern into the grating lobe region. The technique is illustrated by simulations and measurements on a linear four-spiral array.

Correction of near-field effects in phased array element pattern measurements

Differences in phased array element patterns measured in the near-field and far-field are shown to be due to attenuation and distortion of mutual coupling and edge diffraction effects in the near-field. A technique based on the virtual array concept is presented to correct the near-field element patterns. The technique is also used to determine a lower bound on phased array sidelobe performance achievable with adaptive calibrations.

Determination of element locations in a circular array from measurements in the Fresnel region

The electrical locations of the elements in a large circular array are determined from phase measurements in the Fresnel region. A closed-form solution for the element phase center locations is presented which contains the required range correction. The technique is applied to a 108-element, L-band circular array of log-periodic dipoles. It is shown that coupling between the elements and the array structure almost completely obscures frequency dependent effects