F.G. Willwerth

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. >

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>>

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>>

Results of X-band electronically scanned array using an overlapped subarray architecture

The measured results from an X-band electronically scanned array using an overlapped subarray architecture are presented. The 2D architecture uses a 12×12 element subarray with 3 to 1 overlapping. The active electronic scanned array is a receive only implementation consisting of switch, low noise amplifier, phase shifter and attenuator. Measured far-field patterns and excitation at the aperture using near-field scanner demonstrates desired design goals of a 20 degree sector beam with low sidelobes. Finally, the scan performance of the sector subarray beam is measured at 20 and 40 degrees. A three tile implementation is constructed and measured.

An interpolation technique for time-domain phased array measurements

A technique is presented for determining the insertion phase of phased array elements from time-domain spectra. It is shown that the inverse discrete Fourier transform (IDFT), commonly used in obtaining a time-domain spectrum, yields unreliable phase results. A phase compensation to the IDFT is proposed which allows the phase of an array element to be accurately determined without explicit time-gating and without taking a second DFT. The technique is applied to determine the element insertion phases of an L-band array.

3D printed conformal array antenna: Simulations and measurements

A conformal array antenna has been investigated using a combination of 3D printer and copper plating techniques. Circular patch antenna elements were copper plated onto a 3D printed dielectric substrate made of ABS-M30 material. Measured and simulated element reflection coefficient, element gain patterns, and array scanned beam radiation patterns at L band are in good agreement.

Maximum directivity of a series-fed microstrip array antenna for wireless application

An omnidirectional colinear microstrip array antenna is modeled as a ladder network. A formula is derived for the maximum directivity of a series-fed colinear antenna with an infinite number of ladder sections. It is shown that the maximum directivity is asymptotically limited and depends upon the product of the series impedance and shunt admittance of the equivalent ladder network. The result is verified by simulation and experiment.

Polarization ratio determination with two identical linearly polarized antennas

This paper describes a method for determining the complex polarization ratio using two identical, linearly polarized antennas. By Fourier transform analysis of S21 measurements with one of the antennas rotating about its axis a circular polarization ratio is derived which can be transformed into an equivalent linear polarization ratio. A linearly polarized reference antenna is not required. The technique was verified by electromagnetic simulations and illustrated by measurements in an anechoic chamber with two 3.3 GHz square patch antennas.

Low-sidelobe phased array characteristics using planar near-field scanning

A low-sidelobe phased-array antenna was investigated both theoretically and experimentally using the planar near-field scanning technique at a distance of less than one wavelength from the antenna aperture. The theory was applied to a monopole phased array, and it was shown that a theoretical V-dipole probe can accurately model the observed experimental array performance using a waveguide probe. The presence of nonpropagating grating lobes in the plane-wave spectrum has been observed using theory and measurements. Results indicate that the near-field sample spacing must be chosen such that aliases of the grating lobes do not appear in the visible space region. The theory presented can also be applied in the radiating near-field region and to other array thin-wire elements such as dipoles.<<ETX>>