Randall L. Musselman

Direct infrared measurements of phased array near-field and far-field antenna patterns

A thermal imaging technique has been developed to measure electromagnetic (EM) fields. This technique is applied in this paper to measure the EM fields radiated by large phased array radar antennas and to determine the near-field distributions and the far-field antenna pattern. This thermal technique is based on infrared (IR) measurements of the heating patterns produced in a thin, lossy detector screen made from a carbon loaded polyimide film placed in the plane over which the field is to be measured. The temperature rise in the screen material is related to the intensity of the field incident on the screen. An experimental calibration table was developed at NIST/Boulder to convert the temperature rise at any point on the screen into an equivalent incident radiated field strength. This thermal imaging technique has the advantages of accuracy, simplicity, speed, and portability over existing hard-wired probe methods and produces a 2D picture of the near field or the far field. These IR measurements, therefore, can be performed on-site at the remote location of the antenna in-the-field to produce an image of the radiating field of the array, which can be used to determine the overall radiation characteristics of the array, i.e., the radiation pattern of the combined array elements (gain and beam-width) and the condition of the electronic switching circuits (phase shifters and attenuators). Therefore, the overall “state of health” of the array and the need for repair can be determined in-the-field using the IR technique to avoid the expensive and time consuming alternative of dismantling the array and shipping it to a maintenance depot for testing, calibration, and repair on a standard, planar, near-field antenna test range. In this paper, the IR technique is tested in a controlled environment to determine the feasibility of using the IR images as an array diagnostic tool i) to measure the radiated field of large phased array radar antennas (near-field or far-field patterns in a transverse plane parallel to the plane of the array), ii) to measure the transition of the field from the near field to the far field (in an axial plane perpendicular to the plane of the array), and iii) to test the switching of the array from a scan mode to a target tracking mode.

Low-loss negative index metamaterials for X, Ku, and K microwave bands

Low-loss, negative-index of refraction metamaterials were designed and tested for X, Ku, and K microwave frequency bands. An S-shaped, split-ring resonator was used as a unit cell to design homogeneous slabs of negative-index metamaterials. Then, the slabs of metamaterials were cut unto prisms to measure experimentally the negative index of refraction of a plane electromagnetic wave. Theoretical simulations using High-Frequency Structural Simulator, a finite element equation solver, were in good agreement with experimental measurements. The negative index of refraction was retrieved from the angle- and frequency-dependence of the transmitted intensity of the microwave beam through the metamaterial prism and compared well to simulations; in addition, near-field electromagnetic intensity mapping was conducted with an infrared camera, and there was also a good match with the simulations for expected frequency ranges for the negative index of refraction.

Electromagnetic Wave Scattering

The sections in this article are 1 Types of Electromagnetic Scattering 2 The Laws of Spectral Reflection and Refraction 3 Electromagnetic Theorems 4 Diffraction 5 Diffraction Through an Aperture 6 Babinet’s Principle 7 Special Cases of Electromagnetic Wave Scattering 8 Summary

Broadband-interference rejection for mobile satellite communications

The radiation pattern of the null-steering array in Figure 5 agreed well with the simulations in Figure 4. By taking advantage of the jamming signals broader bandwidth, the interference-detection channel controlled the adaptive null-steered array, in order to steer the null in the direction of the interfering signal. The system was able to suppress a 20-Watt, 20-MHz jammer that was six feet from the satellite receiver and that overlapped the satellite signals bandwidth, to a SIR of better than 13dB. Since the system was adaptive, it was also able to suppress jamming signals that were moving relative to the satellite receiver.

Circular array for satcom interference rejection

A useful application of antenna arrays is interference rejection. This paper describes a circular antenna array that was designed to reject interference in a mobile satellite receiver from a close-proximity source of interference. Since this interference source can move relative to the satellite antenna, the antenna array must adaptively adjust its null direction accordingly. The particular design of the circular array necessitated a significant size reduction in the radiating elements. Therefore, size reduction techniques were applied to circular patch antennas to allow them to fit in the circular array.

Adaptive null-steered interference-rejection for a mobile satellite receiver

One of the most useful applications of antenna arrays is interference rejection. This paper describes an antenna array that was designed for the sole purpose of rejecting interference of a satellite receiver from a close-proximity jamming source. Since this jammer can move relative the satellite antenna, the antenna array must adaptively adjust its null direction accordingly. The system consists of a six-element array of circular fractal patch antennas in conjunction with a receiver channel that is dedicated to interference detection and tracking.

Three-dimensional Microwave Tomography: Waveform diversity and distributed sensors for detecting and imaging buried objects with suppressed electromagnetic interference

Microwave tomographic techniques are described in this paper for developing high-resolution images of buried targets using 3D RF CAT Scans with frequency, angular, and polarization diversity and distributed sensors. Surface-contact sensors are used to collect the tomographic data for relay to a circling UAV and transmission to a remote control site (using layered sensing). 3D imaging algorithms have been developed to detect, image, and characterize buried targets. Distributed transmitters and receivers significantly increase unwanted mutual coupling and EM emissions (EMI) that interfere with signal reception, but also increase image resolution. For Ground Penetration (GPEN), reduced mutual coupling and EMI, and improved signal-to-noise ratios (SNR), can be achieved by embedding the transmitter/receiver sensors underground. Simple surface SAR experiments have been performed to detect deep mine shafts at the Zinc Corporation of America. 2D sensor data have been used to validate the 3D processing algorithms. Scale-model lab tests in the DETECT Chamber at AFRL have also been performed to optimize the tomographic images. In addition, WIPL-D models have been used to simulate the embedded and diverse/distributed sensors and to verify the significant enhancement in the received SNR for GPEN obtained by burying the radiating ring under the surface.

Direct infrared measurements of phased array aperture excitations

A thermographic imaging technique has been developed to measure electromagnetic (EM) fields. This technique is applied in this paper to measure the aperture plane fields of large phased array radar antennas and to determine the aperture (source) excitations of the array, i.e., the distribution of the radiated energy over the elements in the aperture plane of the array. These IR measurements, therefore, can be performed on-site at the remote location of the antenna in-the-field to produce a non-distorted image of the field excitations in the aperture plane of the array, which control the overall radiation characteristics of the array. In general, these images can be used for field diagnostics to evaluate the electrical characteristics of the elements of the array, i.e., the state (strength) of the aperture excitations and the condition of the switching circuits (phase shifters and attenuators), which control the radiation pattern of the antenna. The aperture field distribution can be compared to a standard “test pattern” to quickly determine the operational state of each individual element of the array. Individual attenuators and/or phase shifters that produce incorrect element intensities can be easily and quickly identified with this technique. Short-circuited elements at various positions in the array are used to simulate faults in the elements and to test the feasibility of the thermal technique to determine the operational state of the array. Therefore, the overall “state of health” of the array and the need for repair can be determined in-the-field using the IR measurement technique to avoid the expensive and time consuming alternative of dismantling the array and shipping it to a maintenance depot for testing, calibration, and repair on a standard, planar, near-field antenna test range. In this paper, the IR technique is tested in a controlled environment to determine the feasibility of using the IR images as an array diagnostic tool to measure the aperture excitations of large phased array radar antennas.

Patch antenna size-reduction parametric study

Size-reduction techniques are applied to a circular UHF patch antenna, by varying parameters to better predict its desired resonant frequency. Specifically, slits are introduced into the patch, which are parametrically varied to determine the optimum slit dimensions for maximum size reduction.

Size reduction of patch elements for homogeneous perfect absorbing material

Size-reduction of circular patch absorbing elements are analyzed by introducing slits, holes, and cut-out rings. Parametric variation results are presented for slit depth, hole size, and concentric rings to reduce the size of absorbing elements. ANSYS Electronics Desktop, a finite-element method (FEM) solver, was used to simulate the absorption from an array of circular patches, while analyzing parametric studies of the slit depth, hole radius, and ring radius, each of which increases absorption and reduces element size.