John D. Norgard

Infrared Measurements of Scattering and Electromagnetic Penetration through Apertures

An infrared detection technique is used to measure the electromagnetic fields near apertures of planar and cylindrical structures. Qualitative and quantitative results are presented and compared with theoretical solutions where applicable.

An Infrared Measurement Technique for the Assessment of Electromagnetic Coupling

A non-destructive, non-perturbing infrared measurement technique is under development to observe the effects of electromagnetic coupling of energy to the interior of complicated geometrical structures. The applications, advantages, and disadvantages of this new infrared technology are discussed.

Phase Measurements of Electromagnetic Fields Using Infrared Imaging Techniques and Microwave Holography

Complex (magnitude and phase) measurements of the near field of a radiating antenna over a known surface (usually a plane, cylinder, or sphere) can be used to determine its far-field radiation pattern using near-field to far-field Fourier transformations. Standard gain horn antennas are often used to probe the near field. Experimental errors are introduced into the near-field measurements by mechanical probe position inaccuracies and electrical probe interactions with the antenna under test and probe correction errors. A minimally perturbing infrared (IR) imaging technique can be used to map the near fields of the antenna. This measurement technique is much simpler and easier to use than the probe method and eliminates probe position errors and probe correction errors. Current IR imaging techniques, which have been successfully used to rapidly map the relative magnitude of a radiating field at many locations (mXn camera pixels per image captured) over a surface, however, suffer from an inability to determine phase information. Absolute magnitude and relative phase data can be obtained by empirical or theoretical calibration of the IR detector screens (used to absorb the radiated energy over the measurement plane) and by using techniques from microwave holography. For example, magnitude only measurements of the radiating field of an antenna at two different locations (over two different surfaces) in the near field of the antenna can be used to determine its complex (magnitude and phase) far-field radiation pattern using plane-to- plane (PTP) iterative transformations. This paper discusses the progress made to data in determining both magnitude and phase information from IR imaging data (IR thermograms); thus, enabling near-field and far-field measurements of antenna patterns using IR thermal imaging techniques.

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.

Correlation of infrared measurement results of coupled fields in long cylinders with a dual series solution

A theoretical code for determining electric field distributions in long cylinders is validated by an infrared experimental method. A summary of the theoretical approach using the generalized dual series (GDS) solution is presented. The theoretical solutions for selected geometries used for comparison are depicted as cross-sectional views of experimentally generated infrared (IR) images. The IR images are obtained from microwave interactions and are related to the field intensity at their locations. Qualitative comparisons are made between the field strengths measured using the thermal radiation experimental approach and a theoretical prediction for various frequencies. The experimental and theoretical results show reasonable correlation. >

Scattering effects of electric and magnetic field probes

Many electromagnetic measurements of electromagnetic pulse (EMP) interactions with electronic systems use B-dot and D-dot probes. The effects of the measurement probe on the field distribution being measured is considered. An infrared measurement technique is used to determine the field distributions with and without the presence of electric- or magnetic-field probes. Two-dimensional thermogram images of the scattered field patterns are measured. The scattering effects of various probes in the frequency range from 1 to 10 GHz are presented. This frequency range can be used to scale-model many EMP and high-power microwave (HPM) effects. It is shown that wide variations in the response of a probe can occur due to resonant frequency and mutual-coupling effects. These effects are due, in part, to the different measurement configurations of the probe relative to the direction of propagation and polarization of the incident electromagnetic wave to be measured. These differences can be significant at certain frequencies and separation distances for the various probe measurement configurations. >

Measured Internal Coupled Electromagnetic Fields Related to Cavity and Aperture Resonance

Preliminary internal coupling tests using an infrared (IR) measurement technique were conducted with a hollow metal cylinder having a thin slot aperture. Internal field mapping shows dramatically that the energy coupled through an aperture into a given structure is strongly dependent on frequency. In particular, frequencies corresponding to the resonance of the cavity and resonance of the aperture show significant coupling into the cylinder, while those frequencies slightly off a resonance frequency show a greatly reduced internal field confined to the immediate region of the aperture.

Near-field patterns from pyramidal horn antennas: numerical calculation and experimental verification

The electromagnetic field close to a pyramidal horn antenna has been predicted using a numerical technique assuming a spherical phase front in the aperture of the horn. Comparison with experimental results using an infrared imaging technique has shown that the best fit is for a phase front that is 1.25 times more curved than a sphere. The infrared imaging method has been demonstrated to be very useful for mapping the fields in a region that is sensitive to the introduction of metallic probes. >

Dielectric oblate spheroidal model of buried landmines

To approximate buried miens with electrical characteristics similar to their surroundings, an analytical model is chosen over a more computationally time consuming numerical model. A curved volume best approximates some mine types and an analytical model of a buried sphere using the Born Approximation has been developed. When modeling a mine, the sphere offers only one degree of freedom, its radius. The oblate spheroid is a more versatile model since it provides two degrees of freedom: major axis and eccentricity. The analytical solution for the current induced into a dielectric scatterer is developed for the oblate spheroid in the spectral domain and its resulting scattered electric field is determined by solving for all transverse components and transforming the result to the spatial domain via a 2D FFT. Favorable results are achieved by comparing this oblate spheroidal modeled Moment Method results derived by partitioning three different land mines. It is also shown to be superior to the sphere model. A method of inertia is also presented.

Empirical calibration of infrared images of electromagnetic fields

An infrared (IR) measurement technique has been developed to measure electromagnetic (EM) fields. This technique produces a two-dimensional IR thermogram of the electric or magnetic field being measured, i.e., an isothermal contour map of the intensity of the EM field. The intensity levels (equi-color levels) of the IR thermograms are empirically calibrated using standard gain horn antennas at several frequencies, angles of incidence, and polarizations in the near and far fields of the antenna. The results of the initial calibration test for electric field measurements are presented for a lossy Kapton detector screen developed to measure the absolute magnitude of the electric field in the plane of the detector screen. The accuracy of the technique is also discussed.