EMI and EM Energy Transport in the Near Field

D.J. Taylor and M.G. Parent
Radar Division

S. Samaddar
Sachs Freeman Associates

Introduction: The propagation of electromagnetic (EM) energy through stratified dielectric media is a topic relevant to numerous Navy applications. Devices such as multilayer radomes, frequency-selective surfaces, and radar absorbers are designed based on models of how EM waves move through stratified dielectric media. However, the use of such devices in the near field of radiating or receiving antennas requires that knowledge of how electromagnetic waves interact with dielectric media be expanded beyond the simple models used to date. An understanding of the basic physics in this area will allow engineers to successfully design and integrate multiple antenna apertures into composite structures that will be part of next-generation naval vessels such as the DD(X) destroyer.

Research Approach: NRL is developing analytical and experimental methods at microwave frequencies for electromagnetics in the near fields of stratified dielectric media. These methods are aimed at detecting and characterizing wave motions with a noninvasive technique that will allow energy transport in laminated materials to be studied by using only EM wave fields measured in a region external to the structure. The research involves a combination of analytical studies of classic wave motion in laminated media of infinite extent, numerical simulations of electromagnetic wave interactions with dielectric media of finite size, and the development of sub-wavelength sensors to measure the EM fields associated with finite media. The fundamental issue is to extract information from the propagating and evanescent fields that exist near the surface of stratified dielectric media and are intimately coupled to energy transport within the structure. This extracted information can then be used to identify the spatio-temporal "fingerprint" of the transport mechanism involved.

EM Waves in Layered Media: EM wave transport in planar layered media of infinite extent is a classic boundary-value problem. When the source of the EM fields is at infinity and the incident fields are plane waves, the transport is expressed solely in terms of reflected and transmitted plane waves. If the source of the EM fields is a finite distance from the dielectric interfaces, then the solution becomes multimodal in the sense that it contains physical mechanisms other than the reflected and transmitted plane-wave components. The wave-transport physics in the near field include not only the geometric-optics transmitted and reflected modes but also the guided-wave energy trapped in the layers and the interface modes (or "lateral waves") that are excited due to the proximity of the source. It is these EM wave-transport mechanisms that will contribute to the EM interference (EMI) between future embedded phased-array antennas. The distribution of the EM fields associated with these modes is such that they extend into the region outside of the dielectric media. Depending on the nature of the mode, the corresponding component of the field can be propagating or evanescent. Sampling the fields just outside the dielectric, in the free-space region but very close to the interface, allows the detection of field energy from the propagating and evanescent modes. Classification and identification of these modes requires a transform to the temporal-frequency/spatial-frequency domain where dispersion, characteristics of the group and phase velocities, and determination of propagating versus evanescent can be made and associated with EM wave-transport mechanisms in the structure.

Simulating and Measuring Near EM Fields: The physical models for EM wave transport are based on solutions for unbounded, infinite-dimension, layered media; the physical problems encountered by the Navy are naturally finite in size. The analysis of EM fields near finite structures must be accomplished using numerical solutions of Maxwell's equations or direct measurements. NRL is conducting research in both areas to provide EM field data for analysis. Figure 1 shows a snapshot of a time-domain simulation of fields generated by an array of 2-dimensional, electric line sources excited with a Gaussian pulse waveform that interacts with a planar dielectric slab. The array generates an EM wave with a localized planar wavefront that is reflected from and transmitted through the dielectric slab. At the time step shown in the figure, the radiated field from the array has passed through the slab; however some energy remains near the slab. This energy implies the existence of higher-order modes and has two origins: energy trapped in the slab, and diffracted energy from the termination of the structure.

Measurements of the fields very near to such a structure require a different type of sensor—one with high spatial resolution that faithfully captures the evanescent field modes, which exhibit a high spatial frequency component. Figure 2 shows a simple current-loop sensor designed to measure magnetic field strength at 3 GHz. This probe, others like it,¹ and the Radar Division's large planar scanner form the basis for experimental investigations into near-field EM wave phenomena exhibited by planar dielectric structures.

[Sponsored by ONR]

Reference

¹ P.H. Harms, J.G. Maloney, M.P. Kesler, E.J. Kuster, and G.S. Smith, "A System for Unobtrusive Measurement of Surface Currents," IEEE Trans. Antennas Propag. 49(2), 174-184 (2001).