Andrew J. Poggio

Numerical Electromagnetic Code (NEC)

The development and the application of the Numerical Electromagnetic Code (NEC) - Method of Moments are described. NEC is based on a previous method of moments code for thin wires, the Antenna Modeling Program (AMP), and yields improved estimates of the performance of antennas mounted on shore stations, ships, aircraft, and spacecraft. The NEC can model antennas in free space, over a perfectly conducting ground plane, and over finite conduction (lossy) earth.

Evaluation of a processing technique for transient data

A data processing technique, namely Pronys method, is evaluated. The use of the algorithm for EMP and other transient problems is illustrated and the relationship between the waveform parameterization and the singularity expansion method (SEM) is described. The impulse response of a synthesizer network is obtianed using Prony processing of the output of the double exponential excited circuit. Difficulties which arise in this type of processing such as rank deficiency, aliasing, and noise effects are considered and methods for alleviation such as filtering and rank-overspecification are introduced and evaluated. The overall status of waveform parameterization as applied to SEM is considered and suggestions for future research presented.

Time-reversal processing for an acoustic communications experiment in a highly reverberant environment.

Time-reversal (T/R) communications is a new application area motivated by the recent advances in T/R theory. Although perceived by many in signal processing as simply an application of matched-filter theory, a T/R receiver offers an interesting solution to the communications problem for a reverberant channel. In this paper, the performance of various realizations of the T/R receiver for an acoustic communications experiment in air is described along with its associated processing. The experiment is developed to evaluate the performance of point-to-point T/R receivers designed to extract a transmitted information sequence propagating in a highly reverberant environment. It is demonstrated that T/R receivers are capable of extracting the transmitted coded sequence from noisy microphone sensor measurements with zero-symbol error. The processing required to validate these experimental results is discussed. These results are also compared with those produced by an equivalent linear equalizer or inverse filter, which provides the optimal solution when it incorporates all of the reverberations.

The Numerical Electromagnetics Code (NEC) - a brief history

The Numerical Electromagnetics Code, NEC as it is commonly known, continues to be one of the more widely used antenna modeling codes in existence. With several versions in use that reflect different levels of capability and availability, there are now 450 copies of NEC4 and 250 copies of NEC3 that have been distributed by Lawrence Livermore National Laboratory to a limited class of qualified recipients, and several hundred copies of NEC2 that had a recorded distribution by LLNL. These numbers do not account for numerous copies (perhaps 1000s) that were acquired through other means capitalizing on the open source code, the absence of distribution controls prior to NEC3 and the availability of versions on the Internet. We briefly review the history of the code. We show how it capitalized on the research of prominent contributors in the early days of computational electromagnetics, how a combination of events led to the tri-service-supported code development program that ultimately led to NEC and how it evolved to the present day product.

Time reversal and the spatio-temporal matched filter (L)

It is known that focusing of an acoustic field by a time-reversal mirror (TRM) is equivalent to a spatio-temporal matched filter under conditions where the Green’s function of the field satisfies reciprocity and is time invariant, i.e., the Green’s function is independent of the choice of time origin. In this letter, it is shown that both reciprocity and time invariance can be replaced by a more general constraint on the Green’s function that allows a TRM to implement the spatio-temporal matched filter even when conditions are time varying.

Wideband multichannel time-reversal processing for acoustic communications in highly reverberant environments

The development of multichannel time-reversal (T/R) processing techniques continues to progress rapidly especially when the need to communicate in a reverberant environment is critical. The underlying T/R concept is based on time-reversing the Green’s function characterizing the uncertain communications channel mitigating the deleterious dispersion and multipath effects. In this paper, attention is focused on two major objectives: (1) wideband communications leading to a time-reference modulation technique; and (2) multichannel acoustic communications in two waveguides: a stairwell and building corridors with many obstructions, multipath returns, severe background noise, disturbances, and long propagation paths (∼180ft) including disruptions (bends). It is shown that T/R receivers are easily extended to wideband designs. Acoustic information signals are transmitted with an eight-element array to two receivers with a significant loss in signal levels due to the propagation environment. The results of the n...

Techniques for Low-Frequency Problems

In this chapter we survey various techniques that can be used to evaluate antennas in the regime where their physical size is a maximum of a few wavelengths in extent. There are two basic approaches besides experimentation that can be considered for such problems: analytical and numerical.

A methodology for assessing high intensity RF effects in aircraft

Optical components have an inherent immunity to the electromagnetic interference (EMI) associated with High Intensity Radiated Fields (HIRF). The optical technology embodied in Fly-by-Light (FBL) might therefore minimize the effects of HIRF on digitally controlled systems while providing lifetime immunity to signal EMI. This is one of the primary motivations for developing FBL systems for aircraft. FBL has the potential to greatly simplify EMI certification by enabling technically acceptable laboratory tests of subsystems, as opposed to expensive full airplane tests. The authors describe a methodology for assessing EMI effects on FBL aircraft that reduces or potentially eliminates the need for full airplane tests. This methodology is based on comparing the applied EMI stress-the level of interference signal that arrives at a unit under test-versus the EMI strength of the unit-the interference level it can withstand without upset. This approach allows one to use computer models and/or low power coupling measurements and similarity (to other previously tested aircraft) to determine the stress applied to installed subsystems, and to use benchtop cable injection tests and/or mode stirred chamber radiated tests to determine the strength of the subsystem.<<ETX>>

Tools and techniques for estimating high intensity RF effects

Tools and techniques for estimating and measuring coupling and component disturbance for avionics and electronic controls are described. A finite-difference-time-domain (FD-TD) modeling code, TSAR, used to predict coupling is described. This code can quickly generate a mesh model to represent the test object. Some applications as well as the advantages and limitations of using such a code are described. Facilities and techniques for making low-power coupling measurements and for making direct injection test measurements of device disturbance are also described. Some scaling laws for coupling and device effects are presented. A method for extrapolating these low-power test results to high-power full-system effects are presented.<<ETX>>

Evaluation of a Processing Technique for Transient Data

A data processing technique, namely Pronys method, is evaluated. The use of the algorithm for EMP and other transient problems is illustrated and the relationship between the waveform parameterization and the singularity expansion method (SEM) is described. The impulse response of a synthesizer network is obtianed using Prony processing of the output of the double exponential excited circuit. Difficulties which arise in this type of processing such as rank alleviation such as filtering and rank-overspecification are introduced and evaluated. The overall status of waveform parameterization as applied to SEM is considered and suggestions for future research presented.