Time-Domain Fields

Goals

The Time-Domain Fields Project develops basic metrology and measurement techniques for a wide variety of applications such as antenna and sensor calibrations, evaluation of electromagnetic compatibility (EMC) measurement facilities, shielding performance of aircraft, nondestructive testing of electrical material properties, precise generation of standard fields, and detection of signals and threats.

Customer Needs

Time-domain field methods use time windowing to eliminate unwanted signals. These methods find application to problems that cannot readily be evaluated using traditional continuous-wave radiated tests. In particular, time-domain methods allow for localization of large systems and for testing in highly cluttered environments. Customer needs include:

Reflection Properties — EMC test sites, such as anechoic chambers and open area test sites (OATS) use absorbing and low-reflectivity materials to achieve desired performance. The reflectivity of these materials needs to be accurately characterized over a wide frequency range, possibly in situ.

Shielding Effectiveness — Time-domain signals can be used to investigate the shielding effectiveness of large, complex geometries. Aircraft can be tested in situ, either over tarmac or in a hanger.

Propagation in Buildings — Buildings present complicated propagation environments. Communications systems need to know how waves couple through differing building materials and how waves couple from the exterior to interior and between interior locations.

Ultra-Wideband Systems — Ultra-wideband systems are being proposed to increase capacity and to address advanced communications needs. Antenna characteristics and link performance need to be accurately determined.

Technical Strategy

We have developed measurement tools and systems both to generate and receive ultra-wideband radiated signals. We currently have a large cone and ground plane facility. This facility can be used to generate well defined pulse fields for sensor calibration up to 20 gigahertz, plus antenna calibration and system characterization. We used extensive numerical simulations to optimize the feed and cone sections to achieve very high performance levels.

We have been a leader in developing transverse electromagnetic (TEM) horn antennas for receiving ultra-wideband signals. These antennas have very linear phase characteristics and are able to accurately preserve time-domain traces. We will publish a NIST technical note that summarizes our research in TEM Horns.

TEM horns are used in field-deployable systems for transmitting and receiving time-domain fields. The system uses optical fiber links to achieve high isolation and dynamic range. The system has been successfully applied to the evaluation of the shielding effectiveness properties of commercial and military aircraft. These efforts have assisted NASA, U.S. manufacturers, the U.S. Department of Defense (DOD), and the Federal Aviation Administration (FAA), to improve flight safety and reduce the vulnerability of aircraft to electromagnetic interference and threats.

Division staff members perform field penetration
tests on a NASA space orbiter.

We are applying time-domain techniques to develop a database of the electrical properties of building materials in support of homeland security goals. The database will help first responders develop communication systems that will improve performance in emergency situations. We are collaborating on the use of genetic algorithms to accurately extract electrical parameters from time-domain reflection and transmission data. We are also deploying a portable measurement system to measure the propagation characteristics of buildings over ultra-wide bandwidths and provide both time- and frequency-domain channel characteristics.

We have supported industry and standards groups in assessing the performance of electromagnetic facilities such as anechoic and semi-anechoic chambers, shielded rooms, reverberation chambers, and OATS facilities. The performance of microwave absorbers has been measured. Many facilities use low-reflectivity material for test object support and weather protection. We are helping to develop time-domain-based test methods to determine the effects of “reflectionless” materials of the type specified in many radiated test standards on site ability to perform ultra-wideband RCS measurements.

Accomplishments

Aircraft Shielding — We have developed a unique, leading-edge portable measurement system for the evaluation of coupling to complex aircraft environments. The system has been used to evaluate a number of flight vehicles such as the NASA Orbiter Endeavour, a U.S. Air Force earlywarning aircraft, and a Boeing 767 commercial aircraft. Evaluations of a Boeing 737-200 and a Bombardier Global 5000 aircraft at the Federal Aviation Administration (FAA) Technical Center in Atlantic City, New Jersey, were recently performed. The data from this effort are in the public domain and have been disseminated to the aircraft testing community.
Complex Cavity Modeling — Numerical models of complex cavities have been developed to facilitate understanding of measurements performed in aircraft environments. This work is sponsored by the FAA.
Building Materials Characterized — A freefield materials measurement system for the evaluation of building materials has been developed.
EMC Facilities Tested — Several EMC facilities have been tested. A measurement effort was conducted recently at a wastewater instrumentation company to assess the effectiveness of an anechoic chamber retrofit. Significant improvements in chamber performance were realized through a NIST portable measurement system.