David A. Hill

Shielding effectiveness measurements of materials using nested reverberation chambers

The use of reverberation chambers for determining the shielding effectiveness has the advantage over other techniques in that the reverberation chamber exposes the material to a more realistic environment. That is, in a reverberation chamber, the fields are incident on the material with various polarizations and angles of incidence. There are various reverberation chamber techniques found in the literature and international standards. These techniques can give incorrect results because aperture and cavity-size effects are not correctly accounted for. In fact, we show that for no sample in the aperture, these techniques give a nonzero result for shielding effectiveness. In this paper, we review existing techniques and present a new technique for determining the shielding effectiveness of materials from nested reverberation-chamber measurements. The new approach accounts for aperture, cavity size, and chamber loading effects. Various examples are presented to illustrate the utility of the new approach, and a discussion on edge treatments of the materials is given.

Reverberation Chamber Techniques for Determining the Radiation and Total Efficiency of Antennas

Reverberation chambers are becoming a popular alternative testing facility for a wide range of electromagnetic applications. Because of the statistical environment created inside a reverberation chamber, they offer a unique test facility. In particular, these chambers are ideally suited for performing radiated power measurements of either an antenna or device under test, and as such, it is possible to determine the efficiency of antennas. There have been several reverberation chamber techniques proposed over the years for measuring the antenna efficiency; however, these techniques require either the use of a reference antenna (i.e., an antenna with a known efficiency) and/or require the assumption that the two antennas used in the test have identical efficiencies. In this paper, we present three different approaches for determining both the radiation and total efficiencies of an unknown antenna that overcome these limitations and assumptions. We present a one-antenna approach, a two-antenna approach, and a three-antenna approach. We present measured data for three different antennas in order to compare these three approaches. We also discuss the uncertainties related to these types of measurements.

Propagation Along a Braided Coaxial Cable in a Circular Tunnel

The modes of propagation along a coaxial structure contained within a circular tunnel are considered. The primary objective is to develop an approximate impedance boundary condition at the outer surface of the shielded cable that can be used in previously developed formalisms for axial conductors in tunnels. It is assumed that the metal braid can be characterized by a surface-transfer impedance. We also account for the possibility that a Iossy film exists on the outer surface of the dielectric jacket of the cable.

Early Time Behavior in Reverberation Chambers and Its Effect on the Relationships Between Coherence Bandwidth, Chamber Decay Time, RMS Delay Spread, and the Chamber Buildup Time

Reverberation chambers are emerging as a test facility for testing wireless devices and for emulating different wireless multipath environments. The commonly used quantities for characterizing the chambers in wireless applications are 1) the chamber quality factor, 2) the chamber decay time (τRC), 3) the RMS delay spread of the time-domain chamber response τrm s, and 4) the coherence bandwidth BW of the frequency-domain transfer function of the chamber. Analytic expressions that relate τRC and BW and the relationship between τrms and BW are given in the literature. However, these expressions neglect the early-time behavior of the chamber (the time before a chamber reaches a reverberant condition), and hence can give inconsistent results when one is analyzing experimental data. In this paper, we discuss the relationship between BW, τRC, and τrms for realistic chamber behaviors, and we present expressions for these relationships when one takes into account the early-time behavior of the reverberation chamber. This early-time behavior is crucial when one tries to assess and compare these different quantities in experimental data, and as we will see, the relationship between these quantities can be different for different chambers (i.e., different chamber sizes and loading conditions). The model presented here illustrates how the early-time behavior can affect these chamber characteristic quantities for loaded and unloaded chambers, and it also illustrates the problems that can occur when the early-time behavior is not considered.

Use of Reverberation Chambers to Determine the Shielding Effectiveness of Physically Small, Electrically Large Enclosures and Cavities

With the proliferation of small electric devices in recent years, along with various other applications, there is a growing need to test and determine the shielding properties or shielding effectiveness (SE) of physically small (but electrically large) enclosures or cavities. In this paper, we discuss how a reverberation chamber technique can be used to measure the SE of such enclosures. The approach consists of placing the small enclosure inside a reverberation chamber and using frequency stirring to excite the reverberation chamber. A small surface probe (i.e., a monopole) is mounted on the inside wall of the small enclosure to measure the power level inside the small enclosure. We present measured data from various other reverberation chamber approaches obtained from various enclosure configurations. The data from these other reverberation chamber approaches are used to validate the proposed approach. We also compared measured data to theoretical calculations of the SE for two small enclosures with circular apertures. These various comparisons illustrate that the proposed technique is a valid approach for determining the SE of physically small (i.e., cubic enclosure dimensions of the order of 0.1 m and smaller), but electrically large enclosures (that support several modes at the lowest frequency of interest).

Radiated emissions and immunity of microstrip transmission lines: theory and reverberation chamber measurements

The increasing complexity of electronic systems has introduced an increased potential for electromagnetic interference (EMI) between electronic systems. We analyze the radiation from a microstrip transmission line and calculate the total radiated power by numerical integration. Reverberation chamber methods for measuring radiated emissions and immunity are reviewed and applied to three microstrip configurations. Measurements from 200 to 2000 MHz are compared with theory, and excellent agreement is obtained for two configurations that minimize feed cable and finite ground plane effects. Emissions measurements are found to be more accurate than immunity measurements because the impedance mismatch of the receiving antenna cancels when the ratio of the microstrip and reference radiated power measurements is taken. The use of two different receiving antenna locations for emissions measurements illustrates good field uniformity within the chamber.

Propagation Along a Coaxial Cable with a Helical Shield

A leaky coaxial cable is modelled by a dielectric coated conductor shielded by a finite number of unidirectional helical wires. A modal equation is derived and soIved numerically for the propagation constants of both the monofilar and bifilar modes. Numerical results are also presented for the effective surface transfer impedance of the shield. This parameter is found to depend, in general, on the propagation constant.

On the excitation of the Zenneck surface wave over the ground at 10 MHz

Using a planar earth model the infinite vertical aperture with a Zenneck wave variation is found to excite only the Zenneck wave with no radiation field. The finite vertical aperture excites a field which is similar to the Zenneck wave near the aperture but resembles the usual ground wave at large distances. Numerical results for various aperture heights are given for a frequency of 10 MHz for a homogeneous ground path.AnalyseDans l’hypothèse d’une terre plane, une antenne verticale infinie dont la répartition du champ dans l’ouverture est celle d’une onde de Zenneck excite uniquement cette onde de surface sans champ rayonné. Une antenne de dimension finie excite un champ qui ressemble à celui de l’onde de Zenneck près de l’ouverture, mais se rapproche de l’onde de sol classique à grande distance. Des résultats numériques sont donnés pour diverses hauteurs d’antenne à la fréquence 10 MHz en présence d’un sol homogène le long du trajet radioélectrique.

Excitation of monofilar and bifilar modes on a transmission line in a circular tunnel

We consider the excitation of monofilar and bifilar modes on a transmission line in a circular tunnel by a short dipole antenna. As expected, the monofilar mode is excited more strongly by an antenna placed in the tunnel, but the bifilar mode has lower attenuation. The excessive losses in the monofilar or coaxial mode are attributed to the return current flow along the tunnel walls. In the bifilar or transmission‐line mode the fields are more confined to the region of the wire conductors.

Theory of Transmission of Electromagnetic Waves along a Drill Rod in Conducting Rock

We present an analysis of the electromagnetic fields excited by a toroidal cofl that is coaxial with a circular metal rod located in a conducting medium. The integral representation for the induced axial current on the rod is cast in a form that is suitable for numerical evaluation. Some results for the electric current distribution on the infinitely long configuration are given for a fixed magnetic current source. Good agreement is obtained with an asymptotic estimate for this quantity. The effects of truncating the bottom end of the rod and terminating the top in an ideal ground plane are treated by an approximate method. Numerical results for this confilguration are also given.