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EFFECTS OF CORRUGATED WALLS ON THE FIELD UNIFORMITY OFREVERBERATION CHAMBERS AT LOW FREQUENCIES

Emily A. GodfreyScience and Engineering Apprentice Program

Naval Surface Warfare CenterDahlgren VA

Abstract This report documents the effects ofcorrugated walls on the uniformity of theelectromagnetic environment within a small (1.8m x1.2m x 0.8m) reverberation chamber. The frequencyband evaluated was 150 MHz to 650 MHz, taking 5MHz steps. At each frequency 100 tuner steps were

made, resulting in one complete tuner rotation. A smalldual ridge hom antenna, designed for use above 800MHz, was used as the transmit device. Field strengthmeasurements were taken continuously using a three-axis, isotropic field measurement probe. The electricfield intensity was measured in this way at sixteendifferent locations, once with flat, aluminum walls, andonce with corrugated galvanized steel walls. Thecorrugated material was attached to the left, right, andrear walls, with extra corrugation randomly spread onthe bottom. The data shows that corrugated materialincreases the Uniformity of the electromagneticenvironment within a reverberation chamber.

INTRODUCTION

A reverberation chamber, also known as amode-stirred chamber, is used as a method to conductradiated emissions and immunity tests. A reverberationchamber is a shielded enclosure that contains a paddlewheel tuner. The purpose of the tuner is to "stir" themodes present in the chamber and thus help to createan isotropic electromagnetic environment (EME). Amode has resonance frequencies near which thechamber can be efficiently excited. As frequencyincreases, mode density also increases.

The most significant inherent quality of areverberation chamber is its ability to create anelectromagnetic environment equal in strength to the

environment in which the object being tested will beused. The chamber makes it possible to expose anobject to high levels of RF from all aspect angles,similar to the natural environment. This isaccomplished by using the tuner to manipulate themodes. The greater the manipulation of the modes, thegreater the isotropy of the EME within the chamber, andthe more reliable the data. It is not necessary

to manipulate the position of the object being testedwhen the EME within the chamber is isotropic.

This experiment was designed to investigatethe effect that changing the dimensions of the wallswould have on the uniformity of the electromagneticenvironment within the chamber. A total of 32

measurements were taken, 16 with flat walls and 16with corrugated walls. The data collected was thennormalized, graphed, and analyzed to determine if theuniformity within the chamber improved whencorrugated material was placed on the walls of thechamber.

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APPROACH

The frequency band chosen for theseexperiments corresponds to the modal structure of thechamber. The first mode for the chamber is located at149.9 MHz. The ninety-eighth mode for this chamber is

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located at 650.22 MHz. Therefore, by conductingfrequency tests between 150 MHz and 650 MHz a widerange of mode densities can be tested and analyzed.

To conduct the measurements required toassess the impact of cormgated material on theuniformity of the chamber several pieces of equipmentare needed. The chamber in which all measurementsdiscussed in this paper were made measures 1.8m x1.2m x 0.8m. The chamber is a shielded aluminumenclosure. The door of the chamber is lined withcopper coils and wire mesh. It is sealed shut duringtesting by a series of twenty-four latches surrounding allsides of the door.

Antenna Inside Corrugated Chamber

A small dual ridge horn antenna was used totransmit each frequency into the chamber while a three-axis isotropic field measurement probe was used tomonitor the field strength. Also inside the chamber wasa " z" shaped paddle wheel tuner. This tuner reachesfrom floor to ceiling of the chamber and is twelve incheswide. The probe was attached to a monitor using fiberoptic cables and the transmit antenna was attached to asynthesized sweeper and an amplifier. The HP Bussystem was used to control all of the equipment. Theentire setup was run using a HP VEE program, on adesktop computer. The equipment setup is shown inFigure 2. ,

The corrugated material used was galvanizedsteel sheeting. The waves that constitute thecorrugation were approximately two inches long with amaximum amplitude of one-half inch. As shown inFigure 3, he corrugation ran vertically on the back walland horizontally on the left and right side walls. The

corrugation on the floor ran left to right.To correctly map the uniformity of the smallreverberation chamber, it was necessary to carefullychoose sixteen equipment configurations. Theseconfigurations consisted of a series of two transmitantenna positions, four paddle wheel tuner positions,and sixteen different probe locations.

The antenna used in these experiments producesa high VSWR at frequencies below 400 MHz. Tocombat this problem, the program was adjusted so thatwhen the frequency was greater than or equal to 400MHz and less than or equal to 500 MHz, the inputpower to the antenna was reduced by 10 dB. When thefrequency was greater than 500 MHz, the input powerto the antenna was reduced by 20 dB. At all locationsthe probe was oriented the same way, with the x-axisdirected toward the top of the chamber and the y-axisdirected toward the left wall. To test whether orienting

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Figure 4 Maximum Values, Plain Walls

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the probe in the same way for all measurements wasbiasing the data. A second series of experiments wasrun, this series consisted of testing three of thelocations three times - once with the probe normal,then with the probe off axis, and then with the probenormal again.

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Figure 5: Maximum Values, Corrugated Walls

ANALYSIS

The maximum value graphs show themaximum values measured on the x, y, and z axis ateach location, for a total of 48 data points at eachfrequency. Figure 4 is the graph of the maximumvalues taken from the chamber when it had plain walls.Figure 5 is the data taken from the chamber after thecorrugation had been installed. The tighter the groupingof data points the greater the isotropy at that frequency.The groupings in Figure 5 are consistently of loweramplitude and more condensed than those in Figure 4,indicating that the corrugated material 'loaded" thechamber and possibly improved the isotropy of the

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Figure 6: 100% Uniformity, Plain Walls

chamber. The tight groupings at the lowest frequencieswere unexpected and further investigation is necessaryfor a full explanation.

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Figure 7: 100% Uniformity, Corrugated Walls

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When the maximum values gathered wereconverted from volts per meter to decibels, as inFigures 6 and 7, the dB spread was determined.Subtracting the smallest maximum value from thelargest maximum value, after all values had beenconverted to decibels, resulted in the field uniformity indB. Figure 6 shows the uniformity of the chamber withplain walls, using 100% of the data, while Figure 7 isthe uniformity graph using 100% of the data taken fromthe chamber when it had corrugated walls. When thesetwo uniformity graphs are compared, it is apparent thatthe corrugation mproved the uniformity of the chamber.The uniformity measured when the chamber hadcorrugated walls is consistently lower than theuniformity measured when the chamber had plain walls.

On the field uniformity graphs, the line at 6dBrepresents the pass / fail mark according to currentspecifications. Data points below 6dB pass, while datapoints above 6dB indicate unacceptable uniformity.

Figure 8: 75% Uniformity, Plain Walls

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the chamber improves. With the combined data,certain frequencies produce passing uniformity rates,even when using 100% of the data.

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-- I IData Obtained By Comparing Probe DataFrom Two Trans mit Antenna locations

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Figure 13:Combined Data, 75% Uniformity, CorrugatedWalls

After eliminating the allowed 25% of the data,the effective uniformity, of the chamber begins toconsistently produce values below the passing mark of6dB. Figure 12 is the 75% uniformity graph usingcombined data taken from the chamber when it hadplain walls. On this graph 53 out of 100 uniformityvalues are below the passing line (53%). Figure 13 isthe 75 % uniformity graph using combined data takenfrom the chamber when it had corrugated walls. Onthis graph 75 out of 100 uniformity values are below6dB (75% passing rate) and therefore indicateacceptable uniformity. By moving the transmit antennaand averaging the data, a better uniformity isdeveloped.

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Frequency (MHz)

Figure 1 4 Probe Orientations, 100% Uniformity

The second series of experiments consisted ofrunning the same test at three different locations. At

each location the test was run three times, once withthe probe in its normal position, once with the probe off-axis, and again with the probe normal. This series wasconducted only in the corrugated chamber. When thisdata was analyzed in the same manner as the previousdata, the same characteristics emerged in the charts.Figure 14 is the field uniformityegraph for the threeprobe orientations using 100% of the data. Theuniformity points generally follow the same path,regardless of probe orientation. When the uniformity iscalculated using only 75% of the data, all probeorientations have the same characteristics. The trendfor the uniformity to gradually become worse and thenbetter again around 400 MHz was also present in thesetests. These tests prove that the consistent orientationof the probe did not bias the data collected.

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Figure 15: 3 Probe Orientations, 75 % Uniformity

CONCLUSIONS

The effects of corrugated walls on theuniformity of the electromagnetic field within a smallreverberation chamber were investigated. Using aseries of 16 location combinations, consisting of twoantenna positions, four tuner positions, and 16 probepositions, the isotropy within the chamber wasmeasured twice, once with plain walls and once withcorrugated walls. When the results of theseexperiments are compared, it is found that corrugatedmaterial significantly improves the field uniformity of a

reverberation chamber.The tight groupings of data points at the lowfrequencies were not expected. The large passing rateof the data at those same frequencies was verysurprising. Further investigations are necessary toexplain this phenomenon.

It was found that probe orientation does notaffect the data. Whether the probe is always in the

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same position or constantly re-oriented, the resultingdata is the same.

It was also found that by treating twoconfigurations with different transmit antenna positionsas one, improved the uniformity of the chamber.

ACKNOWLEDGEMENTS

I would like to thank my mentors, MichaelHatfield, Mark Johnson, Michael Slocum for all of thetime, effort, and dedication it took to teach me theconcepts discussed in this paper. I would also like tothank Mr. Wil liamLucado for funding this work.

BIBLIOGRAPHY

[l] rawford, M.L.. and Koepke, G.H., "Design,Evaluation, and Use of a Reverberation Chamber forPerforming Electromagnetic SusceptibilityNuInerability

Measurements", NBS Technical Note 1092, April 1986.

[2] Crawford, M.L. and Koepke, G.H., "ElectromagneticRadiation Test Facilities: Evaluation of ReverberationChambers Located at NSWCDD," Dahlgren, Va.:NBSIR 86-3051 June 1986.

[3] Proposed Annex to IEC 6100-4-3 "Radiated, RadioFrequency, Electromagnetic Immunity Test."

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