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An

Sensor and Simulation NotesNote 265

December 1979Experimental Investigation of the

King Surface Current Probing Techniquein a Transient Application

L. Wilson PearsonYoun M. Lee

University of KentuckyLexington, Kentucky

.ifixperimental technique to measure transient surface currents of a scat.-Lererusing a King-type semicircular miniature probe is reported. Theprincipalextension.of the King technique is the use of the 100P probe intransientcurrents measurement. The transient characteristics of the proljeareconsidered based on the Whiteside theory [29] for the loop. The.prob~~sfabricatedwere subjected to transient excitation in a coaxial calibrationjib. Their frequency responses were determined by Fourier transforming theiroutputand deconvolving the excitation spectrum. The agreement between tile=~asuredresults and the Whiteside theory is observed and evaluated. Therobe geometry was tested in an application context, namely, the probing Of.urrenton a cylindrical scatterer. The induced surface currents were rne;~-suredusing the probe along the scatterer, and deconvolution of the probetransferfunction was carried out subsequently. The deconvolved waveform:?Lrereported and the results of numerical predictions by way of a time~O~ainintegral equation technique are reported for comparison. The appl~-c=tionoal for this work lies in the numerical extraction Of the Singularity:~?ansionethod (SEM) description of scattering from a given object, usifig=?zsuredtransient data. The extraction process was carried out on-theCl:lindricalscatterer data. The results repotted here point to the feasi--~ilityof this procedure. Implications of extending the probing method f~,rCga?leX-shaped objects are discussed.

AcknowledgementsThe authors are Erateful to Mr. A. S. Hebert for his able assistanceJGthe construction o; the transient range andthe models used in this 70fi~s

::v- -,..-... ..:.... R. Auton carried out the extraction of the SEM description reported:}&re. The Harrison and King mod&l for the incident field used in the TWTJl+C=Putationswas computed and validated by Mr. B. Wade. Mr. G. B. Melson~;;:&-;elOPedhe data acquisition software for the measurement system.:: ~d Mrs.ttyradshaw and Mr J Des~osiers did the manuscript preparation..@,.


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2LEI 4P

Sensor and Simulation NotesNote 265

December 1979An Experimental Investigation of the

King Surface Current Probing Techniquein a Transient Application

L. Wilson PearsonYoun M. Lee

University of KentuckyLexington, Kentucky

AbstractAn experimental technique to measure transient surface currents of a scat-terer using a King-type semicircular miniature probe is reported. Theprincipal extension of the King technique is the use of the loop probe intransient currents measurement. The transient characteristics of the probeare considered based on the Whiteside theory [29] for the loop. The probesfabricated were subjected to transient excitation in a coaxial calibrationjib. Their frequency responses were determined by Fourier transforming theiroutput and deconvolving the excitation spectrum. The agreement between themeasured results and the Whiteside theory is observed and evaluated. Theprobe geometry was tested in an application context, namely, the probing ofcurrent on a cylindrical scatterer. The induced surface currents were measured using the probe along the scatterer, and deconvolution of the probetransfer function was carried out subsequently. The deconvolved waveformsare reported and the results of numerical predictions by way of a timedomain integral equation technique are reported for comparison. The appli-cation goal for this work lies in the numerical extraction of the SingularityExpansion Method (SEM) description of scattering from a given object, usingmeasured transient data. The extraction process was carried out on thecylindrical scatterer data. The results repofted here point to the feasi-bility of this procedure. Implications of extending the probing method forcomplex-shaped objects are discussed.probes, loops, surface currents, SEM (singularityspectra, waveforms

AcknowledgementsThe authors are grateful to Mr. A. S. Hebert

;JWA-expansion mode), predictions,

for his able assistancein the construction OF the transient range and the models used in this work.Mr. J. R. Auton carried out the extfaction of the SEM description reportedhere. The Harrison and King model for the incident field used in the TWTDcomputations was computed and validated by Mr. B. Wade. Mr. G. B. Melsondeveloped the data acquisition software for the measurement system. Mrs.Betty Bradshaw and Mr. J. DesRosiers did the manuscript preparation.


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TABLE OF CONTENTSChapter1.

11.

111.

IV.

INTRODUCTION. . . . . . . . . . . . .Transient Probing Problem . . . .Ground Plane Considerations . . .Brief Survey of Miniature Probes .Motivation for Adopting King

Transient Application .Survey of the Present Work .

IMPLEMENTATION OF LOOP PROBES .Introduction . . . . . . . .Construction Form . . . . .

Probe. . .. . .. . .. . .. . .

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.Theoretical Analysis of the Probe AssemblyCalibration Jig . . . . . . . . . .Transfer Function of Cable AdaptorsProbe Transfer Function . .

IMPLEMENTATION OF PROBING SCHEMECYLINDRICAL SCATTERER . . .Introduction . . . . . . . .Measurement Configuration .Instrumentation . . . . . .Measured Data . . . . . . .SEM Extraction. . . . . . . .

CONCLUSION . . . . . . . . . . .

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.APPENDIX: CONSTRUCTION TECHNIQUE FOR CURRENT PROBEREFERENCES. . . . . . . . . . . . . . . . . . . . .

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Page77

1010141517171723303638

424242454970767883


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LIST OF FIGURES

Figure 1:

Figure 2:

Figure 3:

Figure 4:

Figure 5:

Figure 6:Figure 7:

Figure 8:

Figure 9:

Figure 10:

Figure 11:

Generic configuration of the measurement of thecurrent waveforms. . . . . . . . . . . . . . . . . . .(a) Center cut side view and end view of thecurrent probe mounted on a carriage. (b) Aclose-up photograph of a probe assembly. . . . . . . .(a) A probe assembly fitted in the slot in acylindrical scatterer. (b) The probe in thescatterer after the slot has been covered withconducting tape to block its influence from themeasurement. . . . . . . . . . . . . . . . . . . . . .A method to adapt 0.035 adaptor for 0.023coaxial cable. The assembly above fits directlyinto the outer conductor assembly on an OmniSpectra OSSM 551-1 connector . . . . . . . . . . . . .TDR traces of probes showing two results ofempirical adjustments of gap between center pinof a 0.035 connector and 0.023 coaxial line. . . . .Geometry of loop and its equivalent block diagram. . .(a) Loop parameters are shown with transmissionline mode current, It, and dipole mode current,ld. (b) An approximate circuit diagram of aclrcularloop. . . . . . . . . . . . . . .. . . . . .(a) A picture of a coaxial jig and a 7/8 toN adaptor used for measurement of known fields.(b) 7/8 rigid coaxial transmission line and7/8 to N adaptors used to calibrate the currentprobe. (c) Approximate junction representationbetween adaptor and transmission line. . . . . . . . .Average of 16 waveforms produced by a pulsegenerator used in this experiment and itsspectrum. . . . . . . . . . . . . . . . . . . . . . .Transfer function of the transition of Type-Nconnector to 7/8 coaxial line . . . . . . . . . . . .Block diagram to compute transfer function ofN to OSM adaptor by measuring V2 and V1: N-female to OSM-male 32: N-female to OSM-female3: N-maletoOSM-male . . . . . . . . . . . . . . . .

Page

8

19

20

21

2424

26

31

34

35

37

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PageFigure 12: (a) Cross-section of the calibration jig used

to calibrate the probe. (b) A probe outputV (t) sampled by a current probe where the8urce of excitation is the pulse shown in

Figure9. . . . . . . . . . . . . . . . . . . . . . . .39Figure 13: Experimental and theoretical transfer functions

obtained for 0.125 probes. (a) theoreticalfrom equivalent circuit - dot-dashed; (b) theo-retic with cable attenuation included - solidcircles; (c) experimental Probe no. 1 - dashed;(d) experimental Probe no. 2 - solid . . . . . . . . . 40

Figure 14: (a) Block diagran of transient antenna rangefacility. (b) The measurement facility photo-graphed from behind the ground plane. . . . . . . . . . 41

Figure 15: A close-up of a scatterer mounted on a brassdisk. Probe is placed and the slot is tapedwith conducting tape. . . . . . . . . . . . . . . . . . 46

Figure 16: (a) A probe carriage is attached behind thedisk where the scatterer is mounted. (b) Theprobe carriage and disk mounted in the groundplane. . . . . . . . . . . . . . . . . . . . . . . . .47

Figure 37. Ground plane for transient antenna system. Thesize of this ground plane is 18~ x 20 . . . . . . . . . 48

Figure 18: Transient current measured 1.5 cm. from groundplane compared with TWTD computation 1.3 cm.fromgroundplane. . . . . . . . . . . . . . . . . . .51




Figure 22: Transient current measured 6.0 cm. from groundplane compared with TWTDcomputation 6.4 cm.fromgroundplane. . . . . . . . . . . . . . . . . . .55


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PageFigure 24:

Figure 25:

Figure 26:

Figure 27:

Figure 28:

Figure 29:

Figure 30:

Figure 31:

IFigure 32:

Figure 33:

Figure 34:

Figure 35:

Figure 36:

Transient current measured 8.5 cm. from groundplane compared with TWTD computation 8.1 cm.fromgroundplane . . . . . . . . .. . . . . . . . . .Transient current measured 11.0 cm. from groundplane compared with TWTD computation 10.7 cm.fromgroundplane. . . . . . . . . . . . . . . . . . .Transient current measured 12.0 cm. from groundplane compared with TWTD computation 12.4 cm.Fromgroundplane . . . . . . . . . . . .. . . . . . . .Transient current measured 13.0 cm. from groundplane compared with TWTD computation 13.3 cm.fromgroundplane. . . . . . . . . . . . . . . . . . .Transient current measured 16.0 cm. from groundplane compared with TWTD computation 15.9 cm.frorngroundplane . . . . . . . . . . . . . . . . . . .Transient current measured 17.5 cm. from groundplane compared with TWTD computation 17.6 cm.fromgroundplane. . . . . . . . . . . . ... . . . . .Transient current measured 18.5 cm. from groundplane compared with TWTD computation 18.4 cm.fromgroundplane. . . . . . , . . . . . . . . . . . .Transient current measured 20.0 cm. from groundplane compared with TWTD computation 20.1 cm.fromgroundplane. . . . . . . . . . . . . . . . . .Transient current measured, 22.0 cm. from groundplane compared with TWTD computation 21.9 cm.fromgroundplane. . . . . . . . . . . . . . . . . . .Transient current measured 24.0 cm. from ground.plane compared with TWTD computation 24.4 cm.fromgroundplane. . . . . . . . . . . . . . . . . . .Transient current measured 25.0 cm. from groundplane compared with TWTD computation 25.3 cm.fromgroundplane . . . . . . . . . . . . . . . .. . . .

Transient current measured 26.0 cm. from groundplane compared with TWTD computation 26.1 cm.fromgroundplane. . . . . . . . . . . . . . . . . .Transient current measured 29.0 cm. from groundplane compared with TV7TDcomputation 28.7 cm.fromgroundplane . . . . . . . . .. . . , .. . . . . .

57

58

59

60

71

62

63

64

65

66

67

68

69

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PageFigure 37: Magnitude and phase plots of natural mode 1

from measured data (solid line) and comparedwith that of Tesche (dot-dashed line) . . . . . . . . 72

Figure 38: Magnitude and phase plots of natural mode 3from measured data (solid line) and comparedwith that of Tesche (dot-dashed line) . . . . . . . . 73



Figure 41: A jig used to make semicircular loop with0.125diameter. . . . . . . . . . . . . . . . . . . 79Figure 42: A notching jig to guide a file to notch the

center of the probe for three different sizes . . . . 80Figure 43: A modified file to notch the semicircular

loop. Note: Not scaled . . . . . . . . . . . . . . 81

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I. INTRODUCTION

1.1

of a

Transient Probing ProblemThe principal goal of the work reported here is the developmentsurface current probing technique which is usable on a transient

(broadband) basis. The probing scheme is to be used ultimately inthe extraction of the Singularity Expansion Method (SEM) descriptionsofin

complex-shaped scatterers through experimental means as describedReference [I].As reported in [I], it is feasible to measure transient surface

current response at many points on a scattering object under study ora model thereof, even when the object is so complex as an aircraft.Figure 1 pictures the generic configuration of the measurement whichis used to derive transient surface current waveforms. Theobject issuspended in the presence of a radiating transient field produced bya transient signal source and a transmitting antenna. For SEM extrac-tion purposes the accurate characterization ofscattering object is an important prerequisiteconfiguration. For present purposes this is a

this field at thefor the experimentalsecondary considera-

tion. The surface current on the object is probed through a multi-plicity of miniature surface current probes, indicated by the smallarrows in Figure 1. Each of these probes provides a transient outputsignal when the object is excited. Typically, magnetic loop probesprovide a voltage response which is proportional to the time

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Figure 1. Generic configuration for the measurement of thecurrent waveforms.

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derivative of the current flowing in the probed direction at thesample location. The present work seeks to explicitly define therealization of these probes and to establish the methodology of theirapplication.

Transient probing is, at times, tedious to carry out, butadvantages result relative to continuous-wave (CW) analysis. Inparticular, we can determine the objects response over a broad rangeof frequency by using transient techniques. The work reported in [I]describes a method whereby one can derive the SEM description of anobject by means of probed transient response data. The SingularityExpansion Method enables us to analyze the transient coupling in sucha way as to understand it better and to represent it compactly. TheSEM representation is quite powerful in data reduction and can beused to predict the response of the object regardless of directionand time history (within band limitations) of the incident wave. Onecan extract the natural resonances and associated modes of the objectfrom which the scattering response may be expanded for new excitations-- transient or CW.

The experimental method to determine transient electromagneticcoupling to metallic objects demands careful consideration of theprobing technique used in measuring the electric field or the mag-netic field. A field probe should be designed such that it sensesthe field quantity of interest and is insensitive to other fieldquantities. Typically, it should be characterizable as electricallysmall in the frequency range of interest so that conventional circuitanalysis can be carried out without too much complexity.


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1.2 Ground Plane ConsiderationsOne must consider eliminating

couplings between the object underor minimizing the effect ofstudy and other bodies such as

measurement instruments and transmission cables. Utilization of theground plane technicpe is a way of avoiding these problems. TO do SOimposes a limitation, however [I]. Namely, only half of the modes onthe object being imaged are antisymmetric with respect to the sym-metry plane. The modes which are symmetric with respect to thesymmetry plane are not recoverable through the ground planemeasurement.

1.3 Brief Survey of Miniature ProbesA great deal of work has been carried out in the ~P community

in the development of the electric field and/or magnetic fieldsensors-- so-called D-dot sensors and B-dot sensors, respectively.Most of the sensors described below were developed for full scaleaircraft measurements.

R. E. Partridge at the Los Alamos Scientific Laboratory hasdevelopedan invisible absolute E-field probemum of disturbance to the field in its vicinityabsolute sensitivity [2]. He uses this idea in

which creates a mini-and has a calculablemodifying the

rectangular loop to measure the E-field and H-field simultaneouslyby employing a common mode amplifier and a dif~erential amplifier E3].Orsak and Whi.tson reported utilization of a capacitive electric fieldsensor developed by C. T. R. Wilson. That sensor was made in theform of an asymmetric plate tohouse a preamplifier including thebatteryand switch. This probe is 4 inches in diameter and stands

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3.25 inches high [4]. Use of an asynunetric dipole made of coaxialtransmission line as a transient probe was discussed by Hall [5].Baum has described twenty-one other probe configurations of impor-tance [6-26]. A current-sampling vertical current density sensorand inductively-coupled vertical current density sensor weredeveloped for the purpose of measuring the vertical component of thetotal current density at a soil or water surface [6]. A discussionabout the design of electrically-small multi-turn cylindrical loopsin the measurement of inhomogeneous magnetic fields and inductancecan be found in Reference [7]. Design and analysis of inductivecurrent sensors which measure the line integral of the magnetic fieldaround an area of interest by using an appropriate array of conductingloops was reported in [8]. m analysis of the circular flush-platedipole in nonconducting media was carried out. This sensor wasanalyzed using the cylindrical vector eigen-function expansions [9].The moebius strip loop has improved upon the conventional coaxialloop in some applications. It has the properties of doubled sensi-tivity to the magnetic field but much less sensitivity to transientradiation effects [1o]. A maximization frequency response of theB-dot loop was examined. Four limitations are considered for theB-dot 100P. These can be summarized as the loop radius, the match-ing of the impedances of the equivalent transmission line of theloop, anddetectingReferencedevelopedbility of

the size of the structure [11]. An investigation ofelectric fields in a dissipative media can be found in[12]. A sensor (which has flat frequency response) wasto measure the electric field in such a medium. A possi-designing a probe so that it can sense the electric field

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associated with the close-in EMP hasconstraints in building such a probeas well. Development of a technique

been investigated. Certainare reported in Reference [13],for measuring electric fields

with internal EMP using wire grids can be found in Reference [141.A generalization of the moebius strip loop is reported for measuringmagnetic fields [151. The effects of radiation and conductivity ona B-dot loop design consideration is considered in Reference [161.Calculations of the frequency response characteristics of the cylin-drical loop are carried out for both nonconducting and conductingmedia for two types of cylindrical loop design [17]. Some varioussensor parameters are defined for electrically-small loops and dipolesfollowing equivalent circuits considerations [18]. Multi-gapcylindrical loopconducting mediaconsidered for a

response characteristics when immersed in the non-have been reported [191. Some design parameters arepulse-radiating dipole antenna as associated with

the high-frequency and low-frequency content of the radiated waveform[20]. Reference [211 provides a technique through which one candefine the geometry of a dipole antenna such that low-frequencyparameters of the antenna are readily calculable. Some electricalparameters of loop sensors for measuring the magnetic field perpen-dicular to the cylinder axis are reported in Reference [22]. Ananalysis of a dipole with two parallel conducting plates, one of thecommon sensors for measuring an electric field, is reported for thecase of two equal conducting plates [23]. Further considerations ofthis circular parallel-plate dipole can be found in Reference [24].Reference C25] describes an analysis of a resistively-loaded dipoleantenna for which the resistance is continuously distributed along

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the antenna such that resistance loading is in series with theantenna conductors. The response of a hollow spherical dipole innonconducting media is considered in Reference [26].. This spherical-ly-shaped sensor with aresistively-loaded.

Most of the probes

slot around the equator is uniformly

developed by the EMP community are large insize. For small-scale objects such as ours, they cannot be scaledso as to be electrically-small. It will be extremely difficult, ifnot impossible, to fabricate these probes when scaled down in sizesince they typically involve several pieces and intricate shapes.Scaling these configurations to a small size introduces severe com-plications regarding the reproducibility of the probes. Current andcharge probes have been implemented on a relatively small scale byEG & G for the Air Force Weapons Laboratory.* These implementationsare costly if considered in a multipleprobe context, such as inFigure 1, and are too large by approximately a factor of two for thescale of models which are convenient for indoor transient measure-ments.

R. W. P. King and his prot~g& have developed a miniaturemagnetic field current probe and thoroughly analyzed its character-istics on a CW basis [27]. In analyzing the magnetic probe, whichtakes the form of a small circular or rectangular loop, they con-sidered two dominant mode currents which the finite loop sustains:namely, the transmission line mode current

*Model ACD-lA(R) D-dot and Model MGL-8B(R)by EG & G, Albuquerque, NM.

and the dipole mode

B-dot probes manufactured


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current. The transmission line mode current is proportional to themagnetic field passing through the loop, and the dipole mode currentsenses an electric field component in the plane of the loop. Byproperly choosing the load point, they eliminate the contribution ofthe dipole mode to the load voltage.

1.4 Motivation for Adopting King Probe in Transient ApplicationThe King-type loop probes described in [27] are well-suited

to the present application. The following features are important tothe multiple-probe transient measurement configuration:

1.

2.

3.

4.5.

The probe must be implementable in a small size (nominally0.100-0.200 inches diameter);The probe must be low in electrical loss for the sake OFsensitivity;The probe and its transmission system must be nonresonantover a broad bandwidth, and its frequency response mustbe characterized for deconvolution purposes;TheThethe

The first ofricated from

fabrication must be reproducible; andfabrication process must be reasonably economical inmultiple-probe context.these requirements is readily met if the probe is fab-miniature semi-rigid coaxial cable of either 0.023 inch

or 0.033 inch diameter. Concomitantly, the fabrication procedureinvolves a few steps and simple tooling. Thus, points 4. and 5.above are honored, as well.

The King designs described in [27] incorporate an electricaljunction interior to the probe body where the loop center conductorjoins the cable leading the measured signal away from the probe.This junction manifests local reactance and, thereby, the possibil-ity for undesirable resonances. At the least, this reactance will


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introduce its own frequency response to that of the probe assembly.liecircumvented this difficulty by fabricating the probe loop andits signal cable from a single continuous piece of semi-rigid cable.*This configuration leads to the honoring of requirement 3. above.

Wnile the King design introduces no intentional loss into the probeassembly, the continuous cable configuration requires a single cablewhich is small in diameter. The 10SS of even a fraction of a meterof such cable manifests appreciable loss at the upper end of thespectrum of interest. This loss is tolerable in terms of sensitiv-ity, but it must be accounted for in thetion of the probe.

frequency-response calibra-

1.5 Survey of the Present WorkThis work describes the results of adopting the King-type probe

for transient measurement. Chapter 2 presents the chosen implemen-tation of the current probe. The configuration is described indetail and the theoretical performance is predicted in terms of thedevelopment in [27]. A calibration fixture suitable for experimen-tally calibrating probes to be used on a cylindrical surface isdescribed. A measured calibration is presented and compared withthe theoretical predictions.

The probe scheme has been implemented on a cylindrical scat-terer in order to assess its viability. The cylindrical objectwas chosen because its transient response is well-characterizedand because the movable probe methods described in [27] can be

*Liepa has independently developed a similar scheme for broadbandCW measurements [28].

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applied. Chapter 3 describes this implementation and presentssignificant results obtained. These results include SEM modeextractions.

Chapter 4 draws conclusionsthe application and usage of thetrays details of the fabrication

the

from the present work and suggestscurrent probe.technique of a

The Appendix por-current probe.

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II. IMPLEMENTATION OF LOOP PROBES

2.1 IntroductionThis section describes the details of the implementation of

the King-type loop probe for measurement of transient currentsinduced on a thin cylindrical scatterer. The cylindrical scattererwas chosen for an initial study of the use of loop probes intransient SEM extraction because its SEM description is alreadywell-known and because it is amenable to a sliding probe configura-tion, therby avoiding, initially, the need for multiple probes.

In the following, the specific mechanical configuration of theprobes used in this work is described. The basis for theoreticallypredicting their response is described and ultimately compared withmeasured frequency response derived through a transient measurement.The coaxial jig used to expose the probe to a known field isdescribed, as well.

2.2 Construction FormThe fundamental problem faced in applying Kings technique to

transient measurements is that the transmission line system must bewell-matched from the probe output to the load. In the presentcase, the input of the oscilloscope which samples the waveform isthe load. Any severe mismatches would introduce resonances intothe measurement system frequency response. These resonances wouldlimit the accuracy with which the probe response could be decon-volved from the measured waveform.

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In this work, 0.023 inch diameter, 50 ohm, semi-rigid coaxialcable was used to construct the probe as shown in the cross sectionview in Figure 2a. The transmission line leading from the probeand the probe itself are made of one piece of coaxial cable, thusdirectly eliminating most sources of mismatch. The probe is semi-circular with a load-gap at the center of the semicircular loopand is mounted on a cylindrical carriage. The transmission lineis secured in 1/16 inch hollow brass tubing. The cylindricalcarriage conforms to the inside diameter of the tubing from whichthe cylindrical scatterer is fabricated. The carriage can slide in

a notched tube so that a single probe can observe the current flow-ing at any location along the scatterer. The 1/16 inch protectivetube serves a second function as the pushrod for this carriage. I?ig-ure 2b shows a closeup photograph of a probe assembly. Figure 3ashows the same probe assembly residing in the slot in a cylindircalscatterer mode. Figure 3b shows the probe in the scatterer afterthe slot has been covered with conducting tape to block its influ-ence from the measurement. Details of the construction technicpecan be found in the Appendix.

To our knowledge, no connector is commercially available whichcan directly accommodate 0.023 inch semi-rigid coaxial cable. Acommercially available connector compatible with 0.035 inch cablewas adapted using the scheme indicated in Figure 4.* Figure 5pictures the time domain reflectometer traces of two probes

*This approach was suggested by Dr. Valdis Liepa of the RadiationLaboratory at the University OE Michigan.

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A5 fold contor conductor backond soldor too carriage and outer conductor.ez -

.1o5-

~ .975 .

(a)

mmm.1111 m 3

ml- . Ill,1

Figure 2. (a) Center cut side view and end view of the current probemounted on a carriage.(b) A close-up photograph of a probe assembly.

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-.

.,.,.

... . .... .. ..,. .. ..- . .:.. .,.l:, - .,,, . . .. - ... . . . . . . . . .. . ..:.. ;.,~+,.>...:: ----

iJ :::-=-.-,.. . ... . _ .- -7. . .. . . . . . . . . . . .. ... . . .. . .. . . ... -. .< . ,,f:.Lw.2L4~.. iMm=> .. a j~ .:~:i:i:~::'-::.4:n'T- ' ';n '':''' ' '''' ' :::,...,...... .... ... .. .,..,..::... . .t \ ---m

.. .~.---- 4--+--- :

..-.~.- : ............ ... -

.

(b)

Figure 3. (a) A probe assembly fitted in the slot in a cylindricalscatterer.

(b) The probe in the scatterer after the slot has beencovered with conducting tape to block its influencefrom the measurement.

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. r

.035 coaxialconnector center pin

outer conductorof . 0358S coax. I

1 t 02.035empirically adjustedto minimize

Figure 4. A method to adapt 0.035 adaptor for 0.023 coaxial cable.The assembly above fits directly into the outer conductorassembly on an Omni Spectra OSSM 5511 connector.


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probe No.2 2100

5

5

1oo

I I I II 2 3 4 5 6 7 8

I

500

-50

lnPI50

.

Probe No. I

I I 1 I I I1 2 3 4 & 6 ? +Time(nsec.)

Fiqure 5. TDR traces of probes showing two results of empiricaladjustments of-gap between center pin of a 0.035connector and 0.023 coaxial line.

61,155.350.0

45.2

40.4

67,661.155,3

50,0

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through theindicated.while probe

adapted connector with the locations of center-pin gapsProbe number 1 shows some mismatch at the connector,number 2 manifests a mismatch which is barely discern-

ible from measurement noise. The sharp peak after 7 nanosecondsin both probes indicates the mismatch at the location of the notchof the probes. The source of the larger mismatch is not evident,but likely lies in fabrication differences. The measurements shownweremade with a 14 GHz bandwidth TDR system. The mismatches arenot appreciable over the 4 GHz bandwidth that we used in measurement.

2.3 Theoretical Analysis of the Probe AssemblyA frequency-domain analysis of our half loop, following that

of Whiteside and King [29], is presented in this section for sake ofcompleteness. Their theory is developed in the frequency domainand thus is counterpart to a Fourier transform domain theory in thepresent application.

Consider the semicircular receiving loop shown in Figure 6.It is loaded with impedance 21. Its equivalent full loop resultingfrom presence of a perfectly conducting image plane lying in theYZ plane is shown in the Figure, too. Of course, the loop is usedon a curved surface. The approximation of a quasi-planar imagingsurface is valid so long as the loop dimensions are small comparedwith the radius of curvature on which it is situated. Assume thatthe loop is symmetrical about the X and Z axes andof the conductor forming the loop is small so thatanalysis of the current is adequate. The validity

the radius (a)a one-dimensionalof this assump-

tion is subsequently %ested Zorrepresent the transmission line

the loop sizemode current,

23

used. Let f.

as indicated in


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z

Figure 6. Geometry of loop and its equivalent block diagram.

, @1


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Figure 7a.* If the loop is small compared with wavelength, it canbe treated as constant. The dipole mode current, Id, is co-directional and the same on both sides of the loop, as indicatedin Figure 7. Therefore, the dipole mode current must be zero atthe load and, hence,response of the loopcurrent it

It is usefulits load by meansof the loop ~0 is

toof

does not couple into the load. The totaldepends only on the transmission line mode

characterize the interrelation of the loop anda Norton equivalent circuit: The admittance

derived by Whiteside [29] using the expressiongiven by King [30].

-j4o


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w

))

Figure 7. (a) Loop parameters are shown with transmission line modecurrent, 7 and dipole mode current, Td .t

(b) An approximate circuit diagram of a circular loop.

26


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magnetic field enclosed by the loop, according to Faradays law

By Assuming that the magnetic field enclosed by the loop

treated as constant over the area of the loop, the abovecan be approximated as

where S is the surface area enclosed by the loop. Thus ,

can be

integral

the no-loadcurrent in the loop can be obtained by multiplying the admittanceof the loop ~. by the voltage ?.

= -(S/L)@l .n

when the loop resides on a thin cylinder, the magnetic field pene-trating the loop is

-i -H = Iz(Z,ju)/2rir,n

where ~z(Z,jw) is the net axial current flowing on the cylinder atthe probe location Z and r is the radius of the cylinder.

Now we can formulate the equivalent circuit of the doubly-loaded loop, as shown in Figure 7b. The output voltage across theload can be expressed as follows:

.Vl ju = Fi ju iz(z,jw),

where

27


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.The transfer function H(jw) relates

rent flowing on the cylinder at the

the output voltage to the cur-

probe point. For frequencycomponents where UL


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function is constructed by accounting for the electrical length ofthe cable in the phase of Hc. The magnitude function is constructedby linearly interpolating the logarithmic representation (dB) ofthe manufacturers attenuation data (Table 1) for the cable. Thus ,the product ~c(ju) ~(ju) is, in practice, deconvolved from ameasured waveform to recover Iz(Z,t) .

Table 1. Attenuation of Coaxial CableManufacturer: Uniform Tubes, Inc.Micro-Coax Part No.: UT - 20Typical Attenuation (dB/100)

0.5 GHz 1.0 GHz 5.0 GHz 10.0 GHz 20.0 GHz51.0 72.0 163.0 233.0 334.0

The electrical parameters of our loop are calculated using theformulas developed above and the dimensional parameters summarizedin Table 2. The electrical parameter results are summarized there,as well. The shape parameter Q = 2 ln(TrW/a)takes on a value of

7.341 in this case. This value represents a relatively fat loopand is near the limit of applicability of the V@iteside theory.The,current in the circuit shown in Figure 7 is.tional to the magnetic field through the loop.~1 complete the circuit description.

seen to be propor-The parameters L and

Table 2. LOOP Parameters (Present Work)

w 0.125 inch 5.213 10-03 ii

na 0.115 inch L 3.817 nHQ 7.341 21 50 ohm

29


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2.4 Calibration JigA probe must be calibrated in order to determine the true

response of the transient signal. A rigid, 45 cm long, coaxialtransmission line was used to calibrate probes in this work. Thecenter conductor is 3/8 inch O.D. hollow brass tubing with an 0.075inch slot. The outer conductor is 0.785 inch I.D. copper tubing.The outer conductor is actually that of EIA Standard 7/8 inchrigid airliner while the center conductor is slightly larger. Thecenter conductor was chosen such that it has the same curvature asthe scatterer of interest. The result of the combination is thatthe characteristic impedance of the coaxial jig is 44.33 ohms, thecapacitance per meter is 75.2 Picofarads, and the inductance permeter is 0.148 Microhenrys. Figure 8 shows a photograph and linedrawing of the jig, along with an adaptor betweenType-N connectors.

Because of the center conductor size chosen,between a 50 ohm Type-h cable feeding the jig and

of the jig. In addition, the adaptor potentially

7/8 EIA and

a mismatch resultsthe 44.33 ohms

introduces alocal junction reactance. The two effects are represented schemati-cally in Figure 8c. This mismatch is conceivably frequency-dependent, thus introducing the possibility of waveform distortionto a transient excitation.

The frequency dependence of the junction is accounted for interms of a transmission coefficient

T10 =


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-------. .-.. ...... ~,

>:%.*&.. ,m:~

l 71

(a)

i

(:)

IPortO Y Port 1 VI i Port Oz8=44,33az c z 50

Figure 8. (a) Photograph of a coaxial jig and a 7/8 to N adaptorused for measurement of known fields.

(b) 7/8 rigid coaxial transmission line and 7/8 to Nadaptors used to calibrate the current probe.

(c) Approximate junction representation between adaptorand transmission line.

31


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Go is the incident wave on the 50 ohm line. This coefficient cannotbe measured directly because of the difficulty associated withmeasuring a voltage on the 44.33 ohm line. The following indirectmethod was used instead.

A measurement is made through the jig using two identical Type-N to EIA 7/8 inch adaptors. The voltages Vin(k) and Vout(t),whose transforms are shown in Figure 8b, are measured.* They areboth defined on 50 ohm cables, so this measurement is straight-forward. The adaptors are presumed to be identical so that thesymmetric transmission line model shown In Figure 8C is valid.

From the model we determine that

where

and

tout = tin -jml/c10 Ole

*10 = 2zl\(z1 -1-ZOZIY + Zo)

(2)

01 = 2zo/(zl + ZOZIY+ Zo) .

The effect of multiple reflections in the jig region is not expressedin (2). In the measurement, the observations of Vout(t) andvin(t)

*Transform data are computed numerically from a 512 sample digitizedrecord of a 5 nanosecond duration waveform. Only every 32nd pointof data is necessary to satisfy the Nyquist criterion for the sig-nificant spectrum of the pulse generator. Therefore, the transformsdisplayed herein were computed as the average of 32 phase adjustedtransforms of subsequences of the data record. Each subsequence isfilled with trailing zeros to a length of 256 in order to maintaingood frequency resolution.

32


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are conducted over a time window of 2-1/2 nanoseconds so that mul-tiple reflection effects are time-gated out of the observation.Thus (2) is the correct model of the observed voltage ~ in theoutFourier transform domain.

We wish to identify T,. in (2) from observation of ~-.-Li,..LuWe observe that

and that-jd/c < OITIO = out in

Thus, time-gated transient observation of

VUL. L

(3)

(4)

Vin(t) and Vout (t),followed by numerically Fourier transforming them, allows computa-tion of (4). Since Zo and Z1 are known, the desired transmissioncoefficient can be computed from (3).

The method described above was implemented using an IKOR IMPgenerator as a transient signal source and a Tektronix P7001

Oscilloscope* with a 7s12/s-6/s-53 sampling scope configuration.The input waveform Vin(t) and its spectrum are shown in Figure 9.The bandwidth of the IMP generator waveform allows determinationof the transmission coefficient to near 2 GHz. The result is shownin Figure 10. It is seen that the coefficient is flat with fre-quency and that the impedance ratio Z1/~ in (3) (0.94 in the

*The P7001 is a digitizing oscilloscope and is interfaced to aTektronix 4051 desktop computer. In this measurement, all wave-forms used result from the averaging of 16 individual waveformobservations acquired by the 4051. This procedure was used con-sistently in all measurements reported in this work, unlessotherwise stated.


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8001

600-%z~ 4oo_2oo-

0 1 1 1 10 { 2Time (nseh703

Figure 9. Averageused in

of 16 waveforms produced by the pulse generatorthis experirnen.tand its spectrum.

34

.

,


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2.0

1.51

=51

5

I I I i I I I I I I i I I 1 I I

I 5 I I1 I1.5 2.0Freq. GHz)

Figure 10. Transfer function of the transition of TypeNconnector to 7/8 coaxial line.


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present case) fully accounts for it.

2.5 Transfer Function of Cable AdaptorsAn OSM to N adaptor had to be employed in order to measure the

i. and ; using the Tektronix Sampling Heads. This adaptor can-ln outnot be ignored in the calibration procedure and, therefore, itstransfer function was determined in the followingdifferent Type-N to OSM adaptors were used in the

N-female to OSM-fernale;N-male to OSM-male;N-female to OSM-male.

fashion. hrmeasurement:

The combination of these adaptors is shown in Figure 11. The threeadaptors are presumed to have identical transfer functions. Thisis, of course, an approximation. It is reasonable except for smalldifferences in electrical length, and khese differences are smallerthan the time resolution of the instrumentation.

Stiject to this assumption, the spectra of the voltage wave-

forms ~2 and 3 (Figure 11)

2and

can be written as follows:=E


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1

-d 3i3EFamplingHead v~Figure 11. Block diagram to compute transfer function of N to OSMadaptor by measuring V2 and V3

1: N-female to OS1l-male;2: N-female to OSM-female;3: N-male to OSM-male.


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[email protected]

2.6 Probe Transfer Function

The precedingfrequency responseis shown in Figure

two sections provide the basis for measuring theof the loop probes. The measurement configuration12a. The slot of the center conductor is taped

with conducting tape to eliminate coupling to the coaxial regionformed by the center conductor and the probe push rod. The inputvoltage spectrum ;


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~ Impedance ZI ~a. .. ..

Vin N Vp~d+ f

a4

2-

0-

-2-

-4-

6 I I I I I I I I I i I I I I I J I I i I I I 2 3 4 5Timehsec.) (b)

Fiqure 12. (a) Cross-section of the calibration jig used tocalibrate the probe.

(b) A probe output V (t) sampled by awhere the sourcepof excitation isshown in Figure 9.

current probethe pulse

39


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-g

I o 1./

0.8 /./././ /,~=-./

0.6z

/E.. ~.

o.2

0,0 I 1 I 1 1 I 1 I r I0 5 I1 0 I 5

Freq. GHzFigure 13. Experimental and theoretical transfer functions obtained for 0.125 probes. (a) theoretical

from equivalent circuit - dot-dashed; (b) theoretic with cable attenuation included - soli~circles; (c) experimental Probe no. 1 - dashed; (d) experimental Probe no. 2 - solid.


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Oscilloscope takes the average of the waveform point by point inthe sampling process, based a different excikakion pulse eachtime. Because of the rapidity of the events being measured, it isnot possible to circumvent the sampling process, of course.

The phase information associated with Figure 13 was notextracted or used explicitly. The various adaptors required incalibrating a phase (time) reference for tie jig preclude doing soin any accurate fashion. In practice, the measured data waveformsare all time referenced, based on a causality criterion.


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III.

3

IMPLEMENTATION OF PROBING SCHEME ON A CYLINDRICAL SCATTERER

IntroductionThe net axial

was measured usingsurface current on a thin cylindrical scattererthe probing method. The currents were induced by

the electromagnetic field radiated by a transmitting antenna nearby.The Singularity Expansion Method description of the cylinder wasextracted from this set of data and compared with Tesches computedresults [31].

A thin cylinder was chosen for the following reasons. The thincylinder has rotational symmetry and the radius is small comparedwith the length; therefore, one dimensional analysis of the currenton it is sufficient. The probe carriage is fitted inside the scat-terer into a longitudinal slot, thereby making it possible to locatethe probe at any point along the structure. Thus, flexibility as toprobing points is provided in evaluating the present probing method.A thin cylinder is relatively easy to solve theoretically, andreliable comparison data is readily available. We ultimately do SEMparameter extraction from the data and the thin cylinder SEM descrip-tion is reliably characterized.

3.2 Measurement ConfigurationA detailed description

presented in this section.A block diagram of the

of the transient measurement system is

measurement facility is sketched inFigure 14 along with a photograph of the inside of the facility.

43


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JL--J= Qno00

DIGITIZINGOSCILLOSCOPEsamp.

Q

ng.r. cog,

75ns9c. 1+] ,,TRiGGERSAMPLE[ ~Gil .omm~

COMPUTER

(al*.,,..:.,.,, .l

-

(b)Figure 14. (a) Block diagram of transient antenna range facility.

(b) The measurement facility photographed from behindthe ground plane.

44

0


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The scatterer is formed from brass tubing 30.0 cm long and 3/8 inchin outer diameter. It carries a longitudinal slot of width 0.075inch. A close up of this scatterer is pictured in Figure 15. Theslot of the scatterer is taped with conducting tape with the probein place. It is mounted on a brass disk so that it can be installedreadily into the ground plane. The location of the probe can bedetermined using the scale on the positioning apparatus shoTmainFigure 16a. This carriage is mounted behind the disk where thescatterer is attached. The pushrod for the probe is attached to acalibrated stop on the movable arm of the probe carriage. Figure16b pictures the scatterers mounting disk from the back side of the

ground plane with probe-positioning jig in place. The probe, isoriented such that the plane of the probe is perpendicular to thedirection of propagation from the transmitting antenna.

The transmitting antenna is a long cylinder made of brass tub-ing. It is 1.71 meters long and 0.25 inches in-diameter at thebase. The radius is stepped down to smaller sizes of brass tubingaway from the ground plane to reduce the weight, and thereby droop,since it is a horizontally-mounted structure. The transmittingantenna and the scatterer are mounted at the center portion of a2 x 8 foot ground plane and they are 79 cm apart. Figure 17 showsthe outside view of the ground plane. The circles in the centerpanel are the locations for the transmitting antenna and thescatterer model, respectively.

3.3 InstrumentationA Digital Processing Oscilloscope (DpO) is used to samPle the


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o

Figure 15. A close-up of a scatterer mounted onis placed and the slot is taped with

. .

a brass disk. Probeconducting tape.

46


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,-

.

.

....-. ...+...... .._. -...-,e>,h,.a%. -- . ___.--+ -... ; :: : : :

a

.. .. ... .,. . . . . ... ,..,,. a

L &aJg

Figure 16. (a)

(b)

(b]A probe carriage is attached behind the disk wherethe scatterer is mounted.The probe carriageplane.

and disk mounted in the ground


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.-,. ..,- -

Figure 17. Ground plane for transientthis ground plane is 18 x antenna system. The Size of20.

48


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waveform. The configuration of the DPO system includes a TektronixD7704 Display Unit, P7001 Processor, 7s12 S~pler, s-6 SamplingHead, and S-53 Trigger Recognize. The excitation source is anIKOR IMP generator whose waveform is shown in Figure 9 . ATektronix 4051 desktop computer is used as a DPO controller and forintermediate data storage. All the data were shipped to the hostcomputer, a DECSYSTEM-10, from the Tektronix 4051 for final numeri-cal processing. The sampler requires a 75 nanosecond pretriggerpulse. Therefore, the excitation signal was passed through a 75nanosecond delay line prior to delivery to the transmitting antenna.A 1.14-meter long and 0.141-inch diameter semirigid coaxial cablewas used as an extension cable in connecting the probe and the s-6Sampling Head.

4 Measured Data

Current waveforms were sampled at 19 locations along the scat-terer for a time period of 5 nanoseconds. Sixteen waveforms wereacquired and averaged as described in Section 2.4. The measuredtransfer function for the probe (as in Figure 13) was deconvolvedfrom the measured waveform by taking a forward Fourier transform,dividing out the probe response, and inverse transforming. This de-convolution fails at zero frequency because the probe transferfunction properly is zero there. Thus , the DC (zero frequency)level of the resulting waveforms is unreliable. This level can becorrected by the appropriate use of interpolation in the frequencydomain. The DC level of the waveform has no bearing on the reso-nant modes derived in SEM extraction. Since the ultimate utility


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of this work is in S extraction, this correction was not carriedout here.

The result of these measurements and the subsequent reductionare given by the solid lines in Figures 18 through 36. The dot-

0dashed lines represent the results of direct integration of themeasured waveforms. The dashed lines provide, for comparison, theresult of a numerical model of the measurement configuration cal-culated with the TWTD transient scattering computer code from

*Lawrence Livermore Laboratories.The TWTD program was employed with some modification in order

to Incorporate the features of the spher%cal wavefront impingingon the scatterer in the experimental configuration. The radiatedeleckric field used as the driving field in the TWTD program wascomputed using the results given by Harrison and King [35]. Theirmodel was for an infinitely long cylindrical transmitting antenna.Our system is equivalent to their configuration since the responsewaveforms were gated off in time domain before the pulse radiatedfrom the end ofthe finite-length antenna arrived at the scatterer.The input waveform in the Harrison and King model was taken to bethe IKOR IMP generator waveform delivered to the transmittingantenna. The TWTD program is sensitive to noise present in thedriving Field data and it can be seen in late-time behavior ofseveral of the Figures. The noise level is heavier when the

*TWTD is a computer code which computes the time history ofthe surface currents of a thin scatterer and details of thedescriptions can be found in References [32] , [331 , and [34] . Q

50


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) . 7

9.

-a .

3.

--4,..

9

I---- TWTD computed Measured 6 deconvolwed

-S -Measured integrated------

time (nsec)Figure 18: Transient current measured 1.5 cm. from qround plane compared with TWTD computation1.3 cm. from ground plane.


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time (nsec,)Figure 19. Transient current measured 3.0 cm. from ground plane compared with TWTD computation

3.0 cm. from ground plane.

I al i , . ,


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F

~,~ ---- TWTD computed Measured ~ deconvolved1 .> L I - Measured G integrated

time(nsec.)Figure 20. Transient current measured 4.0 cm. from-ground plane compared with TWTD computation



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-1-d------

1 1 7---- TWTD computed Measurad ~ deconvolved

z 3 4time(nsec.)Figure 21. Transient current measured 5.0 cm. from ground plane compared with

4.7 cm. from ground plane.TWTD computation

*


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-3T-1

i

E .2?

9.3 1-----1 i 1

---- TWTD computedeasured deconvolwed

--- -Measured & integrated _. ~_ . .I

b* .a.

1 i J 1I r I I IJ i 1 I r 4

time(nsec.1

J

Figure 22. Transient current measured 6.o cm. from ground plane compared with T~D computation6.4 cm. from ground plane.


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~.~ 1 I 4

I---- TWTD computed Measured deconvolwed

R 3 1 L ----- - Measured integrated

i I 1 1

time(nsec,)Figure 23. Transient current measured 7.0 cm. from ground plane compared with ThWD computation


*


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.

-.

E .

5

3

---- TWTD computed Measured deconvolved

--bL

1

1

2

Figure 24. Transient current measured8.1. cm. from ground plane.

time(nsec)8.5 cm. from ground plane compared with TWTD computation


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co

-MeasuredadecOnvOvedi__l

---- TWTII computed

-- Measured ~ integrated

3 4 5time(nsec.1Figure 25. Transient current measured 11. C l cm. from ground plane compared with TWTD computation


@ Ii


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,

Ikor Imp Omni-wave Note265

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