013378-1-F IC0i L 2_ PL RSIT F IC-' D9H." '- C"' "A"?" XCA<sL AND COCXPUTaER ENG aN"'ERING SWXEEP MIEN..,,CY SURFACE FIELD MEASUPREMENTS \adisd V. Li a-i;, Fi.i, epoC:t...tract F295.175-C-0035 I Prepared for: Air Force Weanons Laboratory j- Kiti. ~t._ Ar l or-e ase New.:r 1 J. "-.ne"xi?-I l-"- -0 s... -...j _i New Mexico 1 7 '-..,...1 -. Ann Arzbor, 5 &icigrn

ABSTRACT The research described in this report is primarily experimental and deals with the development of surface current and charge measurement techniques for use on scale model aircraft. The report describes a sweep measurement technique where measurements are made by sweeping a frequency with the sensor fixed and positioned in turn at selected measurement points of interest. Design and performance data of a number of current and charge sensors are presented, including the diode sensors that use high resistance telemetry leads for reduced interaction with the model and the incident field. Results of a current loop probe calibration study, for cases when fields are measured along elements of small radii of curvature, are presented. Measured data are presented for 747 model aircraft and models of various degrees of similarity to the 747 shape to demonstrate the feasibility I - us a 1GU % LLJUJ II L'v"V O" L-L".LO vUoL. cGabLLL.4 l.l4 V. 0=1.4O; iIOJILL1 program. Current measurements are also presented for the so-called wire grid model used by the Air Force in the computation of fields on the B-i aircraft. Amplitude and phase data cover 1.5 to 16.5 MHz full-scale frequency range and explicitly shows the basic resonances of the structure.

PRE FACE The author is grateful to Fred P. Rhine of the Radiation Laboratory for his assistance in performing the measurements and for helpful suggestions and discussions from Professors R. E. Hiatt and T. B. A. Senior. The assistance of AFWL personnel in the course of this study is also greatly appreciated.

CONTENTS Section Page I INTRODUCTION 1 II EXPERIMENTAL FACILITY 3 1. Chamber 3 2. Equipment 4 3. Probes 11 mI EXPERIMENTAL STUDIES 26 1. Introduction 26 2. Surface Perturbations Due to Sensor Leads 26 3. Effect of Monopole Length on Signal Received 30 4. Calibration for Loop Probes - Experiment 34 5. Scale Model Studies 43 IV WIRE GRID MODEL MEASUREMENTS 53 V SUMM:LARY 68

ILLUSTRATIONS Figure Page 1 Block diagram of surface field measurement facility. 5 2 Raw charge data for the 3.133-inch diameter sphere (test object). 7 3 Raw charge data for the 6-inch diameter sphere (calibration sphere). 9 4 Measured (xxA) and theoretical (-) normal electric field at the shadow boundary of a 3.133-inch diameter sphere, 0 = +90~. 10 5 Measured (xxx) and theoretical (-) current in front of a 3.133 -inch diameter sphere. 12 6 Construction details of charge probes. 13 7 Charge measured with a monopole on a 3.133-inch diameter sphere. 15 8 Charge measured with the disc probe on a 3.133-inch diameter sphere. 16 9 Self-rectifyinc charge probes. 18 10 Block diagram of equipment needed when using diode probes. 20 11 Amplifier-filter for 1 kHz signal. 21 12 Linearity test of the diode-disc probe measured on a 6-inch diameter sphere. 23 13 Charge measurements with resistive lead probes on a 3.133 -inch diameter sphere. 24 14 Charge distribution on a 3.133-inch diameter sphere. The level for the measured values has been adjusted for best fit. 28 15 Theoretical and measured phase on a 3.133-inch diameter sphere. The level of the measured data has been adjusted for best fit. 29 16 Received signal vs. monopole length. 33 17 Absorber arrangement for matching the ends of the wire. The match is excellent in the 1-2 GHz range. 36 18 Dimensions of the probes used. 37 19 Measured (Probe 211) and theoretical current amplitude for cylinder. 39 20 Measured (Probe 211) and theoretical current phase for cylinder. 40 21 Measured (Probe A) and theoretical current amplitude for cylinder. 41

Illustrations, continued 22 Measured (Probe A) and theoretical current phase for cylinder. 42 23 Scale models of various resemblance; dimensions in centimeters. 44 24 Current amplitude on the top of the fuselage of the 747 models; top illuminated, E1 parallel to the fuselage. 46 25 Current phase on the top of the fuselage of the 747 models; top illuminated, Ei parallel to the fuselage. 47 26 Current amplitude and phase on the bottom of the fuselage of the 747 models; bottom is illuminated, Ei parallel to the fuselage. 49 27 Current amplitude and phase on the top of the fuselage of the 747 models; top illuminated, El perpendicular to the fuselage. 50 28 Current amplitude and phase on the.bottom of the fuselage of the 747 models; bottom illuminated, E1 perpendicular to the fuselage. 51 29 Wire model used for skin current measurements. Full scale dimensions are in meters; in parentheses are model dimensions in inches. Scale: 1/239.3. 54 30 Current amplitude on top of the fuselage at center, E parallel to fuselage. 56 31 Phase on top oi the fuselage at center, Ei parallel to fuselage. 57 32 Current amplitude on bottom (shadow side) of the fuselage at center, Ei parallel to fuselage. 58 33 Phase on bottom (shadow side) of the fuselage at center, E1 parallel to fuselage. 59 34 Current amplitude near cockpit, Ei parallel to fuselage. 60 35 Phase near cockpit, Ei parallel to fuselage. 61 36 Current amplitude on top of the fuselage, E perpendicular to fuselage. 62 37 Phase on top of the fuselage, E perpendicular to fuselage. 63 38 Current amplitude on top of the wing, Ei perpendicular to fuselage. 64 39 Phase on top of the wing, Ei perpendicular to fuselage. 65 * *

SECTION I INTRODUCTION The requirements under Air Force contract F29601-75-C-0035 included the following: 1. Develop charge measuring techniques and develop techniques for amplitude and phase measurements of current and charge in the sweep frequency mode. 2. Provide experimental data to verify computer codes for wire models. 3. Devise error-correction techniques for measurements near edges. 4. Modify the equipment and/or the techniques as needed to allow reliable measurements to be made at lower frequencies. 5. Determine whether or not it is feasible to use crude models in low frequency measurements. 6. Perform sample measurements to demonstrate the capability of the facility. Tn1in. +r, oniira~ of fthQ Stuody, all of the stated requirements have been satisfactorily completed. The work performed included the development of monopole and disc type charge probes, the development of charge probes using high resistance leads for carrying the detected signal to the receiver, thus avoiding the need for using metallic leads, and the design and construction of the electronics needed for use with the diode type charge probes. A major innovation was the development of techniques for using a swept frequency system for obtaining and recording data as a function of frequency. A swept frequency signal generator was used as a source and the Hewlett-Packard network analyzer was used as a tracking receiver and it provided both amplitude and phase information. As is shown by the data included later in the report, these techniques provide excellent results. The effort required is less than that needed when using the fixed frequency method (ref. 1).' It should be noted, however, even 1. E.F. Knott, Surface Field Measurements, Interaction Application Memo No. 5, Air Force Weapons Laboratory, September 1974 (The University of Michigan Radiation Laboratory Report 012639-1-F). 1

though the swept frequency method provides continuous (raw) data, the data must be reduced or normalized with respect to the incident field. This reduction is done manually, and this accounts for the discrete data points for amplitude and phase data vs. frequency as presented in this report. In the task where the feasibility of using crude models was examined, current data were measured as a function of frequency on scale model Boeing 747 aircraft of various degrees of perfection. Three models were used in this study. The swept frequency range covered included the critical resonant frequencies of the aircraft. This study is discussed in Section III. Measurements were made to provide data to compare with those obtained with computer codes for wire models. Use was made of the so-called wire grid model employed by Taylor et al. (ref. 2)* in their computation of current on the B-1. For the wings in the forward and swept positions, five different measurements were made and data are presented for the 1. 5 to 16. 5 MHz full-scale frequency range. Of particular interest are the data measured on the top and bottom of the fuselage for the case of symmetric excitation with illumination from above. Since these data represent the current for the illuminated and shadow side of the fuselage, they should provide valuable information for determining the validity of various thick wire model computer codes, especially when they are used at frequencies beyond the first resonance peak. The results are discussed in Section IV. In summary, the experimental facility is discussed in Section II; Section mI contains a description of the various experimental studies. The results of the sample measurements are presented in Section IV and Section V contains a summary of the research effort and present capabilities. 2. C.D. Taylor, K. T. Chen, and E.T. Crow, An Improvement on Wire Modeling for Determining the EMP Interaction with Aircraft, Mississippi State University, Mississippi State, MS, 39762, October 1974.

SECTION II EXPERIME NTAL FACILITY 1. CHAMBER The surface field measurement facility has been described in detail in IAM No. 5 by Knott (ref. 1). Briefly, the facility consists of a 40-ft long tapered design anechoic chamber with the transmitting antenna placed at the apex of the tapered section and the receiving or working area, which is rectangular in cross section and is 11 ft deep, 10 ft high and 12 ft wide, at the other end. A probe positioning mechanism that provides by cylindrical and cartesian direction of motion is mounted above the ceiling. A balsa wood tower extends downward, supporting either charge or current probe. The model on which the surface fields are measured is supported by a styrofoam column placed on the floor underneath the probe positioning equipment. Up until the present program, the surface field measurements were made surface of the body to determine the surface field amplitude at selected points on the trajectory. At each point the operator would go into the chamber, position the probe, and then leave the chamber to obtain a measurement of the field amplitude at that point. When another frequency was required, the frequency of the illuminating cw field was changed appropriately and the procedure repeated. Thus, for numerous scans or frequencies, or sampling at closely spaced points, the operating was a very timeconsuming one. With the present need for surface data as a function of frequency, rather than of position along the surface of the body, the cw technique is very cumbersome and rather impractical. Consequently, a swept frequency scheme for measuring the fields was developed and, as the data presented in this report verify, the technique provides excellent data. Nevertheless, there are still areas where improvements can be made, and we would hope to incorporate these changes in future programs of this type.

2. EQUIPME NT Figure 1 shows a block diagram of the chamber and the equipment used for sweep frequency measurements. It differs from the one used previously (ref. 1) primarily in instrumentation. For single frequency measurements, such as done by Knott, the basic instruments required were (1) tunable cw source, (2) microwave receiver, and (3) display indicator, such as a VSWR meter. For a sweep measurement facility, about three times as much equipment is required, and likewise, the cost for such equipment is also about three times as much as for the single-frequency equipment. The basic ingredients of a sweep measurement system are a sweep generator and a tracking receiver (HP 8410S network analyzer). These are supplemented by the additional components shown in Fig. 1. On the transmitter end, a signal from the sweep generator (10 mw, nominal) is first amplified by a traveling wave amplifier (TWT) to about 1 watt. This provides an increase of signal-to-noise ratio at the receiver end. (This amplifier is optional, and probably would not be used when a preamplifier is used between the sensor and the network analyzer.) After the TW;T, the signal goes through a 20dB coupler, an isolator, and then to the transmitting antenna. The coupler provides the reference signal for the receiver and the isolator reduces antenna reflections which may become large as a result of the extended frequency range. On the receiving/recording side a signal (proportional to the current, or charge density) is picked up by a sensor and amplified before going into the signal port of the network analyzer. At present we do not have such an amplifier, and to maintain a sufficiently high signal-to-noise ratio, a TWT amplifier is used in the transmit side to boost the transmitted signal. To obtain a relatively flat phase difference between the reference and signal channels, the two signal paths should be of equal electric lengths. For gross adjustment of the electric lengths, coaxial cables are used, and for fine adjustment a line stretcher and a precision phase shifter are used. When a measurement is taken, the amplitude and phase data are typically recorded by an X-Y recorder on 8.5 x 11 inch graph paper, with amplitude (dB) and

sweep T WT ~~~~~~~~signal l amplifier' - generator reference directional a signal er coupler pretamplifier phaseused a needed shifter ~~~~~~~~~~~~~irecorder ldisplay FIG. 1: Bloc diagram f su probe leasurement facility. network analyzer anechoic chamber X-Y " CRT used as needed recorder display FIG. 1. Block diagram of surface field measurement facility.

phase (degrees) on the vertical scale and frequency on the horizontal scale. The CRT display is used for "tuning up" the system prior to recording. This includes adjusting the reflector of the TWT amplifier, tuning the antenna, setting the power output of the sweep generator, and adjusting the phase shifter. All adjustments are made to obtain a reasonably flat amplitude/frequency response over the frequency band. To present a cursory view of the sweep measurement technique for a typical measurement procedure, the resultant raw data and the final reduced data are presented. In this example, the charge on the model (a 3.133-inch diameter sphere) is measured. In this case the charge probe is mounted (taped) on the model; the model is placed in the chamber and the phase and amplitude data are recorded on the X-Y plotter as the frequency is swept over the 1-2 GHz range. The same measurement is repeated, but this time with a 6-inch diameter reference sphere that will provide the calibration for the incident field. Care must be taken that the two models are placed at the same phase reference plane in the chamber, otherwise an offset in the phase data will result. Figure 2 shows the raw amplitude and phase data of the normal electric field as as measured at the shadow boundary of the 3.133-inch diameter sphere and recorded on the X-Y plotter. Note that even though various tuning adjustments were made prior to these measurements, both the amplitude and phase data exhibit numerous noise-like ripples over the band. These can be attributed, in part, to the transmitting antenna, reflections from the chamber walls and numerous mismatches in the cables, especially in the connectors. The wiggles can be decreased somewhat by adding small attenuator pads in critical places in the system, but these also attenuate the signal and thus may add "real" noise in the data. Since it would be very difficult, if not impossible, to develop a measurement system such as ours in which the absolute incident field strength would be flat, say within 1 dB over a 2:1 frequency band, a calibration run is made to determine the incident field amplitude and phase over the frequency band of the test. This can be done either by measuring the incident field directly, or by measuring the field on a body for which the fields are known in relation to the incident field. Thus, a

Of I -- — ~~~~~~~~~~~~~~~~~~~- ~~~200 1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,,... ~ ' ' r t ' '! I'; I f ' ';~ I ' ', i.I,. 1~~~~~~~~~~~~~~~~~~~~~~ l i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1 ~ ~~~ ~ ~:: a m:. t u d,!.~, I,....-;: ~. ~( ~ I~~~~~.I: ' I i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,~~~ ~ ~~ ': i/ I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I ~ ~~' '. 100 -,3 I!r '% ~ ' t ' I r '. ', ', ' ' i ~ ~...:., Y r %......... ~i. i,, t,,,,.. i,,, i I c(; t -: ' ' ' '~ I '... t, 1 ' ~',' ~~ ~;,I. ' '' Q) 1 I I r~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ ~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~....'.,.'~~ ' i~;,, F 1~~~~~~~~~~~~~~~~~ I I I' I., k~~~~~ ~ ~~ ' ''! I ~ 1 ' ' I,,. ': ~ ~1~ iI I ~~~~~~~~~~~~~~. "-~~~~,, phase;,:.'....'!~,.I. ~~~~~~~~~~~~~~~~~~~~~~~~~~~.... ~ ' ~ i: t~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~: I:.,; '~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~ ~ ~ ~~~,,,,. i:I ' i ' i............. " ~~ ~ ~ ~ ~~~~~~~~~~ '' "I'" 'I i..... I ~~~~~~~~~~~~~I0 ~~~~~~~~~~~~~~~~~~I.1 I 1.0 1.5 2.0 frequency in GHz FIG. 2' Raw charge data for the 3. 133-inch diameter sphere (test object).

measurement of amplitude and phase on a reference sphere can be easily translated to the corresponding incident field values using Mie series computations, for example. Also, in order to keep the experimental errors to a minimum, it is appropriate to use the same probe (i.e., sensor) to measure the surface field for the model as well as for the calibration of the incident field. Expecially when a sensor is of the surface mounted type, some type of surface must always be provided, and for this we have found a sphere to be an excellent choice. The measurement of charge on a 6-inch diameter sphere for incident field determination is shown in Fig. 3. Comparison of Figs. 2 and 3 shows that the corresponding amplitude and phase curves are quite similar, with slowly varying separation between the corresponding amplitude and phase curves. Also, the same small wiggles appear on each of the curves, and since the difference between the corresponding curves is of concern, the presence of the wiggles makes reduction of the data more tedious, but not necessarily less accurate. The results reduced from the raw data are presented in Fig. 4, and these are then compared with the theoretical values as computed from the Mie series for the same diameter sphere. Observe that the measured amplitude is about i dB iow, except for the last point, which is about 3 dB high. This was caused by the loss of track by the network analyzer, as can be seen by the curve jump in the upper right hand corner of Fig. 3, and therefore such a point should not be considered in assessing the accuracy of the measurements. The corresponding phase data of the sphere measurement is shown in the lower part of the same figure and there is, on the average, about a ~5 degree difference between the experimental and theoretical data over the frequency band used. This phase deviation is probably indicative of what can be achieved in our measurements, and we expect no major phase errors as a result of faults in electrical equipment; our concern must be with the accuracy with which a model and the reference sphere can be positioned and aligned in the chamber with respect to the arbitrary reference plane. For example, a similar measurement was made on the opposite side of the sphere where the charge amplitude should be the same, but the phase should differ by 180 degrees. The measured amplitude was again in excellent

i ~ —: ~ -,-, '-~.....-200 100 I..i]. ~': amplitudeo i /,,,,. S ~ I I~.,i I cI I I I ( ~ ~~~~~~~~ 1 9 C, ' f i. s \I i!ir E~-30 *1.~I ~i 100 I~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~.52 r equency, iG FIG. 3: Raw charge data for the 6-inch diameter sphere (calibration sphere). Pc ~i~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. f,~ -~. I,, t, I~, t, I ~~ '',- ( ), ' I '-~., I r " I d) 1 ~~~~~~~ ~~~~~~ ', 3 i I~~I i I~~~" r ~ ~ ~ ~~~ 'i ~ i ',! '., '!~~~~~~ Y r~~~~.....,, 1......,.' - 1. P.. f reunynz

phase reference | Einc( ~~~10 -r~ r- - Probe: monopole, I = 0.240 inch 0 X C\ i x! 2 15 20 1.0 1.5 2.0 -20 b) X X. -30 ct X x -40 1.0 1.5 2.0 frequency in GHz FIG. 4: Measured (xxx) and theoretical (-) normal electric field at the shadow boundary of a 3.133-inch diameter sphere, 0 = +90~. 10

agreement, but the phase was definitely offset by 5 degrees, apparently resulting from a 1/8-inch displacement of either the model or the reference sphere from the phase reference plane. 3. PROBES An indispensable component in any current or charge measurement scheme is a probe or a sensor through which a surface current or charge is transferred into a measurable signal. In the past, most measurements at the facility have been surface current measurements and for these we have constructed and now have a dozen or so loop probes which are basically shielded unbalanced magnetic loops ranging in diameters from about 2 to 10 millimeters. Knott (ref. 1) has described the construction and performance of loop probes in the previous report; more detailed information about construction and characteristics of loop probes can be found elsewhere (ref. 3). Using the measurement procedure described above for charge measurment, a surface current was measured at the specular point for the same model (a 3.133-inch sphere) using a loop probe (No. 221) and repeating the same measurement on a 6-inch diameter sphere for the incident field calibration. The reduced data is shown in Fig. 5, and a comparison with the theoretical values shows that the results here are very good. The amplitude agreement is within 0. 5 dB and the phase difference is less than one degree. The high phase accuracy of this measurement can be attributed, for the most part, to the fact that it is easier to align the front of the sphere than the shadow boundary with respect to the reference plane. Also, the currents at the front of the sphere show less variation with position and frequency than does the charge at the shadow boundary, and consequently a small misalignment of the sensor near the front is not as critical as it would be at the shadow boundary. To supplement the current measurement capability, some charge or D-dot probes were developed.. Figure 6 shows the construction details of two such probes that are expected to be the "workhorses" in future measurement programs. In the upper section the standard monopole is shown. It is made by attaching an OSSM connector (an Omni Spectra, Inc., product) to one end of a 2-foot long, 0.020 inch diameter, 50 ohm semi-rigid coaxial cable. The other end is then used for the probe. First, a piece of the outer conductor, about 0.3 in. long is removed, leaving 3, E. F. Knott, Study of M1icrowave Dosimetry, University of Michigan Radiation Laboratory Report 010531-1-F, July 1972. 11

8 ox x x x 0 csi 1.0 1.5 2.0 frequency in GIIz I 'Ai x phase reference -5 I..I 2. 1.0 1.5 2.0 FIG. 5: Measured (z ~) and theoretical (- ) current in front of a 3. 133 inch diameter sphere 12

I i 0.020" COAX 0.5" (a) monopole 0.5' 5".032" O.02&' COAX (b) disc FIG. 6: Construction details of charge probes.

the insulation intact. The inner conductor forms a monopole which is then bent at right angles where the outer conductor ends. The monopole and coax are then taped with aluminum adhesive tape to the test object and to minimize the discontinuity near the base of the sensor, a piece of tape in which a small hole has been made is placed over the monopole. When such a sensor is transferred from one object to another, or even from one place to another, the tape must be removed and retaped. During the process, the length or the height of the probe changes as much as 0. 010 inch. Consequently, it is difficult to obtain the same length probe when it is transferred, but the resultant length of the mounted probe can be accurately measured to within ~ 0.002 inch with a depth micrometer. Once the lengths are known, the data can be corrected. The study of the monopole length vs. signal received is presented in Section HII, and there also the procedure is given for correcting for changes. in the probe length. The lower part of Fig. 6 indicates the construction details for the disc probe, patterned to some extent after AF Model CFD-IA D-dot sensor, but smaller by a cc,,r f 1 7. Tf onCct+ nf t.xn disc-, one 5/32 inch and the other 1/2 inch in diameter cut out of a G-10 printed circuit board material having a 0. 015-inch fiberglass base and 0. 001 inch copper on one side. The small disc is the active portion of the sensor and the larger disc provides a base through which the. 020 inch coaxial cable passes. The base has radial slots cut for easier flexing when mounting the sensor on a curved surface. When the sensor is mounted, the base disc is covered with tape to smooth the surface. Likewise, the coaxial cable is taped until it leaves the surface, usually on the opposite side of the body. Although the charge data measured on a 3-inch sphere was presented previously (Fig. 4), for probe evaluation purposes it is essential that the measurements be made under similar conditions, preferably in sequence. Such measurements were made with the monopole and disc probes and the resulting data are presented in Figs. 7 and 8 respectively. In each case, the 6-in sphere was used as a reference. Note that the phase data are referenced at the front and not the center of the sphere as is customary. This shift was made to simplify the positioning or alignment of the model

x X Eince 10 phase reference probe: monopole, L = 0.235" o\ l X mS ~ 6 CM 4 _ 1.0 1.5 2.0 x x X measured Mie series 120 100 - I00 80 x x 60 1.0 1.5 2.0 frequency in GHz FIG. 7: Charge measured with a monopole on a 3.133 inch diameter sphere. 15

12 [I iinc 10" phase reference probe: disc x 4 - 1.0 1.5 2.0 1240 x x x measured C i2..Mie series o 80._ 4 \X 40 -. ~ _. 1.0 1.5 2.0 frequency in GHz FIG. 8: Charge measured with the disc probe on a 3. 133 inch diameter sphere. 16

(spheres) in the chamber. Physically this reference is a string stretched horizontally across the chamber perpendicular to the direction of propagation of the incident wave. Note that with the monopole the data measured here is not as accurate as that presented previously in Fig. 4, and measured about two months earlier. The amplitude deviates about 1 dB from the theory and as much as 15 degrees in phase. In this data there appears a definite interference pattern caused by the interaction of two or more signals. The interference could have been caused by an improperly placed piece of absorber in the chamber, or a forgotten metallic object such as a measuring tape in the chamber, but most likely it was caused by the interactions of the incident field with the conductive telemetry lead which may have been improperly led away from the model. The data as measured with the disc probe are much better and deviate only about 0. 5 dB in amplitude and 5 degrees in phase from the theoretical values. In addition to the improved accuracy provided by the disc sensor, there are two other reasons for preferring a disc probe over the monopole. One, the sensitivity of the disc probe is unchanged as the sensor is moved from one location to another as compared to the monopole, whose length and consequently the sensitivity will change; and two, the sensitivity of the disc is usually higher. For example, for the disc as shown in Fig. 6, and monopole of length h = 0. 235 inch, the sensor signal in the 1-2 GHz frequency range for the disc is about 6 dB above that for the monopole. As is suggested by the data of Fig. 7, the charge measurement may be susceptible to errors in telemetry lead interactions. So two charge probes that use high resistance telemetry leads were developed and Fig. 9 shows their construction details. Both are built around a miniature microwave detector diode (PD 0911; Parametric Industries, inc.) which has been designed for low capacitance (0. 15 pF) and high upper cutoff frequency. At 10 GHz, for example, the diode reactance is about 100 ohms. Physically, the diode has an axial lead package, the glass case being 0.70 inch in diameter (max.) and 0. 125 inch long(max.); the leads are 0.014 - 0.016 inch in diameter and about 0. 75 inch long. Both probes have 0. 5 inch diameter circular bases cut out of a soft copper sheet to permit bending to conform with the surface

.526.125 high resistance lead -.50 (a) Diode-monopole probe -i.156 a-.140 '.i50- (b) Diode-disc probe Notes: All dimensions in inches. Diodes: PD0811 (supplied by AFWL). Cables: Resistive lead, twisted pair 40kf2/ft. (supplied by AFWL) FIG. 9: Self-rectifying charge probes. 18

of the test object. For the monopole the active portion of the antenna is simply the diode lead cut to about a half-inch length, while for the diode-disc, the sensing element is a 0. 156 inch diameter copper disc soldered to the upper diode lead right at the glass envelope. The detected signal is removed from the probe by a 6 ft long twisted pair of high resistance leads", one of which is glued with silver paint to the base of the disc and the other to the upper element. When the sensor is mounted on a test object, metallic tape is used to hold the probe to the surface as well as to smooth the surface near the base. When making measurements with these probes the same facility and most of the equipment is used as with the rf probes. Figure 10 shows the block diagram for the new setup, which differs only in that an rf preamplifier has been replaced by a 1 kHz amplifier-filter combination and instead of the network analyzer or receiver, a Scientific Atlanta (LIN/LOG) Display (Series 1830) is used. In addition, the incident cw signal is now square-wvave amplitude modulated at a 1 kHz rate. As the rf frequency is swept the measured signal can be observed on the CRT display or recorded on the X-Y recorder. The circuit diagram for the amplifier-filter module which we designed and built is shown in Fig. 11. It is used with the diode probes to match impedances and filter the noise outside the narrow band centered at about 1 kHz. It consists of two parts: a broadband low noise amplifier (T1, T2) and a 1 kHz active bandpass filter (Ai). In the amplifier section all the gain is provided by the T1 stage and the T2 stage is used to provide low impedance drive for the filter circuit which has about 2.7 kQ2 input impedance. The 5 kQ potentiometer in the filter circuit provides frequency tuning from 900 - 1100 Hz, and the 100 kHz potentiometer provides the signal level adjustment to decrease the signal level so as not to overdrive the LIN/LOG module. When the gain setting of the potentiometer is in the maximum position the overall voltage gain of the amplifier-filter unit is 60 dB. The concept of self-rectifying probes and high resistance leads is not new. In 1962-63 such probes were built at the Radiation Laboratory using then-available Supplied by AFWL; approximately 20k2/ft, single wire. The leads are twisted at about one turn per two inches.

sweep 1 kr~z Amp - modulated TWT signal I 'IAamplifier generator directilona 1'A1Z ~~~~~~~coupler Anil)-fil.ter isolator IN/ LOG ircsistance lead Display anechoic chamber F 1 B dr ep n e n n orCRT recorder display FIG. lo: Block diagr~am of equipraent needed when using diode probes.

24 v 24v 12K 2.7K T1t T2: Motorola, MPF 105 A1: 741 Op-amp FIG. 11: Amplifier-Filter for 1kHz signal.

microwave detector diodes (such as 1N21, 1N23, 1N34, etc), but possibly due to their high inherent capacitance (vs. 0. 15 pF for the diodes used here) and the larger physical size, the sensitivity was poor and the detection characteristics did not follow a logarithmic curve. Such doubts as to performance were still with us as the new probes were being built, but when the initial results became available, our opinion on the use of diode probes changed. In Fig. 12 the measured linearity (actually a logarithmic response as detected with the logarithmic detectors) of the diode-disc probe is shown. The measurement was made using the equipment of Fig. 10 and inserting up to three 10 dB attenuators between the isolator and the transmitting antenna, The signal as received by a diodedisc probe was displayed on the Scientific Atlanta (LIN/LOG) Display. The test model in this case was the 6-inch diameter sphere and the frequency was 1. 5 GHz. From the curve in Fig. 12 it is seen that for t 1 dB tracking, the dynamic response of the probe is 27 dB, but if a higher incident power level were available, either by using a higher gain transmit horn or a higher level power (10 w) amplifier, a larger dynamic range could be obtained. Some additional improvement, perhaps a dB or so, could be obtained by a careful manipulation of the amplifier-filter circuit and the twisted pair resistive line; the line appears to be a source of noise pickup, especially the 60 Hz. This would lower the noise level characterized by the horizontal part of the curve. Of course, a larger signal, and consequently a larger dynamic range, would result if larger probes were used, but this would defeat the concept of "electrically small" probes. The charge as measured with the two diode probes on a 3. 133-inch diameter sphere is shown in Fig. 13. The 6-inch sphere was used as the reference and the measurements were made over a 1 - 2 GHz range. We note that in each case, i. e., for the diode-disc and the diode-monopole, the measurement is within 1 dB of the theoretical value; the disc probe shows consistantly lower values, but the results with the monopole are consistantly higher than the theoretical values. Finally, we remark that the signal received by the diode-monopole was 4dB above that of the diode-disc in the 1 - 1.5 GHz frequency range and gradually increased 22

-10 |/ 1ideal case -20 50 -0noise / ~0 0i _ A -30 -40 -40 -3i -20 -10 I -40 -30 -20 -10 0 Relative transmitted signal, dB FIG. 12: Linearity test of the diode-disc probe measured on a 6-inch diameter sphere. 23

12 oI tfinc [10 Aprobe: diode-disc 8sI CD 4 1,0 1.5 2.0 x x x measured - Mie series probe: diode-monopole x h=.651 inch 10 X \x 0 P4 C) 6 4_ I * I..I 1.0 1.5 2.0 frequency in GHz diameter sphere. 24

to 5 dB at 2 GHz. This is opposite to the behavior of the hard lead rf version counterparts. A possible explanation for the lower signal received by the diode-disc is the higher capacitance of the disc structure as compared with the monopole. 25

SECTION III EXPERIMENTAL STUDIES 1. LNTRODUCTION In this section the results of experimental studies which were designed to improve and simplify the measurement techniques are presented. The first part reports on the st;utdy of the effect of surface perturbations introduced when a lead of a sensor is taped along the surface of the model~ The results show that the random variation of data is larger than the perturbations introdluced by the ca- d, and it is the:re-fore concluded that tapingi the lead alonlg tfhe surface has negligiblle effect on the measurement accuracy. In'the second part the monopole length vs. signal amplituLde received is examined~ The results agree with theoretical predictions and from these results a method for adjusting data taken by different length monopoles is given0 The third topic deals with an experimental study designed to provide correction data for current probes when measuremients are made near edges, thin wires, or surfaces of small radii of curvature. Results are available for the case when the currents flow along the edge or the axis of thAbe Cv]rinder- PlaJns cal] for an i;';uoivat-on,~f f;,:t, transverse case and the development of the related analytical and conmiputer studies as part of a later program. The last topic deals with the scale model measurements using models made to various degrees of resemblance to the actual model. The results indicate that for low frequency (the first resonance and below) a model of the 747 caln be built in about 4 hours out of wood and sheet aluminum on which the induced surface fields will be within the measurement tolerances of the fields on the detailed model. 2. SURFACE PERTURBATIONS DUE TO SENSOR LEADS When measuring surface charge or current on a body, the measuring device ideally should not introduce perturbations to the electromagnetic field either by the sensor itself or the telemetry of signal lead. However, practical sensing devices are of finite size, are usually made of conductive material, and in most cases will have conductive leads to remove the signal away from the object. In this section a comparison of charge data measured with an internally fed monopole and a taped-on monopole on a 3. 133-inch diameter metallic sphere is presented. 26

Mea.su rcments of thze normal electric field (i. e., charge) were mnade on a 3. 133-inDch dialt;er spherLe using various tap ed-on aand a fed-through monopole. For the latter test, a, spherical mniodedl) was Ymnodified to provide the capability of bringing a p-roble out from within the sphere~ The rnodel basically consists of twvo identical solid aluminumn. caps joined togetAher by a threaded shaft at the center, but spaced /i16 inch apart. When a 1/16-inch thick disc of the same diameter as the sphere is ~i se.rted and theL canps tightened, a solid splzhere is obtained. For this spherical model four clscs were maade, one left intact to make a solid sphere, and in the other three slots were cut to allow the eambedding of a coaxial cable0 V-shaped slots were cut at 0-5GO, 0 —C0, and 0-120~ in the discs so that when used in conjunction with the sphere and Othe lead entering- at the top, the probe emerged at either 60, 90, or 120 deg;rees depe-nding upon the disc used. For each of the three probe positions the charce w"-as measured at 1 2 and 1. 8 GIIz as the sphere was rotated through 350 in the azimuthal plane. Then the s p)hnere wras marn-tde solid by replacing the slotted discs with the solid one and the neasujrements were repeated. Bult th'!i.s time thle probe and its coaxial lead was taped to The probe in this case was made by removing a 1/4-inch long section of the outer conductor fromn the coax end, but leaving the teflon dielectric insulation to provide some mechanical support and protection to the 5 mill diameter center conductor. The coax was then taped to the sphere and the exposed quarter-inch center conductor wqas bent normal to the surface. To provide a smoother surface at the base of the monopole, a dime-sized disc of metallic tape with a hole in the center was applied over the monopole. The cable was then taped to the sphere with metallic tape and led away from the surface near the top of the sphere. The same measurements were made with the taped-on probe as with the fed-through one described above. Because of the general similarity of the data for the 60, 90 and 120 degree azimuthal paths and the two frequencies used, results are presented only for the 90 degree or "equatorial" path, and the 1. 8 GHz case. In Figs. 14 and 15 amplitude and phase data along with the theoretical values computed from the expression 27

dB -- Mie series x x taped probe, ~ 0.243 inch o o o internal probe, 0.281 inch x 0 1 0~/ x -~ \ probe: monoIole, hard cable / X L^ t x co -180 -90 0 90 1, FIG. 14: Charge distribution on a 3. 133-inch dit.:eter sphere. The level for thle measured values has been adjusted for best fit.

phase, decg;ees 0i o/ -I Mie series 0 0 0 itei-nal probe, f = 0 243 inch:00 | ~o o o intcrnal iprobe, Q - 0.281. inch too x - O ~,J I0 f 1E 15 freq: 1. 8 GIlz (ka = 1. 5) -. 00 r probe: monopole, hard cable -180 -90 0 90 180 FIG. 15: Theoretical and measured phase on a 3 133-inch diameter sphere. The level of the measured data has been adjusted for best "it,

P (cosO) E (r =a) = - c. 2 in2n+1) r () a '(ka) nn n are presented for 1. 8 GlIzo In the expression, C (x =L I () and P (cos 0) io~t is an associated Legendre function as defined by Strattono The time convention e has been assu:;ued and suppressed. Sincre there is no convlenient nmeans of proviclirdig calibration for the measured data, the level of the lnmlasured curves presented has been arb'itrarily normalized, i. e, moved up or down for the best fit with the thieo-y. As is seen. the measured data matches this theo-ry quitle well, and where there is a discrepancy, it is believed that it is not caused by the probe rlesponlse (incrludin.-g taping. etc.), but rather by inaccurate azinmuthal rotation of the sphere. It is estimated that the azimuthal positionilg a ccuracy was + 3 degreeso The spread of dacta due to inaccuracy of azimutihal positioning of the sphere seems to havre caued larger measurement deviation thI'an. thwt introduced by ta"ping the proob in tofle olvutcr,slnf-e.e of the modael Oone ooDAe.lusioD. hon-ver CPan b,,e drYrv.n from this study, and that is that the main source of measurement error in charge measurements wtill not arise from the taping of the probe but rather from inaccurate alignment of the model in the chamber, and possibly the interaction of the probe lead with the model — and the incident field. Therefore, in charge measurements care must be taken to position or tape the probe as accurately as possible on the body and then to align accurately the body with respect to the incident field direction. 3. EFFECT OF MIONOPOLE LENGTH ON SIGNAL RECEITED A short monopole, made by simply extending the center conductor of the miniature coax, appears to be a good candidate for measuring the normal electric field (i. e., the charge) on perfectly conducting surfaces. Such a sensor is cheap, easy to make, and relatively simple to mount on the surface. To make the sensor, an OSSM connector is first attached to one end of a 50-ohm coaxial cable about 2 feet in length and. 020 inch in diameter. The other end is then used as the probe, as described earlier. The monopole and the coax are then taped to the body with aluminum adhesive tape. 30

\Vhen suchl a sensor is;transferred from one object to anot-her, or even fromr one place to anotlher' on the same mlodel, the tape must be removed and retaped. In so doing, it is versy difficult: o. main;tain the same antennau length or even cut the lengt;h to a desired value. It is, however, possible to measure the length of the mounted prohe to \wiJthin 002 Oillch wsith n a dcepth micromnet —r, and ornce the length is knaown, ithe nmeasured data canw be corrected to correspond to the des.ired m.nonopo].le len gth0 For an electr-i;ically short d'-ipole, the antenna impedl.ce i precdmcinnately capacitive (ref 4)", and is given by)r 22 396 Z ' 183 2h2 -j a where f3 is the incidcent wave number, 21/X, and h is the total height of the dripole. The corresip)onding open circuit voltage is V =E h OO where E1 is the electric field i-niciri-mpingi on the di.pole and -1 ivs the dinle Voctor ihei-.1,j. Translatingo these val-Lues to a monopole geometry of heighat 1h/2, Zt 1(18 3 2h2 396 a 2 and V' -E h oc 2 n where it is assumed that the monopole is normal to the surface, and E is the normal-1l electric field component. Since the monopole is feeding a 50-ohm cable (load), the voltage delivered to the load is 4. ROW. P. King, The Theoy f Linear Antennas, Harvard University Press, Cambridge (1956), p. 190. 31

50 oc 50+ 9.152h2j 198 50 2 n 50+ 1450 2/X2 -j15,85X / Z-jE 3.15f/ p 2 _/A. 9 or simply VL 2 where ~ is the height of the monopole and X the wavelength of the incident field. To verify the above relations experimentally, a monopole was fed through the inside of the 3. 133-inch sphere model and the received signal was recorded as length t changed from a maximum of 12. 8 mm to a minimum of -0 mm. The probe was at the side of the sphere as viewed from the direction of incidence and the measurements were made at 1. 0(0. 2)2. 0 GHz, but at 2 GHz the signal was noisy and unusable. The resultant data for 1. 0, 1.4 and 1. 8 GlIz are plotted in Fig. 16. Since the incident field was not flat with frequency, no frequency dependence on the signal received can be made. Only the antenna length dependence is examined. From the 2 three frequencies examined, it is seen that the voltage is proportional to P for ~ <0. 5 or P <0. 7 cm, Probe lengths commonly used range from 0. 22 to 0. 250 inch (0.56 to 0. 63 cm) and this falls within the region proportional to i e It is straightforward to apply the correction for changes in monopole length. If, for example, for the first measurement 15. 85 mm s = -45 dB but for the second measurement 2= 5.35m, S2 =-49dB then referenced to I = 5.85 mm, the second signal = + 40og,,,1/12 = -49. + 1.58 = -47.42 dB 32

20 x 10 / x x / f=6 1. 8 G H z 20 20 / x c ti o 0 5 f = 1.0 GHz,7X 0 0.5 1.0 1.5 monopole length, X2 cm2 FIG. 16: Received signal vrs. monopole length. 33

4. CALIBRATION FORP LOOP PROBES - EXPERIMENT The response of a small magnetic loop to an alternating magnetic flux density B is characterized by an integral form of Faraday's Law, 5 E.d = — at, B ds (1) C S where S describes the surface of the loop and C its boundary edge. When the loop is a single-gap, one-turn device, such as used in our measurements, the voltage Vg delivered from such a loop can be written in the form (ref. 3) v = -iw p -- B:ds (2) where Z is the intrinsic loop impedance, Z is the load impedance characterized by a 50-ohm load shunted by the gap capacitance, and B. is the flux density to be measured. For ccnvenience a harmonic time dependence e has been adcpted and suppressed. When measurements are made with electrically small loops, it is usually assumed that the magnetic field is constant over the area of the loop and in such case equation (2) becomes VI = -iwBiA Z +Z (3) P where A is the effective area of the loop. It is a valid approximation when the radius of the surface curvature of the body on which currents are measured is much greater than that of the loop. It has been found, for example, that for a 0. 150-inch outer diameter loop (Probe 211), "errors" larger than 1 dB are encountered when measuring current on cylinders of 0. 627-inch diameter. In view of the fact that magnetic loop probes, however small, will tend to respond to the average value of the magnetic flux over the loop, it is desirable to make 34

use of that correction or calibration curve for individual probes when measuring skin currents on surfaces of small radius of curvature. To obtain such curves two probes, four cylinders of different diameters, and three frequencies were used. In all cases the incident electric vector was parallel to the axis of the cylinder. This data is intended to satisfy the calibration requirements for the present program and hopefully will provide verification of a theoretical modeling study to predict calibration curves for other probes. The theoretical study has not been completed and therefore is not included herein. In concept, the procedure for obtaining calibration data for the probes is simple: one measures the current on a body of small radius of curvature for which the theoretical currents are kInowsn or can be readily computed and then the difference between the measured and theoretical values is used for the correction for the particular probe, radius of curvature, and frequency used. Spheres, prolate spheroids, finite cylinders (dipoles) and "infinite cylinders" 'were considered as probable bodies, but after considering various pros and cons of each, an tti!finit cynrl er" wa.S selected. To set up an "infinite cylinder"' in the chamber, eyelets were screwed into the opposing walls under the absorber and to each eyelet a piece of rope about 12 inches long was tied. A No. 14 wire was tied to the ropes and stretched across the chamber. To provide good tension, a small turnbuckle was used at one end between the rope and the wire. Because the rope is essentially nonconductive, there is no need to remove it when the wire is taken out of the chamber; this saves wear and tear on the absorber panels because they do not have to be removed each time the wire is stretched or removed. Four pieces of tubing, five feet in length with diameters of 0. 193, 0.250, 0. 382 and 0. 627 in., were used to simulate the infinite cylinder. Each tube, in turn, was threaded over the wire and a metallic tape was smoothly wrapped around the tubing-wire joint to smooth the surface discontinuity. To reduce current reflections from the ends of the wire where it was tied to the rope, a two-foot length near each end was covered with pieces of absorber. Various combinations and arrangements of absorber materials were investigated to 35

obtain a good match for the currents at the ends of the wire. For the optimum arrangement the standing wave measured along the cylinder was less than 0.75 dB over a 30-cm span as compared to 2 to 3 d \vlhen other arrangements of absorber were used. A sketch of the most successful absorber placement is shown in Fig. 17. The view is fromn the antenna direction, with absorber chamlnber wall i I / i| ' C, ~~rwire _~,~-:* a r D. o pturnu 0e ie eyelet FIG. 17: Absorber arrangement for matching the ends of the wire. The match is excellent in the 1-2 GHz range. pyramids pointing toward thle transmit antenna direction. Measurements of current on the four cylinders were made at three frequencies, 1.0, 1.5, and 1.95 GHz with two probes (see Fig. 18) and the following measurement sequence was used: 1. Set up a cylinder and a probe in the chamber. 2. Measure current along the cylinder for the three frequencies. This was done by switching through three preset cw frequencies on the sweep generator 1.Se u acyineran aprbeinth c36er

iF Probe 21 i a-0. 1 2'? 5. ji c -- 0.272 in, H |oorecr d -- 0. 030 in. s (). 1 Yiln) Probe A a 0.304 il. N t,,_ \c 0. 4 2 0 iln. xs.. I nC SL FIG. 18: Dimensions of the probes used. for each probe position on the cylinder. Data was recorded on tIwo X-Y chart recorders, amplitude on one and phase on the other. The position of the probe along the cylinder was equated to the position on the X-axis on the graph paper. Thus the data points on the graph paper showed the actual current (or phase) standing wave pattern along the cylinder. The current was measured over a span of about 30 cm along the cylinder. 3. Calibrate the incident field. For this, current was measured at the specular points on 3.133 in. and 6 in. diameter spheres carefully positioned in the chamber so that the front of the sphere was in the same plane as the front of the cylinder. Two calibration spheres were used to provide additional accuracy by averaging the incident field values from each sphere. Typically, the difference between the two measurements was 0.1 dB in amplitude and 1 in phase, but discrepancies as large as 1 dB in amplitude and 40 in phase did occur for a few 37

of the measurements. 4. Change the probe and calibrate the incident field (3 above). 5. Measure the current along the cylinder (2 above). 6. Go to 1 and repeat. To ot-i' L the 'nfin.ite curlinder.' current value for the cylinder, minimum and mamum- vXalues of tie current alo-g, the clider were used. Where the dIffexrece wia?3s3 a d7E or JeIss, a direct aPveraging xo vas used, tbut for larger d-i.fferen:C)e=sL; ihr-, (ted valvuCr lU were convirltecl to linear scale before averaging. it-e IlNphasec data -was recordecd on a li-near scale thus m-aaking averaginig straigchtforward. hi Figs. 19 throutgh 22 the results of the n-ea-surenments are presented. No attempt: has been mad.-;e to interpret; thne data in detail. This has been left for. fu~4ULtur xOa.Z whle5 t.nan ' t..em..t- will be, made to corrclate these results with theoretically co-lmpuicd responses of the loops in the presence of these cylinders. jHow-ever, to ma-ke th'e rejsult s iin-mnediately applicable to aircraft moodel nzea sure-. 4- 4- Fl, V 7 11,> /` -"I -r- It I + tr l oats thoollr otial datai for infinit cyI iners is provided. Ths a cmut ed-r — from equationL (ii) on p. 95 of Knott (ref. 1) for which the tinle convention has -iWt iWt ikp cos been changed from e to e and the incident field term cos b e has been added. As expected, cursory observations of the plots show that the larger diameter probe, Probe A, needs a larger correction, but it also needs a phase correction, which the small probe, Probe 211, does not seem to require. In application of these results to compensate for the averaging effect of the probe, it has been asslumed that the needed correction is independent of frequency. This is indicated by the similarity of the three curves as shown in Fig. 19 for the three different frequencies. Analytical studies (to be reported later) using small argument expansions of Hankel functions also confirms this hypothesis. Thus, for example, if the diameter of the fuselage of the model on which the current is measured with probe 211 is 0.5 inches, the needed correction is the average of the three values or about 1.8 dB, a value applicable throughout the frequency range. No phase correction is needed for this probe, as can be seen from Fig. 19. 38

'.apu1t1so IoJ apn!idu-Ie uaJ.no ToTla0JOoql pu-t (iTZ aqoJl) paznsvaWA:61 'DIa soqoui uF aoloawelTp aopulHo 01'T; 0 0 = I —_ ZHI 0'1 9T 8 e xT \ 91 - 9. —ox —_ I- OI zHa Iqr _

-20 theory -40 t 1.95 GH-z -60 a) 0 -30 060 ~)0/ 1.5 GHz -60 -30 1.0 GHz -50 -70 0 0.5 1.0 cylinder diameter in inches FIG. 20: Measured (Probe 211) and theoretical current phase for cylinder. 40

14 1.95 Gliz 10 x x... i_.. 16 lG\ m) '-\ 16 \ 1.0 GHz 12 0 0.5 1.0 cylinder diameter in inches FIG. 21: Measured (Probe A) and theoretical current amplitude for cylinder. 41

-20 1. 95 GHz -40 -60 c -30 1XX 1.5 GHz C).45 -30 1.9 ~~~~~~~-50 1. ~0 GHz -0 05 1.0 cylinder diameter in inches FIG. 22: Measured (Probe A) and theoretical current phase for cylinder. 42

5. SCALE MODEL STUDIES This section is devoted to measurements of skin currents on 747 models built with various degrees of resemblance to the precise shape. The purpose of the study is to provide data to determine to what accuracy the model must be constructed if measurements are to be within a prescribed value of those made on the precise shape model. The emphasis here is on the first resonance and lower, where the precise shape of the model is less critical than at higher frequencies where the skin currents are dominated to a large part, by the local surface geometry. The skin currents were measured, as a function of frequency, on three 747 models. The models were: (1) a detailed 747 model, (2) rough 747 model (A), and (3) rough 747 model (B). Their shapes and critical dimensions, along with scale factors, are shown in Fig. 23. The scale factors are deduced by comparing the model dimensions with those of EC-747 Advanced Airborne Command Post (AABNCP). Thile detailed model that was used was purchases in a local toy store. it is made of metal (cast aluminum alloy) and on the underside had rubber landing wheels mounted on metal ridges that did not resemble those of the aircraft. Before current measurements were made these wheels were removed and the ridges were filed off to resemble the in-flight configuration. The rough models A and B were constructed in the laboratory using wood for the fuselage and 1/32 in. thick aluminum sheet metal for the wings and vertical stabilizers. The models were painted with conductive silver paint. The model A was designed to be of a close resemblance to the detailed model. Its fuselage is a circular cylinder of diameter equivalent to that of a 747 at the midpoint of the fuselage. The wings and tailfin are also shaped to those of the actual model, but do not have the engines nor the broken trailing edge for the front wings. The fuselage ends were rounded to resemble those of the aircraft, but the wing and tailfin edges were left squared. The model B is a rougher model of the 747 aircraft. Its fuselage was 43

2.86 Scale: 1/730.1 (lent:h) (a) Detailed 747 mociel 1/761.7 (xvict; 10.18 Qr7 3.20 (b) Rough 747 model A Scale: 1/674.2 (length) 1 1/ 676. 2 (widCLi) 9.42. 1.35 3.58 \ \7.97 (c) Rough 747 model B \ Scale: 1/728.6 (length) 1/748.3 (width) FIG. 23: Scale models of various resemblance; dimensions in centimeters. 44

made of a larger- d ameter wood dowel whltose ends wTere intentionally left flat. The wings were made of /3a2 in. thick, aluminum sheet, but filed to an elliptic shtpe. Also not/; that the wivgs and stabilizer are noi tapered as in the detailed mor.de l and mluodel A. For tlhe three m,nodeIs th.e skin curre-nts were measured at the mnidpoint of t;he ofuselage, tc)) and boi ttom, and in erich catse the cur:rent was measured on the illuminated side of th;.,' model. n the incident electric vector was par-alt.Tllel to the thuser la. e,: ''J:n the iucseirent else were measured, but vhien.I t-he polarizvation vwas ch-ar,;o:ed, i. e., t-he incident electric vector was normal to the fuselsage, then current flowving aros: ti;e feustela' was measured. The dir: ection of inci den-ce a(nd po lh rization an th ' currenLt components \vhlich wer -reeasuJred are shown by the insert c.tchl on each of the figmures where data ar.:t-i proesen't,eid. The meaXtsurements wre e made byv sweeping 1-2 and 2-4 Gl4z rang-es to cover the 1-6 J`7z rang.Auao d K,-C 2 Gjd r ate; to coverl the 1-3 M\13z range. Th- nrobhe correnCtion dlat'a (Srlio) ITT 3) waSq auCnl<Ie t.he Leease of tom fliselayoe currents excited with the incident electric field parallel to the fuselage (Figs. 23 and 24). The similar cu-rent's measured on the bottom of the fuselage were not corrected because due to the presence of wing roots the effective surface curvature there is almost flat. In the case of perpendicular polarization, amplitude correction was not made since for such the correction data is not yet available. However, due to the flatness of the surface on the bottom of the models it is expected that little or no correction would be needed there, but for the top of the fuselage where a cylindrical shape is definitely dominant, a one or two dB adjustment may be needed. Figures 24 and 25 show the current amplitude and phase on top of the fuselage, illuminated from the top with the electric vector parallel to the fuselage. As seen from the amplitude curves, the detailed model and the model A resonate at 1.75 MHz, but model. B resonates at about 1. 63 MHz. The reason for the lower resonance of model B is its fat fuselage. The behavior here is similar to that of a half —ave resonant dipole - the fatter 45

x x x Detailed 7model o o o aRough model A top CD2 6 frequenicy ir.f MIIz FIG. 24: Current amplitude on the top of the fuseA.,e of the 747 models; top illuminated, E parallel to the fuselage.

tx x Detaitlcd model 40 x o o Rouoh model A - x 1 A, 34totp hi) bO k frequency in Mhiz 4 ~4er 40 FIG. 25: Current phase o the top of the fuselage of thle 747 models top iilumnatedE Parallel to the fuselage.

the dipole the low'er the resonlant frequency, and also the lower The surface current; lensity. The 1.5 d[1 higher curren.t ajmplitude imeasured on model A thzan on the (d-ieLiled. nodel can be explainedl by th1re faXct that lmodel A has a srn.oother shape, im-n-plying a higher Q structure. Figure 26 shows t.he' current in:-;sured on. thlr bottom of the fuselage, wiRth incident el.ectric vector parallel to the futselage. A comp'a.rison cf this data wvith thrat for< Ihe top of ti-ie fuselage showliovs a gs-reat sKi-arity for both the amDllirltde and phiase., t:le only dilffEerence b)e-ing tlhe, 7 cdB (aboutl) loelr amnplitudes for the bottom ne:, easux-ement 'The diffe:' ene is rca1, and results from the fsact; that the flat bottom surface has a tendenlcy to sp.e-ad the currltintstls and thus decrease the current dentsity. Figures 1 7 and 23 slhowv the irmeaisured current on the top) a-nd bottom of the fuselage, resp-ectively., with the inrcidrnt electric vector normal to the fuselarge. Note that niow the curtirents on toy) of the fuJsel.age are about 5 dB lowver than tihos-e for the boti:or-, which is opotisite to th.llai obserTved for the other (parallel) polarization. Tifs behav-7ior can be explained by the fact that for the perpencicul.ar polarizt;ion the donmlinant curre-nts are supportecl by the wings and the bottomn surface beincr flat offcers less opposition to the current flow than does the top of the mrodel which has a cylindrlical hump (fuselage). It should also be remenmbered that since the probe calibrlation data for perpendicular polarization is not available, ino correction has been nmade. It is expected that such correctiorn will raise th1e level of cuirrent measured on top of the fuselage. No attempt is made here to assess the results or to determine a criterion for construction of rough scale models to be used for surface field measurements when commercial or detailed models of required scale are not available. To deduce such criteria one must first know the accuracy (in amplitude, phase and frequency) which the rough model must provide, and one must also know which measurements will be made on the particular model. For examlple, if the fuselage resonance is to be studied, a model such as model A would suffice, but such a nmodel would be inappropriate wrhen wing resonances 48

20 501 * 6 E I N~~~~\ C)~~~~~~; b~~~~ottom C) 0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~C "a0\ o00 x 0 cNC 0 Rough model A a A a Rough model B 0 A - A X~~~~~~~~~~~~~~~ co~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~c 0 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -5 0 1.0 2.0 3.0 1.0 2.0 fre-qu,,-,r-,cy in MIIz FIG. 26: Current amplitude and phase on the bottom of the fuselage of the 747 models; bottom is illum E 1p E o o h fusel Age,~:]oRough '.~__a_ z~ z RougEh model B 1.0 2.0 3.0 1.0 2.0;3.0 frequenrcy in MtlIz FX%, 26: C~rren~ amplitude and phase on the bottom t fslar\ - h 7 wnll~ srtfuv is ililuinatedf

20 50 i -\ x x Detailed model l x o 0 o Rough model A 0 A A Rough model B to t-Oh top f 10 /0\o'>~0 o 07 A O.%.%Q Cod 0?/ N 0 -30. 1.0 2.0 ' 3.0 1.0 2.0 3.0 frequency in MHz FIG. 27: Current amplitude and phase on the top of te fuselage of the 747 models; top illuminated, E perpendicular to the fuselage.

20 500 R0h aXa Rough modeletidme 0 bottom -5..0 ~50. 1.0 2.0 3.0 1.0 2.0 3.0 frequency in MHz FIG. 28: Current amplitude and phase on the bottom of the fuselage of the 747 models; bottom illuminated, Ei perpendicular to the fuselage.

are to be dete rmined. ReI-call th:'a the data for the detailed mlodel showed lower resoanrice than lodei A (see Fig. 27), inicating th.-t for such a case the prese.:n.c. c f eng-)ines clof- s aJffect the resonance, behavior. 52

SECTION IV WIRE:fPRID MODEL AM ET;ASUREMIE NTS In this section, thtke investi.gation of the amnplitde a alci phase of the skin curreents oni wire rid nlodiels is dicused. candu the experi-mental results are presented. The fmzodells use.d are the so-called wire grid. models used by Taylor (-ref. 2) jforj cotipulatio n of currents for a -1 aircraft. The dMeiRensiOdns for thLe t\To iodls used a re shlown1l in_ Pig. 29, with wings for.w ard. and wings sept. The Ia-oriacory )mnodel7s vere Inmade of brass by machi.ning d'ylinders of required clalmeers, Iengt.hs, and end shapes, after hich tle pieces wer'e ojined wit 2l solder to formx thie model. The 1/239.3 scale factor for the model resulted fr:omn selection of 5/8 in. diameter si:andard brass stoclk for tlhe fuselage. For the two m0ord tls.. Nvings forward and vJi.s sept, Lsu T tfce curre t am-nplitude said plh.ase wvas ine-asured. for five cliffrl -reat; cases alnd t0s4-*e art1e isted in Table 1 along- \>1 th corespond figLuLre nu-naber.s (30-39) whIce showr,tL c. LL Vt.."" c.,. r. i ',..L L.,_,jt _. -Ie. J-O C,,, __ Ua L L d (U- -d J- - ~i '0 IT, e S Z! VI III ~ forward and wings swept, with all otlher conditions being the same. The st;ation (STA) numnberLs designate location of the paribcular measurement and those for the fuselage are measured (irn meters) from the nose of the aircraft: and for the wincgs the 3-measurement is made fror the root of thle wing, specifically, from the point wh-ere the axis of the (cylindrical) wing intersects the axis of the fuselage. The (F) or (WV) after the station number refers to the fuselage or wing station, respectively. The above measurements were made using a loop probe (No. 221). Two calibration measurements were made, one with the 3.133 in. diamleter and the other with the 6-in. diameter sphere. The incident field value was obtained by averaging the values obtained from two measurements which, on the average, deviated only 0. 25 dB for the amplitude and 2.3~ for the phase. An amplitude correction factor of 1. 17 dB, appropriate to 0. 625 fuselage diameter, has been added to data in 'igs. 30, 32, and 34, and a 2.27 dB correction factor, appro53

I I~ (Drawing not to scale.) I 1 4C~~~~~~~~~~~~~~) 40 -19. 6m 1 di (3.225)(13) 24. 5m o (4.031 ) co, ~ (.625)F FIG. 29: V/ire model used for sk,,inl cui-:~,u-t measurements. Full scale dimensions are. ii' r in, prenthescs are maodel dimensions in SWe. Scale:- 1/2~39. 3.

TABLE I Inicidenrt polarization wiIh |l lUminti -Ion | Poiu'as wvhero remiCtsuie-. igu.res where res pcct to fuseie m i.n were made: data are presented parallel, top centero of 3fuse e.aie, Figs. 30, 31 top, STA = 22.65 (F) parallel top cente r of fusel.ge c Figs. 32, 33 bottomn, STA -=22. 65(F) parallel top near co.lpit, top, Figs. 34, 35 ~whings forward, STA= 5. 0(F) Ivinfl.s sVwept, ST'i&A 5 ~55 (F) perpendicular top center of fuselage, Figs. 36, 37 top, STA = 22.65(F) perpendicular top center of wings, top, Figs. 38, 39 wings forwvard, STA = 1 0. 8 (XT) wings swept, STA = 1____________________ ____ _12.25 (W)_ Summnary of measurements and data presented for the wire grid model of the B-1 aircraft. 55

25 wmin0S sw-epnt 0 O0 wvia-vs forwNard J STA = 22.65(F) 0 \\ f5~~ N Cj X 0 cJ1 ~ 5 0~~~~~~~~~ 0 ~~X 0~ r4~ ~ ~ ~ ~ ~~X 0,0,O0 0x/0~ % 0~~~~~~~~~~~~~~~~ bD~ ~~v 0 0"0 5 1152 X ~ ~ ~ ~ \ 0 5 1 0 15 2 frequenrcy in MHz FIG. 30: Current amplitude on top o.- the fusella, c at center, E1 parallel to fuselage.

100 of \ -o-o-o-g tQ \ xD x boX-x~. win s ept -100 -200 0 5 10 15 20 frequlrtcy LA M-z FIG. 31: Phase on top of the fuselage at Ci.or,.ll to fuselage.

25 -,~~~~~~~ /x\ ~x x x wings swept,i;~~~~~~~~~ o o o wings forward STA = 22.65(F) 15 " 1S0 5 10 1x fuselagc. \\o.J.~J FIG. 32' Current amplitude on bottom (sh~' -,-': o l'g e Eo -D:l:afl t3 fuselag e.

-100 x X x wings swept o o o wings forward STA = 22. 65(F) -200 k x xc~ax -400 CCfl -400 0 0 5 10 15 frequency in M11z FIG. 33: Phase on bottom (shadow sice) of tile fhe;, r -t *e at e c alter, El parallel to Pisei..re........,~~~~ — partallel to fuseiag'e.

25 xx x ( wings swept OTTO O o c.v~in-s lg forxvard // \&ic~~~~~~~~ SA'A = 5.0G(F) (wilic:s forward) STA = 5.55 (1) (ir.pgs swept) sL '' X 15 ' 045 10 15 2( F.X 0Io 3rr o 10 15 20 frcqu — niy in MiHz FIG. 34: Current ampltud-3 near cockpit, F pirhi tIu fkuhgo.

x x x wings sxvept o 0 XTk....s forward o - 50' ~~,50 STA = 5.06 (F) (wings forward) STA 5.55 (F) (win;Ls swept) 0 k~~~~~~~~~~~~~~~ 0 0 o~~~~~~~ 0~~~~~~~~~~~ '0 a CI1 /,~ C3 -~ x /, o/0 N>o. \0 x, '~?~'x ~. ~ o.6 ~,~ x ~ ----o-< -50 0 5 10 15 20 frcqroufcy in Miz FI~G. 35: Phase near cockpit, FiG. 35: Phase near cockpit, E p-:x~alieI to fuseh, ge.

20 ~( )' >e winges swepit Oi 0 0 0 w% v ings focva rd STA = 22.65(F) 10~~~~1 10 \0 x X~~~ 0.001 ~ ~ ~ ~ ~ /0x 0 0//~ o I / 0`x A - / 7/ 0 -10 1 52 0 5 01 FIG. 36: Cur'-rent. arnpl.'tUde on top of,- I- fu~ste'hlge El pfrmdicu1-r to f`usokige.

50 t ~ o0 STA = 22. 65 (F) C12 ~~Xx CA bo ~ ~ 0 o 0 -50 " / \ 0 x 0 5 15 20 freque'incy in MDI.z FIG. 37: Phase oil top of the i fu el..i;e, i,pal.xidicular to fuiselage.

30 x x x wirins swevpt ~0 o o o wings forward STA = 10.8 (VT) (wings forward) STA = 12.25 (V,) (wings s wept 50120 20 0 FIG. 33: Current amplitude on top of tbc v,- B pcrpondicuThr to fusoSgo.,.90 \ x 0. 0. l0 5.o \ -. o/r c n -,.. in 1vimiz

x x x wings swept 50o 0 o wings forward ) STA = 10. 8(W) (wings forward) STA = 12.25 (VW) (wings swept) 0 C, x -50 x\ x z0 0orI0 —0 0 5 10 15 20 frequc:cy in MIHz FIG. 39: Phase on top of the wing, E p.rpndicular to fuselage.

priate to 0.3'62 in. wing cldiJametcr, has been added to the wvilng data in Fig. 38. The data for the circumferential current measured on top the fusel]ge (Fig. 36) hats nTot beern cadciasted since the correction 'actor is not available. For measurem.e nls presented in this sectio1n top incidence was alwhvays useld wvith thI;e jin cidernt el-etric vector (p-oarizatiorn) parallel or perpzreniculr to the fuscla2e. ai ThJe ph ase refeo.en-ce n was the to pca ide of the f-taseaae. T. measu rem _entos w ere maDLe in four fr equ oncy bands, 0. 35-0.8 CG.-Iz, 0.-.1. 0 G 1z, 1. 0- 2. 0Cz rLn d 2.(0-4 i0G z; usIT i the 1/23. 9 3 mzodel scs'ing factor thIe correspondihng full scale freq~cuency T-ante, is 147-1 6.7 MA11)z. In covering tlhi.s freoql1e.112,y ranoge Cdata is not aivalble for somne frequency ba-n,1lds due to loss of track by tlhe networlk analyzer. This occurred as a result oi operating the, equipment )jeyond ii;ts desi:ned frequency range. In. particular, this is true w:i.th the trn;smsmi.f. n.t antennias. In the 2-4 Giiz ran';e, for example. a standard S-bandci hor- ate-lna. vas, usedwa ad o ndt z a ttracki:g,' dlrop-out wias typically experienced in the 2.0-2.2 GHz range. In tlhe 1.0-.-2.0 GHz bandS, twhere an Ib-band itorn antenna wvas use c, a drop-out occurrec d in the 1. -2. U U-iz range. Likewise, for frequencies below 1. 0 GHz dropouts were experienced, but in most instances these were corrected by retuning the broadband sleeved antenna used in transmitting. In data presented (Figs. 30 through 39) the dropcut ranges are more predominant in the phase data. It should be noted that the lines conecting 'lthe datla points are merely for showin the curve shape and their shape mcay be in question whee lare large gaps in data occur. When making a comparison of measured data and that generated by computer codes one should have a clear understanding of what the fields actually are. The measured current and charge presented here are total fields and include the incident field component. On the other lhand, depending upon the formulations, the computer-generated fields may not include the incident field. For example, it appears that the fuselage current as presented by Taylor et al. (ref. 2) in their Fig. 31 is the net current induced on the fuselage and to find skin current values compatible vith the measurements, 66

the incident field sliould ie added to the 'STtylor salues ftcer threAy thzave been divi-ded by the circumference of the fuselage. To performl such arn addition, the ph-se of the culr-rent and of tOhe iRncident; field is requ:i;:red. Tlherefore, suchI ca.lcutlation Niould lbes t be pferfiorme by, the conlmpu-ter whYen the currents are computerd. Ar aiJternativ e would ib)e Lt su1btract thll -e Sincicdent field fromal the:easured datta, but this pirocedure vwould b-l e m ore comp!licated. Because V-ic re is rno m(ojputtclr data tl pr.Iesenitly avaiil;ble tfor theiS aflrticular wi1re grid1 -models utsed inl thee me;. a JCsureme-nO?-1ets, no coml-pa.-.riso, of Jeia 1sured and conimputed d1aa is lmLde. How&ev-er\, data for the top of the fuselage (P'ig. 30) and for near the cockp:t (.h, ig'. 314), computced using a 1 meter common radius for the model, is avaiabie (ref. 4)'. Tiey sliowr thie same first resonant frequency and thne same generai shape ol: the current curves, but ae 5 to 7 d 3 higher in amp] itude. 5. Letter, T. L. Brown, Dikewood Corporation, Albuquerque, New Mexico, to V.V. Liepa, 27 June 1975. 67

C1 ~"t rl~ Tf ' SDEC rION V SUMMAR1Y A description of a surface field measurement facili;y that uses a sweep frequeLirncr m-neasure lmient; technilqull, and records signals prcpo-prtional to current.ts or chlax.ges as a function of frequency has been presented. Numerous supplementar-v stludies inctluding {cr eesign aI]d performance of charge probe-s and chare probhes with buiti. —jn diodes a.l-td high rets:istance, tek]me etry leads are included. An investigation cof the use o^ crr;-e nt )p oon robaes for meai-;ls-vauremynt>s alolng elenLeilnts of small radius of ctr-vature is des:crib ed aloango wvith calirOLation informi-Lation on;hlese i probes. Of parlticullir intelrest are the current data presez-ted for the 747 aircraft and tIle twirie grid m-olde], used in'1 comnputationf of currents on B-1 aircrtft. The 747 aircraft.l. surface current data a-re p:resented in conrjunct;ion wvith 11rotwlig scaleT mocd.el studies is Twhlich [hej 747 ondel -resuls are com pnared wsith thfe curlren.t neasured on two ronl:h models of h ' —, i47 ai.-crali. Trhe study sugogests that roug'h nodels can 1e5 us.l.edl ati, the first r-cs9OLauee atnd lei freqenieis 1for mea:suring the currents th-a: would r1epresent xWnr -,s r the da,,f iled nlmodel, but cetain miodel dcetaijls mnusc be,. 1. '.,.. e appear to lowier the first resor.iace peak of the cLurren't- excited over tthe wvings and for such mreasurements the engines should be included.' For measurements of fuselage C1urrent, Sym11mrnCetr-iC exSritati;on, engines have no noticeable effect; the length and diameter of the fusehlage are the important dimensions. The data for the wire grid model cover the I. 5 to 16.5 ]MiHz (full scale) frequency.range and include amplitude and phase measuremnelnts for five different stations on the body. The resonances are clearly explicit and agree (in frequency) with the computed results. The development of the current measurel-ment facilil;y described herein is a continuing process and techniques are updated as new techlniques and instruments are developed. Judging from the data of current and charge measured on spheres, the following specifications seem appropriate for the facility. 68

,(,Scae) f.r..er\ny coc'-,,':.a 0. 500 -6 0 G'Iz Maximlum s ze of the medel' 2 feet Expected.ccuracy: G.5 to 1 GtI-z 431 d3 id..n amplituode 10 de'rqees ja phlase 1 to 4 G""z ~ 1 d3 ifn anttplitude 5 coegrees in phalse 4A t1o j G }. '~ 3 cl3 a a-pl:tlde +10 dgee-':s 3:1:i pha'iS i e The._ estinla...c.s are niot finl..:.d as tec':.. s and cqu:.rpmet cha nge acc~uracy ir-;o-vi, l-L-c S.... can be p e:a1 xer -:.l:, theC freq -,-i.ency r -angoe of roperation i.s preesert1ly 1i.lt:e<- Iait t'he iorw f.lt V, erlo:r"t cauLFsed byr the iiteract-ion between the nmei-to by c'o,ii., ti e hfare, orl f; 1-. o. ~ per:.tiotr. The op:eratio n abo ve G6 '-l-u.;.iwl-eC as ths (l:,e cu Hr( oar pr Robes btcomne lecricaalvy hiarge' atd co vc>s5".-W -lht ii it to, 1 iStrcn l " '~ a. i re ev ring cfiects i the merasuremen t-s. j 3'.:., i >'! n' ai s r' G a Rn.! n r i m-.i Ior tztz or' SenlsorS S1noUlCi 'xten.c t' e U..~p: f.cclueCTcy opera'sang J a us. The twvo-fo onaxiLnum size of the modcl is given only as a gtuideline and ha.s bceni (de.idle.d fromn t, h- 2D/ -.far field critelion used in ra-dar cross section baclk scatterig measurnlements, \iere D is the rbmaxijmaulm dimension of the object. if needed, oL)jetc s eslarge as 53 feLet could be placed inside the chamtber, but one lust always 1kee-p in iniz.ad that foi: ob-jeCts of thAt size the inciderit electrom;.ag.etic wave would not be pla-lar. The determin!ing critlerion for seleection of the m:aximum modei size is the 35-foot distance fromn the transmitthar antenna.l to th1re model, the permissible incident wave 1/r amplitude decay over the model along the direction of propagation and the permissible fro.ntal phase error due to the spherical ieature of the wave. Finlally, the measurement accuracy of ~1 dB or ~3 dB (depending upon the frequency range consideredf) has been based on past measurement experience and sample data presented. here. The measurement accuracy depends on many factors, some of whi.ch are the size of the m.odel, the frequency range used, the care used in 69

making the measurements and the general condition of lthe facility and instrumentation. Probes also play an important role, especially the telemetry lead for the charge probe. Tech'anieques of averaging the data over a number of measurements obtained after slight systermatic repositioning of the model and the telemetry leads in the chamber would improve the resultant accuracy, but thie task of data reduction would then become enormous. I such case, an automated data reduction scherrme should be considered. 70