U. 0 j i'll It I Jd l August 1977 Interaction Application Memos Memo 15 Surface Field Measurements on Scale Model EC-135 Aircraft ABSTRACT The surface currents and charges induced on the EC-135 aircraft when illuminated by a plane electromagnetic wave have been measured over a wide range of (simulated full-scale) frequencies, typically 1 to 35 MHz. The measurements were made using 1/447 to 1/114 scale models over the frequency range 450 to 4000 MHz, and data" are presented for aircraft with and without HF wire antennas in configurations simulating (1) free space, topside incidence, (2) near a perfectly conducting ground topside incidence, and (3) near a perfectly conducting ground, oblique incidence.

PREFACE It is a pleasure to acknowledge the assistance of the many members of the Radiation Laboratory who participated in the collection, reduction and presentation of the data presented in this report. Special thanks are due to Mr. Dan Dusette who carried out most of the free space measurements and data reduction, to Mr. Ted Kowalski who developed the software to digitize the data on a PDP-11/20 system and transfer it to the large computer for processing and replotting, and to Mr. Louis Martins-Camelo who spent long hours reducing the data and overseeing the digital plotting. The unfailing assistance and cooperation of the personnel at AFWL/ELPE, and Mr. William Prather in particular, are also gratefully appreciated. 2

CONTENTS Section Page I INTRODUCTION 3 II FREE-SPACE MEASUREMENTS 6 1. Facility and Equipment 6 2. Models 6 3, Sensors 8 4. Data Reduction 12 5. Data 12 m GROUND PLANE MEASUREMENTS AT NORMAL INCIDENCE 51 1. Facilities and Equipment 51 2, Models 52 3. Measurements and Sensors Used 4. Incident Field Calibration 57 5. Reduction of Data 60 6. Data 62 IV GROUND PLANE MEASUREMENTS AT OBLIQUE INCIDENCE 162 1. Facility 162 2. EC-135 Madels and Measurements 165 3. Theoretical Considerations 166 4. Data Recording and Reduction 169 5. Presentation of Plots 169 V CONCLUSIONS 203 3

SECTION I INTRODUCTION In dealing with electromagnetic phenomena there are often cases where information can be obtained using scale models in a laboratory environment that would be difficult, if not impossible, to obtain otherwise. One example of this is the measurement of the skin currents and charges on aircraft to assess their vulnerability to at) electromagnetic pulse (EMP), and this report is concerned with measurements carried out on small scale models of the EC-135 aircraft. The models used ranged in scale from 1/447 to 1/114 and the measurements were made from 450 to 4000 MHz to provide coverage from 1 to 35 MHz at full-scale. Data are presented for the current and charge densities in amplitude and phase at selected stations cn the aircraft as functions of frequency. The configurations treated are the aircraft (1) in isolation (free space) with topside incidence, (2) near a perfectly conducting ground with topside incidence, and (3) near the same ground but with oblique incidence. In the first two configurations, data have been obtained for aircraft with and without HF wire antennas joining the top of the fuselage to the vertical stabilizer. During the course of the contract, there has been continuous development and refinement in the techniques of acquiring, processing and presenting the data. This is particularly true as regards the data reduction and presentation, and the evolution is reflected in the figures presented in Sections II, III and IV. Section II is concerned with the free-space measurements. These data were obtained at the very beginning of the use of swept frequency techniques. As the frequency was swept, typically over a 2:1 band, the sensor output in amplitude and phase was recorded on an analog X-Y plotter. To obtain data at a given station on the aircraft, two measurements are required: one with the sensor mounted appropriately on the model, and the other with the model replaced by a calibrating object, either a sphere or a ground plane, to determine the incident field reference. The ratio of these two is then the current (or charge) relative to the incident field and is the quantity required. In concept at least, such data reduction is easy to perform, but 4

in practice it can be extremely tedious since it should be carried out at many frequency points within the band. For all the free-space measurements, manual data reduction and plotting was necessary, using sampled values generally at 100 MHz intervals. Section III is concerned with measurements for topside incidence on the aircraft near to a perfectly conducting ground plane. At the lower frequencies, 1 to 9 MHz full-scale, the data were again reduced and plotted manually, but by the time the higher frequencies, 5 to 35 MHz full-scale, were reached, we had developed the ability to digitize the X-Y recorder plots, and then process and computer-plot the data. This same technique was then used to process the data contai ned in Section IV for the aircraft near to the ground plane but illuminated at an angle of 180 from the horizontal. 5

SECTION II FREEr-SPACE MEASUREMENTS The surface currents and charges have been measured on the EC-135 aircraft in a simulated free-space environment. The measurements were made over the frequency range 1 to 4 GHz which, for the 1/447 to 1/114 scale models used, corresponds to the full-scale frequency range 2.2 to 35 MHz, but the data presented here are only at frequencies up to 20 MHz. The illumination was topside with the incident field polarized with its electric vector either parallel or perpendicular to the fuselage, representing symmetric or antisymmetric polarization, respectively. Data were obtained for models with and without HF antenna wires connecting the top of the fuselage (near the cockpit) to the vertical stabilizer. 1. Facility and Equipment The measurements were made in the Radiation Laboratory surface field facility using equipment and procedures similar to those described by Liepa (1975)'. Figure 1 shows a block diagram of the facility whose main elements are: (1) anechoic chamber, (2) swept frequency source (sweep generator, power amplifier, isolator and antenna), and (3) receiving and recording equipment (sensors, preamplifier, phase shifter, network analyzer, analog X-Y recorder, and CRT display). 2. Models Seven models were used ranging in scale from 1/447 to 1/114. All are either diecast metal or assembled from plastic kits and purport to model some version of the Boeing 707, with the result that they do not have precisely the same scaled dimensions as the EC-135 aircraft. In translating the measurement frequencies to the full-scale ones, a scale factor was used based on the length when the Liepa, V.V., (1975), "Sweep Frequency Surface Field Measurements," Sensor and Simulation Note-21 0. 6

sweep tve gscerato j refcrcnrp siggalldircctional ipreamplifieia e phase L shifter is 3or probe lead L \1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ neliworKcI analjrzer I anechnoic cham~ber X, Y CRTL\ 21C~~ Kecorde display Figure i.. Surface field measurement facility.

excitation was symmetric, but based on the wingspan when the excitation was antisymmetric. Refueling booms were added to all the models. These were cut out of wood and then spray painted with conducting silver paint. Cockpit and fuselage windows Were also painted, and the landing gear doors and fuselage-wing joints (on metal models) were covered with aluminum or copper tape. On some models HF wire antennas were also attached. These were made of No. 30 wire and installed as per the specifications given in figure 2. Both of the wires are electrically shorted at the joint with the fuselage, but at the vertical stabilizer the upper wire is shorted and the lower one open. The open circuit condition was simulated using a section of string approximately 1/16 inch in length. Table 1 lists some of the properties of the various models. 3. Sensors All of the measurements were made with so-called "hard lead" probes. The surface currents were measured using a bent 3.2 mm diameter shielded loop proberl and the charge measurements were made using an extension of the center conductor of a 0.020 inch diameter coax. This extension or monopole was typically 0.060 inch long and the cable was brought to the surface from within the model. For the charge measurements at the nose of the aircraft, a hole was drilled in the model from the nose (STA:F130)':' into the front landing gear cavity. The sensor lead was brought out from there and taped along the belly of the plane up to STA:F800B from which it was taken in a direction normal to the fuselage and parallel to the direction of propagation of the incident wave. In the wingtip charge measurements at STA:W940T, the cable again joined the model at STA:F800B, from which it was tapled along the underside of the wing and passed to the top through a 0.025 inch diameter hole drilled through the wing.'" Liepa, V.V., (1975), "Sweep Frequency Surface Field Measurements, " Sensor and Simulation Note 210, Fig.- 18. The locations of body station numbers are given in Figure 3. The letter F (or W) in front of the number refers to a fuselage (or wing location, while the letter T (or B) after the number designates the top (or bottom) of the aircraft. 8

ARN-72 Ant, #2 ART-47 #3 AT-741/A AT1076A/A Liaison #1 Irobe?actr Beacon STA 515 WL582.75 AS-52/A ART-47 #2 ART-47 #4 Fin STA 174 AT 1 0 7 6 A /A AT-1076A/A VLF.Receiving Antenna AT 1076A/A STA 610, AS 1909/ARC-06 RBL4.5 ARR-71 #1-4 ARC-58 #4 AT-1076A/A (Fin Sta 143.9 Fixedwire ARC.-58 RCV STA 1216 (WTL410) TA? 425 iFixedwire E(*cfic (ly jSta 64 ST AC 34 1 Ant n STA 08 841.9 AT-2456/ARC _ 85 / ( T1076A/A Pce AT- 6A/A STA S10,T 510 ST LFrst tn Al"N-59 co ~~~ EL~ECTRICALLY --- CONNECCT ED APNT #1 2 ARC34 962 An LiainT-47 2 AT-256A/ARC AT-1076A/A APN- 1 3 3 ARR-71 #5S STA, 995, RiBL48 \r 12245 (4 Places) A/ T-107 6A/A STA 510 VLF Tritrnsmit An~tenna A R,'-7 - ANT #1 AS-1985/1XAIRC96 Liaison #2 Probe at 741/A ARR-71 #1-4 Left Wing BL740 STA 430 AT-1076A1A Liaison #3 Probe STA 450 Right Wing BL740 Figure 2. EC-135 HF wire antenna installations.

Table 1 EC-135 MODELS USED Fuseiage Diameter I i Wing IlWing With Span Thicknes HF HF Model Scale Scale I 400 800 1200 at W600 Wires Wires 447 1/447 1/466 metal 0.93cm 0. 94 cm 0. 86 cm 0. 17 cm X X 325 1/325 1/341 metal 1.15 1.19 1.15 0. 23X X 224 1/224 1/225 metal 1.80 1.77 1.76 0O28 X 129 1/129 1/134 plastic 3.01 t 3. 03 3. 02 0. 59 x 114 1/114 1/117 plastic 3. 50 3.44 3.35'0.30 x

1600 1400 WING STATIONS oc 1200 -'30 W9 T oo 1300 1~,~T~:IF~4i0 Note: Stations W600 and W940 are at the mid8002 O. NACELLE STATIONS sections of t400e wing. Station W94 is located at the center of a circle tangent to the three edges of a wing. Figure 3. EC-135 aircraft body stations, X designates measurement locations. 11

Current measurements were made using an external B-dot sensor or current loop probe. This touched the model only at the point of measurement and even there it was separated by a small dielectric epoxy bead. Since the incident field was always horizontally polarized, the incident electric vector was perpendicular to the sensor lead, thus providing minimal interactions with the incident field. Figures 4 through 7 show photographs of the models used and the implementation of current and charge sensors. 4. Data Reduction For a given frequency scan the plots of current and charge obtained directly from the X-Y recorder show very little resemblance to the behavior that would be expected. This is caused by the nonuniform frequency response of the measuring equipment, and to account for the distortion the model data are normalized to the incident field deduced from data for 3.133 and/or 6. 0 inch diameter spheres obtained under the same measurement conditions. To perform the data reduction, values are read off the model and sphere measurement curves, typically at 100 MHz intervals, and the computation is performed using an electronic pocket calculator. When appropriate, corrections for probe integration effects are also applied. These corrections are largest for the smaller models and can adjust the current amplitude by as much as 33 percent and the charge by 60 percent. 5. Data The surface currents and charges are presented as functions of the full-scale frequency in figures 8 through 31. Corresponding amplitudes and phases are given in figures having the same number but distinguished by the letter a (amplitude) or b (phase). As noted, the amplitudes are normalized to the incident field, and the phase is relative to that of the incident field at the station where the measurement was made. It may also be helpful to note that all figures with even numbers are for aircraft without HF wire antennas, whereas odd numbers are for aircraft with antennas. 12

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A summary of the data presented is given in Table 2, and this may prove helpful in locating results for a particular measurement condition. Thus, to locate the figure giving the current amplitude on top of the fuselage at station 800 for the aircraft with HF wires, excited antisymmetrically, go to the line designated F800T (Fuselage, 800, Top) and then across the table to find the appropriate entry: figure 27a. The graphs themselves are self-explanatory. The insert sketch in the upper right-hand corner of each shows the incidence, polarization and measurement location. The vertical scale is always linear. Because of the different scale models used, some figures have two, or even three, curves which partially overlap, and since each represents a different and independent measurement, the degree of fit provides some indication of the measurement accuracy for that particular station and frequency range. 1-5

Table 2 DATA SELECTION CHART EC-135 without HF wires EC-135 with HF wires Symmetric Antisymmetric Symmetric Antisymmetric Station Number J Q J Q J Q F400T 8a, b 9a, b F800T 10a, b 26a, b la, b 27a, b F1200T 12a, b 13a, b F400B 14a 15a F800B 16a 28a 17a 29a F1200B 18a 19a W600T 22a, b 23a, b W800B 24a 25a F130 20a 21a W940T 30a 3 la Note: Figure XXa refers to amplitude data. Figure XXb refers to phase data.

15 E1 STA:F400T 1Without HF Wires Amplitude 1/224 \ H/HL 1/447 Ir / ~ 1/114 I' 4. I I p0 5 10 15 20 Frequency (Mliz) Figure 8a. Current density at STA:F400T, symmetric excitation, without HF wires.

I 100 k Phase 1/447 STA:F400T /without HF Wires (degrees) 1/224 1/114 0 " -100 0 5 10 15 20 Frequency (MHz) Figure 8b. Current phase at STA:F400T, symmetric excitation, without HF wires.

15 k STA F400T With HF Wires 10 Amplitude IH/HiI 1/447 1/224 i$/r~k: ~\ 1/114 5 0 5~~~~~~ I10 15 2 Frequency (MHz) Figure 9a. Current density at STA:F400T, symmetric excitation, with HF wires.

E 100 k Ph(gse STA:F400T (deg;1ees) 1/224 With HF Wires 1/447, 1/114 O ~~~~~~~~~~I I I ru~~~~~~~~~~~ / I 44 4 4 100 5 10 15 20 Frequency (MHz) Figure 9b. Current phase at STA:F400T, symmetric excitation, with HF wires.

15 A k STA: F t0OT 10 1/447 Without HF Wires Amplitude IH/H'I 1/224 1/114 X\, I~~~ -I 00 5 10 15 20 Frequency (MHz) Figure LOa. Current density at STA:F800T, symmetric excitation, without HF wires.

A 100 k STA:F800T Phas(degrees) Without HF Wires (degrees) 1/447l 1/114 /~~X/ 1/224 -100 0 5 10 15 20 Frequency (MHz) Figure 10b. Current phase at STA:F800T, symmetric excitation, without HF wires.

15 E STA: F 800T 1With HF Wires 10 Amplitude H/H I 1/447 0 5 10. 15I.20 Frequency (MHz) Figure 11a. Current density at STA:F800T, symmetric excitation, with HF wires.

100 k Phase STA:F800T (degrees) With HF Wires 1/447 1/114 0 1/224'~ I -100 0 5 10 15 20 Frequency (MHz) Figure llb. Current phase at STA:F800T, symmetric excitation, with HIF wires.

30 20 STA:F 1200T 1/447 Without HF Wires 1/447 Amplitude IH/Hi 10 1/114 1/224 00 L 5 1-Fq ( 15' Frequency (MHz) Figure i2a. Current density at STA:F1200T, symmetric excitation, without HF wires.

7E~~ 100 k Phase STA:F 1200T (degrees) Without HF Wires 1/447 1/224 0~ 1/114 100 ru3 % I\ I > -100 0 5 10 15 20 Frequency ( MHz) Figure 12b. Current phase at STA:F1200T, symmetric excitation, without HF wires.

15 I I E k 1/447 STA: F 1200T With HF Wires 10 Amplittlde IH/Hi 1/224 \\ 1/114 1/114 10 150 20 - / 0 5 10 15 Frequency (MHz) Figure 13a. Current density at STA:F1200T, symmetric excitation, with HF wires.

E 100 STA:F 1200T With HF Wires Phase (degrees) 1/447 I I N /l S v//224 1/114 -100 I / ~0 5 10 15 20 Frequency (MHz) Figure 13b. Current phase at STA-F1200T, symmetric excitation, with HF wires.

6 k STA: F400QB 1/325. Without HF Wires / \ 4, r Amplitude 1/129 0 5 10 15 20 Frequency (MHz) Figure 14a. Current density at STA:F400B, symmetric excitation, without HF wires.

15 E STA:F400B 1With HF Wires 10 Am plitude IH/HI 1/447 5~~~~~~~~~/4 1/224 5 1/114 0 5 10 15 20 Frequency (MHz) Figure 15a. Current density at STA:F400B, symmetric excitation, with HF wires.

E 6 k STA: F800B Without Wires Am plitude IH/H1I 4 W L I\ l 1/325 2 IN/1/12 0 5 10 15 20 Frequency (MHz) Figure 16a. Current density at STA:F800B, symmetric excitation, without HF wires.

15 A k STA: F800B With HF Wires 10 Amplitude IH/HiI (A) 5 1/447 1/224 1/114 00 5 10 15 20 Frequency (MHz) Figure 17a. Current density at STA:F800B, symmetric excitation, with HF wires.

E 20 STA: P1200B Without HF Wires Amplitud~e H/H1 10 1/325 1/J129 0~~~~~~~~~~~~~~2 0 5 10 15 Frequency (MHz) Figure 18a. Current density at STA:F1200B, symmetric excitation, without HF wires.

30 Ei STA:F 1200B 0With HF Wires 20 Amplitude 1H/H i 1P H/H' 11/447 10 1/224 1/114 0 5 10 15 20 Frequency (MHz) Figure 19a. Current density at STA:F1200B, symmetric excitation, with HF wires.

sensor 60 STA: F 130 1/325 Without HF Wires 40 Amplitude jE/E J 1/129 20 0- ~ ~ ~ ~ ~ ~ % 0 5 10 15 20 Frequency (MHz) Figure 20a. Charge density at STA:F130, symmetric excitation, without HF wires.

to- Ei sensor A k 100 STA:F 130 With HF Wires Amplitude jEn/EiJ 1/325 50 1/114 00 5 10 15 20 Frequency (MHz) Figure 21a. Charge density at STA:F130, symmetric excitation, with HF wires.

E A k 10 STA: W600T WithcMt HIP Wires Amplitude IH!HiI (4 1/341 5 1/134 0 0 5 10 15 20 Frequency (MHz) Figure 22a. Current density at STA:W6OOT, anti-symmetric excitation, without HF wires.

~I I ~~I' E A 50 k STA: W600T Without HF Wires Phase 1/341 (degrees) 1/t34 / l -50 I I I I 0 5 10 15 20 Frequency (MHz) Figure 22b. Current phase at STA:W600T, anti-symmetric excitation, without HF wires.

6 1/466huu.uO..in STA: W600T With HF Wires Amplitude 4 IH/HiI 1/225 (A) Al\ J1/117'IO~~~~~~~~~~1 / 2 I 5 10 15 20 Frequency (MHz) Figure. 23a. Current density at STA:W600T, anti-symmetric excitation, with HF wires.

I I Ei ~~~~~~~50 ~~~~~k 1/466 STA: W600T With HF Wires ~~~~( tl ~ Hiag m ~~~~~~~~..1/2 2 5 0 / ^!/ \\ / I /~ -50 1/117 I 0 5 10 15 2 Frequency (MHz) Figure 23b. Current phase at STA:W600T, anti-symmetric excitation, with HF wires.

Ei k STA:W600 B Without HF Wires 4 Amplitude H/Hi 1/341 2 1/134 0 5 10 15 20 0 5 10 15 20 Frequency (MHz) Figure 24a. Current density at STA:W600B, anti-symmetric excitation, without HF wires.

6 V STA: W600 B With HF Wires 1/466 4 -P Amplitude Na H/H I 1/225 2C An 1/ 117 J -~0 5 0 15 Frequency (MHz) Figure 25a. Current density at STA:W600B, anti-symmetric excitation, with HF wires.

.......... I I I' 6 A k STA: F80OT Without HF Wires Amplitude H/H1 | 1/341 1/134 2 0,I I 0 5 10 15 20 Frequency (MHz) Figure 26a. Current density at STA:F800T, anti-symmetric excitation, without HF wires.

E 50 k STA:F800T Without H F Wires Phase (degrees) 1/341 1/134 0 -50 0 5 10 15 20 Frequency (MHz) Figure 26b. Current phase at STA:F800T, anti-symmetric excitation, without HF wires.

E 6 k STA: F800T With HF Wires 4' 1/466 Amplitude A JIH/HI 2 1/225 1/117 0 5 10 15 20 Frequency (MHz) Figure 27a. Current density at STA:F800T, anti-symmetric excitation, with HF wires.

50 STA: F800T 1/466 With HF Wires Phase (degrees) 0 1/225 1/117 -50 0 5 10 15 20 Frequency (MHz) Figure 27b. Current phase at STA:F800T,' anti-symmetric excitation, with HF wires.

6 E A k STA:F800B 4 Without HF Wires Amplitude IH/HI' 2 1/341 1/134 0 0 5 10 15 20 Frequency (MHz) Figure 28a. Current density at STA:F800B, anti-symmetric excitation, without HF wires.

~~~~~~6 E~~A k STA:F8OOB With HF Wires 4 Amplitude HIH~~~~~~/Hil 1/466 1/117 1/225 ~ 0 5 10 15 20 Frequency (MHz) Figure 29a. Current density at STA:F800B, anti-symmetric excitation, with HF wires.

60 E sensor STA:W940 40 Withaut HF Wires Amplitude IE /Eil 1/341 20 1/134 0 5 10 15 20 Frequency (MHz) Figure 30a. Charge density at STA:W940, anti-symmetric excitation, without HF wires.

30 Ei A^r s ensor - k/ 20 STA:W940T With HF Wires Amplitude 1/466 10 1/117 00~ I1/225 / \ - 0 0 5 10 15 20 Frequency (MHz) Figure 31a. Charge density at STA:W940T, anti-symmetric excitation, with HF wires.

SECTION III GROUND PLANE MEASUREMENTS AT NORMAL INCIDENCE This section presents the measured surface currents and charges on models of the EC-135 aircraft in the presence of a perfectly conducting ground plane. Topside incidence was always used and, depending on the particular measurement, the incident electric vector was either parallel to the fuselage (symmetric excitation) or perpendicular to the fuselage (antisymmetric excitation). The measurements were made using 1/325 to 1/114 scale model aircraft by scanning the frequency over the bands 0. 45 to 1. 1 GHz and 2. 0 to 4. 0 GHz. The data presented here cover 1. 38 to 35 MHz and are for models with and without HF wires stretched from the vertical stabilizer of the model to the top of the fuselage near the cockpit. 1. Facilities and Equipment For these measurements, a vertical ground plane 4 feet high and 12 feet wide was erected in the chamber. A framework of 2 x 4s was used to support a 4 x 12 foot 0. 030 inch thick aluminum sheet whose side edges were then imbedded into the absorber walls of the chanber to reduce current reflections from the vertical edges. Since the incident field is horizontally polarized, the currents on the sheet are also horizontal. The first tests made on the new ground plane were surface scans at a fixed frequency over an area 0. 8 by 0. 8 m square and there the field amplitude was constant to within + 0. 5 dB. However, when the frequency was scanned with the test sensor at the center of the ground plane, amplitude oscillations on the order of 3 dB appeared on the X-Y recorder trace. This was obviously a source of concern, and after numerous tests and measurements (and arguments on how a matched horn antenna can reradiate a part of the received signal) it was concluded that energy was being reflected by the ground plane back into the horn and then reradiated in- or out-of-phase depending upon the frequency. To reduce this interaction, about 60 percent of the ground plane was covered with an absorber with particular attention paid to feathering the absorber edge 51

near to the center of the plate so as not to introduce adverse current reflections. This reduced the ground plane-horn interaction to about 1. 5 dB, and yet a further reduction was obtained by changing the reference signal pickup from a directional coupler located ahead of the transmitter horn to a sensor mounted on the ground plane. This change did not reduce horn-ground plane interaction per se, but did smooth out the recorder traces since both the reference and the test signals increased or decreased simultaneously. Figure 32 shows the frontal view of the absorber-covered ground plane with a model aircraft mounted at the center. The aircraft is out of proportion —the model shown appears over two feet long, although the largest model used was 1/114 scale and about half the size shown. Figure 33 shows a side view of the same and emphasizes the placement of the reference and test sensors. To accommodate the reference and the (bottom) test sensors, holes 3/8 inches in diameter and spaced 5 cm apart were drilled throughout the ground plane in an x-y coordinate lattice whose center is at the center of the sheet. The holes spanned 80 cm horizontally and vertically. By moving the MGL-S8A(R) sensor (the one with connector underneath) through all the mounting positions (holes) and recording the frequency response at each position, vertical and horizontal surface scans were obtained. Figure 34 shows the reduced data for 0. 5 and 1."0 GHz. Measurements were also made in the 2 to 4 GHz band with similar results. Figure 3.5 shows the block diagram of the instrumentation used in measurements of the model aircraft near the ground plane. The arrangement is similar to that used earlier in free-space measurements, the main difference being that the reference signal is now taken from a sensor (MGL-S7A(R)) mounted on the ground plane rather than from a directional coupler located ahead of the horn. 2. Models For the measurements, five different models were used ranging in scale from 1/114 to 1/325. These are the same as previously used for obtaining the free-space data, but because they have been reworked and repainted, some of the scaling factors

SE-A/NSOP Figure 32. Frontal view of the absorber covered ground plane. The ground plane and the transmitting antenna are inside the anechoic chamber (not shown). C/?PL UA A/D.5fiA/.SOR Figure 33. Sideview sketch showing the locations of the sensors (not to scale). 53

* te n:;i i,;;- "1 - I1T-X i....i..fi. lj 1+j -4- - -7f~i 1W - A~~~t TLOW ~ Figure,34. Vertical and horizontal surface scans made with MGL-S8A(R). differ from those originally given. All models are in the "wheels-up" configuration and have wheel wells and doors taped over with adhesive copper tape. There are two 1/325 scale models, one with HF wires and one without.'There is only one 1/216 scale model and for this the wires were attached or removed as required. The 1/129 model is without HF wires and the 1/114 model is with them. Figure 2 shows the locations where these wires are attached and also indicates how they are connected, i.e., open circuit or short circuit condition. To raise the models an appropriate distance above the metal ground plane, four styrofoam blocks were used with each model —two under the fuselage and one under each wingtip. The height of these blocks was chosen to simulate the 1. 22 m fuselage-to-ground clearance on a full-scale EC-135 aircraft. The vital statistics of models are summarized in Table 3. Amongst other things, the relative nose-to-wing and the wing-to-fuselage center distances are given. Such information is helpful in interpreting the measured data and, in particular, explaining the presence of the second resonance peak near 4 MHz. 3. Measurements and Sensors Used The measurements are for the same aircraft body locations or stations used in the'free-space measurements, plus additional measurements for the current on the ground plane under the fuselage of the model. The latter measurements are designated by numbers such as F800G, where t"G" implies ground plane measurement. Figure 3 shows the locations of the stations, and for specifying them the following convention 54

sweep powerr signal power generator i i ~~~~~~~~~~~isolator Ln Ln (3' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ M netwvork ref. analyzer k Sig. / anechhoic chamnber X- Y 1_ - CRT recorder I- idisplay FibJ~re 3 5. gShrafft field n!4n ",mftEjtk WI&-VertjefjL ground plane.

Table 3 EC-135 MODELS USED Wing Fuselage Diameter Wing With Without EC-135 Fuselage Span Materia at Station Thickness HF HF a/L b/L (ab) Model Scale Scale 400 800 1200 at W600 Wires Wires (cm) (cm) (cm) 325 1/324.5 1/340.3 metal 1.15 1.19 1.15 0.23 X X 0.4305 0.5000 0.9305 216 1/216.0 1/225.3 metal 1.80 1.77 1.76 0.28 X 0. 3902 0.5307 0.9209 129 1/129.2 1/134.4 plastic 3.01 3.03 3.02 0.59 X 0.3983 0.4807 0.8790 114 1/114.0 1/117.3 plastic 3.50 3.44 3.35 0.30 X 0.3780 0.5621 0.9402 Full Scale Dimensions of EC-135: Overall length (with boom) 41. 53m Overall length (without boom) 41. 17m Fuselage length (without boom) 39. 27m Wing Span 39.89m Fuselage clearance 1. 22m

has been adopted: F1200T is the Fhselage station, located on the Top, with 1200 (the number used by the Air Force) indicating the distance in inches from the nose, but with the nose already at 130 instead of 0. Other letters used in station designations are W, B, and G, and refer to Wing, Bottom, and Ground positions, respectively. Four different magnetic sensors and one charge probe were used in making these measurements, with the MGL-S7A(A) used for reference. For the top measurements, our own free space 3.2 mm diameter shielded loop probe was used (figure 33), and for the bottom measurements, a similar but straight probe. The MGL-S8A(R) was used to measure the current on the ground plane, and at the nose and wingtips the charge was measured using a 2 mm extension of the center conductor of a 0. 020 inch diameter coax. In all the measurements precautions were taken to minimize the sensor lead interaction with the electromagnetic field. 4. Incident Field Calibration Since it is almost impossible to achieve a flat or even uniform measurement system response over a wide frequency range, the incident field should be measured everytime a sensor or other component in the system is changed. The number of station measurements made per calibration varied. For example, at the F400T, F800T, and F1200T stations, only one calibration run was made for each model and frequency band, while for the bottom stations F400B, F800B, and F1200B, a new calibration run was made for each station. (a) Top Sensor This is the same one used in the free-space model measurements. Its signal lead extends vertically to the ceiling on the illuminated side of the ground plane, but since the lead is perpendicular to the incident electric field, its interaction or disturbance is negligible. For calibration of this sensor and the rest of the system, the loop was moved against the ground plane (with the model removed) and the calibration field H recorded on the X-Y chart recorder. Hence, the incident field H is H = 1/2 H. (1) 57

(b) Bottom Sensor As can be seen in figure 33, this sensor goes through a small hole in the ground plane. Behind the ground plane the sensor is attached to a precision slide which provides accurate positioning. In measurements of the current at STA:F800B, for example, the model is first mounted on the ground plane and then the sensor is moved out until the loop barely touches the model at this station. To record the calibration field, the model is removed and the field recorded. If h is the distance (or clearance) from the ground plane to what was STA:F800B, the incident field can be written as H=: 2 Ho 2 cos (kh) where k is the wave number, Then, H H - (2) -o 2cos(12fh) where f is now the frequency in GHz and h is the distance in centimeters. (c) Wing Measurements The wing measurement data are for anti-symmetric excitation only, for which the incident magnetic vector is parallel to the fuselage. To measure the wing current, the top and bottom loops were both rotated 34 degrees, the estimated average sweptback angle for the wings. The incident field is then given by H H = c 2 cos34 cos (kh) H i.e. H = (3) -o 1. 6581 cos(l2 fh) where f is the frequency in GHz and h is the displacement of the sensor from the ground plane where the calibration measurement is made. For the wing top measurements, the sensor was always moved against the ground plane and hence h = 0. For the wing bottom measurement, the sensor was left at the wing station location (with the model removed) and hence h is the ground to station distance for the particular model used. 58

(d) Charge Sensors Since the 2 mm extension of the center conductor of the 0.020 inch coax requires a base or image plane, a 10 x 10 cm image plane was used to obtain the calibration field. Figure 36 shows its implementation. An L-shaped bracket, made out of 0. 020 inch thick brass, was taped to the ground plane perpendicular to the incident electric vector. A coax with center conductor extended was then inserted through the holes previously drilled and carefully taped with conductive tape to cover the loop created on the underside of the image plane. A loop as seen in figure 36 will respond to the magnetic field, and if not properly hidden, would affect the electric field measurements. O. C2 0,;. COAX -. k /0c' Figure 36. Image plane for calibration of charge probes. To verify that the field measured on this small image plane is the same as that measured on a larger sheet, frequency scans were run as the size of the image plane 2 was increased in four increments to 25 x 25 cm. When compared to the trace for the 10 x 10 cm plane, the recordings typically deviated ~ 0. 25 dB over the 0. 45 to 1. 10 GHz range scanned, and the maximum deviation observed was 0.5 dB. Such variations are within the measurement repeatability tolerances and, hence, a 10 x 10 cm plate is of sufficient size for the present application. When a plane wave of strength E and et time dependence is incident perpendicularly on a perfectly conducting ground plane, the sum of the incident and reflected 59

field is E =2iE sin(kd), where d is the distance from the ground plane where E is observed. The factor i in the equation implies a 90 degree phase shift. If E represents the calibration voltage measured on the image plane, the incident field E is then -O - E -o 2 i sin(kd) E i.e. E (4) -o 2isin(12fd) The charge measurements on the models were made at two stations: the nose (F130, symmetric excitation) and the wingtip (W940T, antisymmetric excitation). For both measurements, the coax lead was brought away from the model at station F870B and then directly through a hole in the ground plane. The station F870B was selected as a result of the following test. A model was mounted on the ground plane as if for measuring the current at STA:1200T, but this time a shorting strap to simulate the lead that is present in charge measurements was connected from the bottom of the fuselage to the ground plane. Measurements were then made as the position of the shorting strap was changed, and of the points tested, the STA:F870B was found to have least effect on the current on the top of the fuselage. In antisymmetric excitations, the charge is zero at any symmetric point on the fuselage and, hence, STA:F870B was also acceptable. 5. Reduction of Data Two methods of data reduction were employed for the data presented here. For the low frequency range, typically 1 to 9 MHz, the data were reduced manually by picking discrete frequency values, writing down the dB and phase data in a table, and performing the necessary mathematical operations using a hand calculator. Points were chosen uniformly spaced in frequency, plus extra points at peaks and other places where the resulting curves were inadequate. The actual points where the data were reduced are 60

shown on the curves. At the higher frequencies the analog data was first digitized using a Computek Tablet interfaced with a PDP-11 computer where the data was stored on the disc. From there it was copied to MTS (The University of Michigan AMDAHL 470 V/6 machine) and further processed using codes based on formulas presented in Section III, para. 4. The reduced data from MTS was then plotted on a desk-top digital plotter covering the frequency range 0 to 40 MHz A correction for the integrating effect of a probe was applied only to the measured data for the charge at the nose (STA:F130). It was assumed that the nose can be approximated as the tip of a prolate spheroid, and using the static solution and the assumption that the 2 mm long monopole responds to the electric field value at its center, i. e., 1 mm from the surface, the following correction factors X were obtained and applied to the data: Scale 1/114 1.621 1/129 1.701 1/216 2.174 1/325 2.755 In contrast to the free-space data previously given for the current, no corrections to account for the integrating effect of the probe have been applied to the present data. The reason is that with a ground plane present, the illuminating signal can be considered as two plane waves traveling in opposite directions and for this case an appropriate correction procedure has not been developed. Judging from the freespace correction data, the correction factor would be at most 1. 2 for the 1/325 model and at most 1. 15 for the 1/114 model. This factor would apply to the scattered component of the field only and is, therefore, applicable to the curves presented only when H scat >>H, i.e., H/H >10. 0 0 61

6. Data The surface ourrent and charge data presented has been normalized to the incident field strength H or Eo and scaled to the full scale frequencies. Figures designated XXa contain amplitude data, and those designated XXb contain the corresponding phase data. Note that the phase is relative to the incident phase at the ground plane surface. Also, the first pair of figures for a given station is for the model without HF wires, followed by the data for the model with wires. Table 4 summarizes the data and can be used as a guide to quickly locate a particular set. All the measurements that were made under this task are included. Most of the results appear excellent and quite consistent, yet there are some "rotten apples" indicating that something abnormal may have happened when the data was taken. Because the reduction of the data lags the measurements by a couple of months, it is impractical to repeat the measurements in question. Where abnormalities in data occur and there is an apparent reason for the effects, comments have been entered on the figures. 62

Table 4 DATA SELECTION CHART Without HF wires With HF wires Anti- AntiStation J or Q Symmetric symmetric Symmetric symmetric Number (Figure) (Figure) (Figure) (Figure) F400T J 37a, b ------- 39a ------- 38a, b 40a, b F800T J 41a, b 61a, b 43a 63a 42a, b 62a, b 44a, b 64a, b F1200T J 45a, b -- 47a -------- 46a, b 48a, b F400B J 49a, b ------- 51a 50a, b 52a, b F800T J 53a, b 65a, b 55a 67a 54a, b 66a, b 56a, b 68a, b F1200B J 57a, b ------- 59a ------- 58a, b 60a, b W600T J ------ 69a, b....... 71a 70a, b 72a, b W600B J ------ 73a, b ------- 75a 74a, b 76a, b F130 Q 77a, b ------- 79a ------- 78a, b 80a, b W940T Q ------ 81a, b ------- 83a 82a, b 84a, b F400G J ------ --—.. ------ F800G J 85a, b 89a, b...... 86a, b 90a, b F1200G J 87a, b 91a, b...... ______ 88a, b 63

— i. E, ----- -24 4 -r-~ - - __: EC-135.-.. STA. F400T, without HF wires 1~..i o~). __ __.. Figure 37a. Currentdensity at STA:F400T, 111 _-_f. -symmetric excitation, without -: HF wires (1-9 MHz). 0~~~~~~~~~.~ x f6 2.4. 9,_____l__ ~ ~ ~ ~ ~ P-QEWYM'R

| —A —A 1/325 E 300 1/216 — o 1/114 200 V / //// 7 EC-135' { STA: F400Twithout HF wires 100 of t~ ~ | j- I' \ IFigure 37b. Current phase at STA:F400T, cn S |..\:>'IX,symmetric excitation, with|/.- k / | ";Alout HF wires (1-9 MHz). \0 A. -100 -200 1F 2 4 6 8 9 FREQUENCY IN MHz

5.0 EC-195 STftFt00OT PAR EBF6.8 64.80 10.0 0.0 10.0 20.0 30.0 00 FREQUENCT (MHZ) Figure 38a. Current density at STA:F400T, symmetric excitation, without HF wires (0-40 MHz). al Figure 38a. Current density at STA:F400T, symmetric excitation, without HF wires (0-40 MHz).

250.0r EC-135 STRxF40T PAR EBF69.8L,.60 150.0 0.0 10.0 20.0 30.0 IO.0 FREQUENCY (M-HIZ Figure 38b. Current phase at STA:F400T, symmetric excitation, without HF wires (0-40 MHz).

24 24 - _- 1/325 — x-s 1/216 ) ~ - n —-7- 1/114 20........ 16 —EC'135 I I STA: F400T, with HF wires 0o 12 - - 0' 0o 1 tFigure 39a. Current density at STA:F400T,, symmetric excitation, with HF wires (1-10 MHz). 81 ~-,....\...I 0:: 8FEUEC INMz-: o - 0 FREQUENCY IN MHz

10.0 EC-135N STRsF4OOT PAR EBF78.81.55 7. 5 5.0 c:'o 2.5 0.0 10.0 20.0 300.0 FREQUENCY [MHil Figure 40a. Current density at STA:F400T, symmetric excitation, with HF wires (0-40 MHz).

OL PHASE LDES3 I 1 o o 0 o o 0o o o o o o O o~~~~~~~~~~ CI'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 00 C 0 0 0 0 CD 0 0d Cr 0~~::1 p 0) CD~~~~~~~~~~~~~~~~~~~~~~c:~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~~, CC mI I < g Z5 C) CD 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 3 0 I( p~~ u11 0~ 01 W | e (O~~~( m @1 Ik ~ j 4. 0T 0 CD ( 0 01 w ~c~~~~~~~~~~~~~~~~~~~~~~~~~~( N. U 0~~~~~~~~~~~~~~~~ o, u (b a~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~1 0~~~~~~~~~~~~~~~~~~~~~~~~~~0

E 48 A —-A 1/325 x-x-x 1/216 o-a-a 1/129 EC-135 STA: F800T) without HF wires 32 Cd Figure 41a. Current density at STA:F800T, symmetric excitation, without HF wires (1-9 MHz). 16 X~~~~~~~ 1 INMH2 4 6 FRE~QUENCY IN MHz

-- 1/324 E X-x -x 1/216 o-o —o 1/129 300"' - EC-135 2-2 ~ ~ ~ ~ iue4.Cretpaea TA.800T. STA: F800T without HF wi're - Nf 100 ( 0 o Figure 41b. Current phase at STA:F800T, symmetric excitation, without HF wires (1-9 MHz). -100 1 2 4 6 8 FREQUENCY IN MHz

EC-135 STARF800T PAR EBF70,65,61 7.5 La iI: E&J 5.0 2.5 0.0 0.0 10.0 20.0 30.0 40.0 FREQUENCT fMHZI Figure 42a. Current density at STA:F800T, symmetric excitation, without HF wires (0-40 MHz).

200. 0 EC-135 STAtFO 0 APR EBF7O.B5.81 100.0 w e 0.0 -100.0 -~200.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY IMHII Figure 42b. Current phase at STA:F800T, symmetric excitation, without HF wires (0-40MHz).

50 -. — 1/325 1/216 E - 1/114 A EC-135 32 r Il STA: F800Twith HF wires 24 24 Figure 43a. Current density at STA:F800T, symmetric excitation, with HF wires (1-10 MHz). 16 0 1 2 4 6 8 10 FREQUENCY IN MHz

10.0, EC-135N STRtF800T PRR EBF79.74.56 30.0 2J cc -J 20. 0 10.0 0.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY (MHZ) Figure 44a. Current density at STA:F800T, symmetric excitation, with HF wires (0-40 MHz).

200. 0 EC-ISSW STRuF800T PAR EBF7S.71L56 100.0 EaJ cn 0.0 cc II 0f O. 0 -200.0 0.0 10.0 20.0 30.0 40.0 FRiEQUENCY( MH!) Figure 44b. Current phase at STA:F800T, symmetric excitation, with HF wires (0-40 MHz)

E "&di /-325 i k ix-x- i/aw — EC-135..II~~. ~ r. STA::...,A F1200T, without HF wires --- - I.. ~ t - ~' I -' -,.-..... i..,.1 Figure 45a. Current density at STA:F1200T, symmetric excitation, without HF wires (1-9 MHz). 1 -s. — r Xr 1~f~I- 4- 4 ~~~~~~.7. 4- -- ~ ~ ~ -..5~ 0 1 4~~~~~~~~X FR$QtJE4CY IN"Mfz

300 A-A-a 1/325 E Eo x-x-x 1/216 1/114 ik 2 oo00 200'///> /'////' EC- 135 f/ STA: F1200Tlwithout HF wires 100 / Figure 45b. Current phase at STA:F1200T, -s~~~~~~~ n I | Isymmetric excitation, without I7 B: | EN HF wires (1-9 MHz). / -100 -200 1 2 4 6 8 9 FREQUENCY IN MHz

10.0 a EC-1SS STRFI1200T PAR EBF71,66,62 7.5 5.0 00 2.5 0.0 0.0 10.0 20.0 30.0 4O0O FRE0QUENCY (MHHZ) Figure 46a. Current density at STA:F1200T, symmetric excitation, without HF wires.(0-40 MHz).

250.0 EC-135 STAtFI200T PAR EBF71,86,62 150.0 LB LU in 50. 0 50.0 ~5Q. 0 -150. 0 0.0 10.0 20.0 30.0 40.0 FREQUENCY (MHZ) Figure 46b. Current phase at STA:F1200T, symmetric excitation, without HF wires (0-40 MHz).

48 X- -x1/216 E 1/114 40 32 AST: F1200Twith HF wires Peak caused by interaction -of the 0 I I probe with HF wires. M3 24 Figure 47az. Current density at STA:F1200T, symmetric excitation, with HF wires (1-10MHz). 16 A xx ~~~o ~ ~0f 0 00 __,j ~ rd-~.~(X_~~,~~'/"/j ~/~O- 0-0-0 gO~0 0-. _____ 0 1 2 4 6 8 10 FREQUENCY IN MHz

150 _ EC-135W STR*F1200T PRR EBF8O.75.57 10.0 LaJ ('3 00 Q 5. 0 0.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY (MHt) Figure 48a. Current density at STA:F1200T, symmetric excitation, with HF wires (0-40 MHz).

200.0 EC-1SS STRAFI200T PRR EBF80,75.57 100.0 La 0 00 a-100. 0 -200.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY TM(HH Figure 48b. Current phase at STA:F1200T, symmetric excitation, with HF wires (0-40 MHz).

120 -as-a 1/325 x-x —x 1/216 o-o-o 1/129 EC-135 STA: F400B, without HF wires 80 oo_.,:I } lllFigure 49a. Current density at STA:F400B, o 1 - - symmetric excitation, without | 4 >4 TI -\ HF wires (1-9 MHz)..40'- 2 4 6 X FRoEQ~ENC 0_ v 1 -- 2 4 ~FREQUENCY IN MHz

100 A-,1 —a 1/325 r-x-x 1/216 pE c —c — 1/129 EC-135;x ~I'STA; F400B3wiLthout HF w-ires -100 M X~~~~~~~X 00 "A -200 Figur 49b- Current phase at STA:F400B, CI)Figurp symmetric excitation, without HF wires (1-9 MHz). -300 -400 1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2 4 6 8 I FREQUENCY IN MHz

30.0 EC-135 STReFUOOB PRR EBFI.11.18 20.0 10.0 0.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY M H Z) Figure 50a. Current density at STA:F400B, symmetric excitation, without HF wires (0-40 MHz).

200. 0 EC-13S STRFIEOODB PAR EBF 1. 1. 8 100.0 0.0 I[ 00 0 oo -100.0 -200. - _..,0 0.0 10.0 20.0 30.0 tO.0 FREQUENC I(MH1) Figure 50b. Current phase at STA:F400B, symmetric excitation, without HF wires (0-40 MHz).

240.. 1/325 1/216 E} 200 4~- 1/114 ~~___/7 EC-135 160160 STA: F400B, with HF wires (X) o 120 Figure 51 la. Current density at STA:F400B, symmetric excitation, with HF wires (1-10 MHz). 80 to l l 40 2 4 6 8 10 FREQUENCY IN MHz

30.0 EC-ISSW STR*FIAOOB PRR EIF22.28.36 20.0 cc "a oa0. 0 0.0 10.0 20.0 30.0 40.0 FREOUENCTI (MMt.) Figure 52a. Current density at STA:F400B, symmetric excitation, with HF wires (0-40 MHz).

L6 PHRSE tDEGS o 0 0 0 oI a a 0 a 0c 0 0 00 0 O C O O O O d Q O,OO d OC O O O t CI~d 0ct CD (O m lH C,, E!, Cd 0 Cj p oi o o F ~~~~~~~~~~~~._._...___._... —-oo H e ~ o ~. Pl o 1U IO~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I Z n 0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ o CD ~' I o to C. 0'ri * 33 0 00 3 E~~~~~~~~~~ $.~~~~~~~~~~~~~~~~~~~~~I cr- L~~~~~~~~~~~~~~~~~~~~~Ig ~~~J' =D~~~~~~~ =C: I -— ~~~~~~~~~~~~~~~~~~~~~ r~~~ 0 ~? 0 1

- - 20 - -a — 1/325 xl- -— 14/216 - o-0-o 1/129 o-. EC-135._.: 1 ----: { - I........ —- STA:-F800B, without HF wires. - -.- -.....- - -' - - -- - r' j' | -o.. Figure 53a. Current density at STA:F800B,.~-' -::I~.... Z Z 1symmetric excitation, with-..40 — 1-' l i x out HF wires (1-9 MHz). o..,, *!............ I..... 2 4 6 8 9 FREQUENCY IN MHz

100 | --- - 1/325 E _-' — 1/216 -o-o 1/129.0 -7,7-77-7. -7-7/ -,LX7 EC-135 ~~~/ / ~~STA: F800Bwithout HF wires j -200 bO i -200i - -400 symmetric excitation, with- _. __ 1-002 4 6 8 1 2 4 6 FREQUENCY8 9 FREQUENCY IN MHz

20.0 EC-135 STA:F800B PRi EBF3.9.15 15.0 us Ea 10. 0 5. 0 0.0 0.0 10.0 20.0 30.0 40.0 Fr t t S F0,NCT IMHti Figure 54a. Current density at STA:F800B, symmetric excitation., without HF wires (0-40 1~)

2t00. 0 EC-135 STRaF800B PAR EBF3S9.16 IL* VI 0. 0 0r. -200. 0 L0.0 10. 0 20.0 30.0 4. FRE-QUENCYT(MHZ) Figure 54b. Current phase at STA:F800B, symmetric excitation, without HF wires (0-40 MN)

120 120 -6- 1/325 |- _-x 1/216 o E 0- O _0 1/114 100 -. EC-135 80t I STA: F800Bwith HF wires s 60 - l I l Figure 55a. Current density at STA:F800B, symmetric excitation, with HF wires (1-10 MHz). 40 x 20 a0o~__. / \' ~.e- — o 0 n 4 —— 0-aA0 \ IIQ - 1 2 4 6 8 10 FREQUENCY IN MHz

10.0 EC-135WN STRF800B PRR EBF21L.32.38 I 5. i a0.0 10.0 20.0 30.0 40.0 FREQUENCY IT(MHZ) Figure 56a. Current density at STA:F800B, symmetric excitation, with HF wires (0-40 MHz).

200. 0 EC-ISSN ST7sF800B PAR E8F24.32.38 100.0 1* Eu mu on 0. 0 cc, -100.0 -200.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY (MHZ) Figure 56b. Current phase at STA:F800B, symmetric excitation, with HF wires (0-40 MHz).

120 Eo A-A-s 1/325 x —X-X 1/216''-" — 1/129 Ile~ 1,1'' ~~EC-135 STA: F1200B,without HF wires 80 40 } ] ] t<VtFigure 57a. Current density at STA:F1200B, r 5a 1 -symmetric excitation, with40 - out HF wires (1-9 MHz). 0 1 2 4 6 8 9 FREQUENCY IN MHz

A A a-4 -6 1/325 -e-x 1/216 ///////// -50 t -o-o 1/129 \ EC-135 1 I STA: F1200Bwithout HF wires _ _ C; x, 0 1- 1, I -150 // o - o Figure 57b.. Current phase at STA:F1200B, -200 symmetric excitation, without HF wires (1-9 MHz). -250 _ _ 1 2 4 6 8 FREQUENCY IN MHz

t5. 0 I EC-13S5 STRF1200B PRR EBF5. 13. 9 tO. 0 -' Ca Cl: 5.0 0. 0 10.0 20. 0 30. 0a. FigurREUENC MH 58a. Figure 58a. Current density at STA:F1200B, symmetric excitation, without HF wires (0-40 MHz).

200. 0 EC-ISS STTAFI200B PAR EBF5.13.19 100.0 Le U.S Lu gn 0.0 cc C? rr -100.0a -200. 0 0.0 10.0 20.0 30.0 40.0 FREQUENCT (MHl) Figure 58b. Current phase at STA:F1200B, symmetric excitation, without HF wires (0-40 MHz).

140 E God- 1/32525\ 120 b- 1/216 >-13- 1/114 EC-135 100 t If i STA: F1200B,with HF wires 11 80 - Figure 59a. Current density at STA:F1200B, _ 4 _1 | ll lsymmetric excitation, with o 0 HF wires (1-10 MHz). 60 40 20 / IFREUENCY IN MHz

30.0 EC-S135 STARF1200B PAR EBF26.33.141 20.0 -=3 I cc.0.0 10.0 0.0 10. 0 20.0 30.0 40.0 FREQUENCY IMHL1 Figure 60a. Current density at STA:F1200B, symmetric excitation, with HF wires (0-40 MHz).

200.0 _ _a EC-135W ST1sFI200B PAR E8F26.33.LL 100.0 CU U -200. 0 0. 0 10. 0 20. 0 30.0 4. FREQUENCYTLMH-t! Figure 60b. Current phase at STA:F 1200B, symmetric excitation, with HF wires (0-40 MHz)

12 ~ 1/1 X-X -AX 1/216 CE 0 0o-o- 1/129 io tEC-135 STA: F800T, without HF wires 0 6 on k e B~C Figure 61a. Current density at STA:F800T, 4 anti-symmetric excitation, without HF wires (1-10 MHz). A A 2 As I x, A A MAA A k~ A ~~A A "A FREQUENCY IN MHz

200 E r x 1/216 A 150 2 1/129 EC-135 STA: F800Twithout HF wires 100 Figure 61bl Current phase at STA:F800T, anti-symmetric excitation, CD P~ 50 I \ without EHF wires (1-9 MHz). 50.I K -50 1 2 4 6 8 9 FREQUENCY IN MHz

10. 0 EC-35 STAFSO0T PERP EBF72.67,63 The scaling for this portion of the curve 7.5 is wrong and is unknown. cLa 5. 0 C) C0 2. 5 0.0 0.0 10. 20.0 30.0 0w.O FRFEUENCY (MHZI Figure 62a. Current density at STA:F800T, anti-symmetric excitation, without HF wires (0-40 MHz).

200.0, EC-135 STRIF800T PERP EBF72,67,63 1 0 0. [ LThe phase offset for this portion of the curve is wrong and is unknown. La ( I w -100. 0 -200.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY (MHZ) Figure 62b. Current phase at STA:F800T, anti-symmetric excitation, without HF wires (0-40 MHz).

12 6-A -A 1/325 |-X-X 1/216 E 1/114 10 EC-135 Cd y STA: F800T,with HF wires 0 o m f L I\6 Figure 63a. Current density at STA:F800T, anti-symmetric excitation, with HF wires (1-10 MHz). 2XX - o > - ^X* \, a, r O _ O _ o O _ x 6\ 1 2 4 6 8 10 FREQUENCY IN MHz

15.0 EC-135W STRgF800B PERP EBF27SSS.39 I10. 0 ~IJJ 5. 0 0. 0 I 0. 0 10. 0 20. 0 3 0.0 4. FflEQUENCT (MHI~ Figure 64a. Current density at STA:F800T, anti-symmetric excitation, with HF wires (0-40 MT)

200.0,.. EC-135W STRF800SB PERP EBF27.95.39 100.0 0oo. 0 Lu Lu Cn 0. 0 -100.0 -200.0 - _ 0.0 10.0 20.0 30.0 4 FREQUENCT (MH7} Figure 64b. Current phase at STA:F800T, anti-symmetric excitation, with HF wires (0-40MHz).

E E 120 -A-A 31/325A kk 1 —xI/216 0-0-o 1/129 EC- 135 - tsA: w8001o, without HF wires - Figure ~65a. Current density at STA:F800B, -C3 o- 0- ~- anti-symmetric excitation, without HF wires (1-9 MHz). 0 9 1 ~~2 4 68 FREQUENCY IN MRZ

A-c-Z 1/325 Eo 100,- - x 1/216 o-o-o 1/129 0 X A-a.-wo — - x Ax Ax c ///////// EC-135 STA: F800B,Without HF wires -100 Ur2 -200 F 5 Figure 65b. Current phase at STA:F800B, anti-symmetric excitation, without HF wires (1-9 MHz). -300 - -400 FREQUENCY IN MHz

10.D -—,,,EC-135 STRtF800B PERP EBF7, 5,21 7. 5 =: B 2. 5 0.0 10.0 20.0 30.0 40.0 FREQUENC (MHHI1 Figure 66a. Current density at STA:F800B, anti-symmetric excitation, without HF wires (0-40 MHz).

200.0 EC-135 STRsFBOOB PERP EBF7.15.21.100.0 tu 0.0 cr. -100.0 -200.0.. 0.0 10.0 20.0 30.0 40.0 FREQUENC IT rIHZ Figure 66b. Current phase at STA:F800B, anti-symmetric excitation, without HF wires (0-40 MHz).

120 1/325 O.-o-7 7 1/216 100 x-x-x 1/114 k EC- 135 80 STA: F800B, withHF wires op-4 960 = l IFigure 67a. Current density at STA:F800B, anti-symmetric excitation, with HF wires (1-10 MHz). 40 20 20 ~ I 0. 1 2 4 6 8 10 FREQUENCY IN MHz

t0., EC-t3SN STAFF800T PERP EBF82.76.58 7. 5 Il. 5.0 -a 2. 5 0.0 0. 0 10.0 20.0 30.0 40. 0 FREQUEiNCT (MHZI Figure 68a. Current density at STA:F800B, anti-symmetric excitation, with HF wires (0-40 MHz).

250.0 EC-1SSH STReF800T PERP EBF82.76.58 150.0'U en 50. 0 -50.0 -150.0 o 0.0 10.0 20.0 30.0 40.0 FREQUENCY TMHI) Figure 68b. Current phase at STA:F800B, anti-symmetric excitation, with HF wires (0-40 MHz).

25 -- - a-~-n 1/325 x -X -X 1/216 k 20COs C 1/129 2 0 EC-135 / I STA: W600T,without 1F wires 15 1 / Figure 69a. Current density at STA:W600T, 0 anti-symmetric excitation, 10 / ox~ without HF wires (1-10 MHz). 5,,~~~~~~~~Y 1 2~~~ 4 6 8 FREQUENCY IN MHz

_-A_ - 1/325 E x-x-6 zx 1/216 30 - -~ o1/129 300 EC-135 200 oa |STA: W600T without HF wires a) 10 ~o Figure 69b; Current phase at STA:W600T, _q i,:'>~ g a'''anti-symmetric excitation, ~~% ~'' / \ without HF wires (1-10 MHz). -100 i 2 4 6 F 9 FREQUENCY IN MHz

3S.0 EC-135 STARWBOOT PERF EW87.81L.89 2. 0 I& N) 0.0 0.0 10. 0 20.0 30. 0 FREQUENCT MHZI! Figure 70a. Current density at STA:W600T, anti-symmetric excitation., without HF wires(04Mz)

200. 0 EC-ISS STRtNBOOT PERP EBWS781&.89 1 00. 0 Na. EL 0.0 a. -100.0 -200.0 0.0 10.0 20.0 30.0 40.0 FREQUENCTY CMH!) Figure 70b. Current phase at STA:W600T, anti-symmetric excitation, without HF wires (0-40 MHz).

12 1 -A-a 1/325 /x x1/216 4E0 ~-O" 1/114 10 EC-135 8 t- II STA: W600T,with-HFwires k xx ru t -a ~0 -" ~ 6 r I i!Figure 71a. Current density at STA:WtOuu, anti-symmetric excitation, with HF wires (1-10 MHz). 0~~~~~~~ 1 2 4 6 4 FRE0UENCY AT MHz

3S.0 EC-135N STRzHBOOT PERP EBN8B.83.91 2. 0:3 (31 1.0 a_ O~ 0. 0 0.0 10.0 20.0 30.0 40 0 FREQUENCT [MM1) Figure 72a. Current density at STA:W600T, anti-symmetric excitation, with HF wires (0-40 MHz).

200.0.0 EC-1SSW STR.U800T PERP EBHN86.83.9 100.0 La WI 0.0 -100.0 -200.0. 0.0 10.0 20.0 30.0 40.0 FREQUENCY (tHZ) Figure 72b. Current phase at STA:W600T, anti-symmetric excitation, with HF wires (0-40 MHz).

0 60 <~~ 1/216 1 k -— o 1/129 -X 50 777777 EC-135 STA: W600B, without HF wires 40 N _ o 30 Figure 73a. Current density at STA:W600B, anti-symmetric excitation, without HF wires (1-10 MHz). 20 10 x ~1 ~2 4 6 8 1 FREQUENCY IN MHz

4-A -A 1/325 300 x-x 1/216 | o-o-o 1/129 200 200 0/ 7//7,.~//// 7 EC-135 STA: W600B,without HF wires -200 C x Figure 73b. Current phase at STA:W600B, \ anti-symmetric excitation, without HF wires (1-9 MHz). I-400 I -600 FREQUENCY4 6 8MHz FREQUENCY IN MHz

EC-135 STAtWBOOB PERP EBW5.1.S 32.0 aIa 1.0 0.0 0.0 10.0 20.0 30.0 0A. FRIEQUENCY MHZ) Figure 74a. Current density at STA:W600B, anti-symmetric excitation, without HF wires (0-40 MHz).

200.0 EC-135 STRIHBOOB PERP EBWS.1.S 100.0 0.0 -100.0 -200.0 4 _ -._ 0.0 10.0 20.0 30.0 40. FREQUENCY tMHI) Figure 74b. Current phase at STA:W600B, anti-symmetric excitation, without HF wires (0-40 MHz).

200 200 i -n 1/326 1/216 E0 1/114 100 k EC-135 80 k ~~~~~~~~~~~~~~~~STA: WG00Bwith HF wires 60 ~- 60 j- I/ Figure 75a. Current density at STA:W600B, anti-symmetric excitation, with HF wires (1-10 MHz). 40 20 0 L ~Jc2 - 1 2 4 6 8 10 FREQUENCY IN MHz

EC1SSW STAUWB8OO PERP EBW479.11 3.0 I I 0.0 0. 0 10. 0 20. 0 30. 0 4 FREQUENCYT(MH) Figure 76a. Current density at STA:W600B, anti-symmetric excitation, with HF wires (0-401~)

200.0 I EC-3SSW STR 6NOOB PERP EBN7.9.t1 100.0 La'Uin 0.0 C,, CA, -100.0 -200.0 0.0 10.0 20.0 30.0 40.0 FREQUENCT (MHiI Figure 76b. Current phase at STA:W600B, anti-symmetric excitation, with HF wires (0-40 MHz).

1200 1". A - 1/325;K-x 1/216 E 0 0 -00 1/129 1000. ~ EC-135 800 STA: F130, without HF wires 0 Figure 77a. Charge density at STA:Fl3O0, - 4 00 400 Fi~re 77a symmetric excitation, withIi out HF wires (1-10 MHz). 200'<0 1 2 4 6 8 10 FREQUENCY IN MHz

— e1/325 200 1/216 0 too~~~~~~ A // /3,r, / X~4 ~~~~~~~~s ~~EC-135 STA: F130,without HF wires bID 0~~~~~~~~ -100 0 ~~~~~~~ i 0 Figure 77b. Charge phase at STA:F130, symmetric excitation, without HF wires (1-9 MHz). -200 0 -300 1. 2 4 6 8 9 FREQUENCY IN MHz

200.0 EC-ISS STRaF1SOO PAR EBQOS2.oI 150.0 IIc 100.0 CO3 50.0 0.0 0.0 10.0 20.0 30.0 FREQUENCY (MHHl) Figure 78a. Charge density at STA:F130, symmetric excitation, without HF wires (0-40 MHz).

200. 0 I EC-ISS STRsF1S0(k PAR EBQ5.2S. 100.0 Su w Ia 0.0 -1 0 0. 0 -200. 0 0. 0 1 0. 0 20. 0 3 0.0 4. FREQUENCY TIll) Figure 7 8b. Charge phase at STA:F130, symmetric excitation, without HF wires (0-40 MHz)

1200 -- - 1/325 t 1/216 E l-0 — 1/114 1000 0 k EC-135 800 -I | oSTA: F130,with HF wires 600 O 600o Figure 79a. Charge density at STA:F130, symmetric excitation, with HF wires (1-10 MHz). 400 200 j AX / 1 2 4 6 8 10 FREQUENCY IN MHz

150. 0 EC-IS5W STAuFiBOC PAR EBQ7.1.B 100.0 I-. cc 50 * 0 0.0 0.0 10.0 20.0 30.0 40.0 FREQUENCT (MM7) Figure 80a.. Charge density at STA:F130, symmetric excitation, with HF wires (0-40 MHz).

200.0 EC-135W STRIF1S30 PRR EBQ7,. 9 100. 0 Lo -100.0 -200.0 I i - 0. 0 10. 0 20.0 30.0 FREQUENCY (MHZ) Figure 80b. Charge phase at STA:F130, symmetric excitation, with HF wires (0-40 MHz).

120 ~- ~-1 120 LL a - 1/325 -X - X 1/216 -o-O-o 1/129 100 /' /7/// IH0,i EC-135 STA: W940T, without HF wires 80 _3 60 4- O lII Figure 81a. Charge density at STA:W940T, anti-symmetric excitation, 40 without HF wires, (1-10 MHz). I \ ~~~~~~~~~FEUN20 FREQUENCY IN MHz

400 1/325 E 400~~~~~~~~~~~~~~~~~~~~~~~~ ~~(Z1/2160 O-o-o- 1/129 300 77 77 n ~~~~~L~~~~~~3 ~~EC-135 STA: W940Twithout HF wires 0 ~~~~~~~~~~~~~0 200 o 1 0~~~~~~~~~~~~ 1 0~~~~~~~~~~~~~~~~ ~~~0 0~~~~~ 100 Figure 8lb. Charge phase at STA: W940T,0 anti-symmetric excitation., without HF wires (1-10 MHz). -100 1 ~~2 4 6 8 1 FREQUENCY IN MHz

I EC-135 STRtW-TIP-0 PERP El 10. 0 5. 0 0. 0 0. 0 10. 0 20. 0 30.0 4. FIEGEUENCT (MHZ) Figure 82a, Charge density at STA:W940T, anti-symmetric excitation, without HF wires (0-40M, )

200.0 EC-135 STR.W-TIP-0 PERP EB1. 1. t 100.0 Uj 0.0 -100.0 -200. 0 0. 0 10.0 20.0 30.0 40.0 FREQUENCY X[M HZ) Figure 82b. Charge phase at STA:W940T, anti-symmetric excitation, without HF wires (0-40 MHz).

200 A-_-_ A 1/325 E x-x —x. 1/216 100 0-0-0 1/114 7/l/////// EC-135 80 r |I:STA: F940T1with HF wires 0 60L -P X Figure 83a. Charge density at STA:F940T, anti-symmetric excitation, with HF wires (1-10 MHz). 40 x 20 / /40 cl._zxd-~Yd ~I J. ----.. I o...1 2 4 6 8 10 FREQUENCY IN MHz

EC-ISSW STAtW-TIP-Q PERP EBOI3,1O,18 10.0 -J _I4 e1 5.0 0.0 0.0 10.0 20.0 30.0 40.0 FIIEOUEnCTI M1HZ) Figure 84a. Charge density at STA:F940T, anti-symmetric excitation, with HF wires (0-40Mz)

200.0 0, EC-135N STRtN-TIP-Q PERP EBQ13.,.18 100. 0 La Ui -200.0 0. 0 10.0 20.0 s0.0 40.O FREQUENCT IMH)t Figure 84b. Charge phase at STA:F940T, anti-symmetric excitation, with HF wires (0-40 MHz).

Eo...48-............ A. -x-X. 1/216 k 1.0......... O-o-o 1/129................-~~ ~ -EC-135 STA: F800G,Without HF wires. - - ~~~~~~~~~~caused by sensor interaction:...... +....................~ -—'......_..... -...:-.. I Figure 85a. Current density at STA:F800!. _f: -.: -1_,f:_-..-...symmetric excitation..............with...-. K ~~~~out HF wires (1-9 MHz). 0 i i_]-~.-._-:...._........: "'.'caused by sensor interaction -.. X~~~~~ 00~~~ ~ ~~: ~~...... X. 2 4 8 9:, —' —:,FREQUENCY IN MHz.. I. —-— ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I —.;c~~~~ —~~-o ~ ~ F P a~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ j~ ~ ~ ~ ~~"_ —''",.!............!'24.~ ~ ~~~~FE&EC MEB~

50 Eo — x —x 1/216 / o-o-o 1/129 0/ I |~~~i I| x / | t, EC-135 I50 i / STA: F800G,without HF wires n-5 0 I I I \' i -100' / W-150| Figure 85b. Current phase at STA:F800G, l -200 symmetric excitation, with- \ out HF wires (1-9 MHz). / \ / -250 1 2 4 6 8 9 FREQUENCY IN MHz

10.0 EC-ISS STAzQSOO PAR EBG8.1 7.5 -J 5.0 2. 5 0.0 1 0.0 20.0 30.0 4. FREQUENCYT(MHIIZ Figure 86a. Current density at STA:F800G., symmetric excitation., without HF wires (0-40Mz)

200.0 EC-135 STAtG00O PAR EBG8.1 100.0 La cn 0.0 a-100.0 -200.0 -...,, 0 0.0 10.0 20.0 30.0 40,O FREQUENCY (MtH[1 Figure 86b. Current phase at STA:F800G, symmetric excitation, without HF wires (0-40 MHz).

Eo — 48-.-.... x-x' 1/216 0;0-0~ 1/.1.29:,.....'-..... Tj~? — T — L'........... EC-135 - ~...'. i.','~~:-'STA FI200Gwithout HF wires.../..... —.......... -.............Figure 87a. Current density at.STA:F1200G,........ -'~,~~~~~~~~~~~symmetricexcitation, without HF wires (1-9 MHz).? ---. -. ~. -— 4 ~~-O —eO-0"-O O O~~~~~~ 00 ~. O'O...0 —"0~~O 0 L 2 4 6 8 FREQUENCY IN MHz

0 V x —X — 1/216 - -507 EC-135 STA: F1200G, without HF wires -100 -150, -200 I200 Figure 87bh Current phase at STA:F1200G, symmetric excitation, without HF wires (1-9 MHz). -250' 1 92 4 6 8 FREQUENCY IN MHz

15.0,. EC-1SS STARG1200 PAR EBG9,.S 10.0 5c5.0 0.0. 0.0 10.0 20.0 30.0 4. FREQUENCY TMHZ) Figure 88a. Current density at STA:F1200G, symmetric excitation without HF wires (0-40 MHz).

200.0 EC-135 STRtG1200 PRR EBG9.3 100.0 I0 0 U, -200.0 1 l 0.0 10.0 20.0 30.0 40.0 FREQUENCY TMHil) Figure 88b. Current phase at STA:F1200G, symmetric excitation, without HF wires (0-40 MHz).

A~~~~~~~~~~ XXrx 1/216 oo 1/129 ill~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0 (J*1 EC -135 - - wiSTA: F800G, without HF wirz) 01 2 46 1 bF U Cn, X-,... 0~ -~ ~~~~~~~~~Figure 89a. Current density at STA:F800G, i — ~; ~~anti-symmetric excitation, without HF wires (1-10 MHz)..2 4 - 8 r: F~~~FEQUENCY IN MHz

Eo 100 x-x-x 1/216 Q-. —-O 1/129 EC-135 STA: F800G, without HF wires -100 " t>^D: =:< -200 ~4 Figure 89b. Current phase at STA:F800G, anti-symmetric excitation, without HF wires (1-9 MHz). -300 L -400 FREQUENCY2 4 6 8MHz FREQUENCY IN MHz

I0.0 EC-135 STRtG800 PERP EBGB.14 7.5 - LI aJ 2.5 0.0 0.0 10.0 20.0 30.0 40.0 FREQUENCY (MHZ) Figure 90a. Current density at STA:F800G, anti-symmetric excitation, without HF wires (0-40 MHz).

200.0 EC-135 STR*G800 PERP EBG6.s 100.0 Cl n 0.0 -100.0 -200.0 0.0 10.0 20.0 30. tO0.0 FREQUENCY {MH7)} Figure 90b. Current phase at STA:F800G, anti-symmetric excitation, without HF wires (0-40 MHz).

48 Eo x-,-) 1/216 — o- 1/129 EC-135 STA: F1200G, without HF wires 32 ( 0o _ Figure 91a.' Current density at STA:F1200G, anti-symmetric excitation, mI1 without HF wires (1-10 MHz). 16 II / 46 / 2 M6 z F2 EQ4 6 8 10 FREQUENCY IN MHz

100 x- -K 1/216' E o-o-o 1/129 0 x X —-,)-, _ EC-135 |e lo/ STA: F1200G, without HF wires -100 / -100 /Figure 91b. Current phase at STA:Fl200G, X/ | xanti-symmetric excitation, as < 1- ~ Jwithout HF wires (1-10 MHz). -200 o 1 2 4 6 8 10 FREQUENCY IN MHz

SECTION IV GROUND PLANE MEASUREMENTS AT OBLIQUE INCIDENCE This section presents the measured surface currents on model EC-135 aircraft in the presence of a perfectly conducting ground plane illuminated at 18 degrees from horizontal. The polarization, i.e., incident electric vector, was horizontal and the models were mounted on the metal sheet so that the fuselage was always parallel to the incident electric field. There were no HF wires present on the models. Both amplitude and phase data are presented and cover the full-scale frequency range 1.385 to 35.1 MHz. 1. Facility A 12 x 12 foot ground plane inclined at 18 degrees to the horizontal was constructed in our tapered anechoic chamber at the Willow Run facility, and figure 92 shows the geometry as observed from the side. In the area where the models were measured, the plane is a 4x12 foot, 0.030 inch thick aluminum sheet, and this was extended another 8 feet toward the transmitting antenna using plywood covered with 0. 005 inch thick aluminum foil. To obtain a uniform ("infinite sheet") field over the measurement area of the ground plane, special attention was given to the front and the rear edges of the structure. To reduce the front edge diffraction, the edge was buried in the chamber absorber and the sheet was partly covered on top with more absorber, whereas for the rear edge a 14 inch diameter cylinder covered with absorber was attached to it. With this arrangement, the surface current was measured along the ground plane in the direction of propagation to determine the purity of the resultant field. To accommodate the MGL-S8A(R) sensor, holes were drilled 5 cm apart in the ground plane over a span of 80 cm. The surface current was then recorded at each of these positions as the frequency was swept over a 500 - 1000 MHz range. Using the center hole data as a reference, the data were reduced at 500 and 1000 MHz and the results are shown in figure 93. In each case, the variation is +0. 5 dB, a value considered to be acceptable for the purposes of this program. Due to time 162

Top Sensor A -CI r =7 Bottom Sensor ~~~~~~~~~~Not to Scale ~Ref. Sensor 51 —--- 35'.-] Absorber Figure 92. Implementation of the 18 degree ground plane.

2 f = 0.5 GHz -40 -20 0 29 40 -2L 2... Distance, cm H 2! o 1 "0 f 1.0 GHz -40 -20 0 20 40 -2._ Distance cm Figure 93. Measured surface current distribution on the ground plane. 164

limitations and the fact that the connector on the sensor broke and had to be repaired, similar measurements in the 2.0 to 4.0 GHz range were not made. However, past experience (supported by theoretical arguments) shows that as the frequency increases, absorber performance usually improves and, hence, field uniformity of ~ 0.5 dB or better is expected in the 2.0 to 4.0 GHz range as well. 2. EC-135 Models and Measurements The measurements were made in two frequency bands using four scale model 707 airplanes modified by adding refueling booms and with styrofoam blocks underneath the wings and fuselage to provide appropriate ground-fuselage clearance. In the 0.45 -1.10 GHz band 1/325, 1/216, and 1/129 scale models were used, while 1/325, 1/216, and 1/114 models were used in the 2.0 -4.0 GHz band to get a higher full-scale frequency coverage. Measurements were made at the following stations on each model: F400T, F1200T top of the fuselage F400B, F1200B bottom of the fuselage W600T(A), W600T(B) top of the right and left wings, respectively W600B(A), W600B(B) bottom of the right and left wings, respectively. The convention for the station designation is the same as before (figure 3), apart from the identification of the right wing (A) and left wing (B) of the model as seen by a passenger sitting in an aircraft facing forward. The model was always placed with the fuselage perpendicular to the direction of the incident wave with the right wing (A) toward the excitation source. For both the top and bottom the measurement procedures were similar. A model was placed on the ground plane with the station to be measured above the "011 position on the ground plane (see insert, figure 93). The sensor was positioned extending either down from the ceiling for the top station measurements or up through the small hole in the ground plane for the bottom station measurements. With the loop just touching the surface of the model, a measurement was made; then, after carefully removing the model without disturbing 165

the sensor, the measurement was repeated for calibration with respect to the incident field. In measuring the fuselage current for both the top and bottom, the loops were rotated so that the plane of the loop was perpendicular to the ground plane and through the center axis of the fuselage. For the wing current measurements, the loop was still perpendicular to the ground plane, but rotated + 56 degrees, the estimated angle between the fuselage and the center of the wings. Such a rotation does alter the loop response to the excitation field, and this is accounted for in the subsequent data reduction process (see equation 11). 3. Theoretical Considerations Consider the geometry shown in figure 94. A plane electromagnetic wave polarized such that the electric vector is parallel to the z-axis is incident at an angle 0 upon the plane y = 0. For convenience, the phase of the incident wave is chosen to be zero at the origin and we can therefore represent the incident electric vector in the form E= BE e-ik(x cos e + z sin ) (5) where the time factor e has been assumed and suppressed. If the plane y = 0 is occupied by a perfectly conducting sheet, the field for y > O can be written as a sum of direct and image waves, with the latter having a minus sign to satisfy the boundary condition E = 0 on the surface. The total field is then E A = -ik~x cos e - y sin 0 e~-ik(x cos 0 + y sin e) i.e. E E2isin(kyi -ikx cos (6) i.e. E-zE 2 isin( ky sin 0)e (6) The corresponding magnetic field can be obtained by applying Maxwell's equation Vx E = iU H to (6) or formulated directly from the diagram as was done for the electric field. Either way, H = -2 H e (Bf sin 0 cos(kysin 0) + iy cos esin(kysine)) (7) 166

Direct wave E -O / -E Image wave Figure 94. Ground plane geometry. 167

where H =ZE E with Z 0 0 0 0 o The case x = 0 is of specific interest since the aircraft model was always positioned so that the station where the measurements were made lay in the plane x = 0. In addition, the loop was oriented vertically and, hence, would not respond to the H component of the magnetic field. Thus, from equation (7) H=-x2H sin cos(khsin 0), (8) where h is the distance from the ground plane to the point of the measurement. This equation now relates the incident field Ho to the calibration measurement H (cal) at the same point in space where the model station is located for the model measurement H(mod). In reducing data, H(mod)/H(cal) is first computed at all sampled frequencies and then renormalized to give H (mod)/H =H (mod)/H (cal 2 sin 0 cos (kh sin 0) (9) The negative sign that appears in equation (8) can be said to apply to both the H (mod) and H (cal) measurements and, hence, cancels out in (9). To measure the axial currents on the wings, the sensor loop is rotated = + 56 degrees, the estimated angle between the fuselage and the center of the wing. H(cal) was also measured with the loop in the rotated position, and to include this effect, equation (9) is multiplied by cos 0 to become H (mod)/Ho =H (mod)/H (cal]2 sin e cos 0 cos (kh sin ). (10) In the computer code, the equation was expressed in terms of more practical variables. With 0 = 18 degrees (the inclination of the ground plane), the frequency in GHz and h in cm, H (mod)/H =[H(mod)/H(cal) 0. 61803 cos 0 cos (0. 06472 fh), (11) where the argument of the cosine function is in radians. Equation (11) was used in reducing all of the ground-plane data presented: for the fuselage stations 0 O 168

and for the wing measurements 0 = + 56 degrees was used, respectively. Of course, the sign in the latter has no effect since cos(-x) = cos x. 4. Data Recording and Reduction The analog signals representing the amplitude (dB) and phase were first recorded on a dual channel X-Y plotter. A calibration measurement was made for each data measurement, so that for eight stations, two frequency bands and three different scale models the recordings totalled 96. For reduction of the data the same procedure was used as described in Section III, in which the analog data plots were first digitized and the data subsequently computer-processed and plotted. The reduction codes were based on formulations presented in Section IV, para. 3. 5. Presentation of Plots The following pages contain 32 plots of amplitude and phase arranged by measurement location, with amplitude data first and then phase data. The amplitude plots are for H/H versus frequency where H is the incident magnetic field without the ground plane. The phase is shown relative to that of the incident field at the location of the station where the measurement is made. To show the results more clearly, the low frequency (1-9 MHz) and the high frequency (5-35 MHz) data are presented as separate plots. Table 5 summarizes the data and can be used to locate quickly any particular measurement. A pair of figures containing corresponding amplitude and phase data are distinguished by (a) and (b), respectively. Note that on each plot in the upper right hand corner there is a somewhat abbreviated title. The code used is as follows: S T A: W 6 0 0 B(A ) 1 2 9 R A B 1, 9File Numbers W-wing B-bottom Scale Rockwell Freq. A(0. 45-1. 1 GHz) B-bottom F-fuselage T-top Freq. B(2 - 4 GHz) T-top 169

Table 5 DATA SELECTION CHART Low Frequency High Frequency Station Number (1 - 9 MHz) (5 - 35 MHz) F400T 95a, b 96a, b F1200T 97a, b 98a, b F400B 99a, b 100a, b F1200B 10la, b 102a, b W600T(A) 103a, b 104a, b W600T(A) 105a, b 106a, b W600B(A) 107a, b 108a, b W600B(B) 109a, b 110a, b 170

t-L L RM1PL! Tu1E tLI1 N..... a. o t O o O 0 o. 0 0, -CD b.U1, ___ I i::I) (0,............_C_ —_......:..............3-......

150.0, -..-.. STRtF400T 325 216 129 A 100.0/ ELJ 50. 0 0.0 A -50. 0 1,,.._..._,. 1.0 3.0 5.0 7.0 FREQUENCY MHZ)! Figure 95b. Current phase-at STA:F400T (1 - 9 MHz).

1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ STRA.tF40OT 325 216 11t8 B 3.0I Z7Z 21,0 2. 0 02 5.0~ 15.C 25.0 35. 0 FReqUENCY tM Ht) Figure 96a. Current amplitude at STA:F400T (5-35MHz).

-200.0..... STRtF4OOT 325 218 114 B -250.0 uJL Cr -300.0 -550.0 -400. _ 5.0 15.0 25.0 35.0 FREQUENCY tMH1v Figure 96b. Current phase at STA:F400T (5 -35 MHz).

9STA F1I200T;25T 218 28 /.4 10.0 II cn l.a ~ 3.30 S.O 7.0 8 0 FREQUENCY (ffHH ) 97a. Current amplitude at STA:F1200T (1 -9 lMHz).

200.0 STRiPI200T 925 216 129 100.0 La IL3 c:3 tn 0.0 cr. -2100.0 1.0 3.0 5.0 7.0 9.0 FREIUENCY (MHZ) Figure 97b. Current pihase at STA:F1200T (1 -9MHz).

LLL AMPLITUDE (LINI 01 o t3 C'"1 CD ND1 c C Im f01 e 01 j X t Id H ~~~~~r.~et~~~~~~~~~~~~~~~~~Crl 0 u0t

t100.0., STR.FI200T 325 2t16 lItA 0.0. -300.0 0o -200.0. 5.0 15.0 25.0 95.0 FREQUENCY fMHt1) Figure 98b. Current phase at STA:F1200T (5 - 35 MHz).

tO 15.....,,..............., - S9TRAtF40B 325 216 129 10.0 o.5. L,-' * 1.0 3.0 5.0 7.0 8.0 F'RBUEFCY (MIe.) Figure 99a. Current amplitude at STA:F400B (1 - 9MHz).

-ST' FRF00B 925 216 129 R 0.0 -50. 0 Lo C' 0 I. -200.0 -200.0 3_,0 S. 7.0 8. 1.0 3.0 5.0 7.0 8.0 FREQUENCY (MHE) Figure 99b. Current phase at STA:F400B (1 -9MHz).

TSPtF400B 325 218 114 B 10.0 5.0 t-~~~~~~~~~~~~~~~~~~i 0.0 5.0 15.0 25*0 35.0 FREQUENCY WMH1Y Figure 100a. Current amplitude at STA:F400B (5 -35 MHz).

400.0 a __,_ _...... S79RF4009 325 216 114 B 200.0..l 0 0.0 -200.0 5.O0 15.0 25.0 S5.0 FREGUENCY [MHZ) Figure 100b. Current phase at STA:F400B (5,35'MHz).

20.0, STRFl2008 325 218 129 15.0 f -5,I 0. 0I~~~I 0. O L;.0 3.0 5.O 7.0 FREQ UENCY (MH l Figure 10la. Current amplitude at STA:F1200B (1-9MHz).

250.0 ST RF1200B 5 2 6 1 29 150.0 50. 0 -50. 0 1.0 3.0 5.0 7.0 9.0 FfREQUENC~Y (MHa Figure lOib Current phase at STA:F1200B (1 -9MHz).

2 0,. 0............... _ _ _.................... - 20.0 - co 0,Of vsIs —5.0 0.0 5.0 15. 25.0 35.0 FREIU EURCY tMHHZ Figure 102a. Current amplitude at STA:F1200B (5-35 MHz).

200.0 -........ —STRs Ft200B 325 216 11t B -1000 -200.0 L f -300.0 5.0 15.0 25.0 5.0 FREQUENCY (MH1) Figure 102b. Current phase at STA:F1200B (5 -35 MHz).

5TRsW800STC 325 216 129 2,1- S, O I 7. 0 1.0 3.0~, 7.o9, F R E UENCY (VIHZI Figure 103a. Current amplitude at STA.-W60OT(A) (I -9 MHz).

250xW6 0r RI 325 218 129 R 150.0 50. 01 tUj 00~~~ 1.0 3.0 5.0 7.090 FRFlQUF-NCYCtltlZ) Figure 103b. Current phase at STA-WGOOT(A-) (1-.9 MHz).

I 0. 0 STAiNBOOT RA) 325 218 114 B 7.5. 2; -:5.0 03 2; 5- 1 H.o F5.o 25.0 35C0 FRE. Lt4CMY YMHEY Figure 104a. Current amplitude at STA:W600T(A) (5 -35 MHz).

STRsWGOOT (A) 925 218 11I 81 0.0. I -100.0 -200.0 l l -900.0L -400.0 L _ I.. __J........ 5.0 15.0 25.0 5.0 FREgQUENCT ~MHI) Figure 104b. Current phase at STA:W600T(A) (5 -35MHz).

STR.sOOT B)1 925 218 129 R I. 0 i 19I0 05.0 1.0 3.0 5.0 7.0 9.0 Figure 105a. Current amplitude at STA:W600T(B) (1 -9 MHz).

250. 0 7TRAsWGOT (B) 325 218 129 A 150.0 50.0:t Cr: 50.0 -50. 0 -250.0.. _ _...._.... _ 1.0 3.0 5.0 7.0 9.0 FREQUENCY (MHE) Figure 105b. Current phase at STA:W600T(B) (1 -9MHz).

9sTA'WNOOT fBI 325 21 11t4 I 0.0..4 2Figure 106a. Current amplitude at STA W600T(B) (5 - 35 MHz). S.O 1S.D 25.0 5. 0 FREQUENCY (MH[) Figure 106a. Current amplitude at STA:W600T(B) (5-35MHz).

200.0. STRtW6S00T tB 325 2t6 114 B 100.0 -100.0 -200.0 0.... -- -.....- _.... — 5.0 15. 0 25.0 5.0 FREQUENCY tMHZ) Figure 106b. Current phase at STA:W600T(B) (5-35MHz).

7. 5 I —-TRt'C R 325 I 29 Ingw~~~~~~~~~~~~~~~~~~~ I Ii * O0.0 I._._....,..... __._. 1.0 3.0 5.0 7.0 9.0 FREQUENCY tIH.) Figure 107a. Current amplitude at STA:W600B(A) (1 - 9 MHz).

250.0 IF. STRt W800B (RA 325 2t6 129 R 150.0 50.0 Uj~~~~~~~jI -50so. 0 I-.1 50.0 -250.0 1.0 3.0 5.0 7.0 8o0 FREQUeNCY tMHt1 Figure 107b. Current phase at STA:W600B(A) (1 -9 MHz).

9TRs W600B (Rf 325 218 l~i t I f.5. ID~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -J.40 ct~~~~i I3 iM CI ~ I 2.5 5.0 15.0 25.0 35.0 FREQUENCY (MHi7) Figure 108a. Current amplitude at STA:W600B(A) (5 -35 MHz).

100.05 STA:W600B (A) 325 216 In I 0 0 E Ict 00 -200.00 5.0 15~~~~I. 0 25.0 5. FREQUENCY CMHII! Figure 108b. Current phase at STA:-W600B(A) (5 - 35 MHz).

STRFtW6OUB CI 325 216 129 R po~oll 20.0'I=O ~ 10.0 0.0 1.0 3.0 5.0 7.0 9.0 FREUUEJNCY (MFH) Figure 109a. Current amplitude at STA:W600B(B) (1 - 9 MHz).

200. 0 8TIRm600 CB) 325 210 120 100.s0 r-0 T CD 100. 0 -200. L. 1 __ 1.0 3.0 5.0 7.0 8.0 FREQUENCY (MHZ) Figure 109b. Current phase at STA:W600B(B) (1 -9 MHz).

30' 0.,.. TRtsW600BC B) 325 216 It) B 20.0 - i A*~~~~~~~~~~AI 10.0 1 0.0 5.0 15.0 25.0 35.0 FRE;UENCY (MH7H Figure llOa. Current amplitude at STA:W600B(B) (5- 35MHz).

500. *O0 _ ___- aY UI - 7I - 1 S9TR W9OOB (B) 32 2, 8 l t B AI 300.0 I LU1310 0. 0 OO. 0 -.. ------ 5.0 15.0 25.0 S5.0 ~FREQUENCY (MHz) Figure 1101. Current phase at STA:W600B(B) (5 -35 MHz).

SECTION V CONCLUSIONS The results presented here are the fruits of an 18-month investigation during which the measurement facility has undergone a drastic change. At the beginning the facility consisted of a relatively small anechoic chamber in which, for example, the frequency was swept using a motor-driven signal generator, whereas now it has a larger and improved chamber with the latest microwave equipment supplemented by an automatic digital data acquisition and processing system. Of course, such changes do create their own problems and none of the data in this report were actually obtained with the full system in operation. As each new piece of equipment became available, the measurement capability increased, with a resulting improvement in the coverage and quality of the data. This evolution is apparent in the data of Sections II, III, and IV. When the EC-135 free-space measurements were carried out, a 450 - 1000 MHz capability was not available, and to cover the critical resonance region of the aircraft using the available 1 - 2 GHz band, it was necessary to use models as small as 1/447 scale. Such small models are difficult to handle and the measurements are more prone to error. However, by the time the ground plane measurements were started, the frequency coverage had been extended to 450 MHz and it was possible to use models which were no smaller than 1/325. The process of data reduction and presentation has also followed a similar evolutionary path. At the start of the investigation, the process was entirely manual, requiring the tedious and time-consuming comparison of measured curves at sampled frequencies, and the subsequent plotting of points adequate to form a graph. The first half of the data in this report was processed in this manner, but during the ground plane measurements a procedure for digitizing the measured curves was developed. All subsequent data were then computer-processed and plotted, leading to a marked improvement in efficiency and accuracy. Although the time taken to digitize the data is by no means insignificant, it would have been almost impossible to present the needed data in the time allotted without this advance. 203

UNIVERSITY OF MICHIGAN 3 9015 03483 2017