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T=H-E 

OHIO m 

UNIVERSITY 

Radar Cross Section Studies/Compact 

Range Research 

W.D. Burnside 

A.K. Dominek 

I.J. Gupta 

E.H. Newman 

P.H. Pathak 

L. Peters, Jr. 

The Ohio State University 

ElectroScience La bora tory 

Department of Electrical Engineering 

Columbus, Ohio 43212 

Final Report 719493-7 

Grant No. NSO 1613 

March 1989 

National Aeronautics and Space Administration 

Langley Research Center 

Hampton, VA 22217 

flASA-CB-185848) BikDAR CROSS SEC!X'IOI 

STUDIES/COHPACT RANGE RESEARCH [Ohio State 

Univ.) 84 p CSCL 171 

389-27066 

Unclas 

G3/32 0224754 
NOT ICES 

When Government drawings, specifications, or other data are 

used for any purpose other than in connection with a definitely 

related Government procurement operation, the United States 

Government thereby incurs no responsibility nor any ob1 igation 

whatsoever, and the fact that the Government may have formulated, 

furnished, or in any way supplied the said drawings, specifications, 

or other data, is not to be regarded by implication or otherwise as 

in any manner licensing the holder or any other person or corporation, 

or conveying any rights or permission to manufacture, use, or sell 

any patented invention that may in any way be related thereto. 

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REPORT DOCUMENTATION 1. REPORT NO. 2. 

PAGE 

4. Title and Subtitle 

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3. Recipient's Accession No. 

8. Report Date 

March 1989 

Radar Cross Section Studies/Compact Range Research 

7. Author(s) W.D. Burnside, A.K. Dominek, I.J. Gupta, 

a. 

8. Performing Org. Rept. No. 

E.H. Newman, P.H. Pathak and L. Peters, Jr. 

8. Performing Organization Name and Address 

The Ohio State University 

ElectroScience Laboratory 

1320 Kinnear Road 

719493- 7 

10. Project/Task/Work Unit No. . 11. Contract(C) or Grant(G) No. 

Columbus, OH 43212 

12. Sponsoring Organiration Name and Address 

(G) NSG-1613 

13. Report Type/Period Covered 

National Aeronautics and Space Administration 

Langley Research Center 

18. Availability Statement 18. Security Class (This Report) 

Unclassified 

10. Security Class (This Page) 

tT--

Final Report 

21. No. of Pages 

74 

21. Price 

__ Hainpton, VA 23665 - ____~ I ~ - ._ 

16. Supplementary Notes 

- . - __ __.. . __ _____~________. ~- ._ - .-- 

16. Abstract (Limit: 200 words) 

This report represents a summary of the achievements on Grant NSG-1613 between The Ohio State 

University and the National Aeronautics and Space Administration from May 1, 1988 to January 31, 1989. The major topics associated with this study are as follows: 

1. Electromagnetic scattering analysis 

2. Indoor scattering measurement systems 

3. RCS control 

4. Waveform processing techniques 

5. Material scattering and design studies 

6. Design and evaluation of standard targets, and 

7. Antenna Studies. 

Major progress has been made in each of these areas as verified by the numerous publications. 

17. Document Analysis a. Descriptors 

b. IdentiRers/Open-Ended Terms 

Unclassified 

(See ANSI-Z38.18) See Inrtructiont on Reverrc 

i 

OPTIONAL FORM 272 (4-77) 

Department of Conimerce 
Contents 

SECTION 

A. Introduction 

B. Past Achievements 

PAGE 

1 

2 

1. 

2. 

3. 

4. 

Electromagnetic Scattering Analysis ............. 2 

a. UTD Extensions for Scattering ............ 2 

b. Flat Plate Scattering .................. 3 

C. Physical Optics Correction .............. 4 

d. Guided Wave Scattering Mechanisms ......... 4 

e. Bistatic scattering from a Cone Frustum ....... 5 

f. External Scattering from Jet Intakes ......... 5 

g. Reflection from Edge Like Structures ......... 6 

RCS Computer Codes ..................... 7 

a. Aerodynamic/EM Interface .............. 7 

b. Flat Plate Scattering Code .............. 8 

C. Scattering Analysis and Codes ............ 9 

d. Low Frequency Scattering Code. ........... 9 

e. Finite Difference Calculations ............. 10 

Indoor Scattering Measurement, Systems ........... 10 

a. Measurement Hardware ................ 11 

b. Compact Range Reflector Developiiient, ....... 14 

Simulation of Compact Range Reflector Syst,ems ...... 15 

e.. 

111 

PRECEDlNG PAGE BLANK NOT FILMED 
5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

a. Target Mounts ..................... 18 

b. Feed Designs ...................... 19 

C. Absorber Scattering Study .............. 20 

d. OSU/ESL Compact Range Improvements ...... 21 

e. Cross Range Processing for Antenna Patterns .... 22 

Control of RCS ......................... 23 

Waveform Processing Techniques ............... 24 

Material Study ......................... 27 

Design and Evaluation of Standard Targets ......... 28 

Inlet Internal Scattering Analysis ............... 30 

Antenna Studies ........................ 32 

a. Array Scan Impedance ................. 32 

b. Microstrip Scattering .................. 33 

Slotline Antenna Analysis ................... 35 

Antenna Cavity Scattering ................... 36 

Propfan Blade Scattering ................... 38 

Miscellaneous Measurements ................. 39 

a. Rough Surface ..................... 39 

b. Resistive Card8 ..................... 39 

C. Contour Rim Inlets. .................. 39 

d. Images. ......................... 40 

Surface Conductivity ...................... 40 

a. Starbody ........................ 41 

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C . Present Program Achievements 

1 . Indoor Scattering Measurement Systems ........... 

a . Compact Range Reflector Development ....... 

b . Measurement Hardware ................ 

C . Target Mounts ..................... 

d . 

e . Mini Range ....................... 

f . Recent Indoor Scattering Measurement Publications 

2 . Material Study ......................... 

Hardware Refinements for Radar Image Enhancement 

a . Theoretical and Experimental Studies ........ 

b . Material Studies Publications ............. 

3 . Waveform Processing Techniques ............... 

a . Three-Dimensional Images ............... 

b . Publications ....................... 

4 . Inlet Scattering ......................... 

5 . Antenna Cavity Scattering ................... 

6 . Propeller Scattering ...................... 

7 . Supercomputer Computations ................. 

a . Electromagnetic Studies ................ 

b . Publications ....................... 

8 . Antenna Studies ........................ 

a . Microstrip Antenna Analysis ............. 

b . Slotline Antenna Analysis and Development ...... 

C . Publications ....................... 

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A. Introduction 

The ElectroScience Laboratory has been conducting a general study of 

evaluating scattered fields. The ultimate goal of this research has been 

to generate experimental techniques and computer codes of rather general 

capability that would enable the Aerospace Industry to evaluate the scattering properties of aerodynamic shapes. Another goal involves developing 

a sufficient understanding of scattering mechanisms so that modification of 

a vehicular structure could be introduced within constraints set by aerodynamicists. Last but not least, a major goal has been the development of 

indoor scattering measurement systems with special attention given to the 

compact range. There has been very substantial progress under Grant NSG 

1613 in advancing the state-of-the-art of scattering measurements, control 

and analysis of the electromagnetic scattering from general targets. 

1 
B. Past Achievements 

. I. Electromagnetic Scattering Analysis 

After a careful review of the analytic techniques available for handling 

the scattering from complex targets such as missiles and aircraft, it was 

very apparent that additional tools were needed. In order to confirm this 

evaluation, various targets were analyzed and measured to illustrate the 

magnitude of the problem. It appeared that presently available codes and 

solutions, although very inefficient in most cases, could be used for most 

basic shapes. However, as one attempted to shape the target, new mechanisms were needed to be added to complete the solution but in many cases 

were not available. Thus, a major effort of this research was to solve these 

problems. 

a. UTD Extensions for Scattering 

The general concepts of the basic GTD and scattering analysis have been 

extended through the advent of the UTD and its extensions which are 

useful for solving a much broader class of problems. First, the equivalent 

current concept has been extended by introducing a creeping wave path 

from the shadow boundary to the diffracting edge. This has been used to 

obtain much better agreement with experimental data for low level scattering analysis of cones at non-normal incidence. This is quite important for 

low scattering missile shapes. 

Next, the concept of corner diffraction was developed. This has been 

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used to analytically obtain the scattering of certain shapes for which rather 

high experiment a1 scattering values have been reported elsewhere. This is 

important in terms of scattering analysis of fins and wings and their control. 

This will be discussed in the control section later. 

We have made a series of measurements for several generations of missile 

shapes provided by NASA and have obtained excellent agreement between 

theoretical and experimental results. Experimental patterns have been repeated after various features (such as fins and the RAM-JET engines) were 

added. 

b. Flat Plate Scattering 

One of the dominant scatterers on missiles and aircraft takes the form of 

fins, wings, stabilizers, etc., that can be represented as flat plates. It has 

been shown by experiments that the scattering can be controlled by shaping 

such structures. Since there are examples in the literature of unexplained 

high scattering from such structures, it becomes important to study such 

structures quite carefully. A dissertation entitled “UTD Analysis of Electromagnetic Scattering by Flat Plate Structures” by F. Sikta [l] has been 

completed on this subject. It makes use of the corner diffraction coefficient, the equivalent currents and a novel edge wave analysis based on 

corner diffraction to treat a variety of plat,e structures. This includes the 

development of a solution which predicts the noli-principal plane scat terecl 

field. This has not been considered by ally realistic approach in the past. 

This analysis not only predicts scattering from such plates, but it shoulcl he 

3 
useful to the designer in two ways: 1) in fixing the shape of the fin, and 2) 

in establishing the parameters and position of absorber required to reduce 

the scattered fields. 

c. Physical Optics Correction 

The physical optics (PO) solution has been known for many years to be an 

approximate solution. In fact, the physical theory of diffraction (PTD) is 

an addition to PO which makes it more correct in terms of scattering from 

edge structures. The PTD has become accepted as a major analysis tool for 

scattering calculations in that it can be easily implemented. Nevertheless, 

both PO and PTD are still approximate results and as such have potential errors. One error is associated with the current abruptly stopping at 

the shadow boundary on a curved surface. This error has been examined 

and found to be quite serious for low observable targets. Thus, a general 

approach to solve such problems has been developed and published [2,3]. 

d. Guided Wave Scattering Mechanisms 

The’scattering from electrically large structures can originate from several 

mechanisms. Scattering from guided waves is one mechanism class that 

has not received significant attention. A common structure to support 

a guided wave is a crack or groove along a surface. The analysis of the 

propagation constant for such a wave was rigorously clone by Sliaiiiaiisky 

[4]. From a scattering viewpoint, it is not merely the exidence of tlie guidecl 

wave but the termination of the crack from which the guided wave scatters. 

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Measurements were performed to verify the analysis in [4]. Work is now 

under progress to analytically study the full three-dimensional scattering 

from cracks. 

e. Bistatic scattering from a Cone Frustum 

Bistatic scattering has become of interest recently in that the scattering 

level is difficult to control for all bistatic angles. With this in mind, a 

research effort has been completed to study the bistatic scattering from 

conical shapes, the cone frustum being the target of interest as described 

by Ebihara and Marhefka [5]. 

f. External Scattering from Jet Intakes 

The external scattering from the jet intake region for many missile geoinetries involves two major mechanisms; first, the direct scatter from the jet 

intake rim, and second the multiple scatter from the surface (using geometrical optics) to the rim. The ogive shape has again been used as a base and 

a circular jet intake rim has been mounted on the surface. Results show 

excellent agreement between measured and computed RCS for this case. 

This work is discussed in detail in a Ph.D. dissertation by Volakis [6]. It 

is observed that many of these tools are also useful for the analysis of tlie 

scattering from externally mounted stores. 

5 
g. Reflection from Edge Like Structures 

A study of the reflection from 2-D geometries that resemble wing profiles 

was undertaken since agreement between theory and experiment revealed 

that the theory generally predicted a value in the side lobe region that was 

low compared to measurement. This study was designed to evaluate the 

scattering as a function of frequency from the low frequency region where 

the leading edge could be represented as a half plane to higher frequencies 

where the reflection from the leading edge could be predicted via geometrical optics. This transition region study failed to give the expected increase 

in the scattered field, because the missing term was associated with the 

edge waves discussed in a previous section. A dissertation by Dominek 

[7,8] has been completed giving this and other results to be discussed in 

the next few paragraphs. 

This study used as canonical targets: 1) circular cylinders, 2) parabolic 

cylinders, and 3) elliptical cylinders. The data for the first and last target 

(1 and 3) was corrupted by the presence of a creeping wave which has 

to be separated from the specular scatter via transient or time domain 

techniques. The second case (2) provided an exact solution for the specular 

scatter mechanism from the parabolic cylinder. 

A model for the specular scatter was developed by Dominek that included not only the radius of curvature at the specular point, but. also the 

distance to the shadow boundary. This model provides a reasonable nieans 

of treating the transition region mentioned above. 

This thorough study has shown that, the scattering from t,he leading 

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edge of the wing profile was definitely & the source of the discrepancy 

between theory and experiment for complex targets. This has since been 

clearly demonstrated to be due to edge waves. 

2. RCS Computer Codes 

We proposed at the onset of this effort to develop a general purpose RCS 

computer code; however, the low observable security guidelines did not 

permit us to do so. Thus, we had to abandon this area until recently. 

The security guidelines have been modified during 1985 which allows us to 

develop such codes but not operate them for classified targets. 

A government consortium (Army, Air Force, Navy and NASA (Ames)) 

has since been formed to guide the development of such codes. Four ESL 

members serve on an advisory group for this consortium. 

a. Aerodynamic/EM Interface 

Careful examination of the various existing computer programs to represent body shape have been disappointing to a certain extent. The PANAIR 

program did not appear to be adequate, Rockwell's CDS system is company proprietary and at one time we were approached by a Northrop Engineer who had suggested their computer program for target shape may 

be available; however, this did not occur. On the ot.her hand, as st,at,ed 

earlier, NASA engineers had been examining their need and focusing their 

attention on the development of a suitable coiiiyuter progra,ni t'o describe 

a general target shape. There were many discussions hetwecPn NASA aiid 

~ 

7 
OSU personnel to accurately pinpoint the needed parameters. 

We have undertaken a joint effort with NASA (Langley, Virginia) to 

develop a general analysis for the scattering properties of complex aerodynamic shapes. Ray Barger [9] with the NASA aerodynamics group is 

basically putting the code together around their new aerodynamic geometry package. This geometry package is very significant in that it is designed 

to accommodate both the aerodynamic and electromagnetic features necessary for an accurate and efficient analysis. Our attention has been focussed 

on the study of the limitation it may possess particularly in the region of 

forward scatter. This is essential when multiple scattering/diffraction occurs. This study has made use of an ogive although it is not restricted to 

'this shape. 

b. Flat Plate Scattering Code 

In terms of shaping, it has been established that flat plate scattering is 

caused by various discontinuities such as an edge, corner, etc. If one curves 

the plate, some of the corners are now replaced by discontinuities in the 

radius of curvature. A study of these mechanisms is now complete. It 

makes use of the concepts of the UTD as applied to this new geometry. A 

dissertation on this topic by Chu [lo] considers additional thin plate discontinuities and extends the analysis to treat the diffraction from a smooth 

junction formed by two curved edges with different. radii of curvature. A 

general computer program has been developed and delivered to NASA for 

such plates. Chu also considers the smooth junction foriiiecl by surfaces of 

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different radii of curvature and has also developed diffraction coefficients 

for this type of discontinuity. 

c. Scattering Analysis and Codes 

The scattering analyses/codes have been extended in two general ways. 

First, the electromagnetic scattering techniques discussed by Volakis have 

been made much more general by using the geometry codes developed by 

Barger of NASA. In a Master’s thesis, Campbell [Ill has evaluated the 

scattered fields from obstacles placed on surfaces described by Barger’s 

geometry code. 

A new technique, designated as an envelope analysis, has been developed 

that should prove very useful to aerodynamicists in that it allows them to 

quickly estimate the scattered fields from a shape under consideration. A 

thesis by Pistorius [12] has been completed showing a number of examples, 

and this effort is continuing. 

d. Low Frequency Scattering Code 

A moment method code and user’s manual [13] has been developed by 

Newman [14] and delivered to NASA which treats general shapes that are 

not larger than ten square wavelengths. This code uses flat plate segments 

to simulate the structure. It is useful in treating objects whose overall 

length is not more than a few wavelengths. For a missile. it could he useful 

up to about 100 MHz; whereas for a fighter aircraft, it could be used to 

about 20 MHz. It has been verified by iiieasuremeiifs taken in our coiiipact 

9 
range. 

e. Finite Difference Calculations 

Finite difference calculations have been investigated for three dimensional 

surfaces using a “March in Time” approach developed by Bennett [15]. 

Results using this approach appears to be comparable to those generated 

by a conventional moment method approach for electrically small structures when tested using an ogive structure. The motivation for this approach is that it can readily calculate the scattered field frequency response 

for a structure and leads itself for vectorization on a supercomputer such 

as a Cray. Calculations using the Cray computer have been performed. 

Presently, the effort is towards plate structure to exanline the edge wave 

scattering from corners. It is hoped to modify the technique into a hybrid 

to include GTD concepts to facilitate the calculation. 

3. Indoor Scattering Measurement Systems 

It was very apparent at the onset of this grant that state-of-the-art measurements were needed in order to verify our theoretical solutions as well as 

identify new mechanisms. As a result, various indoor measurement facilities were evaluated to see their advantages and disadvantages. The compact 

range was most useful because it could handle large t,arget.s at high frequencies. Even though compact ranges were available in the early 19SO’s, they 

did not provide the performance needed for our semi tive nieasureiiients. 

Nevert heless, our analysis showed that. the various features limiting the 

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compact range could be improved. Thus, a major effort with NASA to 

develop such systems was initiated. This required us to develop new feeds, 

reflectors, measurement hardware and target mounts. This, in conjunction 

with the improved compact range reflector, will make the facilit,ies at NASA 

and OSU ainang the best scattering ranges in the world. In this regard, 

we visited the major facilities in Great Britain a few years ago in order 

to evaluate our systems versus theirs, and we can report that they had no 

comparable facilities. 

a. Measurement Hardware 

Originally, a standard CW nulling system was used for our measurements. 

This type of system is not very appropriate for compact range applications 

in that the horn-horn and horn-reflector-horn clutter terms are much larger 

than the target return. Two systems were proposed to solve this problem: 

1) a pulsed/CW radar, and 2) a linear FM system. It was decided that, OSU 

would evaluate the pulsed/CW system and NASA the linear FM one. As a 

result of our effort, a 6-18 GHz pulsed radar [lG] was designed, constructed 

and evaluated. It very successfully eliminated the horn-horn and hornreflector-horn clutter terms. In addition, it uses off-the-shelf components 

and is capable of measuring an 8 foot target. 

More recently, our effort has concentrated on the development of pulsed/CIW 

radar systems for compact range applications. Our 2-18 C4Hz systeiii has 

been designed, constructed, and test8ed and found to he very reliable, efficient, and accurate as shown in [17]. It. is different from our previous 

11 
system because the receive line switches have been moved from the RF side 

of the receive mixer to the IF side. This allows us to reduce the amount 

of RF hardware and still maintain excellent performance. Our sensitivity 

has improved by about 10 dB; in addition, our 5 ns wide receive pulse has 

allowed us to remove more room clutter which makes the system more stable. With our present room arrangement, our background after computer 

subtraction is about 70 dB below a square meter. 

A prototype Ka-bandpulsed/CW radar using Ka-band components has 

been designed which is useful for frequencies between 30 and 35 Ghz. After 

analyzing this system, it was found that it can be improved by using a 

minimum of Ka-band components [18]. Using this approach the transmit 

pulse is generated using our 2-18 transmit box which is then sent to a 

frequency doubler to generate the Ka-band signal. The receive signal goes 

directly to a mixer which converts the Ka-band return to an IF frequency. 

The receive switches are then added to the IF receive system. Using this 

approach one can measure an 8 foot target with better than a 60 dB below 

a square meter noise level. 

More recently we have been developing two pulsed/CW radar transceiver 

systems for NASA (Langley). These systems are based on a prototype design which was also designed, constructed and tested at OSU. The transceiver 

is operational from 2-18 GHz with additional capability from 18-36 GHz 

using a few higher frequency component<s. This is done using a doulde concept which sacrifices some performance because of the lower racliaked power 

(+5 dBm versus 20 dBm). 

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The radiated signal is pulsed with a width of about 5 ns to eliminate 

clutter mainly associated with the horn VSWR and bounce off the reflector. 

The pulses can be controlled to a resolution of 1 ns in terms of their delay 

and width. These signals are fully controlled by an IBMIAT computer 

which is interfaced to the system. 

The receiver subassembly consists of two independent channels which 

can be used to measure the co-polarized and cross-polarized signals scattered by the target. These two signals are coherently detected to a resolution of 0.001 dB and 0.01". In order to provide various performance 

parameters, the system can be requested to hardware average the detected 

signal from 1 to 256 samples. Thus, the operator through the IBM/AT 

controller can tradeoff system performance for speed. 

In that one would like to eliminate the need for taking numerous background measurements, the system must remain very stable with time. This 

stability is built into the radar by shifting the receive pulse gate between 

a fixed or known return and the target. The target return is then divided 

by the know one in order to remove the system variations. This reference 

mode can be selected or not by the operator for each new measurement. 

The software on the IBM/AT computer is menu driven under cursor control to allow the operator to view the receiver settings and simply change 

only selected parameters. It is divided into several pages wliich perform different, functions such as target log and scan parameters page. nieasureiiieiif 

page, processing page, timing page and maintenance page. Through this 

input one can easily specify a measurement sequence and obt aiii the desired 

13 
data. On the other hand, if the system is perceived to have a problem, it 

can be self evaluated to check out each subassembly. If a problem is indicated, the operator can further examine the system using the maintenance 

page. Once the faulty section is defined, it can be removed and tested on 

the bench. 

b. Compact Range Reflector Development 

Most of our effort has been devoted to the improvement of the compact 

range facilities at both NASA and OSU. The OSU designed edges have 

been installed on the NASA reflector, and field probing has indicated that 

these edges are performing as desired. A second set of curved surfaces 

constructed by NASA have also been placed on OSU’s reflector. The results 

of this effort have been reported in Reference [19]. Even greater benefits 

are to be obtained in the future in that a blended surface has been designed 

[20] that will make it practical to operate the compact range reflector at 

lower frequencies and for even larger targets. 

These modified compact range reflector systems have attracted favorable .nationwide attention from both government and industry. In particular, it has attracted the attention of several industrial organizations who 

are installing such a system as well as the manufacturer of the original reflector. For example, Scientific- Atlanta has redesigned t,heir refledor and 

contributed a larger version of said reflector to OSlT. 

In order to properly feed the new blended surface reflector, a subreflectm 

system has been designed for compact range applications [21,22]. In the 

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process of designing this subreflector, it was found that one can correct 

the polarization problem associated with most off-set reflectors. This is 

clone by tilting the subreflector axis relative to the main reflector axis. The 

tilt is rather small and in the off-set plane. This allows one to feed tlie 

iiiain reflector properly to generate a uniform field in the target zone with 

excellent polarization purity. 

The main reflector is still being designed in order to optimize its blended 

surface termination as well as the junction contour between the reflector 

and blended surface. It has been shown that this junction’s diffracted field 

is dependent on the junction contour. Thus, an analysis to generate an 

optimum shape for this contour has been developed by Pistorius [22]. 

4. Simulation of Compact Range Reflector Systems 

It has been our objective to examine the errors associated with the reflector 

system and correct them whenever possible. In terms of the reflector, the 

edge diffractions are the major source of ripple errors. So a major effort 

has been directed toward the reduction of these errors. 

In order to accomplish this task, we have already developed a numerical 

technique to compute the diffraction from blended surfaces. The technique 

is based on the corrected PO solution [2,3]. Using this novel approach the 

total scattered fields in the near zone of the reflector are computed. One can 

then subtract the specular reflection term from the to1 a1 watteiecl fields to 

obtain tlie diffracted fields. Using this numerical technique a computer code 

to analyze semi-circular compact range reflectors 11 as lwen developed in 

15 
References [23,24] and delivered to NASA (Langley). The code can be used 

to compute the total near field of the reflector or its individual components 

(specular reflection, aperture blockage scattered fields and feed spillover) 

at a given distance from the center of the paraboloid. The code computes 

the fields along a radial, horizontal, vertical or axial cut at that distance. 

Thus, it is very effective in computing the size of the “sweet spot” for such 

reflect ors . 

The computer code, as mentioned above, can treat only semi-circular reflectors. The technique, however, is applicable to arbitrary edge paraboloid 

reflectors. For arbitrary edge reflectors, the technique becomes numerically 

inefficient in that computation time to carry out the PO integration becomes a limiting factor. Other methods to compute diffracted fields from 

the blended surfaces therefore need to be developed. 

This code allows us to evaluate the presently-available Scientific- Atlanta 

reflectors and has shown that improvements are needed. Specifically, the 

edge contour of the parabola must be changed as shown by Pistorius [21], 

and the aperture blockage by the feed antenna reduced, the taper must be 

improved and the cross-polarization errors removed. As shown by analysis 

[22], these errors can all be reduced by using a dual chamber concept which 

utilizes a Gregorian subreflector located in a second small anechoic room. A 

small opening is then cut between the two chambers, so that the subreflector 

fields can fully illuminate the main refledor. This systeiii is presently being 

constructed at NASA (Langley) and will be tjestmed this coiiiing sumiiier. 

It was shown by Pistorius [22] t,hat. a concave edge contour reflector 

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provides better performance because it reduces the spread factor or ultimately the diffracted field. In order to simply test that concept, lie used the 

same blended rolled edge around the entire reflector contour. His results 

showed some improvement; however, the reflector rolled edge was not optimized. As a result, we have been developiiig an optimization concept for 

the blended rolled edge for each section around the concave edge contour. 

This is done by optimizing the blended rolled edge along selected radial 

cuts around the reflector. The blended rolled edge at each radial cut is 

then found by interpolating the blended rolled edge parameters from one 

optimized section to the next. The optimization is done by first finding 

the equivalent parabola for each selected radial cut through the center of 

the reflector. Using this equivalent parabola, we obtain an optiinized rolled 

edge based on the lower frequency and height requirements. This approach 

provides even better performance than that, obtained earlier. 

Last year, we developed an improved Physical Optics (PO) solution 

for the rolled edge reflectors which required us to subtract out a shadow 

boundary error term. This error mechanism consists of several terms but 

for most cases can be limited to the first and second order terms. The 

second order term requires us to know the radius of curvature of the curved 

surface at the shadow boundary. This radius of curvature is not that simple 

to obtain but must be evaluated in order to obtain accurate results. We have 

recently evaluated this radius of curvature [25] and fouiicl thal t lie second 

order term has a significant, effect on the results. In fact, the reiiioval of this 

error term from the PO solution allows us to generate even better reflector 

17 
designs such as the optimized shape described previously. 

a. Target Mounts 

A complete low cross-section target pedestal was designed and built at 

OSU and delivered to NASA. In addition, a complete system c,mtroller 

was developed, delivered, and evaluated at NASA by OSU personnel. This 

complete system is a duplicate of the one presently operating at OSU. 

In order to measure conical patterns, a new target mount extension has 

been designed which will be attached directly to the target. It will be able 

to hold several hundred pounds and handle a hundred foot-pounds of torque 

at various elevation angles (0", lo", 20", 30°, and 40"). Using this mount, 

NASA will be able to obtain conical patterns about the rotated axis. 

The studies of absorber scattering have been useful in target mount designs. A target mount extension (designed at ESL) for the ogival pedestal, 

which directly attaches to the target, required treatment to reduce the 

mount scattering. A shroud was built to fulfill the need. The shroud consisted of commercially available layered absorber panel that was properly 

cut tp form a wedge transition between the mount and free space. 

Another mounting scheme has also been finalized. This entails the positioning and mounting of targets that require rigid metal mounts for stability. The mount concept is universal in the sense that, the same hardware 

can be used for a variety of targets without. having to haw a miclue mount 

for each target. 

As shown by Lai [25], the metal ogive pedestal has too large a backscat18 

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ter at lower frequencies. In addition, its bistat,ic scattering is not insignificant especially for scattering centers located off of the pedest8al. To reduce 

the RCS of the pedestal, a treatment study was initiated; however to reduce 

the bistatic scattering, one must consider other mounting techniques such 

as straps hung from the chamber ceiling. 

We have made some great progress using straps to hold a target. In 

fact, it should be kept in mind as a potential future method for holding 

targets during a nieasurement sequence. We have found that the specially 

designed straps from 3M Corporation can hold very heavy targets; yet, 

their cross-section at a small tilt angle relative to the plane wave direction 

can be better than -60 dBsm. In addition, their bistatic scattering level is 

very low so that they don’t interact significantly with the target except for 

the mounting fixture. As a result, we will need to evaluate various strap 

mounting techniques in t8he future. 

b. Feed Designs 

The blended surface design for the compact range reflector allows one to 

measure much larger targets. however, one can’t take advantage of this 

target size unless he is able to feed the reflector properly. Heedy [27] has 

designed an aperture-mat ched horn which is useful for this application; 

however, it is larger than desired and not as frequency frequency agile as 

needed. As a result, other feed designs a,s well as siil)reflec.tor approaches 

are being examined. 

19 
c. Absorber Scattering Study 

A general study of absorber scattering has been initiated. This research 

effort involves both an analytic and measurement aspect in order to understand the scattering performance of commercially available pyramid and 

wedge absorbers. The analysis is based on a UTD corner diffraction solution which is modified to treat material structures. The major aspect, of 

this analytic study is associated with the tip scattering. As this analysis 

was compared with measurements, it was ascertained that the absorber 

scatters incoherently; in which case, all the absorber contributes to the 

clutter throughout the room. Since the wall does not scatter coherently, 

one can’t use a ray trace solution to design the anechoic chamber. Finally, 

the analysis and measurements by DeWitt [28] show that wedge material is 

clearly superior near grazing; whereas, the pyramidal material works better 

near broadside. 

Based on the analysis of DeWitt 1281, the absorber scattering properties 

are dictated by the homogeneity of the material. Attempts were made to 

get better material from absorber manufactures, but they don’t seem to 

have’ the quality control necessary to improve the situation. They claim 

that their thin flat material is as homogeneous as they can possibly achieve 

with their present manufacturing techniques. 

With this input, an absorber study was initiated to exaiiiine this flat 

material. They did appear to be more homogeneons. hut flat material 

has too large a reflection coefficient. Thus, various layered wedge absorber 

designs have been examined using this material. A 10 dR improvement has 

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been achieved using this approach, but more refinement is needed. 

We have also evaluated the absorber treatment needed for aperture 

opening between the two chambers in the Gregorian subreflector system. 

This has involved evaluating various absorber treatments in the aperture 

and recording the resulting antenna patterns. The actual iiieasurements 

have been obtained at NASA (Langley). The absorber performance has 

been critically examined using a new cross range algorithm which identifies 

the various scattering (or radiation) centers associated with a sector of 

the measured pattern. Using this approach, we have been able to pinpoint the significant scattering regions and modify those areas to improve 

performance. Without such a technique, it would be virtually impossible to 

determine which portion of the total absorber treatment caused a l/lOtli of 

a dB ripple. At the present time, these techniques have just about specified 

the absorber treatment for the aperture opening for x-band operation. 

d. OSU/ESL Compact Range Improvements 

The anechoic chamber for the compact range has been drastically improved. 

The improvements entailed the refinishing of the reflector surface to provide 

a smooth rolled edge transition, a complete new absorber treatment of the 

walls, floors and ceiling, a raised floor to accommodate the ogival pedestal 

base and the installation of an Overhead, remobe conbrolled bridge crane to 

facilitate target mounting. 

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e. Cross Range Processing for Antenna Patterns 

We have developed a new algorithm to identify the cross range location of 

the radiation centers based on a small sector of the radiation pattern. This 

is done by coherently processing the pattern data for say plus and minus five 

degrees around a pattern direction of interest as shown in Reference [30]. 

If it can be assumed that the magnitude of a radiation center is uniform 

across this small angle scan the phase indicates the various sources of the 

radiated signal. 

This processing has been used to evaluate the radiating centers for an 8 

foot diameter reflector antenna which was measured in our compact range. 

Simply using the recorded patterns, we could identify the feed spillover, 

feed blockage, edge diffraction and strut scattered field locations and relative radiation levels. In addition, we obtained other terms which were 

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unexpected such as scattering from a small taped hole in the center of the 

reflector. This error term was large enough to destroy the agreement between measured and calculated results because the calculated data didn’t 

include that term. In any event, it is very clear that this type of signal processing will be come more popular in the years to come. In that regard, the 

compact range is most appropriate for this technique in that it can provide 

the required measured phase accuracy as demonstrated by our results. 

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5. Control of RCS 

The introduction of the traditional missile fins caused the scattered fields 

to increase substantially. Based on GTD scattering mechanisms and aerodynamic concepts supplied by C. Jackson of NASA, the fin shape was redesigned to lower its contribution to nearly that of the basic missile fuselage. 

An oversized version was placed on the model, and the experiments 

verified our contention that a substantial reduction can be achieved by 

properly shaping t,he fins. This fin shape has been examined at NASA from 

an aerodynamic viewpoint and appears to have slightly better performance 

as compared with previous designs. 

Based on experimental results, the major source of the scattered fields 

of the basic missile is from the rear of the fuselage. As a result,, it, was 

proposed that the rear be tilted to reduce this contribution which could 

also be used to good advantage from an aerodynamic viewpoint as well. 

The jet intake is another major source of scattered fields. For the present 

model, it is shielded by the missile body from the radar over a wide range 

of aspects looking from below the missile. The success of this shielding has 

been demonstrated by experiment. It has been proposed that this region 

could be extended by tilting the normal to the inlet upward from the axis 

of symmetry. 

23 
6. Waveform Processing Techniques 

In the course of studying the reflection from the leading edge of a wing, 

the time domain techniques have been further developed by Dominek [29]. 

These techniques generally required accumulation of data over a wide range 

of frequencies. This data is then transformed to the time domain, producing a series of scattered waveforms from each scattering mechanism. Unfortunately, when the portion of the spectrum of interest is that where the 

dimensions of the body are small in terms of wavelength, these transient 

signals tend to overlap. Thus various types of windowing were introduced 

to obtain the desired separation. These techniques have proven useful in 

the evaluation and control of scattered fields. 

This study has also produced some additional results for creeping waves. 

The importance of creeping waves arises in low level radar cross section 

(RCS) bodies since they can be scattered back by structures shadowed 

by the main body. A smooth, elongated missile-like body was made with 

several removable inlets to investigate, by measurement, the influence of 

a shadowed inlet in regions of low RCS. The measurements indicated a 

definite low level creeping wave interaction with the inlets. However, the 

level was near our lower measuring capability limit. This limit is being 

substantially reduced at the present time for future measurements. At the 

same time, a computer code was developed to calculate these creeping wave 

interactions based on GTD [30] and incorporating nuiiierical techniques 

developed at NASA [9]. The accuracy of these calculat8ions is depeiident, 

upon how well the GTD creeping wave formulation models surface diffracI 

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tion mechanisms. It is well known that the formulation is very accurate 

for circular and spherical surfaces but very little study has been done on 

elongated surfaces. There has been an effort to coinpare the GTD creeping wave formulation to the true creeping wave values for spheroids and 

ellipsoids by extracting the creeping portion froni the total scatt<ered field 

of these two surfaces. There is as yet no valid asymptotic solution for low 

frequencies in terms of the creeping wave format for elongated bodies; i .e., 

pariaxial diffraction occurs. 

The time domain extraction technique is one of inany possible techniques to obtain the frequency characteristics (RCS) of a particular scattering mechanism by a “filtering” process from the total scattered field of 

a body. Other filtering techniques are also possible such as one developed 

by Ksienski [31]. Ksienski’s approach has been generalized and compared 

to filtering techniques used in time series analysis and geophysical seisniograph research. The literature has been reviewed in those other areas to 

find other applicable filter techniques that could prove useful in scattering 

problems. 

As a result of our studies, a new cross-range measurement algorithiii has 

been developed for specific application to the compact range. Using this 

approach, the target is examined at a few look angles in one plane by simply 

iiioving the feed antenna. Since each feed position can be calibrated, the 

down range image plots from each look angle can I,? ( ~~nihiuwl to rrratr it 

cross-range scatt*ering center location. More will he presented on this f opic 

in the future. 

25 
In our effort to develop new and more powerful tools to extract more 

information from measured data, we are studying various super resolution 

techniques used in spectral estimation. These techniques offer superior 

resolution with the added benefit that far fewer data points are needed in 

the measurement. The high resolution is achieved using the signal-to-noiseratio (SNR). The higher the SNR, the better the resolution. Thus, these 

techniques are quite suitable for present day compact ranges where the 

noise level is typically very low. Specifically, we are looking at the MUSIC 

algorithm [32]. The MUSIC algorithm is an eigenvector base destination 

technique. To use the algorithm, a covariance matrix is formed from the 

samples of the measured data. This measured data can be in the frequency 

domain or in the time domain. The eigenvectors and eigenvalues of this 

covariance matrix are then computed. The eigenvectors are separated into 

a set spanning the signal space and a set spanning the noise space. The 

down range (frequency domain data) or cross range (angle domain data) is 

then obtained by projecting a test vector onto the noise space. When the 

time delay in the test vector equals the time delay for a scattering center 

in the sample data, the test vector is orthogonal to the noise subspace. 

This algorithm has been applied to a number of real world situations with 

great success. For example, in the case of a 10" long ogive, we were able to 

resolve the creeping wave term and the rear t,ip cont,ribut.ion for a 40" look 

angle. The algorithm will be studied in more detail to find the nuinber of 

samples required for a given number of scattering centers. The effect. of 

bandwidth and SNR on the resolution capability will also be st,udied. The 

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ultimate goal of this study is to generate 3-dimensional images of a target, 

using measured data. 

7. Material Study 

The use of materials has proven to be quite rewarding. Both anisotropic 

composite materials and absorbers have been used to eliminate the edge 

wave that is generated at a discontinuity such as corner, propagates along 

an edge and diffracts from a second discontinuity. This has proven to 

be a very significant scattering mechanism for both aircraft and missiles, 

particularly the supersonic vehicles with their sharp edges. This mechanism 

inust be eliminated if the current analysis (and treatment) of leading and 

trailing edges is to be valid. A report has been written describing this result 

[SI. 

The rim for jet intakes has been shown to be a primary scattering source. 

Means of controlling this rim via the anisotropic material approach has been 

considered. a series of measurements have been made for open ended cylinders composed of such materials to obtain a basic data base. The results of 

these measurement have been interpreted. The use of these materials has 

proven to be a diagnostic tool in that diffraction/scattering from a given 

portion of a structure can be modified by its presence. 

A report [34] has been issued suinmarizing the use of coniposi te materials. A second report [35] associated with nearly flat striictiiirs or straight 

edges has also been written. Our work with W.T. Hockes in this area 

has proven to be very rewarding in that we liav? ~aiiiccl :I iiiiicli better 

27 
physical insight into the structural features of composites. Work has been 

focussed on structures that are self supporting and will achieve the desired 

elect romagnetics performance. 

Research on new mechanically strong surfaces with reduced edge waves 

has continued. This structure has been used in the design and development of more practical structures as shown in Reference [36]. Even more 

interesting have been results generated by structures that are more closely 

related to actual aircraft shapes. A preliminary report [37] has been written 

discussing these advances. 

The analytic study of these surfaces has provided an improved understanding of these materials. Thus, it is most important that these studies 

continue. These results suggest that optimum absorption of surface waves 

occurs when fiber radii are of the order of a skin depth. If further studies confirin this concept, and we succeed in adequately matching measured 

and computed results, this will result in substantially improved composites 

from an electromagnetic view point. 

8. , Design and Evaluation of Standard Targets 

A number of standard targets have been developed, constructed and tested 

by NASA and OSU. One, an “almond” shape [38], is a very low echo area 

object and has been used to study perturbations including small intakes. 

This same scatterer is of interest to DoD and industry- as a test shape. The 

second test body called the “peanut”, was used to study multiple scattering. 

A third structure, the generic inlet has been conipared very favorably witah 

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computations using a hybrid solution. 

Canonical shaped bodies like the ogive and sphere-capped cylinder have 

been pursued as targets to evaluate a measurement facility. An effort is 

presently underway to efficiently generate the calculated scattered field from 

these bodies when they are electrically large to obtain an absolute reference 

of what a range should measure. 

The “almond” has proven to be a very desirable support body in terms 

of its scattering characteristics. The almond allows a very large angular 

sector where the scattered field is very low. This feature allows the direct 

ineasurement of a subcomponent on the surface of the body without having 

the nieasurenient corrupted by scattering arising from the support body. 

The almond can also be rigidly supported on the low cross section ogival 

pedest a1 for measurements. 

The evaluation of the almond test body has progressed to the point of 

routing use for subcomponent measurements. The almond version most 

commonly used is a ‘flat) one which on one side has a planar surface to 

facilitate subcomponent mounting. A flush, removable ogival plate which 

covers a cavity region has been incorporated on this flat portion. 

The calculation of the scattering from an almond body is presently 

underway using a “March-in-Time” approach of Bennett’s [ 151. This calculation is being performed on a Cray computer located at Carnegie-Mellon 

University. Such calculations are useful for yerfcmiiancr checks 011 measured results from anechoic chambers. 

29 
9. Inlet Internal Scattering Analysis 

A hybrid technique was used to analyze the general rectangular inlet structure. The agreement of this solution’s results with experimental ones was 

excellent. In addition, it was studied further to determine how much of 

the complexity of the internal fields needs to be retained in order to obtain 

good results. It was surprising to learn that as few as three waveguide 

modes dictated by the angle of incidence were required to generate accurate results even when the rectangular cross section is relatively large 

in terms of the wavelength. Furthermore, this hybrid technique, which 

involves a combination of high frequency (HF), modal and multiple scattering methods, has been developed to deal with a variety of inlet shapes 

which can be approximated by joining together piecewise linearly tapered 

and/or circularly bent sections of waveguides for which the modes can be 

determined. This hybrid technique basically involves expressing the modes 

in terms of an equivalent set of modal rays and then finding the elements 

of the junction scattering matrices in a relatively simple manner, via HF 

techniques, to characterize the junction discontinuities. The interactions 

between junctions (discontinuities) is accounted for by the self consistent 

multiple scattering method to arrive to the total field scattered by the inlet 

model. This work is summarized in References [39,40]. 

A second much simpler ray optics model was also studied by Burkholcler 

[41]. It was found that good results are obtainable ereii from this simple 

geometry. This is especially true for internal structures that are very large 

electrically. 

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In both cases, the analytic results were compared with measured results 

obtained using the compact range system. The inlet modes were supplied 

by NASA. 

An efficient hybrid procedure was developed, as mentioned earlier, for 

analyzing the electromagnetic scattering by a class of inlet shapes that can 

be modeled by joining together piecewise, linearly tapered, and/or circularly bent sections of waveguides for which the modes can be found analytically. This hybrid procedure begins by breaking up the modal fields 

in terms of an equivalent set of modal rays, and then using these modal 

rays in conjunction with asyniptotic high frequency techniques (such as 

the geometrical and physical theories of diffractions) to find the elements 

of the generalized junction scattering matrices in a relatively simple form. 

These scattering matrices fully characterize the reflection and transmission 

properties of the junctions between different- waveguide sections. The int,eraction between different waveguide sect-ions (discontinuities) is readily 

accounted for by the self consistent multiple scattering method. When 

applying this approach to a general rectangular inlet cavity, it was also 

mentioned previously, that, a further simplification was achieved because 

only a set of three consecutive waveguide modes dictated by the angle of 

incidence were needed to get accurate estimates of the RCS for sufficiently 

large waveguides. During the present, period, that, selective modal scheme 

has been studied for other inlet waveguide shapes siirli a.s sf.i:c)iigly 1inea.rly 

tapered as well as circular inlets. While it is f(:nuicl t,lia.t, iiiwe select.ive 

modes are required for the latter configurat.ions t.hau for t,lifi. rectangular 

31 
case analyzed previously, it is true that the selective modes still form a 

far smaller set than the large number of all the propagating modes within 

the inlet waveguide. The work on the hybrid approach for analyzing the 

junction scattering matrices and the evaluation of the RCS of some inlets, 

as well as the selective modd scheme are described in a report and a few 

papers which are listed later in the section on recent publications. 

Before proceeding to analyze the RCS of more inlet shapes, it appears 

appropriate at this time to consider alternative efficient approaches for analyzing inlets with smooth transitions and tapers which could also account 

for inlet wall treatments. Such a study of this complex problem is underway. The hybrid modal ray approach discussed earlier will become more 

difficult and cumbersome for arbitrarily shaped inlets and for treated inlet 

walls, as the modes are very difficult to find for the latter case. Nevertheless, the Studies performed thus far on a class of perfectly-conducting inlet 

models (for which the interior modes can be found easily) provide a useful 

estimate of the upper bound for the inlet RCS values; the treated inlets 

would of course yield mostly lower RCS values. 

10. Antenna Studies 

a. Array Scan Impedance 

The study of scattering by antenna arrays is conbitruitig. The general property of the array that needs to be evaluated and condrollecl is t,he arra,y 

scan impedance. It is very important that. this scan impedance be made as 

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independent of scan angle as possible. To this end our original studies involved techniques for controlling the impedance of an array of “V” dipoles 

over a conducting plane. A reasonably constant scan impedance has been 

obtained for such an array. However, when the feed lines were inserted, 

these desirable results were seriously perturbed. 

b. Microstrip Scattering 

In this section the current and future work on the method of moments 

(MM) analysis of the radiation and scattering from microstrip antennas 

and arrays is described. A microstrip antenna is a metal patch printed on 

an electrically thin grounded dielectric substrate. The main purpose of our 

effort is to develop an accurate and computationally efficient model for the 

radar cross section (RCS) of a rectangular microstrip antenna over a very 

broad frequency range. The RCS of the microstrip antenna is an important 

problem since its scattering can be dominant when mounted on a low RCS 

structure. 

Our past work [42]-[46] (and that of others) on microstrip antennas has 

mostly dealt with the radiation problem. In this case, we were interested 

in the transmitting properties over a frequency range within a few percent 

of first resonance. We are now interested in the scattering properties over 

a frequency range of several octaves. A result of this is that the current, 

distribution on the microstrip patch is very conipljcaterl. as compared to the 

relatively simple current distribution on a transinit ting microstrip near the 

first resonance. Thus, we must use many terms, 1Y. in our NlLl expansion 
of the microstrip current. This presents a problem, since the CPU normally 

increases as N2 in a MM solution. 

We have done two main things to obtain an efficient MM solution with 

reasonable CPU times. Fist, we have formulated the solution so that the 

reasonable CPU time is proportional to N rather than N2. This minimizes 

the CPU time to compute the RCS at a single frequency; however, a second 

problem remains. The microstrip antenna is a highly resonant device. A 

plot of RCS versus frequency would consist of a series of large, but narrow 

peaks. In order not to miss one of these peaks, we must take small steps 

infrequency. Consider the computation of the RCS of a microstrip antenna 

from 1 to 10 GHz. If we take frequency steps of say 10 MHz, then this 

would result in 1000 evaluations of the MM computations, and the total 

CPU time would be prohibitive. To alleviate these problems we have developed a technique of interpolating the MM impedance matrix. In the 

above example, we might compute the MM impedance matrix every 500 

MHz, resulting in a factor of 50 decrease in CPU time. The impedance 

matrix at an intermediate frequency works because the elements in the 

impedance matrix are a much more slowly varying function of frequency 

than the scattered field. 

At present, we have developed a computer code capable of computing 

the RCS of a rectangular microstrip patch over a broad frequency range. 

We have also obtained measurements to verify the accuracj- of the code. 

All of which was reported in Reference [47]. 

The present code does not, consider the microstrip feed. For t,lie ant-enna 

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problem, one can use an extremely simple iiiodel for the feed, which assuiiies 

an extremely thin substrate. However, for the RCS problem we can not 

assume a thin substrate, since the frequency of the incident wave might be 

much greater than the frequency of the first resonance of the patch. Thus, 

our efforts have been directed toward developing a suitable model for a 

microstrip feed. This model will enforce continuity of current at the feed 

line to inicrostrip patch junction, and not be dependent on a thin substrate 

approximation. Basically, this will be done by taking our past work on wire 

to plate junctions [48]-[51] and modifying it for the inicrostrip antenna. 

11. Slotline Antenna Analysis 

A slotline or finline transmission is formed by a slit or aperture in a ground 

plane. If the slit is electrically thin, then a wave can propagate along the 

slit with essentially no radiation. A slotline antenna is formed by gradually 

increasing the width of the slit. When the width is on the order of a 

half wavelength or more, significant, radiation occurs from the open end of 

the line. Slotline antennas are typically printed on a thin (ungrounded) 

dielectric substrate. An advantage of slotline antennas is that they have 

the potential for wide bandwidth if properly fed. Slotline antennas are also 

extremely lightweight and may also have low scattering cross section. Thus, 

they may have application as a dish feed where it is desired to iniiiiinize 

feed blockage. 

To better understand the slotline antenna, we are developing techniques 

for its analysis. These techniques are siinilar to those used for the microstrip 

35 
antenna described above. Basically we are employing a method of moments 

solution for the currents on the metal plates printed on the dielectric substrate. The technique is sufficiently general to be able to treat various 

tapers (;.e., linear, exponential, etc.). At present we have analyzed the 

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scattering from a slotline antenna, and verified the results by comparison 

with a conventional surface patch method of moments solution for plates 

in free space. The next step involves developing feed models for the slotline. When this is done we will be able to compute the input impedance 

and radiation patterns of the slotline antenna. As stated above, we will be 

considering different feeds, in order to find one which maximizes the bandwidth of the slotline antenna. We will also be looking at different tapers to 

see their effects on bandwidth and scattering cross section. 

12. Antenna Cavity Scattering 

The hybrid procedure for analyzing the EM scattering by inlet cavities, 

which was significantly extended under the NASA/Langley grant [39], appears to be potentially useful for the analysis of the scattering by regularly 

shaped antenna cavities as well [52]. The type of antenna cavities treated in 

[52] consisted of rectangular shapes in a ground plane and the cavities could 

be loaded with a dielectric material. This work which provided a highly 

efficient and physically appealing analysis of the 2-D rectangular antenna 

cavities now being extended under NASA/Laiigley support to include the 

effects of simple 2-D antennas with loading, and then subsequently to include 3-D cavities with loaded antenna arrays inside. 

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First, the extension to analyze the radiation by simple electric and magnetic line sources in the 2-D analysis of the dielectric filled rectangular cavity has been performed. The solution for the radiation by sources in such a 

configuration retains the simplicity of the earlier solution obtained in [52] 

without the sources. It is noted that this relatively simple and efficient 

nature of the solution thus obtained is important for use in applications; 

such simplicity is not present in the numerical moment method solutions 

of this problem based on a mode matching or integral equation type formulation. In a sense we have obtained, using the hybrid asymptotic-modal 

procedure, a simple approximate but accurate and efficient Green's function for th problem; i.e., we have obtained the radiation by line sources 

within a dielectric filled rectangular cavity in a ground plane. Many accuracy tests have been perfornied and more tests will be continued further 

for a variety of dielectric parameters, cavity dimensions, and different positions of the line sources. This solution will allow one to directly deal with 

slot or dipole type sources in the cavity. Presently, we are extending this 

solution to include the effects of waveguide fed slot antennas in the cavity 

when the cavity is excited by an external plane wave. The control of cavity 

scattering by loading the antennas will also be studied in the near future. 

All of this work will be extended later to deal with 3-D cavities for which 

the interior modes can be well defined. Furthermore, the dielectric filling 

the entire cavity will also later be replaced lq- a planar rlielri it if- racloii~e 01. 

an FSS which does not, fill the entire cavity. The ultimate goal of this zzwrk 

is to develop a code for predicting the scattering hy antenna cavities and 

37 
for controlling the scattering by such cavities by optimizing the antenna 

loading. 

13. Propfan Blade Scattering 

Significant research has been done and is continuing to analyze the scattering from pusher and puller propfan blades. The effort is two fold being 

both an experimental and theoretical study. The study so far has centered around pusher blade applications but will follow into the puller blade 

applications. 

RCS measurements of planar and three dimensional blades have been 

performed on a special designed pusher forebody to quantify the level of 

returns, the scattering mechanism involved and the similarities and differences between planar and three dimensional blades. The majority of the 

measurements have been static but dynamic (rotating blades assemblies) 

measurements have also been performed. The dynamic measurements will 

be provide an alternative approach to accurately measure the propfan RCS. 

RCS calculations of planar and three dimensional blades are presently 

being performed as a GTD based computer code is being developed. The 

code will be able to calculate the scattering from a single blade or blade 

assembly on a forebody structure. The description of the blades are analytically defined to facilitate the required comput,at,ional requirement,s and 

graphical display of the blades computational requirements and graphical 

display of the blades geometry. The graphical display conveniently indicates 

the geometry of the blade, and its shadowing froiii and t.0 otJier blades as 

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well as the forebody. 

14. Miscellaneous Measurements 

A variety of measurements have been performed during t8his past year. 

a. Rough Surface 

An important angular sector for scattering from rough surfaces is near grazing incidence. Measurements of a plate with periodic sinusoidal variations 

were obtained and compared with simple theory. The peak to peak surface 

variations were .01 and .03 inches. The resonant behaviors of the scattered 

fields were readily apparent and agreed with the calculations for incident 

angles less than 30" from grazing. 

b. Resistive Cards 

Another series of measurements involve effectiveness of resistive cards on 

thick edges. Resistive cards are very useful for scattering reduction from 

thin edges but when the edge becomes thicker, the scattering phenomenon 

changes. The scattering reduction from an elliptic edge was measured over 

a broad band of frequencies at several look angles for single and multiple 

resistive card configurations. 

c. Contour Rim Inlets 

The scattering reduction possible on shaping the rim of inlets was experiinentally studied. Four different rim cont.ours were tested: a straight cir39 
cular rim, an outwardly expanding rim, an inwardly contracting rim and 

rim contour that oscillated about a plane. It was found that no significant difference was observed among a series of pattern and swept frequency 

measurements. Conceptually, an oscillating rim would reduce the axial 

backscattered field if enough phase difference could be achieved. 

d. Images 

A new scheme to generate two-dimensional images of scattering centers has 

been developed. This technique entails the processing of downrange, bandlimited impulse responses from two look angles which are symmetrically 

offset a slight distance from the downrange symmetry axis of the compact 

range reflector. The returns from the two look angles are paired together 

and the downrange location of the scattering centers is the average downrange distance of each pair. The crossrange displacement of the scattering 

center is obtained from the relative downrange offset between the average 

downrange location and the individual returns. To determine whether the 

scattering center is located to the left or right of the down range symmetry 

axis, the measurement which precedes the average downrange location is 

used. This algorithm is well suited for reflection and diffraction mechanisms. 

15. Surface Conductivity 

RCS model measurements often have to be coated t,o provide a conductive 

surface. The conductive quality of these coatings can affect. such measure40 
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nients. A series of rectangular cross section inlets were built to measure 

tthe influence of conductive coatings on tlie RCS from a structure. This 

structure was chosen because an accurate GTD solution exists for both the 

structure and cavity scattering to provide a perfectly conducting reference 

solution. This structure provides an accurate indicator of tlie loss characteristics of the paint because the larger off axis angles involve many model 

reflections. The inlets were fabricated from fiberglass and had an external 

surface coating of silver. The internal surfaces of the inlets were coated with 

air sprayed paints or arc sprayed with metal. Tlie metals examined were 

silver, copper, nickel and zinc. The silver pain was superior and agreed 

very well with the calculation with the nickel paint being the next best 

followed by tlie copper paint. The zinc arc sprayed coating had a similar 

performance as did the nickel air sprayed paint. 

a. Starbody 

RCS measurements were performed on a new forebody design for high speed 

aircraft. This forebody could be crudely represented by a cone with finds 

radially expanding from the tip to the base. Two versions were tested with 

one having the fins expanding in a plane containing the cone’s axis and tlie 

other having the finds spiraling outward. The scattering performance of 

either design was less than desirable. Tlie expanding fin stmicture at the 

tip of tlie cone created a larger scattering center. The spiral tin tealure of 

one design presented the existence of specular returns much earlier than 

the other design. 

41 
C. Present Program Achievements 

1. Indoor Scattering Measurement Systems 

a. Compact Range Reflector Development 

During the present period, we have been developing techniques to design 

blended rolled edges for arbitrary rim shaped compact range reflectors 

which may be centerfed or offset-fed. If one wishes to add a blended rolled 

edge to an arbitrary rim shaped reflector, he should make sure that the total surface of the reflector is smooth and continuous. Therefore, the choice 

of the rolled edge plane for various points on the rim (junction contour) is 

very important. Also, one should make sure the method outlined by Gupta 

et al. [53] to obtain the optimum rolled edge parameters is applicable in 

the rolled,edge plane; i.e., the section of the paraboloid which lies in the 

rolled edge plane is a parabola. Such a rolled edge plane was found. If 

blended rolled edges are added in these planes, one obtains a reflector with 

minimal surface discontinuities. Also, the whole reflector can be defined 

analytically using simple expressions. In the rolled edge plane, it is trivial 

to find the equivalent parabola used in the design of blended rolled edges. 

Thus, the optimum rolled edge parameters can be obtained relatively easily. 

In fact, one can write a computer code to obtain the optimum rolled edge 

parameters for various points on the junction contour. Such a comput,er 

code is being developed. 

During this year, we also continued our effort, in developing efficient 

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t,echniques to analyze a compact range reflector with blended rolled edges. 

We have been using a corrected PO technique [3] to analyze such reflectors. 

However, since PO involves the integration of surface currents, the technique is inefficient for large reflectors (larger than 20X x 20X) even using a 

super computer. Nevertheless, one can use vectorization capabilities of the 

super computer to increase the speed of integration. We have developed 

such a code. The code can be used with scalar as well as vectorized machines and does not require extraordinary space. On a vectorized machine, 

however, the code uses very little CPU time. For example, on the CRAY 

X-MP/24, the code is at least 50 times faster than our VAX 8550. 

We are also testing a numerical GTD analysis of such reflectors. In the 

case of a blended rolled edge compact range reflector, various mechanisms 

contributing to the scattered fields in the target zone are: specular reflection 

from the parabolic part of the reflector and diffraction from the junction 

between the rolled edge and the paraboloid. In the case of the paraboloid, 

specular reflection can be computed quite easily. However, since the junction between the rolled edge and the paraboloid is a higher order junction, 

the junction diffraction can not be computed using conventional GTD. As 

a result, we are using numerical methods to compute the junction diffraction. Initial results obtained using this numerical GTD approach are quite 

promising. Once the technique is tested t,horoughly, a computer code will 

be developed and delivered t,o NASA (Langley). 

43 
b. Measurement Hardware 

The two pulsed/CW radar systems have been built for NASA (Langley). 

The dual channel system has been delivered, tested at their site and is 

operating properly. The only significant problem encountered with the 

Langley system was the pedestal controller. The controller used a few wire 

wrap circuit cards which had intermittent problems. These cards have since 

been replaced with printed circuit cards in order to avoid these types of 

problems. More recently, a Ka-band subassembly has been sent to NASA, 

and they are presently evaluating its performance in their range. 

We are now in a mode of fully developing the capability of this new radar 

system. This year, we have mainly concentrated on the software needed to 

use all the hardware features available based on the original design. For 

example, it was determined that the radar was so automated that it could 

be programmed to perform a series of operations on its own. Thus, we 

developed a “programming page” for the AT system software which allows 

the operator to actually define a full measurement sequence without further 

input. Using this approach, we have been able to generate large data sets 

over’night. In most cases, these data sets have been used to generate down 

range/cross range images of a complex target. 

With the radar system running more in an automatic mode, it became 

clear that various feed antennas would have to be selected under co~iiputer 

control. For example, one might use two antenna feeds, one for liorizontal polarization and one for vertical. These would both be located near 

the focal point of the main reflector. In order to interfare these feeds to 

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the software, an “antenna page” has been added to the menu list. Using this page, one can control various digital lines which can be connected 

to switches that in turn select the appropriate antenna as the feed. The 

operator can then simply identify these digital line settings on the “antenna page” and distinguish one feed selection from another by using an 

antenna type character. For instance, the “H” antenna may stand for the 

horizontally polarized feed and the “V” for the vertically polarized one. 

With all these features available, it became clear that we could generate 

data faster than we could process it. In order to solve this problem we 

had to develop a more automatic processing procedure. This required us 

to change the original file name structure in order to have it indicate the 

appropriate calibration files. Using this approach, the AT operator actually 

generates the appropriate file names automatically. Then our processing 

program on the VAX calibrates the measured information, manipulates the 

data, writes the output files and plots the results. This is a very highly 

automated procedure because the file names indicate how each measured 

data set should be processed. 

The hardware aspects of the system were then evaluated to determine 

what subassemblies limited the performance of the radar. It was very clear 

from these studies that our WJ synthesized source limited the radar’s speed. 

For example, the radar can collect more than one thousand data points 

per second; however, the WJ source took as long as IS iiiillisecontls to 

synthesize one new frequency. As a result, we cannot operate tlie radar 

in its fast mode without changing the synthesizer. With this in mind, we 

45 
designed a whole new synthesizer concept which will actually be built under 

next year's grant. 

In reviewing our Ka-band subassembly we found that the radiated power 

was not sufficient for some applications. As a result, we reviewed other 

power generation concepts and found that AVANTEK had developed a 

new amplifier Ka-band doubler which outputs more power. We ordered this 

unit and verified that it does perform at least 10 dB better than traditional 

diode doublers. At the present time, this new doubler is being integrated 

into our Ka-band subassembly. 

c. Target Mounts 

The metal ogival pedestals are commonly used as a target support because 

they can hold heavy targets and have a relatively low RCS, especially at 

high frequencies. Lai and Burnside [54,55] have shown that the RCS of 

a metal ogival pedestal is directly proportional to the square of the wavelength of the incident field and is approximately 20 dB below a square 

meter at 1 GHz. Thus, metal ogival pedestals do not appear to be a good 

choice for supporting targets at low frequencies; i.e., below 1 GHz. For a 

horizontally polarized incident wavefront, the RCS of Styrofoam columns 

is approximately equal to that of metal ogival pedestals; while for vertical 

polarization, the RCS of Styrofoam columns is higher [5q. Furt'herinore, 

to support heavy targets the volume of t,he st.yrofoaiii coluiiiiis should be 

rather large. Thus, Styrofoam columns do not appear t,o be a goocl choice 

either. Gupta, Lai and Burnside [55,56] studied the scat,tered fields froin 

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dielectric straps and concluded that they perform much better at lower 

frequencies. One advantage of the straps is that when the plane wave illuminates the straps at or near edge-on incidence, the RCS of the strap 

is quite low at low frequencies. Even though its RCS increases as the frequency increases, it is still lower than that for metal ogival pedestals. As 

a result, it is possible to align the straps with the lower RCS direction of 

the target, so the minimum RCS of the target coincides with the minimum 

RCS of the strap. In addition, the straps can be built using composite 

materials so they can handle very heavy targets. Therefore, the dielectric 

straps appear to be a good choice for supporting targets at low frequencies. 

The straps can be used to hold a target using at least two basic methods. The first one is by connecting the straps directly to the target. The 

second method is by wrapping the strap around the target. The second 

method appears to be easier in that the target does not have to be modified. Both methods are expected to have a relatively low RCS when used 

as a target support. If the strap is directly attached to the target, the RCS 

of the support will be dependent on how well the strap is connected using 

a low scattering attachment. On the other hand, the second approach is 

dependent on the scattering level of the strap as it surrounds the structure. 

In order to evaluate the scattering level for this type of attachment, the 

scattering performance of a thin dielectric strap surrounding a perfectly 

conducting structure was st,udied. A iiioiiieiit met hot1 technique was tisetl 

to find the currents excited within the strap by an incident. plane wave. 

Then, the Uniform Geometrical Theory of Diffraction along with the sta47 
tionary phase integration method was used to compute the fields scattered 

by the strap. The method was used to evaluate the RCS of a strap wrapped 

around an ogive. The computed RCS showed good agreement with the measured RCS results. It was found that for low frequencies, below l GHz, the 

strap scattering is quite low. Thus, straps are an excellent choice for low 

frequencies. 

Some empirical design formulas, which can be used to choose the straps 

for a given application, are also developed. These formulas are good for 

general convex structures and are expected to give a reasonable estimate of 

the RCS of the dielectric straps when used as a target support structure. 

d. Hardware Refinements for Radar Image Enhancement 

Based on the diagnostic value of radar images, it is clear that this type 

of information will be more in demand in the future. As a result, we have 

been evaluating various schemes which will allow us to collect this data more 

accurately and efficiently. For conventional down range cross range images, 

the measured data must be collected and stored more rapidly. Using the 

present radar system, we can collect the image for one full 360" azimuth 

cut (800 frequencies and 1800 angles) in about 30 hours. This is clearly 

too slow. To solve this problem, we started the development of a new 

synthesizer as stated earlier. It is expected this new source will be able 

to provide the same data in approximat,eIy 3 hours. In addit,ion, we are 

planning to develop new processing systems which will allow us to outsput 

image results while the data is being measured. Using this concept,, we plan 

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This will be achieved by overlapping measurements and processing with 

specially designed hardware and software. 

We have also been developing a new image processing procedure using 

niultiple feed antennas. This technique will be described in the waveform 

processing section; however, it is also considered here because it requires 

adclitional measurement hardware. With this approach our new radar must 

collect measurement results simultaneously from various feeds. These feeds 

provide different look angles relative to the target,. In order to provide 

this data, the radar has been modified to allow us to use multiple feeds as 

described earlier. This approach is useful for testing this concept, but it is 

not very efficient in terms of the radar capability. The radar can potentially 

measure three channels at the same time such that the data for the three 

image feeds can be acquired simultaneously. This feature was not originally 

built into the radar, but is has been subsequently added during the present 

reporting period. This hardware and software update allow us to collect 

the backscatter data simultaneously from the three feeds. This means that 

we could eventually output these new image plots in virtually real time. 

e. Mini Range 

The mini range radar system has been completed and is now operational at 

The Ohio State University. A new Ka-band sul~asse~i~l~l~s heing tlevelopecl 

for this radar and should be completed during the next repc)rt,ing period. 

For the present time, the radar is simply functioning in an open room 

49 
without absorber. The mini range chamber has been designed and will be 

built during the next reporting period. It is being constructed such that it 

can be completely torn down and moved from one laboratory to another. 

The absorber will be mounted on each panel using Velcro so that it can be 

replaced if damaged. The chamber will be about 7 feet high, 8-1/2 feed 

wide and 20 feet long. The reflector will be mounted at one end, and the 

target and access areas at the opposite end. Note that the far end of the 

chamber will be left open. Although we are considering using a garage 

door concept which could be used to close off the back wall of the chamber. 

With this approach, a garage door opener would be used to automate the 

opening access. In any event, a cart will also be designed in the future 

to allow for easy access to the target zone. It is expected that this whole 

system will be in operation during the summer of 1989. 

f. Recent Indoor Scattering Measurement Publications 

1. K.P. Ericksen, I.J. Gupta and W.D. Burnside, “Design of Blended 

Rolled Edges for Compact Range Main Reflectors,’’ Technical Report 

716148-33, The Ohio State University ElectroScience Laboratory, prepared under Grant NSG-1613, for NASA-Langley Research center, 

Hampton, VA, August 1988. 

A procedure to design blended rolled edge terminations for arbitrary 

rim shape compact range main reflectors is presented. The reflector may be center-fed and offset-fed. The design procedure leads to 

a reflector which has a continuous and smooth surface. This procedure also ensures small diffracted fields froiii tlie junction het,ween the 

paraboloid and the blended rolled edge while satisfying certain constraints regarding the maximum height of t,he reflector and minimum 

operating frequency of the system. The prescribed procedure is used 

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to design several reflectors and the performance of these reflectors is 

presented. 

2. I.J. Gupta, K. Ericksen and W.D. Burnside, “Design of Blended 

Rolled Edge for Arbitrary Rim Shape Compact Range Reflectors,” 

AMTA Symposium Proceedings, Atlanta, Georgia, September 1988. 

A procedure to design blended rolled edges for arbitrary rim shape 

compact range reflectors is presented. The reflector may be center-fed 

orr offset-fed. The design procedure leads to continuous and smooth 

rolled edges and ensures small diffracted field from t*he junction between the paraboloid and the rolled edge. The performance of a 

compact range reflector designed using the prescribed procedure is 

also presented. 

3. P. Swetnam, M. Poirier, P. Bohley, T. Barnum and W. Burnside, “A 

Wide Band Instrumentation Radar System for Indoor RCS Measurement Chambers,” AMTA Symposium Proceedings, Atlanta, Georgia, 

September 1988. 

An instrumentation radar system suitable for collection of backscatter 

characteristics of targets in an indoor chaniber was built and installed 

in The Ohio State University ElectroScience Laboratory. The radar 

is a pulsed system with a continuous coverage from 2 to 18 GHz, 

and spot coverage from 26 to 36 GHz. The system was designed to 

have maximum flexibility for various test configurations, including 

complete control of the transmit waveform, H or V transmit polarization, dual receive channels for simultaneous measurement of like and 

cross polarization, greater than 100 dB dynamic range, and convenient data storage and processing. A personal computer controls the 

operation of the radar and is capable of limited data reduction and 

display functions. A mini-computer is used for more sophisticated 

data storage. This paper will present. details of the radar along with 

measured performance capabilities of t.lie syst.em. 

4. Z. Al-Hekial and I.J. Gupta, “Scat,tering froni Thin Dielect,ric Straps 

Surrounding a Perfectly Conducting Structure,” Tecliiiical Report 

719493-5, The Ohio Stat,e lJniversit,y ElecbroScience Lahorat,ory, pre51 
pared under Grant NSG-1618 for NASA-Langley Research Center, 

Hampton, VA, January 1989. 

A method to calculate the electromagnetic scattered fields from a 

dielectric strap wrapped around a convex, conducting structure is 

presented. A moment method technique is used to find the current 

excited within the strap by the incident plane wave. Then, UTD 

is used to compute the fields scattered by the strap. Reasonable 

agreement was obtained between the computed and the measured 

results. 

The results found in this study are useful in evaluating straps as a 

target support structure for scattering measurements. 

5. C.W.I. Pistorius and W.D. Burnside, “Scattering from a Lossy Dielectric Wedge,” International APS/URSI Symposium, Syracuse, New 

York, June 1988. 

The problem of scattering from a lossy dielectric wedge is a complicated one to solve, due in part to the difficult nature of the boundary 

conditions. Many of the published solutions are either too limited or 

too,complicated for practical use. The Uniform Theory of Diffraction 

(UTD), for instance, cannot be used since the diffraction coefficients 

for a lossy wedge are not known. In this paper a technique [l] based 

upon ray tracing and aperture integration is proposed; whereby, the 

scattering from a lossy wedge can be calculated. 

6. H. Shamansky and A. Dominek, “Target Mounting Techniques for 

Compact Range Measurements,” AMTA symposium Proceedings, Atlanta, Georgia, September 1988. 

The compact range provides a means to evaluate the radar cross section (RCS) of a wide variety of targets, but successful measurements 

are dependent on the type of target mounting used. This work is 

concerned with the mounting of targets to a iiietal ogival shaped 

pedestal, and in particular focuses on two forms of niounting tecliniques; the “soft” (non-metallic) and the “hard” (metallic) mounting 

configurations. Each form is evaluated from both the iiiechanical and 

electromagnetic viewpoints, and the liiiiitations associated with each 

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type are examined. Additional concerns such as vector background 

subtraction and target-mount interactions are also examined, both 

analytically and thorough measurements performed in the ElectroScience Laboratory’s Anechoic Chamber. 

7. T.H. Lee and W.D. Burnside, “Cross Range Processing of Patterns 

for Large Reflector Antennas to Obtain Radiation Centers,” AMTA 

Symposium Proceedings, Atlanta, Georgia, September 1988. 

A technique to determine the radiation centers of large reflector antennas in a given direction is presented in this paper. Coherent processing is used to deternline various radiation centers based on far 

zone pattern data of the antennas provided that adjacent centers are 

separated far enough so that their locations can be resolved. Nunierical results for processing of two reflector antennas, a prime focus fed 

and a Cassegrainian, are presented to validate this technique. The diagnostic value of this technique for reflector antennas is demonstrated 

by processing the actual measured pattern and identifying some unexpected radiation centers. One can also use this technique to fine 

tune numerical pattern simulations of reflector antennas. 

8. T.H. Lee and W.D. Burnside, “A Diagnostic Technique Used to Obtain Cross Range Radiation Centers from Antenna Patterns,,” Technical Report 716148-30, The Ohio State University ElectroScience 

Laboratory, prepared under Grant, NSG-1613, NASA-Langley Research Center, Hanipton, VA, May, 1988. 

A diagnostic techiiique to obtain cross range radiation centers based 

on antenna radiation patterns is presented in this report. This method 

is similar to the synthetic aperture processing of scattered fields in the 

radar application. Coherent processing of the radiated fields is used to 

determine the various radiation centers associated with the far-zone 

pattern of an antenna for a given radiation direction. This technique 

can be used to identify an unexpected radiation ceiitw that ci~ates 

an undesired effect in a pattern; on the other hand. if can iniprove 

a numerical simulation of the pat tern by identifying other significant 

mechanisms. Cross range results for two 8’ reflector antennas are 

presented to illustrate as well as validate this tecliriirliie. 

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9. A. Donlinek and T. Nguyen, “The Ogive as a RCS Compact Range 

Standard,” Technical Report 719493-3, The Ohio State University 

ElectroScience Laboratory, prepared under Grant NSG-1613, NASALangley Research Center, Hampton, VA, January, 1988. 

E- and H-plane RCS patterns at 4 and 10 GHz are provided (based 

upon moment method calculations) for a perfectly conducting ogive 

to be used as a compact range verification standard. The dimensions 

of the ogive are 36” and 9.546” long with half tip angles of 15” and 20”, 

respectively. Comparison between the calculations and measurements 

are also provided. 

10. G. Clerici and W.D. Burnside, “Geometrical Optics Design of a Compact Range Gregorian Subreflector System by the Principle of the 

Central Ray,” Technical Report 719493-4, The Ohio State University 

ElectroScience Laboratory, prepared under Grant NSG-1613, NASALangley Research Center, Hampton, VA, January 1989. 

In recent years, the compact range has become very popular for measuring Radar Cross Section (RCS) and antenna patterns. The compact range, in fact, offers several advantages due to reduced size, a 

controlled environment, and privacy. On the other hand, it has some 

problems of its own, which must be solved properly in order to achieve 

high quality measurement results. For example, diffraction from the 

edges of the main reflector corrupts the plane wave in the target zone 

and creates spurious scattering centers in RCS measurements. While 

diffraction can be minimized by using rolled edges, the field of an offset single reflector compact range is corrupted by three other errors: 

the taper of the reflected field, the cross polarization introduced by 

the tilt of the feed and the aperture blockage introduced by the feed 

itself. These three errors can be eliminated by the use of a subreflector system. A properly designed subreflector system offers very little 

aperture blockage, no cross-polarization introduced and a minimization of the taper of the reflected field. A Gregorian configuration has 

been adopted in order to enclose the feed and the ellipsniclal subreflector in a lower chamber, which is isolated by absorbers from the upper 

chamber, where the main parabolic reflector and the target zone are 

enclosed. The coupling bet8ween the two rooms is performed through 

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a coupling aperture. The first cut design for such a subreflector system is performed through Geometrical Optics ray tracing techniques 

(GO), and is greatly simplified by the use of the concept of the central ray introduced by Dragone. The purpose of the GO design is to 

establish the basic dimensions of the main reflector and subreflector, 

the size of the primary and secondary illunlinating surfaces, the tilt 

angles of the subreflector and feed, and estimate the feed beaniwidth. 

At the same time, the shape of the coupling aperture is initially determined. The design of the subreflector system is performed through 

some design equations which have been derived from geometrical considerations and from the zero cross polarization equation. These design equations are used to specify a subreflector system for the next 

generation conipact range with improved reflected field taper, crosspolarization and aperture blockage performance. The design of the 

coupling aperture is performed by considering the reflection shadow 

boundaries of the over extended subreflector. 

2. Material Study 

a. Theoretical and Experimental Studies 

A continuing study of composite structures has a goal of controlling the 

scatter caused by edge waves. Both a theoretical and experimental program 

is being pursued. The ogival shape reported last year has been constructed 

by NASA and will be available for measurements soon. A new model for 

the 1/8th scale stabilizer has been constructed and tested. This model 

incorporated an available R-card which was symmetrically arranged. This 

R-card was 2-D with a 1-D resistance profile and while it shoiv-etl substantial 

promise, it still did not have an optimum design. This led to considering 

concepts for constructing the appropriate R-card with a 2-D profile to re55 
place the ones available commercially. The availability of such R-cards 

should be useful in controlling both direct and edge wave diffraction from 

corners. 

Several larger composites sheets have been constructed and used for 

transmission measurements. The material performed as expected except 

there seemed to be different amounts of insertion loss for the transmitted 

polarization. These experiments are part of an overall goal to characterize 

these anisotropic materials as anisotropic resistive sheets. This will simplify 

the analysis of such materials. The next task here will be to associate 

the physical parameters of the composites to the electrical properties of 

the R-card. Several types of measurements are underway and are to be 

correlated theoretically, at least for the present, to reduce the measured 

data to ohms/O for particular structures. 

These same techniques should prove useful in the design of the propfan 

blades once the multiple reflections involving such reflections have been 

controlled. 

The theoretical studies are now being focussed on a dissertation that 

involves the control of scattering by various loss mechanisms. One that was 

studied from the start involves the general mechanisms for a lossy straight 

wire [36]. This should be very similar to the case of an edge wave on a 

carefully designed composite. Asymptotic analyses have been generated 

that give the trapping, loss and reradiation froiii a lossy wire. This ties 

very neatly with existing theories and is an almost perfect. agreement with 

the more complex moment method solution which requires iiiuch greater 

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computation time. Other related topics that are now being investigated 

include loss on an imperfectly conducting strip and two dimensional ogive. 

Finally, this dissertation will also evaluate scattering from cracks. The 

control of such scatter by use of lossy materials will be covered in a separate 

report,. 

b. Material Studies Publications 

1. A.K. Dominek, L. Peters, Jr., H.T. Shamansky and W.T. Hodges, 

“Edge Wave Control Using Composite Material on a Three Dimensional Airfoil,” Report 716148-32, The Ohio State University ElectroScience Laboratory, prepared under Grant N S G - 16 13, NASA- Langley 

Research Center, July 1988. 

No Abstract (Classified Report) 

3. Waveform Processing Techniques 

a. Three-Dimensional Images 

As discussed in the measurement systems section, the radar hardware has 

been modified to simultaneously obtain data from three compact range 

feeds located near the focal point of the reflector, one at the focus, one horizontally offset and one vertically offset. These three feeds provide slightly 

different look angles relative to the target reference system. Each feed data 

set is then transformed from the frequency domain to the time domain. 

Since each set of data can be calibrated separately. one can iise the ceiifer 

and horizontal feeds to evaluate the horizontal offset of a given isolated 

scattering center. Then the center and vertical feeds can provide the ver57 
tical offset scattering center information. This approach works well if each 

of the scattering centers are isolated in time so that one can evaluate the 

slight time shifts between the different look angles. As a matter of fact, it 

has been shown to be very efficient and accurate if the scattering centers 

are clearly isolated. 

This past year, we have been evaluating the use of this new image 

algorithm for more complex targets. As expected, for such targets, the 

scattering centers aren’t always isolated such that the algorithm can’t distinguish how to equate the scattering center information seen in one look 

angle to another. To correct this situation, we have been developing tracking algorithms for individual scattering centers. Using this approach, the 

scattering centers are tracked in the regions where they are clearly isolated. 

If they tend to cluster at certain look angles, they are flagged and then 

tracked sgain after they become isolated as the target continues to rotate. 

The tracking algorithm is then used to fill-in the regions where the scattering centers clustered and couldn’t be isolated. The actual implementation 

of this tracking algorithm is still being evaluated. Nevertheless, it is clear 

that this new image approach will be very efficient and effective in that it 

can provide image data in virtually real time. 

b. Publications 

1. A. Dominek, I. Gupta and W.D. Burnside, “A Novel Approach for 

Two-and Three-Dimensional Imaging,” AMTA S’yinpu.~iuin Proceeding, Atlanta, Georgia, September 1988. 

Conventional radar imaging requires large amounts of data over large 

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bandwidths and angular sectors to produce tlie location of the doniinant scattering centers. A new approach is presented here which utilizes only two swept frequency scans at two different look angles for 

two-dimensional images or three swept frequency scans at three different look angles for three-dimensional images. Each swept frequency 

scan is the backscattered response of a target. A different plane wave 

illumination angle can be conveniently obtained by offsetting the feed 

horn from the focus of a compact range reflector without rotating the 

target. The two-and three-dimensional target information for the location of the dominant scattering centers is then obtained from tlie 

bandlimited impulse responses of these swept frequency scans. 

4. Inlet Scattering 

A hybrid asymptotic high frequency-modal procedure was significantly extended under NASA/Langley support, in tlie past,; that work provided tlie 

crucial foundation for further development of analysis and codes, under separate support, for treating the scattering from more general cavity shapes 

(than those dealt with previously) which could again be modeled by joining together piecewise separable waveguide sections (e.g., straight, curved 

annular, or linearly tapered sections). While this hybrid method has been 

found to be highly useful, even as a check on other more approximate 

methods, it nevertheless cannot treat inlet shapes with arbitrarily varying 

cross-sections for which modes cannot even be defined in a conventional 

sense. A generalized ray expansion met hod has therefore been developed 

to deal with more arbitrarily shaped inlet cavities. In the latter approach, 

one shoots rays in a prescribed manner to track the fields clown an inlet 

cavity and back from a simple int.erior terniination. This ra\- niethod, also 

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developed under separate support, is different from the more commonly 

used geometrical optics ray shooting method, and it has the advantage 

that the rays need to be tracked only once independent of the incident 

angle; just the initial ray amplitudes change with the incident ray direction. It is of course necessary to validate the various analytical methods, 

as well as the codes based on such analysis, which are being developed for 

treating the EM scattering by inlet cavities. For this reason, it had already 

been proposed to have experimental models, built by NASA/Langley, for 

the purposes of the aforementioned verification. At present, the design information required for the construction of the experimental models is still 

being gathered. It is very important to note that this type of information 

will be largely dictated by the codes being developed for use in the near future. The codes in turn are dependent on the use of analytical procedures 

which have only recently been developed and which are still undergoing 

refinements, modifications and extensions. One specific geometry which 

will be of prime interest for experimental verification shall consist of a reasonably large inlet cavity with a rectangular cross-section at the open end 

which then transforms to a circular cross-section at the closed back end via 

an s-type transition. The design details of this and few other geometries 

will be provided to NASA/Langley in the next few months after the code 

for analyzing such geometries is completed and running. A list, of papers 

related to the inlet cavity scattering work which werc. ~~ul~lisliecl during the 

past period are given below. 

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1. Ayhan Altintas, Prabhakar H. Pathak and Ming-Cheng Liang, “A 

Selective Modal Scheme for the Analysis of EM Coupling Into or 

Radiation from Large Open-Ended Waveguides,” IEEE Trans. on 

Antennas and Propagation, Vol. 36, No. 1, January 1988. 

The problem of electromagnetic (EM) coupling into or radiation from 

open-ended waveguides is addressed here. Of particular interest is the 

high-frequency range where a large number of propagating waveguide 

modes can be excited and conventional procedures requiring a summation over a large number of propagating modes can be become 

cumbersome and inefficient. A selective modal scheme is proposed 

based 011 the observation that the modes which can contribute most 

significantly to the fields coupled into the waveguide are those whose 

modal ray directions are most nearly parallel to the incident wave 

direction. This concept is illustrated by calculating the EM radiation and backscattering froin open-ended parallel-plate, rectangular, 

circular, and sectoral waveguide geometries. The calculations employ the usual geometrical optics, aperture field, and Ufinitsev edge 

current techniques. Also included are some measured results which 

further verify the accuracy of the above computations. 

2. P.H. Pathak and A. Altintas, “An Efficient High-Frequency Analysis of Modal Reflection and Transmission Coefficients for a Class of 

Waveguide Discontinuities,” Radio Science, Vol. 23, No. 6, pp. 1107- 

1119, Nov.-Dec. 1988. 

A relatively simple and efficient high-frequency analysis of electromagnetic modal reflection and transmission coefficients for waveguicle 

discontinuities which are formed by joining different waveguide sections is presented. The analysis extends the concept of geometrical 

theory of diffraction based on equivalent edge currents and utilizes 

it in conjunction with the reciprocity theorem to describe interior 

(waveguide) scatatering effects. It. is noted t,ha.t t,he previous use of 

equivalent edge currents was mostly rest,rict,etl 1.0 ex1.cvior scal,t.eriiig 

hy edged bodies, and it,s applicat,ioii to deal wit,li int,evir)r sca.t,t,ering 

was limited to those guide geoiiiet,ries for which iinage t.heory could 

be used effectively to account for the interior wall efFect.s. The present. 

extension allows one 1.0 treat, more geiiera.1 t.n-c-)- atid t.hree-tliineiisiona1 

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waveguide geometries provided the waveguide modes and their associated modal rays can be found explicitly. In particular, expressions 

for two-dimensional reflection and transmission coefficients are developed, and numerical results are shown for a flanged, semi-infinite 

parallel plate waveguide a.nd for the junction between the two linearly 

tapered waveguides. One sample result is also shown for the reflection coefficient of a three-dimensional open-ended circular waveguide. 

Detailed expressions for three-dimensional waveguide discontinuities 

are being reported separately. 

5. Antenna Cavity Scattering 

By extending, in a simple fashion, the already completed work on the prediction of EM plane wave scattering from dielectric filled, two-dimensional 

rectangular cavities in a ground plane, the related problem of EM scattering by the same cavity when it contains a slot antenna fed by a waveguide 

behind the cavity has been solved. It is recalled that the analysis of this 

cavity problem is based on the hybrid asymptotic-modal approach which 

was discussed briefly in the work dealing with the analysis of inlet cavities. 

Such a hybrid analysis is relatively simple and provides an approximate but 

accurate and efficient analytical form of the Green’s function for a source 

within the cavity. 

In contrast, other approaches would generally provide a less efficient 

numerical Green’s function which has to be recomputed by modal matching 

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or the moment method each time the geoiiietzical or electrical para.meters of 

the cavity are changed. This presently developed hybrid form of t.lie ca,vit.y 

Green’s function can be employed in a moment iiiedliod formulation to 

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evaluate the unknown currents only over tlie antennas in the cavity. Again, 

the other approaches would generally require one to treat the cavity as 

well as the antenna in the cavity by the moment method thereby making 

those approaches more cumbersome and inefficient. It is noted that the 

analysis of the EM scattering by a waveguide fed slot in a cavity has been 

completed presently for both the TE and TM polarizations. The next step 

in the present study is to use an optimization procedure to determine the 

load in the waveguide which would control the scattering by the waveguide 

fed slot in the cavity to achieve tlie desired goals. Following that step, 

t,lie scattering by an array of waveguide fed slot,s in a cavity will also be 

analyzed and loaded via an optimization algorithm. Subsequently, this 

study will be extended to deal with slot,s and other antennas placed wit,liin 

three-dimensional antenna cavities. 

6. Propeller Scattering 

Measurement and code development in the propeller scat teriiig work have 

Loth advanced during this past period. A series of iiieasureiiients were 

performed to determine the blade interaction between blades on single and 

dual hubs for both static and dynamic blade configurations. The blade 

interactions can be categorized into simple geometric shadowing and blade 

to blade scattering. It was observed for some configurations that hlade 

to blade scattering is present but not. dominant. Dyiiaiiiic c-ouiiter blade 

rotation measurements were taken for the first time and compared to single 

blade rotation measurements. The power spectrum for t lie.;^ nieasureiiieits 

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indicated a stronger frequency spread as expected due to the larger rate in 

the change of the scattering mechanisms. 

The GTD code presently being developed has graphic and scattering 

prediction capabilities for a single blade configuration. The scattering prediction is based upon either individual edge and corner contributions or an 

equivalent current concept. An improvement to the code has been the incorporation of the scattering from the junction of the blade to the forebody. 

The code is presently being modified to simulate counter blade rotation 

conditions. An additional software package is being developed to define the 

required parameters for user defined planar blade contours. This software 

will also incorporate parameter generation for three-dimensional blade capabilities. Comparisons between measurements and calculated results are 

good for the scattering mechanisms that have been included. Terms that 

are still absent from the solution are body to blade and blade to blade 

interactions. 

7. Supercomputer Computations 

a.. Electromagnetic Studies 

A general study of a particular form of time domain scattered field solutions 

[15] is currently being performed. There are a variety of reasons to pursue 

this topic. The reasons are (1) the results give physical insight iiit,o the 

scattering properties of a target (2) the general foriii of tlie solut,ioii is 

amenable to vectorisation for faster computation and (3) bhey are easy 

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to implement. Three dimensional, perfectly conducting structures have 

been analyzed such as a finite length wire, a flat rectangular plate and 

surfaces with volume. The solution involves the determination of the surface 

currents on the structures which are then iiitegrat<ed to obtain the scattered 

far field. The only major limitation appears to be the stability of these 

solutions. 

Both the electric and magnetic integral field equations show some instability. Only the solution for the wire seems to be robust. Techniques 

are presently being examined to condition the solution which improves its 

stability. 

b. Publications 

1. H. Shamansky and A. Dominek, “The Determination of the propagation Constant for the Travelling Wave in an Infinite Ground Plane,” 

IEEE Transactions on Antennas and Propagation, AP-36, No. 10, 

pp. 1455-1463, October 1988. 

The propagation constant for the travelling wave in a trough in an infinite ground plane is examined. The electromagnetic scattering from 

the infinitme geometry is closely related to the finite extent geomet,ries, 

which are common structures on aerodynamic bodies of interest. The 

traveling wave behavior is an essential element of the overall scattering from structures with joints that form troughs. The null field 

integral equation is used to determine the electromagnetic fields in 

the trough structure, and pulse basis functions give the distribution 

of the aperture fields. From this, t.he propagat.ion const,ant. is solved 

using the Newton-Raphson it,erat,ive scheme. T:a,i.i(:)iis sizts of ~eoiiietries are examined. The far field ra.cliat.ec1 pat i.eri,s a1.e ca.Icula.terl 

and compared with other solubions, t,lierehy ditla.t.in,q t,lie inbegral 

formulation w hi ch sub sequeri t 1 y p r ovi ded t. he pro 1) a.g a. t, i 011 cons t. ant. . 

Measurements of t,wo t.rougli geoiiiet,ries a.re perf~:~riiiecl t (-) valida.t,e the 

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theoretical results. 

2. A. Dominek and L. Peters, Jr., “RCS Measurement of Small Circular 

Holes,’’ IEEE Transactions on Antennas and Propagation, AP-36, 

No. 10, pp. 1495-1497, October 1988. 

The Rayleigh scattering from small holes in a thin screen was studied. The backscattered fields calculated from a dipole expansion were 

compared to those from an exact solution and measurements for a 

single, electrically small hole. 

3. H. Shamansky and A. Dominek, “The Determination of the Propagation Constant for the Travelling Wave in an Infinite Ground Plane,” 

IEE/URSI Symposium, Syracuse University, June 6-10, 1988. 

The propagation constant for the travelling wave in a trough in an infinite ground plane is examined. The electromagnetic scattering from 

the infinite geometry is closely related to the finite extent geometries, 

which are common structures on aerodynamic bodies of interest. The 

travelling wave behavior is an essential element of the overall scattering from structures with joints that form troughs. The null field 

integral equation is used to determine the electromagnetic fields in the 

trough structure, and pulse basis functions are used to determine the 

distribution of the aperture fields. From this, the propagation constant is solved using the Newton-Raphson iterative scheme. Various 

geometrical sizes are examined. 

Measurements of two trough geometries are performed to validate the 

theoretical results. It is also shown that the technique can be suitably 

applied to characterize higher order travelling wave modes in a finite 

extent trough geometry. 

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8. Antenna Studies 

a. Microstrip Antenna Analysis 

In the previous reporting period, we developed efficient techniques, and 

associated computer codes, for analyzing the scattering from a rectangular 

microstrip antenna over a broad frequency range. In brief the technique involves a method of moments solution of an integral equation for the current 

density on the microstrip patch. The accuracy of the method was verified 

by comparison with measured RCS results versus frequency over a 5 to 1 

bandwidth. It was found that the RCS of the microstrip patch consists of 

a number of large but narrow peaks. The RCS at the peaks can be larger 

than the physical area of the patch. The first- peak is centered around the 

resonant frequency of the lowest order mode, however, there are other peaks 

centered around the resonant frequency of higher order modes. The results 

of this research are documented in a journal article [47], a masters thesis, 

a technical report and a user’s manual for the coinputer code. 

b. Slotline Antenna Analysis and Development 

A slotline or finliiie transmission line is formed by a slit or aperture in a 

ground plane. If the slit is is electrically thin, then a wave can propagate 

along the slit with essentially no radiation. A slotline antenna is formed by 

gradually increasing the width of the slit. Wheli 1 lit- ivifli 11 ic on tlie OI.CIPI’ 

of a half wavelength or more, significant, radiatioii (ICCIII’S froin tlit open end 

of the line. Slotline antennas are typically printed on a thin ( iingrounded) 

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dielectric substrate. An advantage of slotline antennas is that they have the 

potential for wide bandwidth if propertly fed. Slotline antennas are also 

extremely lightweight and may also have low scattering cross section. Thus, 

they may have application as a dish feed where it is desired to minimize 

feed blockage. 

To better understand the slotline antenna, we are developing techniques 

for its analysis. These techniques are similar to those used for the microstrip 

antenna described above. Basically we are employing a method of moments 

solution for the currents on the metal plates printed on the dielectric substrate. The technique is sufficiently general to be able to treat various 

tapers (i.e., linear, exponential, etc.). At present we have analyzed the 

scattering from a slotline antenna, and verified the results by comparison 

with a conventional surface patch method of moments solution for plates 

in free spice. 

The slotline antenna hs the potential for very wide bandwidth. In order 

to study the effects of various tapers on the bandwidth, it is necessary that 

the method of feeding the antenna does not limit its bandwidth. For this 

reason, our feed is a semi-infinite slotline. Currently we are developing 

methods for analyzing the slotline antenna with this very wideband feed. 

An experimental study has begun to understand and optimize the performance of slotline antennas. This study includes the slot,line antenna and 

its wideband feed. 

Using the results of [57], a wideband slotline feed has been designed 

and tested. The feed is a microstrip t,o slotline t,ransit3ion and has been 

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measured to have a VSWR less than 1.25 from 0.5 to 20 GHz. 

A slotline with a linear taper has been built and tested. The average 

VSWR is about 2.3. At present slotlines with more gradual tapers are 

being built in an effort, to reduce the VSWR. 

c. Publications 

1. D.P. Forrai, E.H. Newman, “Radiation and Scattering from Loaded 

Microstrip Antennas Over a Wide Bandwidth,” The Ohio State University ElectroScience Laboratory, Technical Report No. 719493-1, 

prepared under Grant NSG-1613 for NASA/Langley Research Center, Bampton, VA 23665, September 1988. 

The integral equation and moment method solution is developed for 

two different antennas in the presence of an infinite grounded dielectric substrate. The first antenna is a rectangular microstrip patch 

antenna. This antenna will be analyzed for excitation by an incident 

plane wave in free space and a vertical filament of uniform current in 

the dielectric. This antenna can be loaded by a lumped impedance 

in a vertical filament of uniform current extending from the patch 

through the dielectric to the ground plane. The radar cross section 

of the microstrip antenna is found from the plane wave excitation 

and shows good agreement to measurement for both an unloaded and 

loaded antenna. The input impedance is found from the current filament excitation. This is compared to the measured input impedance 

of a coaxially fed microstrip antenna and shows good agreement for 

both unloaded and loaded antennas when the dielectric substrate is 

much less than a wavelength. The second antenna is a vertical thin 

wire monopole extending from the ground plane into or through the 

dielectric substrate. The mutual impedance between two imbedded 

monopoles is compared to a previous calculation. 

2. D.P. Forrai, E.H. Newman, “A User’s Manual for the Loaded Microstrip Antenna Code (LMAC),” The Ohio State University ElectroScience Laboratory, Technical Report No. 719493-2, prepared under 

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Grant NSG-1613 for NASA/Langley Research Center, Hampton, VA 

23665, August 1988. 

This report describes the use of the Loaded Microstrip Antenna Code. 

The geometry of this antenna is shown and its dimensions are described in terms of the program inputs. The READ statements for 

the inputs are detailed and typical values are given where applicable. The inputs for four example problems are displayed with the 

corresponding output of the code given in the appendices. 

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References 

[l] F.A. Sikta, “UTD Analysis of Electromagnetic Scattering by Flat 

Plate Structures,” Report 711930-2, The Ohio State University ElectroScience Laboratory, prepared under Grant NSG-1613 for NASALangley Research Center, Hampton, Virginia, December 1981. 

[2] I.J. Gupta and W.D. Burnside, “Physical Optics Correction for 

Backscattering from Curved Surfaces,” IEEE Trans. on Antennas and 

Propagation, May 1987. 

[3] I.J. Gupta, C.W.I. Pistorius and W.D. Burnside, “An Efficient Method 

to Compute Spurius End Point Contributions in PO Solutions,” IEEE 

Trans. on Antennas and Propagation, Vol. AP-35, No. 12, December 

1987. 

[4] H. Shamansky, “The Determination of the Propagation Constant for 

the Travelling Wave in a Trough in an Infinite Ground Plane,” Master’s 

Thesis, The Ohio State University, Department, of Electrical Engineering, 1987. 

[5] Waturu Ebihara and R.J. Marhefka, “Bistatic Scattering from a Cone 

Frustum,” Report 716148- 16, The Ohio St ate University ElectroScience Laboratory, prepared under Grant NGS-1613 for NASA- Langley Research Center, Hampton, VA, August 1986. 

[6] J.L. Volakis, “Electromagnetic Scattering from Inlets and Plates 

Mounted on Arbitrary Smooth Surfaces,” Report 711930-5, The Ohio 

State University ElectroScience Laboratory, prepared under Grant 

NGS-1613 for NASA-Langley Research Center, Hampton, VA, August 

1982. 

[7] AX. Dominek, “A Uniform Electroiiiagnetic Reflection A nsatz for Surfaces with Small Radii of C)urvature,” P1i.D. Dissertation, Tlie Ohio 

State University, Department of Electrical Engitieeritig. 1984. 

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[8] A.K. Dominek, L. Peters, Jr., and W.D. Burnside, “A Reflection 

Ansatz for Surfaces with Electrically Small Radii of Curvature,” IEEE 

Trans. on Antennas and Propagation, Vol. AP-35, No. 6, June 1987. 

[9] R.L. Barger, “A Procedure for Designing Forebodies with Constraints 

on Cross-Section Shape and Axial Area Distribution,” NASA Technical Paper 1881, Langley Research Center, 1981. 

[lo] T.T. Chu, “First Order Uniform Theory of Diffraction (UTD) Analysis of the Scattering by Smooth Surfaces,” Final Report 711930-6, 

The Ohio State University ElectroScience Laboratory, prepared under 

GTant NGS-1613 for NASA-Langley Research Center, Hampton, VA, 

April 1983. 

[ll] R.H. Campbell, “Electromagnetic Backscatter from a Flat Plate in the 

Vicinity of a General Curved Surface,” Master’s Thesis, the Ohio State 

’ University, Department of Electrical Engineering, 1984. 

[12] C.W.I. Pistorius, “An Envelope Description of the Backscattered 

Power Patterns of Conducting Bodies,” Master’s Thesis, The Ohio 

State University, Department of Electrical Engineering, 1984. 

[13] E.H. Newman and R.L. Dilsavor,” A User’s Manual for the Electromagnetic Surface Patch Code: ESP Version 111,” Report 716148-19, 

The Ohio State University ElectroScience Laboratory, prepared under 

Grant NSG-1613 for NASA-Langley Research Center, May 1987. 

[14] E.H. Newman, P. Alexandropoulos and E.K. Walton, “Polygonal Plate 

Modeling of Realistic Structures,” IEEE Trans. on Antennas and 

Propagation, Vol. AP-32, No. 7, July 1984. 

[15] C.L. Bennett and R. Hieronymus, “Space-Time Integral Equation Approach to Solution of Plane Symmetric Scat teriiig Prol)leiiis,” Report 

SCRC-CR-75-9, Sperry Research Center, May 197.5. 

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[16] R. Whitacre, “The OSU Pulse/CW Radar for Compact Range Radar 

Cross Section Measurements,” M.Sc. Thesis, The Ohio State University, Department of Electrical Engineering, August 1985. 

[17] D.N. Jones, W.D. Burnside, “A Very Wide Frequency Band Pulsed/IF 

Radar System,” Report 716148-28, The Ohio State University ElectroScience Laboratory, prepared under Grant NGS-1613 for NASALangley Research Center, Hampton, VA, March 1988. 

[18] W.D. Burnside, P. Bohley and D. Jones, “A Broadband, Inexpensive, 

Ka-band Pulsed Radar System,” AMTA Symposium Digest, Ottawa, 

Canada, September 23-25, 1986. 

[19] W.D. Burnside, M.C. Gilreath, B.M. Kent and G.L. Clerici, “Curved 

Edge Modification of Compact Range Reflector,” IEEE Trans. on Antennas and Propagation, Vol. AP-35, No. 2, February 1987. 

[20] C.W.I. Pistorius and W.D. Burnside, “The Design of Blended Rolled 

Edges for Compact Range Main Reflectors,” Fifth International IEE 

Conference on Antennas and Propagation, University of York, UK, 

March 30-April 2, 1987. 

[21] C.W.I. Pistorius and W.D. Burnside, “A Concave Edged Refledor 

with Blended Rolled Surface Terminations for Compact Range Applications,” AMTA Symposium Digest, Ott,awa, Canada, Septeiiiber 

23-25, 1986. 

[22] C.W.I. Pistorius, “New Main Reflector, Subreflector and Dual Chamber Concepts for Compact Range Applications,” Ph.D. Dissertation, 

The Ohio State University, December 1986. 

[23] I.J. Gupta and W.D. Burnside, “IJser Manual for Seini-(’ircttlar (’0111- 

pact Range Reflector Code,” Tech. Report 716148-18, The Ohio Si ate 

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University ElectroScience Laboratory, prepared under Grant NGS1613 for NASA-Langley Research Center, Hampton, VA, January 

1987. 

[24] W.D. Burnside, R.C. Rudduck, I.J. Gupta and J. Clerici, “Electromagnetic Performance Study of Scientific- Atlanta’s New Compact Range,” 

AMTA Symposium Digest, Ottawa, Canada, September 23-25, 1986. 

[25] S. Ellingson, “Curvature of Blended Rolled Edge Reflectors at the 

Shadow Boundary Contour,” Technical Report 716148-29, The Ohio 

State University ElectroScience Laboratory, prepared under Grant 

NSG-1613 for NASA Langley Research Center, Hanipton, VA, April 

1988. 

[26] A. Lai and W.D. Burnside, “Monostatic and Bistatic Scattering by 

Metal Ogival Target Support ,” AMTA Symposium Digest, Ottawa, 

I Canada, September 23-25, 1986. 

[27] D.J. Heedy, “An E-Plane Analysis of Aperture-Matched Horn Antennas Using the Moment Method and the Uniform Geometrical Theory 

of Diffractions,” Master’s Thesis, The Ohio State University, Department of Electrical Engineering, 1984. 

[28] B.T. DeWitt, “Analysis and Measurement of Electromagnetic Scattering by Pyramidal and Wedge Absorbers,” Ph.D. Dissertation, The 

Ohio State University, Department of Electrical Engineering, 1986. 

[29] A.K. Dominek, L. Peters, Jr. and W.D. Burnside, “A Time Domain 

Technique for Mechanism Extraction,’’ IEEE Trans. on Antenna3 and 

Propagation, Vol. AP-35, pp. 305-312, March 1987. 

[30] P.H. Pathak, W.D. Burnside and R.J. h,larheflm. “A TTiiifcmii GTD 

Analysis of the Diffraction of Electroiiiagnet,ic Waves by a Siiiootli 

Convex Susface,” IEEE Trans. on Antennas and Propagation, Vol. 

AP-28, pp. 631-642, September 1980. 

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[31] D.A. Ksienski, “A Method of Resolving Data into Two Maximally 

Smooth Components,” Proceedings of IEEE, Vol. 73, No. 1, January 

1985. 

[32] R.O. Schmidt, “Multiple Emitter Location and Signal Parameter Est,imation,” IEEE Trans. on Antennas and Propagation, Vol. AP-34, No. 

3, pp. 276-280, March 1986. 

[33] L. Peters, Jr., R.T. Swann and W.T. Hodges, “An Experimental Investigation of Electromagnetic Scattering Properties of Flat Composite 

Structures,” Tech. Report 716148-2, The Ohio State University Electroscience Laboratory, prepared under Grant NGS- 1613 for NASALangley Research Center, Hampton, VA, October 1984. (Classified RePort) 

[34] L. Peters, Jr., A.K. Dominek, R. Swann, W.T. Hodges and S. Wozniak, “Edge Wave Control Using Absorber on Composite Material,’’ 

Report 716148-15, The Ohio State University ElectroScience Laboratory, prepared under Grant NGS-1613 for NASA-Langley Research 

Center, Hampton, VA, July 1986. (Classified Report) 

[35] E.H. Newman, R.J. Marhefka and L. Peters, Jr., “Scattering from 

Nearly Flat Surfaces or Nearly Straight Edges,” (‘Report 716148-17, 

The Ohio State University ElectxoScience Laboratory, prepared under 

Grant NG S- 161 3 for NASA- Langley Research Center, Hampton, VA, 

October 1986. (Classified Report) 

[36] T. Hodges, (NASA/Langley Res. Ctr., Hampton, VA), E. Newman, L. 

Peters, Jr., (OSU-ElectroScience Lab., Cols., OH), “Preliminary Version of an Electromagnetic Model for Graphite Composites,” Technical 

Report 716148-25, The Ohio State TTniversity FlectroScience Lal>oratory, prepared under Ci r an t N S C: - 1 G 1 3 for N A S A - La 11 g l e)- H esea I. cli 

Center, Hanipton, VA, November 1987. 

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[37] A.K. Dominek, L. Peters, Jr., “Edge Wave Control on ThreeDimensional Fins,” Technical Report 716148-27, The Ohio State University ElectroScience Laboratory, prepared under Grant NSG-1613 

for NASA-Langley Research Center, Hampton, VA, September 1987. 

[38] A.K. Dominek, H.T. Shamansky, R. Wood and R. Barger, “A Useful 

Test Body,” AMTA Symposium Digest, Ottawa, Canada, September 

23- 25, 1986. 

[39] A. Altintas, P.H. Pathak and W.D. Burnside, “Electromagnetic Scattering from a Class of Open-Ended Waveguide Discontinuities,’’ Technical Report 716148-9, The Ohio State University ElectroScience Laboratory, prepared under Grant NGS-1613 for NASA- Langley Research 

Center, Hampton, VA, March 1986. 

[40] A. Altintas, “Electromagnetic Scattering from a Class of Open- Ended ‘ Waveguide Discontinuities,” Ph.D. Dissertation, The Ohio State University, Department of Electrical Engineering, 1986. 

[41] R. J.’Burkholder, “High Frequency Backscatter Analysis of Electrically 

Large Inlets,” Master’s Thesis, The Ohio State University, Department 

of Electrical Engineering, 1985. 

[42] E.H. Newman, “Strip Antennas in a Dielectric Slab,” IEEE Trans. on 

Antennas and Propagation, Vol. AP-26, pp. 746-653, September 1978. 

[43] E.H. Newman and P. Tulyathan, “Analysis of Microstrip Antennas 

Using Moment Methods,” IEEE Trans. on Antennas and Propagation, 

Vol. AP-29, pp. 47-54, January 1981. 

[44] E.H. Newman, J.H. Richmond and B.W. Kwan, “RiIutual Iiiiyedaiice 

Computation Between Microstrip Anteiiiias,” IEEE Trans. Mic~o~uau~ 

Theory and Tech., Vol. MTT-31, pp. 941-945, Noveiiiber 1983. 

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[45] E.H. Newman and J.E. Tehan, (‘Analysis of a Microstrip Array and 

its Feed .Net,work,” IEEE Trans. on Antennas and Propagation, Vol. 

AP-33, pp. 397-403, April 1985. 

[4G] B.W. Kwan, “Mutual Coupling Analysis for Conformal Microstrip 

Antennas,” Ph.D. Dissertation, The Ohio State University ElectroScience Laboratory, Dept. of Electrical Engineering, Cols., OH, December 1984. 

[47] E.H. Newman and F. Forrai, “Scattering from a Microstrip Patch,” 

IEEE Trans. on Antennas and Propagation, Vol. AP-35, March 1987. 

[48] E.H. Newman, and D.M. Pozar, ‘(Electromagnetic Modeling of Coniposite Wire and Surface Geometries,” IEEE Trans. on Antennas and 

Propagation, Vol. AP-26, pp. 784-789, November 1978. 

[49] E.H. Newman and D.M. Pozar, ‘(Considerations for Efficient 

WireISurface Modeling,” IEEE Trans. on Antennas and Propagation, 

Vol. AP-28, pp. 121-125, January 1980. 

[50] D.M. Pozar and E.H. Newman, (‘Analysis of a Monopole Mounted 

at or Near the Edge of a Wedge,” IEEE Trans. on Antennas and 

Propagation, Vol. AP-29, pp. 488-495, May 1981. 

[51] D.M. Pozar and E.H. Newman, “Analysis of a Monopole Mounted Near 

an Edge or a Vertex,” IEEE Trans. on Antennas and Propagation, Vol. 

AP-30, pp. 401-408, May 1982. 

[52] M.C. Liang, P.H. Pathak and C.W. Chuang, “An Efficient Hybrid 

GTD- Modal Analysis of the Scattering by an Open-Ended Dielectric 

Filled Cavity in a Ground Plane,’’ Report No. 7166 11-2, The Ohio 

State University ElectroScience Laboratory, preparecl uuder Oont rad. 

F33615-84-K-1550, Air Force Systenis Chimand. Aeronautical Division, Wright-Patterson AFB, OH 4.5433, July 1987. 

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[53] I.J. Gupta, W.D. Burnside and C.W.I. Pistorius, “Design of Blended 

Rolled Edge for Compact Range Reflectors,” AMTA Symposium Proceedings, Seattle, WA, September 1987. 

[54] Kin-Yue Albert Lai and W.D. Burnside, “A GTD Analysis of Ogive 

Pedestal,” Technical Report 716148-8, The Ohio State University, 

ElectroScience Laboratory, prepared under Grant No. NSG-1613, for 

NASA-Langley Research Center, hampton, VA, March 1986. 

[55] Albert Lai and W.D. Burnside, “Optimal Mounting Methods in Cornpact Range Application,” Technical Report 719267-1, The Ohio State 

University ElectroScience Laboratory, prepared under Grant GU-0160 

for Boeing Aerospace Company, Seattle, WA, February 1987. 

[56] Inder J. Gupta, Albert K. Lai, Walter D. Burnside, “Scattering by 

Dielectric Straps with Potential Application as Target Support Structures,” submitted for publication to IEEE Trans. on Antennas and 

Propagation. 

[57] B. Schiek and J. Kohler, “An Improved Microstrip to Microslot Transition,” IEEE Trans. Microwave Th. and Tech., Vol. MTT-24, , pp. 

231-233, April 1976. 

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