DR. J.W. ROCKWAY & DOUGLAS W. DU BRUL

PERFORMANCE PREDICTION ANALYSIS FOR SHIPBOARD ANTENNA SYSTEMS

THE AUTHORS

Dr. John W. Rockway was born in Tacoma, Washington, on 25 March 1944. He received his BS and MS degrees from Washington State University. q l lman. Washington. in 1966 and 1968, respectively, and he completed his PhD degree in Engineering Science at same University in 1971. Currently, he is employed in the RF Communications Division of the Naval Ocean Systems Center, San Diego. Calijbrnia. and his present interests include the computer simulation and development of antenna systems. Dr. Rockway is a member of Sigma Tau and Tau Beta Pi.

ABSTRACT

Dlrcwion coven the we of brau wale rhlp modehg md numerical mo&lhg u hutrumeats for pdicthg the performmce potential of rhipboud antennu o m the

lion8 of each are reviewed, .ad the radb of a recent corn- puiron study are presented In whlch both methodologlea were applied to the u m e rhlp ddgn mdyrlr problem.

2 to 30 MW&&Z h d . &rmt-&y @ l l l t i ~ md U m h -

Mr. Douglas W. Du B d received his BS degree in Electrical Engineeringfrom the University of Texas, Austin, Texas. in 1959. For the past seven years he has been employed in the RF Communications Division, Naval Ocean Systems Center. Previous employment includes five years with General Dynamics on Space Progmms and five years with Bechtel Corporation. Refinery and Chemical Plant Division. Mr. Du Brul is a member of the Institute of Electrical and Electronic Engineers.

INTRODUCTION

s INCE MODERN COMBATANT SHIPS are crucially de- pendent on electromagnetic (EM) systems, it is particu- larly important that the very best available means be employed for estimating the performance potential of candidate antenna arrangements on proposed topside configurations prior to design freeze of the basic super- structure. Because unavoidable antenna site compro- mise and intersystem interaction are rapidly becoming limiting factors on the number of EM systems which can be used simultaneously, antenna site requirements must be driving functions throughout the entire ship design process. These concerns are equally important during the planning of alterations to existing ships.

Given this design environment, it naturally follows that the available antenna performance prediction methodologies need to be constantly reviewed in an effort to improve the quality of the predictive capability

and to reduce the analysis time and cost. To these ends a study was recently conducted at the Naval Ocean Systems Center (NOSC), San Diego, California, to determine how the performance of the newer numerical modeling approaches compares with the older approach of making actual measurements on carefully scaled brass ship models. This paper outlines the capabilities and limitations of each approach and summarizes the findings of a comparison study in which both methods were applied to the same design task.

An overview of the design problem is included for the benefit of the reader who is interested in the subject but unfamiliar with its background. Also, a brief descrip- tion is included of the advantages and limitations of both the numerical and scale model measurement ap- proaches prior to discussing their relative usefulness.

Overview of the Design Problem

In the search for optimum antenna arrangements, the design engineer is constantly faced with two basic facts:

1) No rigorous algorithms can be developed for se- lecting antenna locations until all the factors in- fluencing the desirability of an antenna site have been identified and their influence on the system associated with the candidate antenna determined.

2) All factors influencing antenna site desirability cannot be determined until all antennas are located.

The fact that these two statements are incompatible indicates that it is not possible to develop rigorous methodologies for locating antennas. However, once the antennas have been tentatively located by considering the individual requirements of each, it is possible to analyze the complete antenna arrangement to deter- mine the net influence of the topside structural con- figuration and antenna arrangement on the potential performance of individual systems. By an iterative process, candidate topside configurations and antenna arrangements can be analyzed to determine their relative desirability. When properly executed this approach to topside design synthesis by means of iterative analysis can lead to near-optimum configura- tions. Such an approach requires reliable means for predicting the antenna performance degradation at- tributable to antenna siting factors.

Since nearly every system is assigned an antenna location which is less desirable than that which would be assigned if it were the only antenna on the ship, an

Naval Engineers Journal, October 1977 33

ANTENNA PERFORMANCE PREDICTION ANALYSIS ROCKWAY/DU BRUL - elemtd of system performance degradation due to antenna site compromise will likely occur. The degrada- tion will be definable in technical terms and will imply a reduction in the system performance value. To predict this form of performance degradation it is necessary to determine the radiation patterns and feedpoint im- pedance of each antenna for given siting conditions.

In a similar manner, each system may suffer per- formance degradation due to the existence of radiation components of shipboard origin in the unique electro- magnetic environment at the specific site of its antenna. This degradation will also be definable in technical terms and will imply a reduction in the performance value of the system. Analysis of this form of EM system degradation requires (among other things) estimates of antenna-to-antenna coupling.

The nature of the impact of topside configuration on antenna performance varies with the radio frequency at which a particular antenna operates. Frequencies from 2 MHz to 30 MHz are a special challenge because large elements of topside structure (even entire masts) may act as parasitic resonators and cause unacceptable alteration of the radiation patterns and/or feedpoint

impedances of antennas. For antennas operating at higher frequencies, radiation patterns are influenced primarily by the radiating characteristics of the basic antenna structure, height above water, and line-of-sight blockage by structure. Antenna-to-antenna coupling is influenced by the radiation patterns of the antennas involved and the physical separation distance between them.

ILLUMINATING ANTENNA ON ARCH

ANTENNA UNDER TEST

MODEL snip^^ TURNTABLE

GRWNOPLANE CAPACITIVELV COUPLED TO

ROTARY TURNTABLE

I 1 I RECORDER 1 1

ROCKWAYIDU BRUL ANTENNA PERFORMANCE PREDICTION ANALYSIS

ANTENNA UNDER TEST

MODEL SHIP

A

The speed with which antenna performance predic- tion analysis can be executed is very important. Topside design and antenna arrangement is an iterative process in which the quality of the final design may well depend on the number of iterative cycles that can be executed in a given time frame.

BRASS SCALE SHIP MODELING

The basic approach to scale modeling is to make a series of measurements on scale model antennas installed on scale ship models. Frequencies are selected so that the ratio of wavelength to model ship size is the same as would exist on a full-sized ship. With accurately scaled models the measured antenna param- eters will closely match those of antennas installed on full-sued ships.

Radiation patterns are obtained with the model on a turntable so that the azimuth angle can be varied while a radio link between a model antenna and a range facility antenna is monitored (Figure 1). The elevation of the illuminating facility antenna must be variable. At the NOSC Model Range this is accomplished by posi- tioning it along a track on the inner face of a curved tower having a constant radius relative to the center of the turntable (Figure 2).

Feedpoint impedance and antenna-to-antenna isola- tion is measured on a separate facility at NOSC. The model is placed on a specially prepared groundplane over an underground instrumentation mom (Figure 3). This arrangement avoids having equipment and per- sonnel in the vicinity of the antennas where their presence could degrade the accuracy of the measure-

Computational Application/Extrapolation

Basic Data of EmpiricaVCalculated Exact E-M anal@^

Approximate EM Solutions (Numerical Techniques)

.

Block #l

Computational Approach to Antenna Performance

Prediction

1

1

Block #5 . Solution of

Maxwell's Equations (Method of Moments)

Optical Methods, i.e. (GTD, PTD, etc.)

1 .

Fig-4. I d ~ ~ t i f h t h o f C o m p o t . t i o l u l ~ S - A ~ ~ P - - * AMlyEil3.

Naval Engineer8 Journal, October 1977 35

ANTENNA PERFORMANCE PREDICTION ANALYSIS ROCKWAYIDU BRUL

tribution on conducting surfaces, and those elements of interference (intermodulation products) involving the unique, often accidental electrical characteristics of full-scale ships which are related to non-linear im- pedances in the topside structure. Without these measurements, it is not possible to make complete HERO (Hazard to radiation-sensitive ordnance), RAD- HA2 (Radiation hazard to personnel), or RFI (Radio Frequency Interference) investigations.

Antenna parameters routinely measured on scaled ship models include far field radiation patterns, feed- point impedances, antenna-to-antenna coupling, and the attentuation of radiation by lattice type structures. In addition, scale ship models can be used to determine the line-of-sight blockage and/or percentage of sky coverage associated with candidate locations for scan- ning antennas.

Brass modeling is very reliable if the scale factor is not too high and the modeling is sufficiently accurate and detailed. Model construction time may be a matter of months for initial construction, but once a basic ship class model is in inventory it is possible to analyze rapidly many antenna arrangements.

NUMERICAL MODELING

A number of subdivisions exist within the general field of antenna-related numerical modeling as illus- trated in Figure 4. This discussion is limited to the “Method of Moments” subdivision (Block 6 of Figure 4).

Electromagnetic radiation problems can always be represented by an integral expression with an inhomo- geneous source term. However, until the advent of the high-speed digital computer, such representations were often academic. They could not readily be solved. The unifying concept in this mathematical treatment of radiation problems is known as the “Method of Moments. ”

The “Method of Moments” essentially involves a re- duction of the associated integral equation to a system of linear algebraic equations where unknowns are coefficients in some appropriate expansion of the current. The resulting matrix equation can then be solved for the current by a high-speed digital computer. The steps involved in applying the “Method of Moments” are:

1) Formulate thin-wire integro-differential equation. 2) Reduce the integro-differential equation to a sys-

tem of linear algebraic equations (“Method of Moments”).

3) Specify boundary conditions. 4) Solve the system of linear equations for current. 5) Use the current to solve for: Far field. Near field,

and Impedance.

Computer programs based on the “Method of Moments” have been developed [2] [3] [41 [S] . The POCKLINGTON INTEGRAL FORMULATION is used which is valid only for thin linear antennas.

The performance of an antenna located on a con- ducting body, such as a ship, depends on the currents induced on the whole structure. Thus, it is not sufficient merely to model the antenna; the adjacent conducting surfaces must also be modeled. Extension of thin wire modeling techniques to the representation of conducting surfaces involves use of wire-grid models of the surfaces involved [6]. Figure 5 illustrates a wire-grid represen- tation of the exterior surfaces of a PGG class ship. To test the validity of the wire-grid approach to

modeling of conducting surfaces, a simple experiment was performed at NOSC. Two models were constructed; one a simple box-lie structure, and the other a wire-grid equivalent of the tirst. Both were constructed to the same scale. Identical antennas were mounted at equivalent locations on each model and then identical measurements were made of several antenna perform- ance parameters on each model. There was good corre- lation between the two sets of data. Finally, a mathe- matical modeling routine was run based on the con- figuration of the wire-grid model. The computed data was in close agreement with that measured on the two models [6].

Capabilities and Limitations

In greatly simplified language, antenna numerical modeling accounts for both the physical configuration of an antenna and the structure on which it is mounted, and it yields data of interest to antenna engineers. These output data include feedpoint impedance, the strength of “near” and “far” eleccrOmagnetic fields, RF surface currents, and the degree of coupling between antennas. The output data are functions of frequency, physical location, and topside configuration.

The computer time required to solve a particular radiation problem is the primacy limiting factor in numerical modeling. Some problems require an in- ordinate amount of computer time and the cost may simply outweigh the advantages in such instances.

The amount of computer time required is dependent on the number of linear algebraic equations which must be solved. At the present time these equations are solved by some form of GUASSIAN Elimination. The cost

36 Naval Engineers Journal, October 1977

ROCKWAY/DU BRUL ANTENNA PERFORMANCE PREDICTION ANALYSIS

1 FABRICATE B R A S MODEL 1

of a GUMSUN Elimination solution increases as the cube of the number (n) of algebraic equations. The number (n) is dependent on the size of the conductlug structure in wavelengths and the desired accuracy of the electric current specification. For instance, radiation pattern calculations require a less accurate electric current solution than do impedance calculations. Thus, if accurate impedance calculations are desired, more algebraic equations must be used than if only accurate radiation pattern calculations are required. Finally, for each frequency of interest a separate system of algebraic equations must be solved for each antenna included in a particular investigation.

As another consideration, since it is desirable to limit the number of unknowns, the number of structural elements to be included in a given numerical modeling analysis must be limited. Those to be included are chosen in the order of decreasing importance. Except for very basic structures there exists the possibility that a number of smaller structural items and/or antennas will be omitted for cost reasons.

A Corsp~arsoo S ~ Y

PREPARE NUMERICAL MODEL INPUT DATA

In a recent study conducted by NOSC, both numerical and scale modeling analysis methodologies were applied to the same shii design analysis problem [9]. The problem involved analysis of proposed high frequency antenna arrangements on a PGG Class ship (Figure 6). The objective was to determine the present day relative capabilities of the two approaches. Comparison parameters included analysis time, cost, ability to respond to design changes, and accuracy of the data obtained.

Generally speak& the basic tools and skills had been developed prior to the study for both method- ologies so that the efforts expended on the PGG Project were routine except for the fact that the numerical modeling procedures were still being rdlned at the implementation detail level. For the b w s model approach, the analysis effort included the planning and fabrication of the model, measurement of the perform- ance of the antennas on the model, and analysis of the measured data. The numerical modeling effort included creation of a wire-grid representation of the ship, identi- fication of the individual wire segments, preparation of the data for entry into the computer, and analysis of the computer output data. A styrofoam ship model was made to aid in creating the wire grid representation of the ship (Figure 5). The wire sections were represented by colored yarn attached to the model surf.ce.

Time and Cost Considerations

several analysis procedure steps are executed in the same manner regardless of whether the brass or numerical modeling approach is selected, as illustrated in Figure 7. Costs associated with these steps have been omitted from the comparative cost data. Only those costs associated with the uniquely different steps in the analysis approaches are discussed (steps in the split paths of Figure 7).

RF SYSTEM DESIGN ANTENNA TYPE SELECTION

I 1

ANALVZE MEASURED DATA ANALYZE CALCULATED DATA

I I I

C Z I b PREPARE FINAL RE-T

Figure 7. Fuactiod Block Diagram of the Anteam @dam

Performance prediction data produced by the two approaches differ parametrically, as well as in volume, as shown in TABLE 1. Alternatively, an effort could have been made to compute comparative data only for those analysis procedures which produced identical types and volumes of data. However, it was felt that such data could not be as accurately determined, would imply equal capability for the two approaches, and would needlessly de-emphasize the advantages and limitations of each. Consequently, the reader must bear in mind that the capabilities and scope of the analysis, as well as the casts, vary for the two approaches.

There are other cost-related considerations. The cost of measuring the performance of an individual antenna on a brass scale model is not directly related to the complexity of the topside structure. The impact of the conducting elements of ship structure, equipment, and other antennas are accounted for simply by the manner in which their mere presence is dected in the measured data. In numerical modeling. cost and analysis complexity is directly related to the number of conducting elements to be accounted for in a given investigation.

37

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Naval Englnwn Journal, October 1971

ANTENNA PERFORMANCE PREDICTION ANALYSIS ROCKWAYIDU BRUL

NUMERICAL MODELING

BRASS SCALE SHIP MODE LING

Near Field Prediction Capability I Frequency Range 2 to 10 MHz

Not a Present Capability I A Present Capability I 2 to 25 MHz*

Far Field Calculations

Frequencica Measured 2,4,6,8 MHz 43Freqs aaotr2to25MHzband ,

5". 10". 15'. 20". 25" I 5". 15". 30' I Elevations Analyzed I

Antenna-to Antenna Coupling

216 for each antenna in each I plots obtained Polar radiation I candidate antenna arrangement I candidate antenna arrangement

15 for each antenna in each

Memured and plotted for all pairs of antennas for continuous sweep of frequency from 2 to 30 MHz.

Calculated for all pairs of antennas at 2,4,6,8 & 10 MHz

Measured and plotted for each antenna for a continuous sweep of frequency from 2 to 30 MHz.

Calculated for each antenna at 2.4.6 & 8 MHz Fedpoint Impedance

+For a scale factor of 1%

Another cost-related consideration is that once the design approach has been formulated, the model construction work and radiation measurements required under the brass modeling approach follow well- established routines at NOSC. No significant changes in the costs associated with these activities are anticipated in the near future. However, numerical modeling analysis work has not yet become truly routine, and the execution of the PGG task required the attention of highly trained personnel. It is anticipated that the cost of numerical modeling will decrease in the near future.

TABLES 2 and 3 contain man-hour cost summaries for those steps which were distinctly different. Again,

the reader is cautioned not to draw general conclusions from these data. As indicated, there were several non- typical aspects to the project.

Sensitivity to Design Change

Greater time and expense are involved when incor- porating major ship design changes into a brass model than a numerical model. The disparateness is significant if the modifications involve alterations to the overall hull size, the number and/or size of deckhouses, or alterations to primary mast structure. This point is well illustrated by an incident which occurred during the

SUMMARY OF COST DATA FOR TEE A " N A PERFORMANCE ANALYSIS - PGG CWSS SHIP USING BRASS SCALE MODEL SHIP APPROACE

TASK LABOR ( h 4 ~ HOURS) NON-LABOR EXPENSE (S)

Model Fabrication Impedance Measurement Antenna-to- Antenna Coupling Measurement Radiation Pattern Measurements Materials

TOTALS

38 Naval Engtneers Journal. October 1977

ROCKWAY/DU BRUL ANTENNA PERFORMANCE PREDICTION ANALYSIS

SUMMARY OF COST DATA USING NUMERICAL MODELING TECHNIQUES - PGG CLASS SHIP ANTENNA PERFORMANCE ANALYSIS

TASK LABOR (MAN HOURS) NON-LABOR EXPENSE (S)

Model Fabrication Fmt Model Second Model

Reparation of Computer Input Data and Actual Input to Computer

Computer Processing Costs Terminal Charges (data input) Computational charges:

Run No. 1 Run No. 2

Engineering Overview

Interpretation of Results Associated ADP Charges

Computer Rental during analysis period

TOTALS

2o 2o 66

8

- 40

execution of this study. A decision was made to change the length and width of the hull (in order to accommo- date larger propulsion equipment) after three weeks’ effort had been spent on fabrication of the brass model. The changes could not be incorporated into the first model, and so it was scrapped at a loss of approxi- mately 20% of the estimated total model fabrication cost and three weeks of schedule time. The same changes were incorporated into the numerical model in one week.

Generally speaking, the numerical model can be altered in less time and at a lower cost than the brass model if the changes involve major alterations to the hull dimensions. However, once a brass model has been completed, it is quicker and cheaper to analyze changes to an antenna arrangement on the brass model. Ap- parently the numerical modeling approach is most useful for supporting new ship design efforts where candidate design review time is too short for fabrication of brass models. But even this advantage for numerical modeling must be qualified because at the present state-of-the-art, numerical modeling is not practical for large ships due to its cost.

Accuracy

The accuracy of brass modeling has been demon- strated by comparison of model prediction data with measurements made on full-sized antennas [l]. Corre- lation is good as long as model fabrication is accurate and sufficiently detailed. The accuracy of numerical modeling (at NOSC) has been judged primarily by comparison with brass model measurements or meas- urements made on full scale antennas under controlled antenna range conditions.

In the case of several complex antenna structures it has been demonstrated that numerical modeling

40

388 471 - - 24

150 - 150

1199 - -

provides reliable engineering data on feedpoint im- pedance, coupling, “near” and “far” fields. In the case of the PGG, engineering decisions regarding the topside antenna arrangement were supported by both the mathematical calculations and the brass model meas- urements.

It is concluded that the accuracy of both modeling approaches is acceptable for the type of analysis work required in this task. This conclusion would not neces- sarily be valid for a larger ship or one containing a much larger number of small complex topside struc- tures (the PGG ship is approximately 180 feet long). For the larger and more complex ships, the brass model would likely have the greater accuracy.

CONCLUSIONS OF COMPARISON STUDY

Brass modeling techniques have been in use for a longer period of time than numerical modeling. The boundaries of its range of usefulness are fairly well known. On the other hand, antenna numerical model- ing capabilities are still being expanded and the ulti- mate potential is not known. Following are some tentative conclusions drawn from experience to date.

1) Brass modeling and numerical modeling are not necessarily two ways of doing the same thing. Frequency range, superstructure complexity and/ or limitations of measurement or computational capability may preclude consideration of one or the other of the two approaches for a given task.

2) For the numerical modeling approach used in this study there is a ratio of cost to ship size which becomes less favorable as ship size is increased.

3) Brass models have a higher initial cost but lower cost per unit of measurement data than numerical models.

Navel Englneen Journal, October 1977 39

ANTENNA PERFORMANCE PREDICTION ANALYSIS ROCKWAY/DU BRUL

NAVAL ARCHITECTS AND MARINE ENGINEERS

4) For analysis of the performance potential of an- tennas during preliminary design of new ships, numerical modeling is the only acceptable ap- proach at present. Inaccuracy caused by omission of structural detail will likely result in manageable redesign requirements later. The impact of major structural features will have been meaningfully scoped. Construction of brass models of every can- didate design under study is an unacceptable ap- proach because of the time and expense involved. Model fabrication time for the larger ships is about two months.

5) Brass modeling of operational ships and ships in the final stages of design is unquestionably justi- fied because it will accommodate any degree of superstructure detail without a corresponding in- crease in the cost or complexity of measuring the performance of individual antennas. Also, once a detailed brass model is in inventory, the model measurement approach is a quick, reliable, and accurate means of determining the impact of pro- posed alterations to the ship topside structure or antenna arrangement.

6) Numerical modeling is capable of yielding near- field data and current distribution on the ship structure. Brass modeling techniques presently used in routine analysis work do not have this capability.

7) Numerical modeling capability is reasonably port- able. The software is easily transported and can be utilized wherever properly trained personnel have access to a suitable computer. By comparison, most brass ship models are difficult to transport. Facilities for making model measurements are fewer in number than computer installations suitable for running numerical modeling pro- grams. Most model measurement facilities are ex- posed to the weather which can have severe im- pact on measurement schedules unless they are lo- cated in mild climates.

8) The cost of measuring the performance of an indi- vidual antenna on a brass scale model is not di- rectly related to the complexity of the topside structure. The impact of conductive elements of ship structure (i.e., externally mounted equipment and other antennas) is accounted for simply by the manner in which their mere presence is reflected in the measured data. In numerical modeling. cost and analysis Complexity is directly related to the number of conducting elements to be accounted for in a given investigation. However, in prelim- inary design, the impact of small superstructure details need not be considered.

An overview of the preceding conclusions indicates that numerical modeling and brass scale ship modeling are, at present, complementary rather than competitive methodologies. However, numerical modeling is in its infancy and it is expected to undergo considerable change in capability and c a t in the years ahead.

REFERENmS

[l] Jasick, Henry, Antenna Engineering Handbook, First Edition. New York, N Y McGraw-Hill Book Company, Inc.. 1%1 (pp. 2-51).

[2] Harrington. R.F.. Field Computation by Moment Meth- ods. New York. NY: The McMillan Co., 1%8.

[31 MB Associates, “Antenna Modeling Program. Supple- mentary Computer Program Manual (AMF+2),” MP-R- 7514, Office of Naval Research Contract No. N0014-71-C- 0817, January 1975.

[41 Richmond, J.H., “Computer Program for Thin-Wire Structures in a Homogeneous Conducting Medium,” NASA Technical Report 2902-12, Grant No. NGL 364Q8- 138, August 1973.

[S] Chao, H.H. and B.J. Strait, “Computer Rograms for Radiation and Scattering of Arbitrary Configurations of Bent Wires.” Syracuse University Scientific Report No. 7 on Contract F19628-68-C-0180, AFCRL-70, September 1970.

[6] Logan, J.C. and J.W. Rockway, “Computer Techniques for Shipboard Topside Antenna Modeling Below UHF,” NELC Technical Note 3284, January 1977.

[7] Hansen, L.S., “Measurement Techniques for Scale Modeling HF and UHF Shipboard Antennas.” NELC Technical Note 3283, January 1977.

[8] Mittra, R., Editor, Numerical and Asymptotic Techniques in Electnnnagnetics. New York, N Y Pergammon Rtss, 1973.

191 Du Brul. D.W. and J.W. Rockway, “Antenna Mathe- matical Modeling vs. Brass Scale Modeling.” NELC Technical Note 3137, March 1976.

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40 Naval Engineers Journal, October 1977