Computer modeling of tactical high frequency antennas

Calhoun: The NPS Institutional Archive

Theses and Dissertations Thesis and Dissertation Collection

1992-06

Computer modeling of tactical high frequency antennas

Gregory, Bobby G.

Monterey, California. Naval Postgraduate School

http://hdl.handle.net/10945/23585

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COMPUTER MODELING OF TACTICAL HIGH FREQUENCY ANTENNAS12. PERSONAL AUTHOR(S)

GREGORY. Bobby G. Jr.

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Master's Thesis13b. TIME COVERED

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the Department of Defense or the U.S. Government.17. COSATI CODES

HELD GROUP SUB-GROUP

18. SUBJECT TERMS (Continue on reverse if necessary and identify by block number)

NEC-3, PAT7, HT-20T, ELPA-302, Horizontal Dipole

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

The purpose of this thesis was to compare the performance of three tactical high frequency antennas to be used

as possible replacement for the Tactical Data Communications Central (TDCC) antennas. The antennas weremodeled using the Numerical Electromagnetics Code, Version 3 (NEC3), and the Eyring Low Profile andBuried Antenna Modeling Program (PAT7) for several different frequencies and ground conditions. Theperformance was evaluated by comparing gain at the desired takeoff angles, the voltage standing wave ratio ofeach antenna, and its omni-directional capability. The buried antenna models, the ELPA-302 and horizontal

dipole, were most effective when employed over poor ground conditions. The best performance under all

conditions tested was demonstrated by the HT-20T. Each of these antennas have tactical advantages anddisadvantages and can optimize communications under certain conditions. The selection of the best antenna is

situation dependent. An experimental test of these models is recommended to verify the modeling results.

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COMPUTER MODELING

OF TACTICAL HIGH FREQUENCY

ANTENNAS

by

Bobby G. Gregory, Jr.

Captain, United States Marine Corps

B.S.E.E., University of Colorado, 1985

Submitted in partial fulfillmentof the requirements for the degree of

MASTER OF SCIENCE IN ELECTRICAL ENGINEERING

from the

NAVAL POSTGRADUATE SCHOOLJune 1992

11

ABSTRACT

The purpose of this thesis was to compare the performance

of three tactical high frequency (HF) antennas to be used as

possible replacement for the antenna system of the Tactical

Data Communications Central (TDCC) . The antennas were modeled

using the Numerical Electromagnetics Code, Version 3 (NEC-3)

,

and the Eyring Low Profile and Buried Antenna Modeling Program

(PAT7) for several different frequencies and ground

conditions. The performance was evaluated by comparing gain

at the desired takeoff angles, the voltage standing wave ratio

(VSWR) of each antenna and its omni-directional capability.

The buried antenna models, the ELPA-3 02 and horizontal dipole,

were most effective when employed over poor ground conditions.

The best performance under all conditions tested was

demonstrated by the HT-20T. Each of these antennas have

tactical advantages and disadvantages and can optimize

communications under certain conditions. The selection of the

best antenna is situation dependent. An experimental test of

these models is recommended to verify the modeling results.

111

C.I \

TABLE OF CONTENTS

I. INTRODUCTION 1

A. OVERVIEW 1

B. BACKGROUND 1

II. PROBLEM DISCUSSION 3

A. PROBLEM STATEMENT 3

B. ANTENNAS 3

III. MODELING SOFTWARE 8

A. NUMERICAL ELECTROMAGNETICS CODE-3 8

1. STRUCTURE MODELING 9

2. WIRE MODELING 10

B. PAT7 PROGRAM 12

C. TAKEOFF ANGLE CALCULATION 14

IV. MODELING RESULTS 17

A. GENERAL 17

B. HT-20T MODEL 18

C. ELPA-3 02 MODEL* 3 3

D. HORIZONTAL DIPOLE MODEL 63

V. EXPERIMENTAL VERIFICATION 92

IV

kmwa, I™U* LIBRARY

asSWS5S"«VI. CONCLUSIONS 94

A. MODEL COMPARISON 94

B. RECOMMENDATIONS 94

APPENDIX A. SAMPLE NEC-3 DATA SET 96

APPENDIX B. SAMPLE PAT7 PROGRAM DATA SET 97

LIST OF REFERENCES 98

INITIAL DISTRIBUTION LIST 99

LIST OF FIGURES

Figure 1. The hub-spoke deployment of the TDCC. ... 4

Figure 2a. HT-2 0T Antenna 6

Figure 2b. ELPA-3 02 Antenna 6

Figure 2c. Horizontal Dipole Antenna 6

Figure 3. Skywave Transmission chart developed by Prof.

R.A. Helliwell, Stanford University, 1964 16

Figure 4. HT-2 0T Antenna 19

Figure 5. VSWR for HT-2 0T over good ground 2

Figure 6. Elevation pattern for HT-2 0T over good ground

for f=2, 4, and 8 MHz 21

Figure 7. Elevation pattern for HT-2 0T over good ground

for f=12, 20, and 30 MHz 22

Figure 8. Elevation pattern for HT-20T over fair ground

for f=2,4, and 8 MHz 23

Figure 9. Elevation pattern for HT-2 0T over fair ground

for f=12, 20, and 30 MHz 24

Figure 10. Elevation pattern for HT-20T over poor ground

for f=2, 4, and 8 MHz 25

Figure 11. Elevation pattern for HT-20T over poor ground

for f=12, 20, and 30 MHz 26

Figure 12. Azimuth pattern for HT-2 0T for f=2 MHz over

good ground, varying takeoff angle (theta) 27

VI

Figure 13. Azimuth pattern for HT-20T for f=4 MHz over

good ground, varying takeoff angle(theta) 28




good ground, varying takeoff angle (theta) . ... 30





Figure 18. ELPA-302 Antenna 34

Figure 19. VSWR for ELPA-3 02 over good ground, varying

heights 36

Figure 20. VSWR for ELPA-302 over poor ground, varying

heights 37

Figure 21. Elevation pattern for ELPA-3 02 for f=2 MHz

over good ground 38

Figure 22. Elevation pattern for ELPA-302 for f=2 MHz

over fair ground 3 9


over poor ground 40


over good ground 41


over fair ground 4 2

VII


over poor ground 4 3


over good ground 44


over fair ground 45


over poor ground 4 6


over good ground 47


over fair ground 48


over poor ground 49


over good ground 50


over fair ground 51


over poor ground 52


over good ground 53


over fair ground 54

Figure 38. Elevation pattern for ELPA-302 for f=30 Mhz

over poor ground 55

viii

Figure 39. Azimuth pattern for ELPA-3 02 for f=4 MHz,

h=0.5m, varying takeoff angle (theta) 56

Figure 40. Azimuth pattern for ELPA-302 for f=12 MHz,





constant takeoff angle (theta) , varying heights. . 59





Figure 45. Horizontal Dipole 64

Figure 46. VSWR for horizontal dipole for varying heights

over good ground 65

Figure 47. VSWR for horizontal dipole for varying heights

over poor ground 66

Figure 48. Elevation pattern for horizontal dipole for

f=2 MHz over good ground 67


f=2 MHz over fair ground 68


f=2 MHz over poor ground 69




ix










f=8 MHz over poor ground 7 5

















x



Figure 66. Azimuth pattern for horizontal dipole for f=4

MHz, h= 0.5m, varying takeoff angle (theta) . ... 85


MHz, h= 0.5m, varying takeoff angle (theta). ... 86


Mhz, h= 0.5m, varying takeoff angle (theta). ... 87


MHz, constant takeoff angle (theta), varying

heights 88



heights 89



heights 90

XI

I . INTRODUCTION

A. OVERVIEW

Computer modeling of wire antennas provides an inexpensive

and efficient means for predicting the performance of

antennas. There are several software packages available which

use various numerical methods to predict antenna

characteristics. The Numerical Electromagnetics Code, Version

3 - Method of Moments (NEC-3) uses the method of moments for

analysis of the electromagnetic response of antennas and other

metal structures. NEC-3 is capable of modeling antennas

located above ground and buried or near the earth. The Eyring

Low Profile and Buried Antenna Modeling Program (PAT7) is

based on equations developed in Reference 1 which also uses

the method of moments and field integral equation techniques.

PAT7 is used primarily to model near-earth or buried antennas.

Each program has its advantages and limitations for

applications based on antenna geometry and location.

B . BACKGROUND

The Marine Corps Tactical Systems Support Activity

(MCTSSA) at Camp Pendleton, California is exploring the

possibility of replacing an existing antenna system. Two

antennas have been proposed as possible replacements. MCTSSA

has requested that a computer model of both antennas be

generated so that a theoretical comparison of characteristics

and capabilities can be accomplished. This thesis discusses

the computer models used for the comparison, analyzes the

results of the models and proposes an additional model for

consideration.

II. PROBLEM DISCUSSION

A. PROBLEM STATEMENT

The antenna to be investigated is used for the Tactical

Data Communication Central (TDCC) which provides an HF radio

communications hub for the centralized computer processing of

data for the tactical command and control of Marine Air Wing

assets. Operational employment of the TDCC normally dictates

that the distant stations are within a 600 mile radius of the

hub as illustrated in Figure 1, so the antenna must be omni-

directional. The transmitters operate with one kilowatt of

power output. The current system uses an antenna which

requires a coupler that has a poor maintenance history. It is

desired that the new antenna not require a coupling device to

eliminate this problem. The antenna must also be field

expedient and require no more than two men to erect it. It

must be broadband so that trimming antenna dimensions for each

frequency is not required.

B. ANTENNAS

Two antennas which fit the criteria above were chosen for

further analysis. The HT-2 0T, designed by the Antenna

Products Corporation, and the ELPA-302, designed by the Eyring

Corporation, were decided upon by MCTSSA as antennas that met

the requirements above. The HT-20T is a back-to-back sloping,

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Figure 1. The hub-spoke deployment of the TDCC,

horizontal dipole configuration as shown in Figure 2a. The

ELPA-3 02 is a buried or near-earth horizontal dipole array

antenna as shown in Figure 2b.

Additionally, a simple horizontal dipole antenna (Figure

2c) was proposed as a result of this thesis which would

satisfy these requirements as well as requirements for manpack

radios in the Marine Corps inventory. The reason a

horizontal dipole was chosen is that it is a common, field

expedient antenna that is familiar to Marine Corps

communicators. Its advantages are that it can be erected

above ground, as it is now most commonly used, or it can be

buried or employed near earth, which has not been a common

practice, but is practical. The horizontal dipole is ideally

suited for Marine Corps type missions since buried or near

earth antennas are more easily concealed, easy to employ, and

lightweight.

The length of the horizontal dipole was chosen after

considering test results from Eyring, Inc. on the ELPA-3 02

antenna design. These results showed that when operating at

the lowest desirable frequency, 2 MHz, the electrical length

of a 91.5 meter buried antenna is approximately 0.7 5

wavelengths. Above ground, this antenna would be 0.6

wavelengths long. The difference in length is due to the

conductivity and permittivity of the soil conditions, which

are frequency dependent, that change the electrical length of

the buried antenna. The desired power gain for an antenna in

Figure 2a. HT-20T Antenna

Q1 Sm -^ y

6.2m 1 O y^_Jz

Figure 2b. ELPA-302 Antenna.

Figure 2c. Horizontal Dipole Antenna.

contact with the soil occurs at an electrical length of one

wavelength. As the operating frequency of the antenna

increases and additional wavelengths are formed on the

element, a higher antenna gain results and produces a higher

signal-to-noise ratio. Considering this information and the

practical aspects of carrying and burying an antenna, a length

of 91.5 meters was chosen as optimum. The input impedance of

a buried antenna will be discussed later. [Ref. 2]

Comparisons between normalized radiation power output over

good, fair and poorly conducting ground conditions at several

heights of employment, required takeoff angles for propagation

paths, and a discussion of the results and recommendations are

discussed in this thesis.

III. MODELING SOFTWARE

A. NUMERICAL ELECTROMAGNETICS CODE-

3

The Numerical Electromagnetics Code-Version 3 (NEC-3) was

developed by the Lawerence Livermore National Laboratory

located in Livermore, California. NEC-3 is a user-oriented

computer code for the analysis of the electromagnetic response

of antennas and other metal structures using the numerical

solution of integral equations for the currents induced on the

structure by sources or incident fields [Ref. 3]. This

approach avoids many of the simplifying assumptions required

by other solution methods and provides a highly accurate and

versatile tool for electromagnetic field analysis.

The code combines an integral equation for smooth surfaces

to provide convenient and accurate modeling of a wide range of

structures. A model may include nonradiating networks and

transmission lines connecting parts of the structure, perfect

or imperfect conductors, and lumped element loading. A

structure may also be modeled over a ground plane that may be

either a perfect or imperfect conductor. NEC-3 also has the

capability to model antennas with elements that touch or are

buried in the ground using the Sommerfeld method.

The excitation may be either voltage sources on the

structures or an incident plane wave of various polarizations.

The output may include induced currents and charges , near

electric or magnetic fields, and radiated fields. Hence, the

program is suited to either antenna analysis or scattering and

Electro-Magnetic Pulse (EMP) studies.

The integral equation approach is best suited to

structures with dimensions up to several wavelengths.

Although there is no theoretical size limit, the numerical

solution requires a matrix equation as the structure size is

increased relative to one wavelength. Hence, the modeling of

very large structures may require more computer time and file

storage than is practical on a specific machine. In such

cases, standard high-frequency approximations such as

geometrical optics, physical optics, or the geometrical theory

of diffraction may be more suitable than the integral equation

approach used in NEC.

1. STRUCTURE MODELING

The basic elements for modeling structures in NEC are

short straight segments for thin wires and flat patches for

surfaces. An antenna and any conducting object in its

vicinity that affect its performance must be modeled with

strings of segments following the paths of wires and with

patches covering closed surfaces. Proper choice of the

segments and patches for a model is the most critical step in

obtaining accurate results. In this thesis, thin wire

modeling by NEC is used.

2. WIRE MODELING

A wire segment is defined by the coordinates of its two

end points and its radius. Modeling a wire structure with

segments involves both geometrical and electrical factors.

Geometrically, the segments should follow the paths of

conductors as closely as possible, using a piecewise linear

fit on curves. The following are electrical considerations

for wire segment modeling:

The segment length A relative to the wavelength X:

- Should be less than about O.lA in order to getaccurate results in most cases.

- May be somewhat longer than O.lA, on long wires with noabrupt changes while a shorter segment, 0.05A. or less,may be needed in modeling critical regions of theantenna, such as feed points and connections ofmultiple segments.

- For A less than O.OOlA. should be avoided since thesimilarity of the constant and cosine components ofthe current expansion can lead to numerical inaccuracy.

- The wire radius, r, relative to A. is limited by theapproximation used in the kernel of the electric fieldintegration equation. The segment radius a relative toboth segment length A and wavelength X requires that:

- a should be less than O.lA.

- a should be less than 0.125A.

- a can be less than 0.5A, but this requires theextended thin-wire kernel option by placing the EKcard in the input data set. The limits on an EK cardare straight-line segments and no multiple connectionpoints.

- Connected segments must have identical coordinates forconnected ends. NEC-3 connects two end segmentsif the separation between the end of the segments isless than 0.001 times the length of the shortestsegment.

10

Segment intersections other than at their ends do notallow currents to flow from one segment to another.

Large wire radius changes should be avoidedparticularly for adjacent segments. If the segment hasa large radius, then sharp bends should be avoided aswell.

When modeling a solid structure with a wire grid, alarge number of segments should be used.

- A segment is needed at the point where a networkconnection , a voltage or a current source, is going tobe located. Current sources are limited to isolated,very short constant current sources located at theorigin, (i.e., excitation of tiny constant currentdipole)

.

Base fed wires connected to ground should be vertical.

- The segments on either side of the excitation sourceshould be parallel and have the same length and radii.

Parallel wires should be several radii apart.

- Before modeling a structure , the limit of the numberof segments and the number of connection points shouldbe checked for the particular version of NEC.

Once the antenna geometry and ground plane are specified,

the type of excitation must be declared. The excitation can

be either a voltage source or an incident plane wave of

various polarizations.

The wire radius, a, relative to A, is limited by

approximations used in the kernel of the electric field

integral equation. The thin-wire kernel and the extended-wire

kernel approximations are used to approximate the current

distribution for each wire segment. From these current

distributions, far-field radiation patterns are calculated.

11

NEC-3 requires that for the thin-wire kernel

approximation, the segment length to wire radius ratio, A/a,

must be greater than 8 for errors less than 1% and A/a for the

extended-wire kernel approximation must be greater than 2 for

the same accuracy. Segments with bends in the wire should

avoid using small values for A/a. The angle of intersection

of wires is not restricted in any manner. Segments cannot

overlap since the division of current between two overlapping

segments is indeterminate. A large radius change between

connected segments should be avoided since this will introduce

errors in calculations. A segment is required at each point

where a network connection or voltage source will be located.

B. PAT7 PROGRAM

The PAT7 program is specifically designed to model the

performance of buried or near earth antennas and traveling-

wave antennas in the 1 kHz to 100 MHz frequency range. The

model is based on the theory developed in Reference 1 which

uses the formal solution of Maxwell's equations to calculate

the electromagnetic fields and associated distributions of

current and charge in bounded regions. The electrical

properties and characteristics of the material media in which

the antenna is located and its effects on fields and currents

are the focus of the analysis. [Ref. 4]

PAT7 tabulates and plots both pattern and feedpoint

characteristics for models in real earth environments. Ground

12

conductivity and dielectric constants as a function of

frequency are used to characterize the burial or underlying

environment of the antenna. Radiation patterns are described

as a function of elevation, azimuth, and polarization.

Groundwave patterns are described in terms of field strength

at a specific far field distance over real ground conditions.

PAT7 possesses an NxM multi-element array modeling

capability. The assumed geometry of the model consists of

from 1 to N parallel, linear elements arranged in a subarray.

From 1 to M of these subarrays can be combined in-line to form

an array.

The program very accurately describes the performance of

real antennas when the input model uses measured data in

describing the various input parameters used in making

calculations. Buried antennas are most effective when they are

insulated from their environment. The following are electrical

considerations, characteristics, and capabilities the user

must be concerned with when modeling with PAT7:

- Desired elevation and azimuth angle of view, in degrees,to obtain desired antenna radiation pattern.

- Transmit frequency

- Soil conductivity and permittivity

- Antenna wire insulation conductivity and permittivity.The complex permittivity of the wire insulation shouldbe 1.2 times less than that of the surrounding ground.

- Antenna element length and height (negative values areused for buried antennas) Height must be less thanO.lX and cannot be zero because PAT7 does not calculateelements touching the air-ground interface.

13

- Wire radius must be small compared to antenna length.(Same conditions as NEC-2)

- Conductor radius and insulation radius. (The insulationradius must obviously be larger than the conductorradius.

)

- Number and spacing of elements used in array modeling aswell as number of and distance between subarrays.Subarrays cannot overlap.

- Main beam steering. (The user can steer the main beam ina desired direction. This is done in practice bycontrolling the feed signal and using phase sensitivebaluns.

)

- The actual impedance of the feed element may be used,otherwise a matched, perfect feed element is assumed.

- The desired wave type (ground or skywave) to becomputed.

- E-field polarization (vertical, horizontal or mixed)

.

- Designation whether element is end or center fed.

- Antenna may be either terminated by a user given loadimpedance or open termination.

A graph of the antenna radiation pattern is then produced. A

sample input screen is shown in Appendix B.

C. TAKEOFF ANGLE CALCULATION

The takeoff angle for High Frequency communications is an

important characteristic in antenna design. In order to

obtain the desired signal strength at the receive station, the

maximum power radiated at a specific azimuth and elevation, or

takeoff angle, must be calculated so that the signal

reflected off the ionosphere to the receive station is

optimal. The takeoff angle for the problem discussed in this

14

thesis was determined using Reference 5 and the skywave

transmission chart as shown in Figure 3. The takeoff angle

was calculated for two ranges, 300 and 600 miles. A one hop

path is considered optimum since signal attenuation and path

losses are minimized. For daytime communications, a one hop

propagation path was used with a reflection off the ' E' layer

at an altitude of 110 km. For nighttime communications, a one

hop path reflected off the F2layer at an altitude of 3 00 km

was used. Using Figure 3 and the conditions above, for

daytime communications, a one hop path requires a takeoff

angle of 39 and 20 degrees for the ranges of 300 and 600 miles

respectively. For nighttime communications, again using a one

hop path, the takeoff angles are determined to be 49 and 28

degrees for the ranges of 300 and 600 miles respectively.

These takeoff angles will be used to compare the results of

the computer models. [Ref. 5]

15

Figure 3. Skywave Transmission chart developed by Prof,R.A. Helliwell, Stanford University, 1964.

16

IV. MODELING RESULTS

A. GENERAL

In order to make an accurate comparison of the antennas,

some common conditions were imposed. Each antenna was modeled

over the three types of grounds with different conductivities,

a, and relative permittivities, er

. The values used for the

various ground conditions were: for "good" ground, a = 0.01

S/m and er= 30; for "fair" ground, a = 0.005 S/m and e

r= 12;

and for "poor" ground, a= 0.001 S/m and er= 6 [Ref. 1] . The

HT-20T was modeled at the fixed height and dimensions as

described in Reference 6. The ELPA-302 and horizontal dipole

were modeled for dimensions given in Reference 2 at four

different heights to allow for comparisons of non-traditional

employment. Radiation patterns were computed for each of the

antennas under the various height and ground conditions.

Antenna performance was analyzed using the maximum gain

achieved at the desired takeoff angles and direction, and each

antenna's voltage standing wave ratio (VSWR) over the 2 to 30

MHz frequency spectrum. The transmission line impedance was

taken as 50 ohms, a standard value for most radio frequency

(RF) applications. The frequencies chosen to make comparisons

were 2, 4, 8, 12, 20 and 3 MHz. These frequencies were

chosen after analysis showed that these frequencies most

17

accurately represented the antenna's characteristics over the

frequency spectrum.

B. HT-2 0T MODEL

The HT-20T is a omni-directional , "sloping vee-like"

,

orthogonal dipole antenna designed primarily for High

Frequency (HF) skywave propagation. It consists of two dipole

antennas mounted mutually perpendicular in azimuth as

illustrated in Figure 4. Each element is terminated with a

4 00 ohm resistor. The center of the antenna is 27 feet above

the ground.

The VSWR varies between 1.02 and 1.78 over good ground

conditions as illustrated in Figure 5. The VSWR of fair and

poor ground is also within these limits but not illustrated

because the change in ground conditions had little effect on

VSWR in this case. The antenna radiation patterns are

illustrated in Figures 6 through 17. Each figure shows the

pattern for a fixed ground condition for frequencies under

consideration. The antenna is oriented for transmission of

the main beam along the positive and negative x-axis. The

reference plane from which plots were taken was from the xz-

plane and azimuthal plane through the origin. From

examination of these antenna radiation patterns, it is evident

that the HT-2 0T is omni-directional and demonstrates good

skywave propagation gains at 39 and 49 degree takeoff angles.

The radiation patterns show a maximum gain of -2 db at 20 Mhz

18

Figure 4. HT-20T Antenna.

19

Figure 5. VSWR for HT-2 0T over good ground.

20

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21

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22

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Figure 8. Elevation pattern for HT-20T over fair ground forf=2,4, and 8 MHz.

23

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24

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Figure 10. Elevation pattern for HT-20T over poor groundfor f=2 , 4, and 8 MHz.

25

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26

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32

over good ground.

A change in ground conditions does have some effect on the

radiation pattern. As the ground conditions vary from good to

poor, peak gain varies 2 to 3 dB at takeoff angles greater

than 30 degrees, depending upon frequency. For takeoff angles

below 3 degrees, however, there is virtually no change.

These results are expected because, although the ground has

some effect, the antenna as designed is high enough to

minimize ground interactions. The antenna radiation patterns

also match those predicted in Reference 6, which validates

this model.

C. ELPA-302 MODEL

The ELPA-3 02 antenna is a broadband HF skywave antenna and

is illustrated in Figure 18. It does not require tuning

either electronically or by trimming the antenna length. The

antenna is fed by an Element Feed Unit (EFU) which acts as a

balun and phase controller. By using the EFU, the main

antenna beam can be steered in elevation and azimuth. The

ELPA-302 can also be terminated so reflections on the

radiating elements are reduced or eliminated. The ELPA-302

was modeled in PAT7 using a matched termination to minimize

reflections on the antenna and an elevation steering at 34

degrees, the median takeoff angle, to achieve the best

possible results. [Ref. 2]

The VSWR for the ELPA-3 02 was calculated for good and poor

33

Figure 18. ELPA-302 Antenna

34

ground conditions at each height as illustrated in Figures 19

and 20. For good ground, the VSWR is somewhat higher for

heights above ground. For poor ground, the VSWR is

significantly lower for heights below ground. This can be

explained by examining the electrical characteristics of the

various types of ground. Ground conductivity and permittivity

are functions of frequency. The electrical characteristics of

poor ground effectively reduce the input impedance of the

antenna to a value closer to the 50 ohm transmission line

impedance. This results in a reduction in VSWR for poor

ground. Good ground does not have this effect because the

conductivity and permittivity are higher and do not

significantly help the antenna to match the transmission line

impedance. Although the VSWR is reduced for the antenna

buried in poor ground, the negative consequences are that the

gain is also reduced by burying the antenna. [Ref. 3]

The antenna radiation patterns for the ELPA-302 are

illustrated in Figures 21 through 44. Each figure shows the

pattern for a fixed ground condition for the heights of 0.5

meters, 0.006 meters, -0.5 meters, and -1.0 meters. These

heights were chosen because they are the most practical when

using the ELPA-302 for the TDCC mission. The patterns are

oriented with the main beam along the positive and negative

x-axis. The reference plane from which plots were taken was

from the xz-plane through the origin. The radiation patterns

35

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36

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38

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39

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59

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60

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61

show a maximum gain of -16 dB at 2 Mhz for h= 0.5 meters over

poor ground. The patterns illustrate that changes in height

and ground condition have a great effect on antenna gain.

For all ground conditions, a height of 0.5 meters provides

optimal gain at the desired takeoff angles while the buried

antennas have approximately 12 dB less gain. The buried

antennas, however, have an increase in gain as ground

conditions degrade. One explanation for this can be related

to image theory. As the ground becomes more lossy, the

distance between the image plane and the antenna

increases. This allows for more constructive addition of the

transmitted and reflected wave. If the image plane was exactly

A/4 distance from the antenna, then constructive interference

would be at a maximum. In poor ground, the image plane to

antenna distance is closer to A/4 than in good ground. [Ref . 1]

A height of 0.5 meters provides optimal gain at the desired

takeoff angles.

Azimuth patterns for the ELPA-302, shown in Figures 35

through 41, were modeled in two ways. First, the height was

kept constant at 0.5 m (the height which provided the highest

gain) and the takeoff angle was varied to illustrate the

change in azimuthal gain versus takeoff angle. Next, the

takeoff angle was held constant at 34 degrees, a median value,

and the height above ground was varied to illustrate the

change in azimuthal gain versus height.

The VSWR and various radiation patterns agree with those

62

in Reference 2, which lends validity to the results obtained

here.

D. HORIZONTAL DIPOLE MODEL

The horizontal dipole is an omni-directional antenna

designed primarily for HF skywave propagation. It consists of

two wires of equal length connected via a balun to a

transmission line as illustrated in Figure 45. The horizontal

dipole modeled is not terminated because, in some tactical

environments, trimming and terminating antennas is not always

practical. The overall characteristics of the horizontal

dipole are similar to those of the ELPA-302, but with some

exceptions that will be discussed later. PAT7 was used to

model this antenna.

The VSWR for the horizontal dipole was taken for good and

poor ground conditions at each height and is illustrated in

Figures 4 6 and 47. For good ground, the VSWR is not

significantly affected by changes in height. For poor ground,

the VSWR is relatively constant for heights above ground but

significantly lower for heights below ground. The comments on

the ELPA-302 also apply to the horizontal dipole with respect

to electrical effects of the ground.

Antenna radiation patterns for the horizontal dipole are

illustrated in Figures 4 8 through 71. Each figure shows the

pattern for fixed ground conditions for heights of 0.5 meters,

0.006 meters, -0.5 meters, and -1.0 meters. The patterns are

63

Figure 45. Horizontal Dipole

64

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Figure 66. Azimuth pattern for horizontal dipole for f=4MHz, h= 0.5m, varying takeoff angle (theta)

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Figure 68. Azimuth pattern for horizontal dipole for f=30Mhz, h= 0.5m, varying takeoff angle (theta)

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Figure 69. Azimuth pattern for horizontal dipole for f=4MHz, constant takeoff angle (theta) , varying heights.

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90

oriented for transmission of the main beam along the positive

and negative x-axis. The reference plane was from the xz-plane

through the origin. The radiation patterns show a maximum

gain of -5 dB at 2 MHz for h= 0.5 meters over poor ground.

The patterns illustrate that change in height and ground

conditions have a great effect on the radiation pattern. As

ground conditions vary from good to poor, the peak gain

actually increases a maximum of approximately 7 dB at 2 MHz.

When the antenna is buried, there is an increase in peak

gain as ground conditions degrade. This can be explained

using image theory as discussed for the ELPA-3 02. Also, as in

the case of the ELPA-302, the height at 0.5 meters provides

optimal gain at the desired takeoff angles for all ground

conditions. The buried antenna's gain in the direction of the

takeoff angle is 15 dB less than the gain of the antenna above

ground.

Azimuth patterns for the horizontal dipole are shown in

Figures 55 through 65 and were modeled to illustrate the

change in azimuthal gain versus takeoff angle and the change

in azimuthal gain versus height.

The results the various radiation patterns obtained are

similar in shape but not in gain to what would be expected of

a dipole in free space. This allows a rough check of the

model used here and suggests the results are within reason.

91

V. EXPERIMENTAL VERIFICATION

The results of this thesis should be verified by

experiment before a final decision to procure a specific

antenna is made. A proposed experiment is included for

continuation of this thesis by another party. The experiment

should contain five sites in the hub-spoke configuration

discussed earlier to test the performance of the antennas.

Each of the three antennas would be operated at the hub site

with the four spoke sites located from 300 to 600 miles

incrementally away in radius from the hub and 90 degrees

apart. This will test the omni-directional capabilities and

the short to medium skywave range of the antennas. Ground

constants at the sites should be measured accurately and

compared with model results. The antennas should operate for

24 hours per day to test frequency response for day and night

communications. The test should continue from several days to

a week to ensure that a fairly normal ionosphere activity

period was used. The VSWR for each antenna should be measured

over the entire spectrum. The antenna elevation and azimuth

radiation patterns should be measured via a helicopter

flyover.

The importance of accurate measurements under the

identical conditions used for the models is paramount for a

92

valid comparison of results.

93

VI. CONCLUSIONS

A. MODEL COMPARISON

A direct comparison between the models discussed is

dependent upon the user's criteria. All of the antennas

demonstrate broadband characteristics and omni-directional

azimuthal capability. The HT-20T has the highest gain at each

of the required takeoff angles and the lowest VSWR when

compared to the ELPA-302 and horizontal dipole. The ELPA-302

and horizontal dipole provide adequate gain at the required

takeoff angles at heights above ground and for buried antennas

under certain conditions. The VSWR for the ELPA-3 02 is

practical for use with transmitters that require a VSWR of 2.5

or less. The VSWR for the horizontal dipole is too high for

most frequencies below 7.0 MHz. The ELPA-302 and horizontal

dipole are much easier to construct and erect than the HT-2 0T,

and provide a much less visible battlefield target.

B. RECOMMENDATIONS

From the results of this thesis, the HT-20T antenna is

recommended for use whenever there is adequate space, time, a

minimum requirement on displacement, and when concealment is

not a priority. An example of this type of employment would

be for physically large communications systems, like the TDCC,

that do not displace frequently. The ELPA-3 02 is recommended

94

when concealment is a priority, ground conditions are poor,

and when deploying vehicle-mounted radios which move more

frequently than the TDCC type systems, but are sometimes able

to stay in position long enough so that burying the antenna

would be practical. The horizontal dipole is recommended for

limited use for manpack, mobile employment, and where mobility

and ease of construction and concealment are a priority. It

is important that, in this method of employment, the limited

frequency range of the antenna is considered for frequency

allocation and mission planning. A typical application would

be a reconnaissance unit or an infantry unit which moves

constantly but could stop in order to employ a simple, field

expedient tactical antenna in a desert environment where poor

ground prevails.

Each of these antennas have advantages and disadvantages

as outlined in this thesis. Each antenna can optimize

communications under certain conditions. The user must make

the final decision on which antenna to use in order to

maximize communications.

95

APPENDIX A. SAMPLE NEC- 3 DATA 8ET

**COMMENT CARDS**

CM THIS IS AN HT-20T ANTENNA, f=2 MHz OVER GOOD GROUNDCE

**GEOMETRY CARDS**

GW10,15, .5,0.25,9.2,26.4,12,95,1, .001GW20,1,26.4,12.95,1. ,26.4,12.95,0., .001GW30,6,26.4,12.95,0. , 26. 4 , 12 . 95,-2 . , .001GW5, 10, -.5, 0.25, 9. 2, -26. 4, 12. 95,1, .001GW15, 1,-26. 4, 12. 95,0. , 26 . 4 , 12 . 95, 0. , .001GW25,6,-2 6.4,12.95,1. , -26. 4 , 12 . 95,-2 .

, .001GW2,l,-.5,0,9.2,-.5,0.25,9.2, .001GW1,1, .5, 0,9. 2,. 5, 0.25, 9. 2, .001GX0,010GE-1,0,0

**PROGRAM CONTROL CARDS**

EXO, 1,1, 00, 1.0, 0,0EXO, 1,2, 00, -1.0, 0,0EXO, 2, 1,00, 1.0, 0,0EXO, 2, 2, 00, -1.0, 0,0FRO ,1,0,0,2.0,1GN, 2, 0,0, 0,30.0, .01LD4, 20, 0,0, 400,0.LD4, 15, 0,0, 400,0.RPO, 91, 9, 1501, 0,0, 1,10, 0,0PL3 ,2,0,4RPO, 4, 18 1,1000, 20. ,0.,0.,2.,0.,0.PL3 ,2,0,4RP0,4, 181, 1000,28. ,0.,0.,2.,0.,0.PL3 ,2,0,4RPO, 4,181, 1000, 39. ,0.,0.,2.,0.,0.PL3 ,1,0,4RPO, 181, 1,1000, -90, 0,1, 0,0,0

This is a sample data set for NEC-3 input to obtain data forelevation and azimuth patterns and input impedance.

96

APPENDIX B. SAMPLE PAT7 PROGRAM DATA SET

, ELPA-302 ANTENNAT Elevation (deg) , Theta [0-180]P Azimuth (deg), Phi [0-360]F Frequency (MHz) 30C Conductivity of Soil (Siemens/m) 0.01H Permittivity of Soil (relative) 30E Permittivity of Insulation (relative) 5X Conductivity of Insulation (Siemens/m)L Length of Element (m) 462 Height of Element (m) 0.5A Radius of Conductor (m) 0.001B Radius of Insulation (m) 0.005N Number of Elements 2

S Spacing of Elements (m) 6.2J Azimuth Steering (deg)K Elevation Steering (deg) 90M Number of Subarrays 1

D Distance between Subarrays (m)

I Matching Impedance (ohms) [0=ignore]W Wave/Polarization/Symmetry/Termination SVBOG Graph Gain (dBi) vs T = to 180 by 5

Enter symbol of variable to change, or;0=0ptions, Q=Quit, R=Run:

This is a sample data set for the antenna radiation patternsused in the PAT7 program for the ELPA-302 antenna using goodground

.

97

LIST OF REFERENCES

1. King, R.W.P. and Smith, G.S., Antennas in Matter . The MITPress, 1981.

2. Eyring Inc., Operations Manual for the 302A Eyring LowProfile Antenna . 1990.

3

.

The Numerical Electromagnetic Engineering Design System .

Version 3.0, The Applied Computational ElectromagneticsSociety, 1989.

4. King, M.B., Gilchrist, R.B., and Faust, D.L., Eyring LowProfile and Buried Antenna Modeling Program . Eyring Inc.,1991.

5. Boithias, L. , Radio Wave Propagation . McGraw-Hill BookCompany, 1987.

6. Antenna Products Corp., HT-20T Transportable HF Antenna .

1988.

98

INITIAL DISTRIBUTION LIST

No. Copies

1

.

Commandant of the Marine Corps 1

Code TE 06Washington, D.C. 20380-0001

2. Defense Technical Information Center 2

Cameron StationAlexandria, VA 22304-6145

3. Library, Code 52 2

Naval Postgraduate SchoolMonterey, CA 93943-5002

4. Chairman, Code EC 1

Department of Electrical and Computer EngineeringNaval Postgraduate SchoolMonterey, CA 93943-5000

5. Director, Research Administration 1

Naval Postgraduate SchoolMonterey, CA 93943-5000

6. Dr. Richard W. Adler, Code EC/Ab 5


7. Dr. David C. Jenn, Code EC/Jn 1


8. Mr. Wilbur R. Vincent, Code EC/Ab 5


9. Captain Bobby G. Gregory Jr., USMC 5

P.O. Box 59 3

Frisco, CO 80443

10. Marine Corps Tactical Systems Support Activity 1

Attn: Captain R. Smith, USMCCamp Pendleton, CA 92055

99

DUDLEV ,<

NAVAL PvMONTEREY CA

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GAYLORD S

Computer modeling of tactical high frequency antennas

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