Design of VHF and UHF Comm Antennas.pdf

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Report No. FAA-RD-79-7

DESIGN OF VHF AND UHF COMMUNICAnONS

AIR/GROUND ANTENNAS

A. SINDORIS, F. FARRAR, D. SCHAUBERT

U.S. Army Materiel Development and Readiness Command

HARRY DIAMOND LABORATORIES

Adelphi, Maryland 20783

. . e • -

RECUVED -~-

• MAR 9 1979

Interim Report February 1978

Document is available to. the U.S. public through . the National Technical Information Service, Springfield, Virginia 22161.

·Prepared for

LIBRARY

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL AVIATION ADMINISTRATION

Systems Research & Development Service

· Washington, D.C. 205•

NOTICE

This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof.

Technical Report Documentation. Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

FAA-RD-79-7

4. Title and Subtitle 5. Report Date

February 1978 Design of VHF and UHF Communications Air/Ground 6. Performing Organization Code

Antennas Laboratory 50, Branch 150 8 . Performing Organization Report No.

7. Author/ sl

A. Sindoris, F. Farrar, D. Schaubert

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) u.s. Army Materiel Development and Readiness Command Harry Diamond Laboratories 11. Contract or Grant No.

Adelphi, Maryland 20783 DOT-FA78WAI-851

13. Type of Report and Period Covered

1_;2. Sponsoring Agency Name and Address Design Study of Federal Aviation Administration Communications Antennas;

Systems Research and Development Service Feb - July 1978 2100 2nd Street, s.w. 14. Sponsoring Agency Code

Washington, D.C. 29591 ARD-220

15. Supplementary Notes

This study report completes the design phase for new antennas to be subsequently built and evaluated in the follow-up phases, and to have ultimate application in improving the R.F. voice communications coverage used in air/ground traffic control.

16. Abstract

The Harry Diamond Laboratories (HDL) has undertaken the investigation of improved communication antennas for the Federal Aviation Administration (FAA). These antennas are used in VHF (118-136 MHz) and UHF (225-400 MHz) frequency spectrums in air/ground air traffic control operations. The FAA is looking for improved antenna standards in terms of space coverage and environmental durability, in the process of planning replacements for several thousand current operational antennas.

HDL has devoted an intensive design search into available technology, and conducted a concurrent analysis of the key variables affecting the performance of FAA antennas during the first six months of this effort. In doing so, it has concentrated on six types of antennas specifically designated for primary considera-tion by the FAA, namely VHF and UHF dipoles, VHF and UHF medium gain

· 'omnidirectional types, and high-gain VHF and UHF directional antennas. Certain constraints were also specified such as limiting sizes andweights, and power capacities. Analysis was also required in related areas such as lightning protection • .

The two dipole designs were deleted by the FAA, but the remaining results include design recommendations for four of the FAA applicati~s and should prove very useful in establishing future antenna design standards and performance . specifications.

17. Key Words 18. Distribution Statement

Communications Document is available to the U.S. public VHF Antennas, UHF Antennas through the National Technical Information Omnidirectional Patterns, Directional Service, Springfield, Virginia 22161.

Patterns

19. Security CICissil. (of this report) 20. Security Classif. (of this poge) 21· No. of Pages 22. Price

Unclassified Unclassified 124

Form DOT F 1700.7 l8-72l Reproduction of completed page authori zed

Acknowledgement

This work was performed by the U.S. Army ERADCOM Harry Diamond

Laborator:i,es under Interagency Agreement No. DOT-FA78WAI-851. · The authors wish to express a special thanks to the Project Manager, Mr. Leonard .Bosin, and to Messers Newell Anderson and James Coyle of the FAA for their advice and assistance during this program.

Thanks also to Dr. Howard Jones and Mr. Whilden Reinard of HDL for their assistance.

ii

Title

1.

2.

TABLE OF CONTENTS

•· INTRODUCTION

1.1 Background

1.2 General Criteria for Improved Antennas

BASIC DESIGNS

2.1 Omnidirecti~nal Antennas

2.2 Directional Antennas

3. __ ANTENNA SPECIFICATIONS AND OTHER CONSIDERATIONS

4.

5.

3 .. 1 Omnidirectional Antennas

3.2 Directional Antennas

3.3 Other Design Requirements

3.3.1 3.3.2 3.3.3

Mast Coupling Reliability and Maintenance Lightning and Static

3.4 Other Considerations

DESIGN CONCEPT FOR GAIN OMNIDIRECTIONAL ANTENNAS

4.1 Prelminiary Designs

4 •. 2 Final Designs

4.2.1 VHF Omnidirectional Designs

4.2.2 UHF Omnidirectional Designs

4.3 Simulated Operational Performance

4.3.1 4.3.2 4.3.3

Ground Reflection Model Vertical Lobing Flight Simulation Data

4.4 Mechanical Properties

DESIGNS FOR DIRECTIONAL ANTENNAS

5.1 VHF Directional Design

5.2 UHF Directional Design

6. CONCLUSION

iii

TABLE OF CONTENTS (Contin~ed)

Title

;REFERENCES

~PENI)IX A ISOLATION BETWEEN DIPOLES

APPENDIX B MEASUREMENT OF ANTENNA VSWR THROUGH LOSSY CABLE

APPENDIX C ARY4 -- FORTRAN PROGRAM FOR CALCULATING FREE

SpACE RADIATION PATTERNS

APPENDIX D FIXEDR -- FORTRAN PROGRAM FOR CALCULATING ANTENNA

RADIATION PATTERNS OVER SMOOTH SPHERICAL EARTH .

APPENDIX. E FLTSIM -- FORTRAN PROGRAM FOR CALCULATING RECEIVED

SIGNAL AS FUNCTION OF DISTANCE

i v

Page

104.

A-1

B-1

C-1 ·

D-1

E-1

1. INTRODUCTION

Design concepts for new VHF and UHF communications antennas have been developed to provide air/ground communications. These antennas are intended for use by the Federal Aviation Administration (FAA) as standardized equipment at ground facilities of air/ground communications operations. These improved ·antenna designs provide state-of-the-art electrical and mechanical performance while meeting

. the important practical considerations of size, weight, and cost.

1.1 · Backgrotmd

Commtmication between gronnd-based controllers and aircraft pilots during air-traffic-control operations is by radio. The radio communications link operates on a double sideband, amplitude modulated carrier in either the VHF band of 118 to 136 MHz or the UHF band of 225 to 400 MHz with channel allocations separated by 25, 50 or 100 kHz.

Channels in the VHF band are intended primarily for communications with civilian aircraft--both commercial and general aviation. Military aircraft communicate on channels in either the VHF or the UHF band.

Since controllers must communicate with all aircraft in each assigned airspace sector, simultaneous VHF and UHF radio links must be provided.

Two distinctly different types of antennas are required at the grotmd facilities to establish the radio communications link. The omnidirectional antenna is the most prevalent type of antenna since it can provide communications coverage over a very large airspace sector without directional preference. Where aircraft are constrained to fixed air routes or where high effective radiated power (ERP) over a certain portion of the airspace sector is required, directional antennas are employed. Both types of antennas are typically located on

19-m-high towers for en route control operations although some towers are as high as 28 m. For te~inal control operations, the antennas may be located on smaller 13-m towers near the runways or on the roof of the airport control cab.

· With the recent improvements in performance and reliability of the radios and telephone lines connecting the controllers to the remotely located radios, it is now possible to significantly improve the performance of the coumiunications link by an improvement in the ground antenna portion of the system. The heavy work load imposed on the radio connnunications link by increasing air-traffic density is partially responsible for this need to improve communications.

1.2 General Criteria for Improved Antennas

The important criteria · for improved electrical performance from these antenna concepts are complete radiation pattern coverage and antenna gain. To provide continuous radio connnunications, the antennas must provide adequate radiated power density or electric field strength at all locations :i.n the assigned airspace sector.

The radiation patterns of the omnidirectional antennas must be sufficiently uniform in all directions around and above the antenna so as to provide the necessary radiated power density. The radiation patterns of the directional antennas must be sufficiently uniform only in a specific direction over a much more limited airspace sector.

Both the omnidirectional and the airectional antennas must have gain in the angular portion of the radiation pattern that is in use for communications with aircraft at long ranges the horizon to provide the necessary radiated power acceptable, noise-free connnunications link.

omnidirectional antenna may at first seem in and angles low to density and thus an

Gain from an violatio.n of the definition--an omnidirectional antenna is one that radiates uniformly over all space and is by definition of zero gain. However, the omnidirectional antennas discussed in this report radiate uniformly in the azimuth plane only, with substantial gain roll-off in the elevation plane. A vertical dipole is an example of an antenna that gives gain--2.1 dB relative to isotropic (dBi)--yet is classified as omnidirectional in the context of this study. Decreased gain .and nonuniformity in the vertical plane of the radiation pattern is acceptable over angular portions where the aircraft is high over head and therefore much closer to the radio site. However, deep, broad nulls in the radiation pattern are undesirable since they can cause holes in the coverage of the conmuriications system.

The most important criteria for improved mechanical performance is the ability of these antennas to perform electrically without degradation in the operational environment. Since most of the antennas are located on towers fully exposed to the extremes of all weather conditions and in many situations at radio sites far from maintenance .

personnel, environmental durability and a long maintenance-free lifetime are important requirements affecting the mechanical designs.

Wind loading · during ic'ing~ vibration, moisture, and temperature extremes are specific environmental · factot's . .. . ··. that · are · .. considered .in .·the development of the design concepts.

Other criteria considered in the design of the improved antennas ·are these: (1) impedance and gain bandwidth, (2) VSWR, (3) maximum input power, (4) polarization, (5) size, (6) weight, and (7) unit cost. Certain peripheral criteria, though not antenna design criteria per se, are also considered since it is in some cases possible to adjust antenna performance factors that will maximize overall per formance of the radio communications link. These are the peripheral effects considered: (1) isolation . from other antennas, (2) influence and shadowing effects of nearby towers, (3) vulnerability to static, power line transients, and lightning, and (4) ground reflections and multipath interference.

Antenna design criteria are discussed in detail in section 3.

2. BASIC DESIGNS

(\j

Four antenna design concepts have been developed to .satisfy there quirements of the improved . antennas. The antennas are these: (1) a

. gain omnidirectional antenna for VHF and one for the UHF band, (2) a directional antenna for the VHF band and one for the UHF band.

2.1 Omnidirectional Antennas

The omnidirectional antennas are three- or four-element, linear phased array antennas.

The array has a gain maximum

Each element in the array is a dipole.

on the horizon and .omnidirectional azimuthal coverage with no holes in the vertical plane coverage. The

VHF and UHF designs are electrically very similar. The two designs differ in physical size by approximately the ratio of the operating freque~cies with small differences in element spacing.

2.2 Directional Antennas

· The VHF directional antenna concept is a yagi antenna of 6 to

10 elements. The exact number is chosen to give the required high gain over the VHF bandwidth. Because of the large bandwidth required of the

UHF antenna, a log-periodic antenna is needed. To obtain the required high gain from a log-periodic antenna, a pyramidal design is chosen.

3. ANTENNA SPECIFICATIO~S AND OTHER CONSIDERATIONS

These antenna concepts that are designed to meet certain specific electrical and mechanical specifications represent improvements in performance over existing antennas. Also, peripheral design criteria are considered in the design of the improved antennas to insure that there is overall improvement of the radio communications link.

3.1 Omnidirectional Antennas

The sl>ecifications in table 1 have been applied in the development of the design conceptsfor the omnidirectional ·antenna. A gain of 5 dBi at the .maximum in the radiation pattern is obtaine4 by vertically arraying dipole antennas. This large vertical aperture produces the gain by narrowing · the radiation pattern in the elevation plane. Sufficient gain in the vertical plane is, however, maintained to give adequate ERP for good communications with aircraft located overhead. (The calculations of received signal level discussed in section 4.3 show this to be true.) The antennas are designed to produce 5-dBi gain at the low frequency end of th~ band; because the electrical size of the antenna aperture increases with frequency--the physical aperture is fixed--the gain can be expected to increase slightly with' increasing frequency. This effect is more evident in the

UHF antenna design because of the large frequency range-:--225 · to 400

MHz--0 f the UHF band. A 2:1-VSWR impedance match to a 50-ohm system is needed to minimize loss in gain (0.5-dB loss for a 2:1-VSWR mismatch) and minimize power reflected to the transmitter.

The above specifications are met over a 14-percent bandwidth around 127 MHz for the VHF antenna design and over a 56-percent bandwidth around 313 MHz for the UHF antenna design. Since the gain omnidirectional antennas in some cases may be connected to 50-W transmitters, 50 W of continuous wave (CW) 'input power handling capability is provided. This requirement impacts the choice of components such as power dissipating resistors in the feed network of · the antennas. Vertically polarized radiation is required from all ground communications antennas to match the polarization of the aircraft and to mitigate vertical lobing of the radiation pattern due to multipath reflections from the ground. (There is a slight advantage

· in that the ground reflection coefficient in general is smaller for vertical polarization than it is for horizontal.)

Table 1. Omnidirectional antenna specifications

Property Specifications

Electrical

Gain

Az~uthal pattern coverage

Vertical pattern maximum

Impedance

VSWR

Bandwidth

CW power

Polarization

Mechanical

Height

Weight

Wind

Ice

Altitude

Humidity

Cost

5 dBi min

Uniform

At horizon

50 ohms

2:1 max

14% VHF (118 to 136 MHz) 56% UHF (225 to 400 MHz)

50 W max

Vertical

6 m max

9 kg max

157 lan/hr max

1.25 em max

3.8 Ian max

5 to 100%

$1000 max ll

A maximum aperture height of 6 m "is the practical limit imposed on the mechanical design. At VHF, this aperture size limits the maximum theoretical gain available to approximately 7 dBi. When losses in the feed network, cable losses, nonuniform current distributions, and radiator efficiencies are taken into account, the specified actual gain of 5 dBi for the VHF antenna of this restrictive height is possible to achieve, but the available aperture must be considered marginal. Figure 1 shows the theoretical gain of an antenna

. as a function of aperture height. The calculation is based on the ideal situation of a 100-percent efficient uniform current distribution radiating into free space to give the dipole radiation pattern of

E (8, q,) = sin sin (kL/2 cos 8) kL/2 cos 9

The directivity is then calculated by pattern integration from

4 1T IE (8 <1> ) 1

D = o' o ffjE (8, <!>) j 2 dS

At UHF where the wavelength is 1/2 to 1/3 of the wavelength at VHF, the

6-m maximum height is not restrictive and 5-dBi gain is easily achieved. Light weight (9 kg) is a practical consideration based on the need to have one man install the antenna on a tower. The· wind loading of 157 km/hr with 1.25 em of ice at a maximum altitude of

3.8 km in htm1idity of 5 to 100 percent are the extremes of the physical environment to which the communications antennas are exposed. A $1000 unit cost, which affects the choice of materials and the construction technique, has been considered in the following design concepts.

m i=

8 w a: -c a:

0 z

CJ 5

...J

.... w a:

w :::t: 4

I zl ~I ~I ~I ~I C1

~I ~I I I I

1 2 3 4

APERTURE(WAVELENGTH)

2.5 5.0 7.5 10.0

APERTURE (m) AT 120 MHz

Figure 1. Directivity of wire antenna with uniform current distribution.

Jt

5.

t

12.5

3.2 Directional Antennas

The specifications in table 2 have been applied in the development of the design concepts for the directional antennas. A gain value cf 10 dBi can be achieved by concentrating the radiation pattern over a small airspace sector. The beam widths are consistent with the 10-dBi gain value but will change with frequency especially over the UHF band. The beam widths in table 2 are only approximate values. By limiting the backward directed radiation to 18 dB below the forward peak, coupling to towers and support structures can be held to a minimum. The other electrical specifications are the same as those of the omnidirectional antennas.

The mechanical specifications of the directional antennas are the same as those of the omnidirectional antennas except for height.

Since the directional antennas are end-fire types, the appropriate specification is length along the boom. This length is set at . a maximum of 6 m.

3.3 ·other Design Requirements

3.3.1 Mast Coupling

When an antenna couples radio frequency (RF) currents to the mast supporting the antenna or to any metallic structure nearby, the radiation pattern, gain, and impedance of the antenna may be degraded. Although isolation from the support mast is not normally an antenna specification, such coupling and means of minimizing it are an important part of the design concepts. For example, coupling to the mast, as the following data indicate, distorts the radiation pattern of the antenna. The problem of mast coupling applies only to the gain omnidirectional antenna. Because of the high gain and the high front-to-hack ratio, the ma~t coupling problem for the directional antenna is minimal and the radiation pattern is not perturbed significantly.

Table 2. Directional antenna specifications

Property Specifications

Electrical

Gain

Pattern Coverage

Elevation

Azimuth

Front-to-hack ratio

Impedance

VSWR

Bandwidth

CW power

Polarization

Mechanical

Length

Weight

Wind

Altitude

. Humidity

Cost

10 dBi min

-so0 VHF (118 to 136 MHz) -450 UHF (225 to 400 MHz)

18 dB min

50 ohms

2:1 max

14% VHF

56% UHF

50 W max

Vertical

6 m max

9 kg max

157 km/hr max

1.25 em max

3.8 Ian max

5 to 100%

$1000 max

One method of measuring mast coupling is to first operate the antenna under test in the free-space environment of an anechoic chamber with no support mast. The antenna is supported by a

Styrofoam dielectric tower. Radiation pattern and gain are measured with a miniature, battery-powered transmitter connected directly to the antenna RF terminals. A simulated mast is then attached and the measurements are repeated. Any change of the.pattern is caused by RF coupling to the mast. The solid curve in figure 2 is a mast-free pattern measurement made on an FAA model FA-8955 (R. A. Miller,~nc), UHF discone antenna. When a mast is attached to the antenna, the radiation pattern distorts slightly and the gain decreases a few decibels at the +90-deg points on the pattern. The portion of the pattern around 90-deg is critical for good, long-range communications because at these angles the antenna illuminates the horizon. A loss in gain at the horizon can affect communications with distant aircraft that are located low to the horizon. The effects of mast coupling are frequency sensitive. Figure 3 shows data from the mast-coupling test taken at 400 MHz instead of 225 MHz. Pattern distortion caused by mast coupling is clearly evident.

Coyle [1] has performed similar mast-coupling tests on numerous FAA and commercial antennas and has reported similar distorted radiation patterns and losses in gain. The test of the FAA model

FA-79571A (R. A. Miller, Inc.) VHF coaxial dipole antenna is important.

It showed that for a specific length of mast a 20-dB deep null was formed at the horizon. If this antenna happened to be mounted on a tower with that specific length of mast, communications with aircraft at the horizon would be, at best, very poor.

225 MHz oo T + OdBi

I

I

180°

DISCONE ANTENNA

R.A. MILLER, INC.

(FA-8955)

--- NOMAST

--1-m MAST ---- 2-m MAST

Figure 2. Elevation plane radiation patterns of discone antenna with and without mast.

400 MHz

OdBi

I - ------~--------1800

DISCONE ANTENNA

R.A. MILLER, INC.

(FA-8955)

---NOMAST

--1-mMAST ----2-m MAST

Figure 3. Elevation plane ·radiation patterns of discone antenna with and without mast.

3.3.2 Reliability and Maintenance

Since air-traffic control operations depend on the antennas for a continuous radio communications link, reliability is an important design consideration. The antennas are passive devices and normally provide very high short-term reliability. (In addition, backup radio communications links are available to the controllers and relieve the safety burden from any one antenna.) Because many of the antennas are in remote locations with limited access, the ability to operate for very long periods of time--15 years, if possible-without periodic maintenance also is important. Careful choice of materials, accelerated lifetime testing by an environmental test laboratory, and high quality control during production will be necessary to meet these long-term requirements.

3.3.3 Lightning and Static

Grounding of the antenna mast, the antennas in the phased array, and the elements in the yagi and log-periodic antennas provide in most cases protection for the antennas, transmitters, and receivers from the effects of lightning. A spark gap is a common means of providing this protection for antennas that utilize de open-circuit elements. Antennas and elements that present de short circuits offer lightning protection as well as protection against static buildup without an auxiliary spark gap. Grounding and using antenna elements which have low impedance at de also reduce the vulnerability of the radio communications link to power-line transients and other sources of

EMI. When the resistive surface antennas are protected by dielectric radomes, coating or impregnation of the dielectric with a a lossy material may be necessary to dissipate static electric charges that may tend to buildup on the radomes. The above lightning and static-protection techniques are included; where possible, in the conceptual designs of the imp~oved antennas.

3.4 Other Considerations

Multit>ath interference from ground reflections affects the signal strength in the radio communications link. As shown in figure

4, as the aircraft approaches the communications antenna, both a direct wave and a·ground reflected wave are received by the antenna on the aircraft. Depending on the path length differences and the phase of the ground reflected wave, constructive and destructive interference

.occurs along the aircraft flight path. The same phenomenon takes place when the aircraft is transmitting and the receive antenna is on the ground, i.e •. , the radio communications link is reciprocal. (The theorem of reciprocity states that the signal level does not change when receive and transmit antennas are interchanged.)

Multipath interference makes the antenna look as if the radiation pattern has lobes in the elevation plane. In developing the conc~pts for the omnidirectional antennas, this lobing is taken into account effect.

because antenna parameters can be adjusted to lessen its

Eliminating elevation-plane lobing, however, is not possible.

To minimize multipath interference, vertical polarization is preferred over horizontal. Multipath interference is minimized because the reflection coefficient of the ground is typically lower for vertical polarization. The reflection coefficient is, however, still large enough that lobing of the vertical radiation pattern must be considered·when the performance of the antennas is evaluated in the multipath environment of the radio communications link. (A computer modelling code--FLTSIM--has been developed for this evaluation and is discussed in section 4.)

1-' 0\ T~

H = 5 TO 15X

H

I / I //

<r;MAGE

I ANTENNA

I

<c,Q~

<c,V'{.

<c,<c\V

~Q<?' 6<?·.0-J

Figure 4. Ground reflections and multipath interference

• J

The improved electrical performance of these design concepts can be destroyed by improper mounting on FAA towers or by poor tower-top design and layout. The antennas should be mounted higher than all railings and other ancillary structures. lYhere possible, railings and structures within 1 or 2 wavelengths (A) of the antenna should be made of nonconducting material. The mast and its associated support structure may be composed of conducting material since the metallic coaxial cable to the antenna is always present and since the illumination lev~l of structures below the antenna is minimal because of the pattern null in the direction of the t:ast. The installation of other antennas, beacon lights, and weather equipment should be avoided whenever possible. Complete plastic tower tops could minimize the effects of nearby conductors on antenna performance. The illumination of the hardware on the tower tops can C.istort the radiation pattern

[2], change the antenna VSWR, and decrease gain severely enough to produce a hole in the radio communications link. It is important to take precautions before the installation of the improved antennas in order to guarantee high performance from these antennas.

Coupling among antennas on the same tower causes a small amount of pattern distortion depending on separation distance, but because of limited space available normally four antennas must be placed on each tower. Besides the pattern distortion, interference between channels because of interantenna coupling may be a special consideration that dictates another type of antenna design. Campbell and Arnold [3] have studied this coupling phenomenon and have provided practical design information on vertically stacked antennas that provide low interantenna coupling. Their data for coupling between horizontally and vertically separated dipole antennas are reproduced in

Appendix A.

Appendix B describes a method for mon!toring the VSWR of a communications antenna from the transmitter or receiver end of the cable. This method includes a correction factor for cable losses. For example, the curve in figure 5 shows how the measured VSWR at the transmitter end of the cable needs to be corrected for the 3-dB cable loss to obtain the VSWR at the antenna. Loss in antenna gain due to the impedance mismatch is indicated. Performing the perodic tests described in Appendix B helps insure the specified performance from the antennas in the radio communications system.

4. DESIGN CONCEPT FOR GAIN OMNIDIRECTIONAL ANTENNAS

Linear phased array antennas have been designed to give omni directional radiation patterns in the azimuth plane and sufficient gain in the elevation plane to provide continuous communications coverage.

A preliminary design of a mast-supported antenna has been rejected as unsatisfactory. The performance of each antenna design has been evaluated ~y a computer code that simulated the flight of an aircraft through an airspace sector illuminated by the antenna.

4.1 Preliminary Designs

The basic antenna is a vertically oriented linear array of dipole elements for both the VHF and the UHF bands as shown in figure 6a. The orientation of this linear array with respect to the horizon and in a spherical coordinate system is shown in figure 6b.

The data in figures 7 though 12 show the changes in elevation radiation pattern and gain of the array as the number of elements, or dipoles, is increased from one to six for a constant element spacing of 0.6 A and an excitation of equal amplitude and phase on the elements. The calculated patterns are applicable to both the VHF and the UHF designs although the physical size of each antenna is different. By substituting meters for wavelength (A is 2.36 mat 127 MHz and 1.0 mat

300 MHz), the physical distance between elements and the length of the array can be obtained. The gain of the array is calculated by pattern integration and is more correctly interpreted as directivity, which is equivalent to gain if the antenna is 100 percent efficient. A copy of the computer code for these calculations is listed in Appendix C.

e:;.

\0

200:1 a:

<( z 37:1 z w z

14:i

6:1

4:1

,, t' ~' •

~--------~----------~----------~----------y---~20

3dB CABLE ATTENUATION

L ::T I 1

Ill :9.

10 ~ ...J

1:1 2:1 2.5:1 3:1 1.5:1

I o

VSWR MEASURED AT TRANSMITTER

Figure 5. Effect of 3-dB cable attenuation on VSWR measurements

(a)

T

ELEMENT

~I

I N

(b)

1 8=00 z

(/1= 270° 6=900 l". I ~VI

8= 180°

N

Figure 6. Linear array (a) of half-wavelength dipoles and (b) in spherical coordinate system q, =goo y 6=900.

-----190°

4> ARBITRARY

Figure 7. Calculated radiation pattern of dipole array • one element

ELEMENT SPACING= 0.6>-

Figure 8. Calculated radiation pattern of dipole array, two elements

5.93 dBi oo/

ELEMENT SPACING = 0.61.

Figure 9. Calculated radiation pattern of dipole array, three elements

150°

ELEMENT SPACING= 0.6~

Figure 10. Calculated radiation pattern of dipole array, four elements

ELEMENT SPACING= 0.6 X

Figure 11. Calculated radiation pattern of dipole array, five elements

_..............8.73 dBi oo~ --1- .. ··-----------.

--... ,_ 30°

ELEMENT SPACING= 0.6~

Figure 12. Calculated radiation pattern of dipole array, six elements

As the number o£: elements in the array increases~ the radiation pattern in the elevation plane becomes narrower with the main beam and gain maximum remaining on the horizon. The azimuth plane pattern is uniform because this is an omnidirectional antenna. The gain exceeds 5 dBi when the array is composed of three elements.

Adding more elements · increases the gain. but at a slower rate. Also, for an array of number of lobes

·in gain with the figure 13.

two or more elements~ vertical lobing is present, the increasing with the number of elements. The increase increase in the number of elements is summarized.in

Figures 14 through 16 show the effects of an increase in element spacing on the radiation pattern and gain of the array. As element spacing increases, the main beam of the pattern narrows and the gain increases. The antenna is presenting a larger aperture and gain increases in proportion to spacfug approaches ). , the grating have been suppressed by the dipole aperture. However, as the element lobes near +20 and +160 deg, which element pattern, begin to greatly increase.in amplitude. This increase causes a less rapid increase in main beam gain (see figure 17) and causes the gain to decrease as the element spacing is increased beyond A • (A grating lobe the equivalent of a main lobe, begins to appear in phased array radiation patterns whenever the element spacing exceeds )./2.)

In the phased array radiation pattern, the nulls caused by the elevation plane lobing are undesirable because, if the nulls are very deep, communications might be lost as an aircraft flies through them.

The technique of defocusing the phased array--perturbing the .

equal-phase condition applied to each element--can be used to fill the nulls.

9~----~------r------r------~--~~------~

£D :2 z

N 00 <.:1

ELEMENT SPACING= 0.6 >-

1 2 3 4 5 6

NUMBER OF ELEMENTS

Figure 13. Dipole array gain versus number of elements

':t' .. '"

0° 7.09 dBi

--------~ 30° '-

Figure 14. Calculated radiation pattern of dipole array, four elements spaced 0.6A

_.......,....s.13 dBi oo..........-

180°

Figure 15. Calculated radiation pattern of dipole array, four elements spaced 0.8A

~8.63dBi oo~

Figure 16. Calculated radiation pattern of dipole array, four elements spaced l.OA

VJ

N

9~--~-----r----~----~----r---~----~----~

-·-al .:E. 7 z z z w

6 1-z

4 0.9 . 1.0 0.3 0.4 0.5 0.6 0. 7 0.8

ELEMENT SEPARATION .(WAVELENGTHS)

Figure 17. Antenna gain versus element spacing. Four element dipole array

~· ·"' ..... iJ'

Figure 18 compares the pattem of a six-element, equal-phase array with the pattem calculated after a defocusing phase taper of ex2 has been applied.* Peak gain has decreased by 0.3 dB, but the nulls are no longer deep enough to cause a loss in communications. The results of defocusing a four-element J?hased array are shown in figure 2 19. For ex ==25 deg at the end elements, the first null fills in with a · main beam loss in gain of only a few tenths of a decibel. By applying 2 a perturbation of aK ==50 deg, the first null can be made to almost disappear, but main beam gain decreases by i dB. The null is filled by the sacrifice of main beam gain. The second null in the four-element array is affected only s~ightly but is not of as much concern as the first null because an aircraft would be much closer to the antenna at the angle of the second null. (The results of the simulated performance discussed in section 4.3 more fully explain the need for null filling.) Defocusing is applied carefully in the final designs to optimize the performance of the radio communications system.

*The phase taper of sx2 = 4> is applied by advancing the phase of the end elements + deg relative to the center of the array. The phases of the other elements are adjusted to approximate a parabalic phase front across the array aperture.

w -ll-

0 I I I I [ '7 '7 ~c': l I l I

--IN~HASE

--- {3X DISTRIBUTION

-5

ELEMENT SPACING= 0.5A

! -1t a..

w 1- ..J w a:

-40 " 0 15 30 45 60 75 90 105 120 135 150 165 180

ELEVATION ANGLE FROM VERTICAL (8)

Figure 18. Calculated radiation pattern of six-element array showing effect of non-linear phase distribution

VI

-r:c :g a:

w s:

Q.

w 1- ...J w a:

Or-----.------.-----.------.---~~--~-r----~------~----~

- - EQUAL PHASE

-- ~x2=25° ---- ~x2 = 50° fll/1"---- / ' /J / ,_

I

,'V \ If/ \

/v \ " \ I

\ I \ I \ I \ I \I ll

ELEVATION ANGLE FROM VERTICAL (DEG)

Figure 19. Beam defocusing for various phase distributions

Based on the above analysis of the concept of a linear.

phased-array antenna, two preliminary designs have been developed: (J) a fiberglass-radome-supported array and (2) a mast-supported array.

These concepts are depicted in figure 20. Because of higher potential for meeting weight and maintenance-free lifetime requirements, the fiberglass-Tadome-supported array is the concept chosen for the final design. The mast-supported concept, although inexpensive, has been rejected. A description of various mast-supported concepts considered and some of the reasons foT rejection are given below.

Since the mast is being illuminated by the dipole elements in a mast-supported a.rray, an attempt is tr.ade to incorporate the mast as part of the radiating dipole element. For example, the mast is included as part of the conductor in a folded dipole [4] or a loop antenna. The mast-supported phased-array antennas depicted in figure

21 are some of the numerous configurations conceived in which the mast is made part of the antenna.

The radiation characteristics--pattern, gain, and input impedance(Z)--of these antennas are obtained by computer analysis. The method of moments is applied to solve the differential-integral equations that represent a formulation of Maxwell's equations for thin-wire structures. The computer code used is a modified version of the Lawrence Livermore Laboratory WF-LLL2A code, which is a derivative of the WAMP code [S].

None of the antenna configurations in figure 21 performs satisfactorily. All bad low gain, directional radiation patterns, a highly reactive input impedance,·or some combination of these factors.

VJ

RADOME-SUPPORTED

DIPOLE ARRAY

FIBERGLASS

RADOME

ISOLATION

CHOKES

METAL

MAST

MAST-SUPPORTED

DIPOLE ARRAY

Figure 20. Two design approaches

(a)

Z VERY LARGE

(e)

(b)

Z HIGHLY

REACTIVE

LOW GAIN

(f)

Z NOT GOOD Z VERY LOW

PATTERN VARIES

WITH FREQUENCY

(LOW GAIN ON HORIZON)

0) . U)

Z HIGHLY

REACTIVE

PATTERN IS

Z VERY LARGE LOW GAIN

(c)

Z VERY LARGE

(g)

(d)

Z HIGHLY REACTIVE

LOW GAIN

(h)

Z AND PATTERN Z AND PATTERN

NOTACCEPTABLE NOT ACCEPTABLE

(k)

NON RESONANT

ATTACHED

ELEMENTS

Z HIGHLY

REACTIVE

(I)

RESONANT

DETACHED ELEMENT

Z NARROW

BANDWIDTH

Figure 21. Mast-supported antenna structures: (a} to (f) lA structures and (g) to (1) folded dipole structures

A more conventional type of mast-supported antenna· that performssatisfactorily is shown in figure 22. The dipole elements are attached to the mast by an insulating support material. The radiation characteristics, which are again analyzed by the method of moments computer code, are good. These antennas have gains of 5 to 7 dBi, input impedances of 50 to 100 ohms, and omnidirectional radiational patterns with the four-element.design showing more azimuthal variation than the eight-element design. The analysis shows a serious deficiency, however--the antenna is narrow band. This limits the eesign concep.t to only the VHF band. Acceptable performance over the UHF band has not been obtained from this design or from designs of other mast-supported antennas.

Although not a phased array, a simple radome-supported antenna called a Franklin antenna [6] (shown in figure 23) has been analyzed because of its potentially low cost. Since the wire antenna modelling code· can analyze reactively loaded wires, both an inductively loaded and a capacitively loaded Franklin antenna have been investigated. The calculated performance in both cases is poor. The capacitively loaded version has a very highly reactive input impedance and low gain. The inductively loaded version shows moderate gain levels but has a highly reactive input impedance. Because of these problems, the Franklin antenna has been discarded from further consideration.

4.2 Final Designs

The final design concept is a linear phased array of dipole elements in the radome-supported configuration. Dipole elements, isolation chokes, the feed network, and interconnecting cables are all.

of lightweight construction and provide no mechanical support. The small-diameter, fiberglass tube surrounding the dipoles supports the array. A potting material fixes the parts firmly inside the tube and protects against vibration, shock, and weather. The dipole elements, their spacing, and their excitation amplitude and phase are adjusted to optimize each design for each operating frequency band.

.1::-

:a:

N cD

• 0 • • 0 • t f f

4 ELEMENTS 8 ELEMENTS

t f ~ t ~ . l--- 25 TO 50 em 25T050 em~ r--

Figure 22. Possible VHF antenna designs

1.

I I I

:E ~ ~ oct

I I I T

LARGE Z

VERY LOW GAIN

L = 0.01 j.tH

L:::: 1 j.tH

L = 100 j.tH

NO EFFECT

HIGHLY REACTIVE Z

GAIN LOSS AT HORIZON

INDUCTOR STOPS CURRENT

GAIN LOSS DUE TO REDUCED

APERTURE

Figure 23. Franklin-type antenna

4.2.1 VHF Omriidirectional Deslgns

The antenna patterns and gain values shown in figures 14 to 16 were calculated by assuming perfect lossless dipole antennas fed in phase with equal-amplitude signals. The gain of each single dipole element is· 2.15 dBi. In practice, the gain of any array is reduced by element inefficiencies, losses in the coaxial cables that feed each element, radome inefficiencies, and losses in an imperfect power

. divider. In general, it is possible to construct a single dipole element that is very nearly lossless and has a gain of approximately +2 dBi over a narrow bandwidth. However, when the element is constructed so that the bandwidth spe~ifications are met, the gain of the element is reduced. That is, the efficiency of the radiator over the narrow bandwidth must bandwidth. ·An be reduced when the element is operated over a wider actual element gain of +1 dBi over the VHF band is a more realistic goal.

incoming isolation

An isolated power divider is used to distribute the

RF energy equally to the various dipole elements. The is needed to insure that the antenna will continue to function if an element fails. Also, the power divider must be designed to operate with 50 W of input power. Components that meet these specifications are commerically available ·and typically exhibit approximately 0.5-dB loss.

Since each element in the array is to be excited in phase or very nearly in phase for the defocused design, the cables that connect each element to the power divider must be the same length.

Unequal-length cables will cause the beam to tilt away from the horizon at some operating frequencies and thus cause a loss of communications at long ranges. Since the cables that feed the upper elements must pass through the lower elements, small-diameter cable is desirable to minimize the degradation of the performance of the lower elements. However, because insertion loss of coaxial cable is inversely proportional to cable diameter, large-diameter cable is desirable to reduce cable loss. A semi-rigid coaxial cable with 0.141

in. (0.36 em.) · outer diameter is · a good compromise. At VHF frequencies~ this cable typically exhibits a loss of 0.118 dB/m (3. 6 dB/100 ft) and causes only minimal degradation of lower element performance.

The total of these loss factors dictates that the antenna be designed for approximately 7-dBi directivity (by using ideal elements) so that the actual realizable gain reaches +5 dBi. A maximum antenna height of 20 ft (6.1 m) and an element length of 4.25 ft (1.29

m) (a A/2 dipole at 116 MHz) indicate that a maximum of four elements can be used. These elements are stacked vertically with 0.62 A (1.6 m) spacing between .element centers as shown in figure 24. The directivity

("gain" for ~ lossless antenna) is at least 7 dBi over the VHF band.

A three element design, also shown in Figure 24, has the same total height but the element centers are spaced 0.93A (2.4 meters) apart. Although the calculated directivity is 0.1 dB less than the desired 7 dBi at some frequencies, the realizable gain should still exceed +5 dBi.

The radiation patterns for the four- and three-element VHF designs are shown in figures .25 and 26. The elevation angle (9)

~easured from vertical is plotted on the horizontal axis so that 0 deg corresponds to directly overhead and 90 deg corresponds to the horizon.

Relative power is plotted on the ·vertical axis. (Radiation patterns show E 9 (a).) If ideal lossless radiators were used, the 0 dB relative power level would correspond to a gain above an isotropic point source equal to the directivity of the antenna. Maximum power is directed at the horizon to insure good communication at these long ranges. High gain is not required at the low elevation angles since the aircraft is nearly overhead, and the distance between transmitter and receiver is small. In figure 25, the nulls at 68 and 112 deg have been filled by using the defocusing technique discussed in section 4.1. The nulls appearing at 42 and 138 deg will be partially filled by scattering from rough terrain.

f 2.36X

6.1 m f fr t_I_ ~

I'

2.36X

4 ELEMENTS

3 ELEMENTS

DIRECTIVITY:

7.05 dB at 116 MHz

7.30 dB at 125 MHz

7.59 dB at 136 MHz

DIRECTIVITY:

6.98 dB at 116 MHz

7.00 dB at 125 MHz

6.91 dB at 136 MHz

Figure 24. Radome-supported VHF omnidirectional gain antenna

-m a:

w t.:L.

w

\.J1 1-

..J w a:

,., ti

01 I I I I I :;;;oar;;;.: I I I I I I

125 MHz DIRECTIVITY= 7.3 dB

-40 I I I I - I I I I I I I I I II

0 15 30 45 60 75 90 105 120 ..... .,. ... ,.,.... .. .. ,., '1ft

ELEVATION ANGLE FROM VERTICAL (0)

Figure 25. Radiation pattern of four-element VHF omnidirectional antenna

.o- 0\ a:

w w w a:

0 I I I I I I ·; I C: I I I I I I

-30

-35

125MHz DIRECTIVITY= 7.0 dB

·40' I I I I I I I I I I I I

0 15 30 45 60 75 90 105 120 135 150 165 180

ELEVATION ANGLE FROM VERTICAL (0)

Figure 26. Radiation pattern three-element VHF omnidirectional antenna

The radiation pattern of the three-element VHF design is.

shown in figure 26. The larger spacing between element centers. in this design causes the increased size of the grating lobes at 30 and 150 deg. Although the gain of the three-element design is not as high as that of the four-element design, its inherent lower cost warrants further consideration.

4.2.2 UHF Omnidirectional Des.i~s

Basic antenna theory states that the gain of an antenna is proportional to the physical aperture (in this case, height), measured in wavelengths. Therefore, to achieve a specified gain, a UHF antenna will be proportionally smaller than a similar VHF antenna.

Figure 27 indicates the relative sizes and the element spacings for the three- and four-element designs. The large UHF bandwidth (225 to 400

MHz) dictates that certain compromises be nade. For example, in the thre~-element design, if the interelement spacing were increased. to

1. 33 m to achieve 7-dB directivity at 225 MHz, then at 400 MHz the grating lobes would be so large that the directivity would decrease well below 6 dB. Reducing the element spacing to 0.8 m decreases the size of the grating lobes and thereby increases the directivity at 400

MHz, but the overall array is smaller and no longer has sufficient gain at 225 MHz. The radiation patterns at a compromise spacing of 1.07 m are plotted in figures 28 to 30.

The four-element design has sufficient gain at 225 MHz and, since the elements are spaced only 0.8 m apart, the grating lobes are relatively small at 400 MHz (figures 31 to 33). Between 425 and

300 MHz the directivity increases 0.92 dB, but from 300 to 400 MHz it increases only 0.16 dB at 30 and 150 deg.

due to the increased size of the grating lobes t t t t t t 0.8~ 1.07m l t 0.6~

0.8 m

I

2.10~ ·2.8m

2.3~

3.1 m

3 ELEMENTS

4 ELEMENTS

DIRECTIVITY

6.44 dB at 225 MHz

6.94 dB at 300 MHz

6.67 dB at 400 M Hz

DIRECTIVITY

7.00 dB at 225 MHz

7.92 dB at 300 MHz

8.08 dB at 400 MHz .

Figure 27. Radome-supported UHF omnidirectional gain antenna

.p.

\0

~' r •J

0~--~--~----r---~~-T--~~--~--~----~---r--~----~

-5

-10

~ -15 a:

LIJ

3:

2 -20

LIJ

1- ..J -25

LIJ

a:

DIRECTIVITY= 6.44 dB -35

-40 o-~;---;tn-~--=--~--:f.:---:-:~~-----!----_L _ _L__j] 15 30 45 60 75 90 105 120 135 150 165 180

ELEVATION ANGLE FROM VERTICAL (0)

Figure 28. Radiation pattern of three-element UHF omnidirectional antenna at 225 MHz

-CD

'a -a:

w

Q.

VI w 0 2: ...

..J w a:

0~--~---r--~~--~--~--~r---~--~---r---,----~--~

-30

-35 DIRECTIVITY= 6.94 dB

D' o 1 5 J o 4~ ~o is ~o 1bs 1Jo 1~s ,Jg 1~ mn·

Figure 29.

ELEVATION ANGLE (THETA) FROM VERTICAL

Radiation of three-element UHF omnidirectional antenna at 300 MHz

VI

I-'

¥1 ,. . , ..

0~--~--~----r---~--~---,~--~---,----~---r--~~--~

-5

-10.

iQ.

:!! ·15 a:

Ul 3:

a.. -20 Ul ..J Ul ·25 a:

-35

DIRECTIVITY= 6.67 dB

15 30 45 60 75 90 105 120 . 135 150

ELEVATION ANGLE {THETA) FROM VERTICAL

Figure 30. Radiation pattern of three-efement UHF omnidirectional antenna at 400 MHz

165 180

-ID , -a:

w

> VI ;:: N

w u:

0, I I I ;. I >I< I I I I I I

DIRECTIVITY= 7.0 dB

-40 I I I I II I I I I I I I II I I I •

0 15 30 45 60 . 75 90 105 120 135 150 165 180

Figure 31.

ELEVATION ANGLE (THETA) FROM VERTICAL

Radiation pattern of four-element UHF omnidirectional antenna at 225 MHz

Vt w

0 I I I I I I 7N I I I I I I m- ~ -15 a:

w w ..J w -25 a:

-35 DIRECTIVITY= 7.92 dB

60 75 90 105 120 135 150 165

ElEVATION ANGlE (THETA) FROM VERTICAl

Figure 32. Radiation of four-element UHF omnidirectional antenna at 300 MHz m-a:

w 3':

CL.

w l.n 1-

..J w a:

Ot I I I I I rK I I I 1·. I I

DIRECTIVITY= 8.08 dB

-4o L____l__,__j____;,.__l_-:':----:~~~-fn;---;;;,*-~~~---;s 0 45 60 75 90 105 120 135 150 165 180

ELEVATION ANGLE (THETA) FROM VERTICAL

Figure 33. Radiation pattern of four-element UHF omnidirectional antenna at 400 MHz

4.3 Simulated Operational P~formance

4.3.1 Ground.Reflection Model

The free-space antenna patterns do not provide complete information about the antenna's performance in the real environment.

The fact that the antenna is operated a certain height above a dielectric sphere (the earth) means that there will be reflections that will significantly modify the · free-space radiation patterns. The geometry for this situation is shown in figure 34. The antenna is located;a height h1 , above a spherical earth with relative permittivity er and conductivity cr. The wave that travels &long the direct path, Rd, to. the aircraft is · combined with the wave that follows the reflection path, Rr. The amplitude of the signal received at the aircraft depends on the separation distance d, the ground reflection coefficient, and the direct and reflected path length difference. Some other characteristics of the model are listed in figure 35. The received voltage can be calculated by using the equations shown in figure 36.

4.3.2 Vertical Lobing

A computer code called FIXEDR was developed to model the effects on the radiation patterns of reflections from a lossy spherical earth. ' This computer code is listed in Appendix D. The reflected signal magnitude and phase are dependent on the distance traveled and the magnitud~ and the .phase of the ground reflection coefficient. The ground reflection coefficient is a function of the incidence angle as shown in f.igure 37.. Complete cancellation occurs at the horizon

( 9 • +90 deg) because the reflected signal is equal in magnitude

(I Rv I =1) and 180 deg out of phase with the direct signaL As the elevation angle decreases, the reflected signal adds or subtracts as the path length difference ·varies the relative phase. At some elevation angle, the magnitude of the reflection coefficient reaches a minimum and the angle changes from out of phase to in phase. This is called the pseudo-Brewster angle.

V1 0\ \

-'----d ----

I

I I

I I

I I

Figure 34. Earth reflection of radio signals

I

I I

I O',fr

V1 "-A

I

SMOOTH~ CURVED EARTH

I 4/3 RADIUS EARTH

I HALF-SPACE REFLECTION COEFFICIENT

I DIVERGENCE FACTOR INCLUDED FOR SPHERICAL EARTH REFLECTION

I DIRECT AND REFLECTED PATH ANr,LES AND LENGTHS

I ANTENNA PATTERN NULLS LIMITED TO -25 DB

Figure 35. Characteristics of earth reflection model

V = RECEIVED VOLTAGE

E = INCIDENT ELECTRIC FIELD

= E + E DJl~-JK(RR - Ro> D R

ED = ELECTRIC FlELD \'IA DIRECT PATH

~ F(90 ) t/GTr ;/4"NRD

ER = ELECTRIC FIELD VIA REFLECTED PATH

= F {9R) VGTPT • /41'r RR

D = DIVERGENCE FACTOR

= rl + 2 RlR2 ~ -1/2

L RD TANY J

R 7 REFLECTION COEFFICIENT

= J:J.2 SIN~- \JN2 -…