UNCLASSIFIED

AD NUMBER

AD853486

NEW LIMITATION CHANGE

TOApproved for public release, distributionunlimited

FROMDistribution authorized to U.S. Gov't.agencies and their contractors; CriticalTechnology; 20 MAY 1969. Other requestsshall be referred to Naval AirDevelopmental Center, Warminister, PA.

AUTHORITY

USNADC ltr 22 Dec 1971

THIS PAGE IS UNCLASSIFIED

CENEjahttsvlll., Wormintstere Pernnsylvania

Report No, NAeC-AE-685?7 20 MAY 1969

Ka-BAND RADOME DESIGN

j PHASE REPORTAIRTASK NO. A3533E12021/RO08-01-0l

Work U~nit No. 48

This doownent ia auhject to 8pec/tal exportcontm~ls anid each txwzerattaZ to foreigngoernenta or foreign nationaZs miay be madeonlkj with prior approval of COMNAVAIRDEVCLWor. COWIAVAIRSYSCOMv (APJ-6022)

DDCJ 1 IN 4 1 1 11

JI 7iTT rTIT

iU41F -DEPARTMENT OF THE NAVY

NAVAL AIR DEVELOPMENT CENTERJOHNSVILLE

WARMINSTER. PA. 18974

Aero-Electronic Technology Department

REPORT NO. NADC-AE-6857 20 May 1969

Ka-BAND RADOME DESIGN

PHASE REPORTAIRTASK NO. A31533EO1/2021/RO08-Ol-O1

Work Unit No. 48

Standard and nonstandard types of radome wall structures arediscussed to aid in determining practical design concepts'for radar systems operating at Ka-band frequencies. Trans-mission efficiencies of selected radome panels are illus-trated and compared for a hypothetical radome designproblem.

Reported by

Radar Division

Approved by: /. 1 7 , (-> '- --M. J. NlMaigan, SuperintendentRadar Division

D. W. Mackiernan \Technical Director

This docwnent is subject to special expoit controlsand each transmittal to foreign goverrnents or foreignnationals may be made only with prior approval of COM-NAVAIRDEVCEN or COMNAVAIRSYSCOM (AIR-6022)

F.! NADC-AE-6857

S -U M M A R, Y

INTRODUCTION

AIRTASK NO., A31533EO1/2021/ROO8-O1-O1, :Work Unit No. 48, was estab-lished for the purpose of conducting research on coficepts for Ka,,bandradome-.design

The Ka-band of frequencies represents the lower end of a large groupthat operates with millimeter wavelengths. Because of the difficulty inbuilding conventional radomes of£ufficient strength and transmissionproperties for ,radar systems opetiting in these bands, new radome designtechniques must be found. Standdrd type radomes have some limited usagefor Ka-band radar systems. This report presents an analysis of selectedstandard and nonstandard approaches to the design of Ka-band radomes..

RESULTS AND CONCLUSIONS

Results of this study indicate that:

1. First' drder* Ka-band radomes do not have the physical strength forgeneral usage on aircraft.

2. A full-wave solid wall of reinforced plastic can provide a radomefor some Ka-band systems. A third-order*, solid-plastic radome wouldprovide a marginal electrical design.

3. The A-sandvich, thin-skin radome is not compatible with Ka-bandradar. An A-sandwich radome with half-wave skins can be used with radaroperating at Ka-band and higher frequencies.

4. The B-sandwich concept (limited to low-die'lectric skin materials) canbe applied to the design of Ka-band radomes.

S. Bi-layer and tti-layer radome wall constructibn using half-waveveneers can be used with radar systems operating in narrowband millimeterwavelengths.

•6. A multi-layer radome wall structure could extend the operating fre-quencies of a Ka-band radar system.

• "Transmission of power is of a periodic nature and varies as the

wall thickness (or any veneer) of the radome is expanded. Theequation to optimize transmission through a solid homogeneous

radome is: Wall thickness = nx/(2 / - sin2 81). The orderrefers to the choice for the value of n.

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f

NADC-AE-6857

RECOMMENDATIONS

The full-waVe wail of a low-loss dielectric material is recommendedin the design of Ka-band radomes. It offers the simplest solution, pro-vided the strength is adequate and that approximately 0.75- to l-db lossin transmission (one Way) can be tolerated. Composite types usingquarter-wave and half-wave veneers can provide better electrical perform-ance at a'higher cost.

For designs of radomes for millimeter wavelengths above Ka-band, newtechniques should be explored'.

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NADCrAE-6857

TABLE OP CONTENTS

SP a g c

t dMAc . . . .. . . . . . . . . . .i

Introduction .ii..i

Results and Conclusions illRecommendations .... . . . . . iv

LISt OF FIGURES .... . . . . vi

LIST OF SYBOLS ... ...... ....... . vii

SCOPE OF ANALYSIS .... ... ........ .

DESIGN APPROACHES .. ................. 3

Solid Dielectric Panels . 0 . . . . . . .Dielectric Sandwich Panels. . . . . . . . . . 5Bi-Layer Panels . . . . . . .. . .. . . . . . . . 20Tri-Layer Panels., ............. 23

DISCUSSION O RESULTS ................. 26

TABLES

I Typical Radome Dielectric Materials. . . . . .. 2II Summary of Ka-Band Radomd Wall, Types

Investigated . . . . . . . . . . .. . . . . . . 31,

e o o e e ~ t e e e e eev

NADC-AE-6857

L I S T 0I F 1 G U',R E S

,Pg u r'e i t I e P a g e

1 Transmission, Through a Solid Dielectric Panel. . 4'2 Transmission and P Measurements Through Solid

Plastic Panels . . . . . . . . . . 63 Transmission Through an A-Sandwich Dielectric

Panel ... .. . . .............. . . . 8

A Transmission Through Dielectric 'Sandwich" PanelWith llalf-Wave Skins (Skin Dielectric 4.15)• . . 9

5- Transmission T1irough ielectr'lc Sandwich PanelWith Half-Wave Skins (Skin Dielectric 5.55) ... 10

6 Transifission and IPD Measurements Comparing Two1lalf-Wave-Skin )ielectric Sandwich Panels. . .. 12

7 Transmission and iPD Measurements ThiroughQuarter-Wave-Skin Sandwich Panel (CoreDielectric 2.5). .... .......... . .. . 13

8 Transmission- and IPD Measurements ThroughQuarter-Wave-Skin Sandwich Panel ,,CoreDielectric 5.0). . . . . . . . . . . . . . . . 14

9 Transmission and IPD Measurements ThroughQuarter-Wave-Skin Sandwich Panel (CoreDielectric 7.5). ............ . . . . . 15

10 Transmission and IPOD Measurements ThroughQuarter-Wave-Skin Sandwich Panel (CoreDiel-ectivic 10.0) . . . . . . ........... 16

11 Transmission and IPD Measurements ThroughQuarter-Wave-Skin Sandwich Panel (CoreDielectric 12.5) . . . . . . . . . . . .. . . . . 17

12 Transmission Through a Ba-Sandwich DielectricPanel. . .... ...................... .... 19

13 Transmission and IPD Measurements Through a Bi-Layer Dielectric Panel (Thickness of Substrate= 0.083 Inch) .... .................. 21

14 Transmission and IPD Measurements Through <a B i .=Layer Dielectric Panel (Thickness of Substrate= 0.166 Inch). . . . . . . . . . . . . . . . . 22

is Transmission and IPD Measurements Through a Tri-Layer Dielectric Panel (Thickness of Substrate= 0.083 and 0.100 Inch) ............ . .. .. 24

16 Transmission and IPD Measurements Through a Tri-Layer Dielectric Panel (Thickness of Substrate= 0.089 and 0.112 Inch) .... ............. 25

17 Transmission Through a Variable Seven-layerDielectric Panel . . . . . ............... 27

18 IPD Measurements Through a Seven-Layer Dielec-tric Panel ........................... 28

19 Transmission Through a; Seen-Layer DielectricPanel as a Function of Fequency ......... .... 29

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L IS T 0 P S Y' BO L 'S

d = Thickness of variable layer (ifichesl

IPD' = Insertion phase delay (radians)

Tj2 = Power transmission coefficient (perpendicular polariiation)

X z Panel wall thickness (inches )

tan i,= Dielectric loss tangent

C = Dielectric constant

0 = Angle of incidence (degrees)

= Free-space wavelength

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NADC-AE-6857

SCOPE OF ANALYSIS

In an airborne radar system, the choice of millimeter wavelengths canprovide a high-resolution system in a relatively small space on the air-frame. The design and fabrication of a radome for such a system are moredifficult than for systems operating at lower frequencies. The scope ofthis report is limited to the design of radomes to be used at the lowerend of the millimeter wavelength region of the electromagnetic frequencyspectrum, specifically the Ka-band, which has free-space wavelengths lessthan a centimeter.

Transmission mea&urements through flat panels of standard and non-standard types of radome wall structures are plotted herein as aids inselecting usable Ka-band radomes. Selected dielectric materials appli-cable to radome construction are to be found in table I. The shape ofthe radome depends on the choice of position on the airframe and therequired aerodynamic fairing. This implies: that the wavefront willusually strike the radome surface at varying angles of incidence. There-fore, the examples of reasonable radome wall designs do consider mini-mizing the difference in the insertion phase delay (IPD) for the spreadof incidence angles anticipated for the radome geometries. If a curvedradome provides good transmission of power, and the phase alignmentapproaches that of the initial wavefront, the pattern distortion is min-imal. A frequency of 35 ghz (free-space wavelength of 0.337 inch) isassumed in the theoretical transmission plots comparing the differentwall types.

Throughout this report, the more critical case of incidence at per-pendicular polarization is assumed in determining transmission. If theradome and the placement of the antenna can be restricted to permitincidence at parallel polarization oniy, excellent transmission canusually be obtained over a broad frequency band and wide range of inci-dence angles. A broader tolerance in construction is available underthis restriction.

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NADC-AE-6857

TABLE I

TYPICAL RADOME DIELECTRIC MATERIALS

Relative DielectricConstant Material

1- _Air

1 Plastic Foam (Polyurethane)1.2 Plastic Honeycomb (Phenolic)

2- Teflon

2.3 Ceramic Foams2.6 Polyphenylene Oxide2.7 Plexiglas

3- Neoprene; Vinyl Molding Compound

3.8 Fused Silica

4-4.2 Polyester Fiberglass Laminate4.5 Polybenzimidazole (Imidite) Fiberglass Laminate

4.9 Furan Epoxy Fiberglass Laminate

S-

*Corning (9606) Pyroceram (Glass-Ceramic);Cordierite

6- Beryllia

6.6 Corning (0120) Glass

7-

8-8.1 Coors (AD-85) Alumina

9-

9.6 Coors (AD-995) Alumina

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NADC-AE-6857

DESIGN APPROACHES

SOLID DIELECTRIC PANELS

Glass-reinforced plastic is a commonly used radome material. Thismaterial forms a homogeneous composite that has a dielectric constantof approximately 4.15 and a loss tangent that remains close to 0.015.Figure 1 illustrates the percentage of generated power that is transmit-ted through panels of various thicknesses of this reinforced plasticmaterial for several angles of incidence of radiation. Note that therecurring peak values for transmission at each angle of incidence remainclose together out to the third order, and that there is a reduction inpower with an increase of the order as a result of heat losses in thematerial.

Thin Wall

The so-called thin-wall panels (zero order) are extremely thin andhave very little strength for aerodynamic loadings. It is conceivable,-however, that electromagnetic windows this thin (0.015 to 0.020 inch)could be used in side- and aft-looking radar systems on an aircraft.For radiation near normal incidence, the expected transmission of powerthrough these electrically thin windows would be approximately 70 to 80percent.

Half-Wave Wall

The half-wave wall (first order) is also thin. The effectiveness ofits use as a window on high-speed aircraft would depend on the windowbeing in a protected location. The design thickness of such a radomewould be 0.085 to 0.095 inch, according to the variation of incidenceangles that must be accommodated in allowing full mobility of the radarantenna. The power transmitted would exceed 84 percent of incident powerfor a wall span of 0.095 inch for angles up to 70 degrees. The IPD ofzero incidence and that of the maximum angle differ by 0.75 radian forthe 0.095-inch panel, and by only 0.3 radian for the 0.085-inch panel.The design for a specific radome shape for half-wave wall would have aminimum pattern distortion if the 0.085-inch wall were used, providedof course, that little power is transmitted at incidence angles above50 degrees.

Full-Wave Wall

The full-wave wall (second order) of reinforced plastic has suffi-cient strength for most Ka-band radome applications. The transmissionis slightly lower compared to a half-wave wall, because of the increasedheat loss through the additional material. Also, it is more difficultto design a constant wall thickness to provide adequate transmissionthrough a broad spread of incidence angles. Nevertheless, a wall designof 0.185 inch would permit transmission through angles of incidence from

NADC-AE-6857

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NADC-AE-695S7

0 to 70 degrees at a loss of only 1.5 db or less. There is also anincreased difference in the IPD between minimum and maximum angles overthe half-wave design.

Solid plastic radomes of increasing orders are much more difficult todesign, and their effectiveness is marginal. Transmission at the peakthicknesses (optimum for specific incidence angles) declines graduallyfor higher orders, and a given choice of wall thickness will permit lessvariation in the incidence of radiation.

Figure 2 compares four usable, uniform, solid.-plastic radome walldesigns for a hypothetical case. it assumes the shape of the radome issuch that the angles of incidence are between 0 and 60 degrees. Thepermissible loss in transmission (one way) is assumed to be 1.5 db.Clearly, the optimum electrical design is the half-wave wall (0.090 inch),wherein approximately 90 percent of the power would be transmitted regard-less of the orientation of the antenna enclosed in the window. In thisdesign, the IPD curve is relatively flat over the span of incidence con-sidered. The second choice is the full-wave wall, which is obviouslyphysically stronger, and permits transmission greater than 82 percent ofthe power out to an incidence angle of 50 degrees. Here, the minimumtransmission of power would occur at the maximum angle of incidence,which would be 70 percent (- 1.5 db). In this design, the slope of theIPD curve is not severe; therefore, a small distortion of the wave frontis anticipated. Both the half-wave and the full-wave wall designs aresuperior to the thin-wall (0.015-inch) design in transmission effective-ness. The 0,015-inch wall has the minimum spread in IPD. It is physi-cally thin for plastic construction, but it is not electrically thin.Figure 2 also shows a third-order radome, which illustrates a strongwall that is a marginal electrical design.

DIELECTRIC SANDWICH PAINELS

A-Sandwich

The majority of the sandwich radomes built for aircraft are of thesymmetrical A-sandwich type. They have two thin outer skins of a soliddiele tric material separated by a lightweight, low-dielectric core mate-rial. This type of construction provides a high strength-to-weight ratioand good electrical qualities for X-band and lower frequencies. Usually,the A-sandwich is highly competitive with the solid-wall type radomesdiscussed previously. In the classic A-sandwich, reflections from thetwo skins cancel when they are spaced an integral number of quarter-wavelengths. The skins cannot remain exactly a quarter-wave apart overa frequency band or a range of angles. Thin skins and low-dielectriccores produce reflections small enough that the skins need not be exactlya quarter-wave apart to produce an acceptable transmission level.

5

NADC-AE-685i7

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NADC-AE-6857

The skins should be very thin electrically so the reflections fromeach face of a skin are nearly 180 degrees out of phase. (For Ka-bandthin electrically means extremely thin physically, on the order of 0.010inch or less. Even this is not satisfactory at high angles of incidence,where electrical thickness is a rapidly changing function.) The corcdielectric constant should be low so the reflection from the air-skininterface is nearly equal to the reflection fromn the skin-core interface.

To evaluate the use of the standard A-sandwich radome in the Ka-band,it is reasonable to assume and analyze a conventional construction usinga reinforced plastic skin material with a dielectric constant of 4.15and a core of plastic honeycomb construction with a dielectric constantof 1.25. Figure 3 illustrates transmission thrcugh a panel of suchdesign with a skin thickness of 0.033 inch. The abscissa is the vari-ation of the core thickness. Unlike the transmission plot of a solidwall with variable span, only the first-order peak values remain closeenough to choose arbitrarily a unique core span to accoimmodate variableincidence of radiation. Assume that an A-sandwich radome is to bedesigned to satisfy the hypothetical problem investigated in the solid-wall study. The choice of a core span of 0.020 inch would transmit from86 to 92 percent of the power generated for variable incidence from 0 to60 degrees. The overall physical thickness of the sandwich, however, isonly 0.086 inch, and it would have less strength than the first-ordersolid wall built of the same material as the skin of the sandwich. Asurvey of the second order clearly indicates that no unique core spanwill transmit sufficient power over the span of incidence considered forthe design. The curves of figure 3 show clearly the penalty imposed bythe 0.033-inch skins that are not electrically thin.

1lalf-Wave-Skin Sandwich

The concept of using a sandwich-type radome structure to obtainstrength by increasing the wall span while maintaining low weight, neednot be abandoned. A half-wave wall of the reinforced plastic designedfor transmitting Ka-band at high incidence is approximately 0.090 inch.If the skins of the A-sandwich radome are replaced by half-wave spans,the radome will have greater strength and can be applied to Ka-bandand higher frequencies. Figure 4 is a transmission plot through thistype of wall construction. It is interesting to note that "his plotshows that transmission at most angles of incidenec is rather insensi-tive to the choice of core size. The peak values of the curves areaffected by the absorption of energy passing through the skins. The lowvalues of transmission for each angle of incidence through this typesandwich match closely the transmission through a homogeneous span ofthe core material itself. Another exar'ple is to replace the plastic skinswith half-wave walls of a relatively lossless media. Figure 5 displaysthe transmission through such a sandwich usin g pyroceram (F- = 5.55,tan 6 = 0.0002) for the half-wave skins. This extremely low-loss materialpermits the peak values of transmission for each incidence angle toapproach unity coefficient.

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NADC-.\L-0857

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NADC-AE-6857

PERPENDICULAR POLAPIZArIONiFREQ.: 35ghz1.0

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FI1GURE 4 -Transmission Through Dielectric Sandwich Pane]

h'ith i1alf-Wave Skins (Skin Dielectric 4.15)

91

NADC-AE-6857

PERPENDICULAR POLARIZATION; FREQ.=35 qhz

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FIGUREi 5 r ransmission Through Dielectric Sandwich Panel

With 1half-Wave Skins (Skin Dielectric 5.55)

-10 -

NADC-AE-6857

Again, assume the hypothetical design problem used in the solid-wallanalysis. A core span of 0.250 inch is a convenient choice. Figure 6illustrates the transmission through a panel of this core with half-wave pyroceram skins. Two other core sizes are plotted for comparisonto reveal the small change in transmission that occurs if the core toler-ance is ±0.025 inch. A phase delay difference plot is also included.In comparing both sandwich designs, the low-loss quality of the pyroceramskin sandwich is preferable.

B-Sandwich

The B-sandwich has intrigued the radome engineer since its inception1

in 1948. It consists of a high-dielectric core faced with low-dicIcetricskins, it is the reverse order of the A-sandwich. The excellent electro-magnetic properties of the B-sandwich are obvious to anyone familiar withthe use of lens coatings on optical instruments. Ilie low-dielectric skinsact as quarterwave matching transformers; herein lies the major objectionto most B-sandwich proposals, because the low-dielectric materials of theskins are usually less durable when exposed to the flight environment.

A Chebyschev or Butterworth transformer synthesized to match freespace to a panel made of reasonable dielectric materials will have the low-dielectric-constant layer at the surface. Its dielectric constant willapproach unity as the number of sections increases. Unfortunately, mostdurable materials have dielectric constants above 2. Because of thismaterial limitation, transformer matching with dielectric layers is limitedto one or two sections. One-section Chebyschev and Buttcrworth trans-formers are identical. The structure is known as a 1I-sandwich. Othertransformer types would behave similarly.

Although the classic filter synthesis, when applied to radome design,leads to a fragile structure (primarily because of the use of low-dielec-tric skin facings), the filter theory can provide direction to radomedesign efforts. Low-dielectric quarter-wave skins improve the electricalperformance of a dielectric wall (core) over a rane of frequencies andangles of incidence as the core-skin relationship approaches the classicfilter. For figures 7 through 11, a wall span of 0.140) inch is chosen,and the core (0.020 inch) is permitted five discrete dielectric values:e = 2.5; S.0; 7.5; 10.0; 12.5. It is apparent that the best broadbandcondition occurs when the dielectric constant of the core is some valuethat lies between 7.5 and 10.41. ( 'see figures 9 and In.) The dielectricsin these graphs are neoS the classic matching network (iP-sandwich radoie)

It is interestixig to note that for a given basic wall material, the

shlns (matching transformers) can either he used with the basic materialas a core, or the basic Iaterial can be uscd as a quarter-wave transformer

11.45C Perozal '.'ote4- on Larly IajoP'c .istor?,:Technique for Airl"or'ze Radorc Dccign, 1o0 I: Air ForccAvionics Laborato i ;V'PAF Report :.o. AFAL-TR-66-301 Up 11and

- 11 -

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NADC-AU-6857

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FIGURE 7 - Transmission and IPD, Measurements Th'lrough (?uartcr-l'.avc-Skin Sandwich Panel (Core Dielectric 2.5)

-13

NADC-AH-68S 7

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FIGURE 8 - Transmission and IPI) Measurements Th1rough Quartcr-W\avc-Skin Sandwich Panel (Core Dielectric S.0)

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NADC-AFi-6857

1.0 PERPENDICULAR POLARIZATION

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FIGURE 9 -Transmission and IPI) Measurements Through Quarter-Wave-Skin Sandwvich. Panel (Core Dielectric 7.5)

isI

NADC-AE-6857

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170

NADC-AE-6857

with a higher dielectric material used as a core (load). This is con-sistent with the proof by Fano2 that ideal matching structures are idealfilters. Increased performance in the pass band is accompanied byincreased reflections in. the stop band.

The dielectric constants of the B-sandwich skins and core are sochosen that the characteristic admittance of the skin material is thegeometric mean of the admittances of the environment (air, eo = 1) andthe core, all of which are functions of the angle of incidence (0). (Seeequation (2).) The oft stated 'square root of the core" is the specialcase of normal incidence. The normalized characteristic impedance (Z)in a medium (neglecting reflected waves) has been defined3 as:

Z = 1/' ( -sin2 & (1)

and the admittance (Y) equation after squaring becomes:

= C-sin 2 6 /c sin2 0)(:E-sin 2 e)= /Y y (2)sC c air

where subscripts

o = free space,c = core, ands = skin.

Note that for normal incidence (6 = 00), equation (2) is simply:

C (3)

The graphs of transmission are similar to the plot of some filterperformance. This suggests application of the extensive literature onfilters (and matching) to the radome design problem. Thus far, the B-sandwich remains the sole example, primarily because of the lack ofphysically rugged materials with low dielectric constants for the skins.

For most radome designs, the flexibility in the choice of dielectricconstants is limited to the list of durable, inexpensive materials readilyobtainable on the commercial market. 'The quarter-wave skins for Ka-bandare reasonably thin for low-dielectric materials. The core, constructedof a higher dielectric material, can be varied in thickness and made suf-ficiently strong to be the prir,.e load-bearing member. An example of apractical B-sandwich radome wall to accommodate the hypothetical Ka-bandradome design is illustrated in figure 12. The quarter-wave skins

2. Matthaei., Young, and Jones; M4icrowave Filters, Impedance-Matching Networks and Coupling Structures; I9 4; I/cGrcmo-Hill; Chapter 1.

3. Snow, 0. J., 6 Apr 1953; Application of ,he I-pediance Conceptto Radome Wall Design; iRAVAIRDEVCE17 Report iRo, h'ADC-EL-521J6,

- 18-

NADC-AE-6857

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-19-

NADC-AE-6857

(transformers designed for 50 degrees incidence angle) are assumed to beof Duroid (reinforced Teflon) or of Polyphenylene oxide (PPO), bothhaving a dielectric constant of approximately 2.5. The core dielectricconstant determined by equation (1) should be 9.499. A material nearthat value is alumina (e'-9.0) and is chosen for the core.

The one-way power transmitted will remain equal to or above 82 per-cent, regardless of the choice of core thickness. Inasmuch as the skinsare quarter-wave for SO degrees incidence angle, the power remains con-stant at 87.7 percent for this incidence angle. There is some oscillationfor the radiation incident at angles above and under the design angle ofSO degrees. Alumina has a low loss tangent (tan 6 - 0.0002), and istherefore assumed lossless in figure 12. It can be seen that preferredchoices of core thickness occur to optimize transmission. The weightrequirement and the need to minimize the phase distortion, direct thatthe choice of wall span be a minimal value commensurate with the struc-tural and aerodynamic demands.

BI-LAYER PANELS

An unusual but workable design is a radome wall made from two lami-nations of different dielectric materials each a half-wave in thickness.Because half-wave spans are relatively thin for most radome materials atKa-band, such bi-layer radomes will not be excessive in weight for air-borne application.

An example of such a design is to have an outer half-wave-wall shellof pyroceram bonded to an inner half-wave shell of glass-reinforcedplastic. Figure 13 illustrates the transmission through a wall panelbuilt to this configuration. 'The plastic wall thickness is 0.083 inchand is half-wave for normal radiation; the thickness of the p'yroceramlayer is varied and would depend on the anticipated variation of inci-dence of radiation (radome shape). The pyroceram veneer of 0.077 inchbacked up by the 0.083-inch glass-reinforced plastic is a good choiceto accommodate the hypothetical design problem previously considered.

Essentially, the half-wave bi-layer is a second-order radome, andwhen compared to that of the full-wave homogeneous plastic wall (figure14), the transmission properties of the bi-layer are slightly superior.Also, the curve of the difference in the IPD shows that the phasingproblem of the bi-layer is less critical than that of the full-wave wall.The total wall span is 0.160 inch, and is competitive physically with thefull-wave wall.

An extension of this technique is to compare a wall design of a half-wave pyroceram veneer bonded to a full-wave panel of glass-reinforcedplastic with the third-order plastic wall illustrated in figure 2. Abi-layer radome consisting of an outside pyroceram shell of 0.079-inchthickness and a glass-reinforced plastic substrate of 0.166 inchappears to be a good choice for the hypothetical design problem assumed

- 20 -

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in this report. The total wall span is approximately a quarter of aninch. Figure 14 indicates that this bi-layer design is electricallysuperior to the third-order plastic design, with regard to both the trans-mission and the phase alignment.

It is to be noted that relatively high-dielectric materials can beused to advantage in the bi-layer construction, especially with materialshaving very low heat losses.

TRi-LAYER PANELS

An extension of the bi-layer design technique is to add an additionalhalf-wave panel. Three half-wave panels of dissimilar dielectric mate-rials form the tri-layer panel. For example, assume that a panel isconstructed of pyroceram (el), glass-reinforced plastic (C2), and a vinylmolding compound (c3). Figures 15 and 16 illustrate the theoreticaltransmission of 35 ghz power through such a panel. The thickness of theouter pyroceram layer is a parameter in both figures. The thickness ofthe other layers are fixed, half-wave spans (for normal incidence in fig-ure 15 and for high incidence in figure 16). It is apparent that a fewmils added or subtracted to any of the veneers can adjust significantlythe transmission plot as a function of the angle of incidence.

Assume the tri-layer wall is to be applied to the hypothetical radomedesign problem. In figure 15, the second and third layers are half-wavepanels for normal incidence, and a choice of 0.082 inch for the pyroceramlayer will permit an average power transmission of approximately 78 per-cent. The minimim transmission is 72 percent and exceeds the minimumtransmission required for the hypothetical radome. A look at figure 16,with the second and third layers half-wave for high incidence, suggeststhat a good choice for the pyroceram layer is 0.073 inch. This wallconstruction will also provide an average power transmission of approx-imately 78 percent. The IPD curves for both are comparable to the third-order homogeneous reinforced plastic wall (see figure 2).

To design a constant-wall, tri-layer radome that will accommodateradiation at variable incidence, the following technique can be applied.First, choose the veneer material with the lowest heat loss (or lowestdielectric constant if loss tangents are nearly similar) to be half-wavefor the maximum angle of incidence. Then, fabricate at least one ofthe other veneers as a half-wave for the lowest angle of incidence. Thechoice of half-wave of the third veneer can be flexible, depending onwhere the emphasis is required to accommodate the majority of incidence.Ordinarily, if all angles of incidence are of equal importance, then thechoice is to make the third veneer also a high-incidencc, half-wavestructure. Note that a tapered wall can be made by machining just asingle veneer in the radome construction. The use of tri-layer radomesis limited to narrow bandwidths.

-23-

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MULTI-LAYER PANELS

One radome concept relatively easy to study with the matrix form ofthe equations of transmission is that of the periodic structure. Here,a panel or set of panels is repeated to form the total structure. Someinteresting phenomena occur in the transmission of perpendicularly oolar-ized signals through these periodic structures. The dielectric wallbehaves as a low-pass, equal-ripple filter, with the number of ripplesequal to the number of repeated panels. The sharpness of the cutoffincreases with increasing order, but the cutoff frequency is a functionof the basic cell structure.

This report is not intended to cover an extensive study of multi-layer radome designs. However, a seven-layer panel is included to illus-trate a broadband structure for the Ka-band frequencies. The designphilosophy is to use two A-sandwich walls bonded together with an addedlayer of the core material. Figure 17 illustrates the change in trans-mission of power as this middle core (d) is expanded. The IPD data isalso plotted (see figure 18) for a discrete choice of the core (d = 0.100inch).

The dielectric materials used in this multi-layer panel are thefamiliar plastic wall structures. The A-sandwich segments are first-order designs optimized for an angle of incidence of 50 degrees. Thethickness of each skin is 0.020 inch. The outer core spans are 0.053inch. (Note that the standard phenolic honeycomb core material is usuallycut to thickness values of one-eighth inch or larger.)

This seven-layer wall structure demonstrates the feasibility of multi-layer application to Ka-band radar. Figure 19 displays the transmissionof power through the seven-layer panel (d = 0.100 inch) as a function offrequency. For the familiar radome design problem assumed in this report,such a wall design could be used for radar operating in the frequencyband from 33.0 to 37.2 ghz.

DISCUSSION OF RESULTS

Table II lists in summary form the Ka-band radome wall types inves-tigated during this analysis, and presents facts pertinent to theirapplicability.

The thin-wall, half-wave-wall, and first-order A-sandwich radomescannot be used to house Ka-band radar antennas on an aircraft at posi-tions exposed to severe air loadings. Second- and subsequent order thin-skin A-sandwich designs are not practical for Ka-band or higherfrequencies.

- 26 -

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- 29 -

NADC-.AE-6857

The simplicity of construction and reasonable electrical propertiesof . full-wave wall design (provided it is physically adequate for theproposed position on an aircraft) make this a favorable choice for a .Ka-band radome. More complicated designs employing half-wave veneers canprovide better transmissibility for a narrow choice of frequencies. Theseinclude half-wave-skin A-sandwich walls, as well as bi-layer and tri-layer designs.

The D-sandwich radome is severely limited by the materials that maybe used in its construction. However, this design permits a broad toler-ance in its manufacture; can be made stronger by increasing the corewith negligible effect on the electrical properties; and allows a broadbandwidth.

Any reasonable veneer of dielectric material used in the design ofelectromagnetic windows for millimeter wavelengths exceeds thin wall.An electrically thin wall is assumed to be -much less than a quarter-wave-length through the material. Therefore, if airborne radar systemsemploy extensive use of these frequencies, it will be necessary toinvestigate new radome wall concepts.

Further study on the applicatiou of the multi-layer concept is recoil-mended. Other studies of interest include investigation of tuned react-ance walls and polarization controlling devices.

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NALC-A-6857

SU1MIARY OF Ka- BAND RAL)ONL' WALL. TYPEiS INVESteIGATEI1)

t~ypi calI Phys ical L onstruct i on Plrue r i-roadl',adlype of Ihail 1taterials sticngth 1olicen I rauni ss io In, Ipal I Ity

lain hal I 1aIss-rcipeorced(zero order) plastic Very weak Loose lioN I wted

- hiass-ceranics. Vcry weak Loose 'srjod LimintedtCrami es Ve r) Weak~ Loc~o F.,i r !Ii I ted

nal f-dave Plast ics heaL Average erv d'oo L'i-; Led(first order, lass-ccrantics lWeaL Averae ter% -,o-,d L,:.itud

(eranics hcak Avcerage Ier% fod iiit ed

Full-havc Plastics Good Average NZIf.1t.

isccond orderl W.ass-ceramics Good Iit 1.00t.,I oLe raini cs (,ood I P. It 00d 1.) i I t cd

Ifirst order) (.lass-reitiforcedplastic skillshoneycomb core 4 cak K erap'e (W 1 i r

(second, order) tlass-reinforcedplastic skins-honeycomb core G ood I gat 0or'

lial f-Si've-S.za1 Glass-reinforcedban d'sich plastic skins %,cry Good Averige & I td

(dJass--ceramicSWIaS %cry good Average 1r%: ,ooi. ij r

ll-Sandwicia Plastics,laminatedfibierglass,and ceramics (:ood Loose ~ o

IL51-Layer Vlastic andglass-ceramics Good light 11ond li

Iri-Layer I lastic andglass-ceratmics Good Tight i a ir - ou 1.11 mu,

! cvcn-Laycr Laminatedfiberglass -

laioeycomb core Good Lxcel Icitt for )ot, any~les ot aiscideiite

Skins should lie of a material to withstand rain and heat erosion.

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4

UNCLASS-I FIEDS ii'i ;tv Cta 4 ia'ion 7

DOCUMENT CONTROL DATA.R&DSet.4(ttv Cl.'bfic ation of 1tu10 body of nbstraCI asd indexing annotntiun nw,, . ,ered when tho overall report 1. .tlacsthed

ORIGINATING ACTiVITY (Corpora(e author) 12a. REPORT SeCURITY CLASSIFICATION

AERO-ELECTRONIC TECHNOLOGY .DEPARTMENTJ UNCLASSIFIEDNAVAL AIR DEVELOPMENT CENTER,, JOIINSVILLE 2b. GROUP

WARMINSTER, PENNSYLVANIA 189743 ,REPORT TITLE

.

Ka-BAND RADOME DESIGN

4. DESCRIPTIVE NOTES (Type of report and-inclusie dates)

P1 ASE REPORT5, AU THOR(SI (First name,middle Initial, last name)

P. J. CARROLL AND J. R. CIESNUT

6- REPORT DATE 7a. TOTAL NO, OF PAGES 7b. NO. "OF REFS

20 MAY 1969 38' 3 (FOOTNOTES)6a. CONTRACT OR GRANT NO 9a. ORIGINATOR'S REPORT NUMBE'R(S)

NADC-AE-6857b. PROJECT NO.

AIRTASK A31533E01/2021/R008-01-01 -

cWORK UNIT NO. 48 9b. OTHER-REPORT NO(S) (Any other numberx that may be asslgnedthis report)

d.

10 DISTRIBUTION STATEMENT "

THIS DOCUMENT IS SUBJECT TO 'SPECIAL EXPORT CONTROLS AND LACI! TRANSHITAL TO FOREIGN

GOVERNMENTS OR FOREIGN NATIONALS MAY BE MADE ONLY WIT!! PRIOR APPROVAL OF CONAVAIR-DEVCEN OR COMNAVAIRSYSCOM (AIR-6022)

11. SUPPLEMENTARY NO'TES 12. SPONSORING MILITARY ACTIVITY

NAVAL AIR SYSTEMS COMMANDDEPARTMENT OF TI!E NAVY

13 ABSTRACT

Standard and nonstandard types of radome wall structures are

discussed to aid in determining practical design concepts

for radar systems operating at Ka-band frequencies. Trans-

mission efficiencies of selected radome panels are illus-

trated and compared for a hypothetical radome design

problem.

DD ,FORM 4-73 (PAGE 1)DINOV J 473UNCLASSI FI EDS/N 0101.807-6801 Security Classification

U N C L A S S I F I E D0,kcurity Classification

KEY WORDS LINK A LINK 0 LINK C

ROLE WT ROLE WT ROLE WT

RADOMESDIELECTRICSMI LLIMEITER WAVELENGTIIS

DD INOV651473 U N C L A S S I F I E D(PAGE 2) Security Classification