2.4_Lehman Chambers_Technical Proposal.pdf

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DAHLSRFJX,-,?'JISION

TAPERED

ANECHOIC CHAMBER

REFURBISHMENT

Volume ll r tff"llical Pronosal

PRESENTED BY

I.EHIhAT GHAITIBER$ Solicitation #NOOI 78-98-R-301 6

January 30, 1 998

NSWC - Dahlgren Division Volume ll - Techniel Proposal Table af ContentsTapered Anechoic Chambe r Refurbishment

TABLE OF CONTENTS

INTRODUCT|ON............ ...'.......'

SECTION I- MANDATORY REQUIREMENTS.... "......"...."..' 6

1.0 0.A. PLAN AND PROCESS CONTROLS ...........'

1.1 GeneralQuality Standards.... ..'......-.....'

1.2Key Process Controls...... ....'...........".'

1.3 Reflectivity Testing -...-.."..-"".7

1.4 Fire Retardancy Testing '-.'.'...7

1.5 Conc1usion................ """""7

2.0 CONICAL AREA ABSORBER ."..........'.'.8

3.0 SCHEDULE........... .'...."...'.' I

SECT]ON II- TECHNICAL UNDERSTAND]NG AND APPROACH.......... .........."..""' 10

1.0 TNTRODUCTTON ...............'.10

2.0 ELECTROMAGNETIC DESIGN CONSIDERATIONS AND CHITERIA.. ............1 1

3.0 CHAMBER LAYOUT ,...."..,.17

3.1 Absorber Layout......". """"'17

3.2 Receiving End Wa11...... .'--"..17

3.3SideWallsAndOeiling....."... ....."""""""17

3.4 FloorAbsorber........... .."'""'18

3.5 ChamberTaper............ """'18

3.6OonicalTaper.......... """""'18 3.TGeneralDescriptionOfTheC-RamSFCTypeAbsoherUsed....'.'...' .....'.'.'."'...."18

4.0 CHAMBER DESIGN .'.........' 19

5.0 PERFORMANCE VER|F|CATION............ ...'..'.-..'...'22

6.0 CONSTRUCTION PRACTICES .....'.'...".23

6.1 Project Management """"""23

6.2 Chamber Refurbishment,"............... """"24

6.3 Testing and Drawings

SECTION III - CORPORATE EXPERIENCE AND PAST PERFORMANCE

sEcTtoN lv - PERSONNEL QUAL|F|CATIONS.......... .......' 38

1.0 DESIGN PERSONNE1................ ....-

2.0 SITE PERSONNE1................ ...-..'

sEcTloN v - coRPoRATE RESOURCES...... "'....'..-.......40i IEH]NAH GHA]fIBERS

Paul E Lehman, lnc

January 30, 1998 PaP#2

NSWC - Dahlgren Division Volume ll - Technial Proposal lntroductionAnechoic Chamber Ref urbishment

INTRODUCTION

I*hman Chambers has closeiy analyzed the requiremmb of the NSWC - Dahlgren

Division, Tapered Anechoic Chamber Refurbishment project and has proposed a complete tum-key design/construction solution compliant to the specifications of this project. Lehman Chambers is prepared to execute the requirements of this project with an experienced team of known industry leaders having a proven record of past performance.

Lehman Chambers offers a single'source project team fully capable of providing all labor, materials, equipment, tools, supplies, and incidentals required to design, layout, construct, fabricate, install, integrate, test and otherwise make ready for use the existing tapered anechoic c-hamber with an absolute minimum risk to the govemment.

Achieving the high absorber performance requested is no simple matter. The successful accomplishment of such a task requires optimized absorber materials, absolute design and installation accuracy, intensive absorber quality control, precision calibration of test equipment and perhaps most importantly Proven project performance.

I€hman Chambers has successfully completed to date over forty anechoic chamber projects to include; antenna measurement chambers, compact ranges, 3 & 10 meter EMC chambers, relocations, and reftubishments. We have worked successfully with the U.S'

Govemment on projects at NADEP ]acksonville, NADEP Pensacola, Letterkenny Army

Depot, NASA Langley Research Center, the Naval Researdr Laboratory, and currently at the National Institute of Standards and Technology (NIST) on a chamber refurbishment and on a compact range/host building at Wamer Robins Air Force Base. l€hman

Chambers has never had a contract terminated or failed to meet a promised performance specification.

I-ehman Chambers will work with the Cuming Corporation of Avon, MA and Advanced

Testing and Design Services (ATDS) of Decatur, GA to accomplish the anechoic absorber design and testing. These companies are proven leaders in the field of anechoic drambers with over 40 years of combined experience in the design, testing, and installation of anechoic absorber.

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Pap#3

Tapered Anechoic Chamber Refurbishment lntroduction

Cuming Corporation was founded in 1980 by Dt. William R. Cuming noted engineer, industrialist, and expert in radar absorbers and composite materials. Dr. Cuming is also the founder of Emerson & Cumin& the well known supplier of plastics for electronics. Dr.

Cuming sold Emerson & Cumingin1978 and left to pursue other interests. His experience as a consultant to many of the countries leading electronics and aerospace companies led him to the conclusion that an urgent need existed for a knowledgeable and responsive vendor of radar absorbers, syntactic foam, dielectric plastics, and other highly specialized materials.

Today Cuming Corporation is a welkstablished supplier of a broad array of high-tech products to many customers in the United States, Canada, Western Europe, Israel and the Far East. Customers indude General Dlmamics, Hughes, Lockheed-Martin, McDonnell/Douglas, Sanders Associates, Shell Offshore, Mobil Oil, John Hopkins

University, and the Armed Forces. Although still classified as a small business, the firm looks forward to continuing its dynamic growth. Additionally, Cuming Corporation has contracted with Mr. Leland H Hemming, an anechoic design consultantwith over 17 years experience, to complement their design staff.i

IEHMAH GIIAMBERS

Paul E Lehman, lnc

Page # 4

Tapered Anechoic Chamber Refubishment lntroduction

ATDS (formerly lnown as the Howland Company) was founded by Ray Howland in

L975. ATDS is an independent mgirrcering firrr staffed with registered, .graduate engineers. ATDS provides design analysis, development, installation assistance, trouble' shooting, evaluation, training, and feasibility study services in the fields of antenna ranges, anechoic chambers, and radar cross section test facilities. ATDS also specializes in electromagnetic interference/compatibility, shield design, and nonionizing radiation hazard studies and the design and installation of instrumentation and measurement systems for test facilities. ATDS has worked with numerous industrial companies such as

L-ockheed-Martin and Motorola, as well as the goverrunent, to design anechoic test facilities. ATDS is currently working with l*hman Chambers on the compact range project at Warner Robins Air Force Base.

As a summary, t"ehman Chambers has assembled a top-notch project team for this tapered chamber refurbishment. We have strived to provide the "Greatest Value" solution-for the govemment and back up onr proposal with proven performance records.

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IEHHAI{ GHAMBERS

Paul E Lehman, lnc

Pap# 5

NSWC - Dahlgren Divisian Tapered Anechoic Chamber Hefubishment

Volume ll - Technial Propasal Mandatory Requirements

1 .O Q.A. PLAN AND PROCESS CONTROLS

1.1 General Quality Standards

l€hman Chambers will contract to Cuming Coqporation for the manufacture and delivery of the anechoic absorbing material for this refurbishment. Cuming Corporation manufactures C-RAM SFC broadband anechoic material, and its related products, according to a Standard Manufacturing Plan, which is in accordance with MIL-1-45208 quality standards. All inmming raw materials are controlled, in-process work is bgged and identified on the shop floor, and finished goods are labeled by lot. There is full traceability of raw materials and process measures to every finished piece.

Following a contract award, the Product Manager, Quality Manager, and Manufacturing

Supervisor will establish a written Quality Assurance Plan specific to the job. This will be reviewed and agreed to by all parties, ensuring that the product is manufactured, tested, and packaged in accordance with the terms of the contract.

For the purpose of sampling for testing, a production batch (or lot) is defined as comprising all parts saturated in carbon solution on a given workday. The batch numh will be coded to the date of saturation. A production batch may typically range from 60 to 500 parts 2' x 2'.

1.2 Key Process Controls

[r order to assure consistent loading of the absorber material, both with respect to fire retardant material and dielectric lossy materials, a series of process controls are maintained. Batch travelers follow the product on the shop floor, and key process controls are recorded.

1. The size, geometry, and weight of the raw foam parts are checked for uniformity prior to saturation.

2. The saturated (wet) weight of each unit is checked for consistenry and compliance with the production procedures.

3. Prior to us€, the safurant is checked for specific gravity to assure the presence of the required amount of fire retardants and RF lossy materials"

I.EHMAII GHA]TIBERS

Paul E Lehman, lnc

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NSWC - Dahlgren Division Volume ll - Technial Proposal Tapered Anechoic Chamber Befufuishment Mandatory BEuirements

4. Prior to removal from the drying ovens, the product is probed for proper dryness.

5. After drying a sample of pieces from each lot is probed intemally, measr.ring for DC conductance, to ensure there are no unsafurated spots within the pieces.

6. Final product painting (if requested) is performed as a hand spray painting operation. To achieve consistent and uniform appearance, a constant sprayrng angle is maintained and a master color samples is used for color comparison.

1.3 Reflectivity Testing

For the material quoted, each production batch will be tested for reflectivity at L Band.

The measurement is made against a 6 ft. x 6 tt. reference plate; 9 pieces of 24 n. x 24 rn.

absorber are tested at a time.

1.4 Fire Retardancy Testing

Cuming Corporation, as a matter of policy, tests a sample from every production lot according to NRL-8093 Tests 'J., 2, and 3. As this is a destructive test, a single sample from each lot is chosen for testing. Written records are kept on file, and will be submitted to the goverrunent upon request as a deliverable.

All C-RAM SFC-7zand C-RAM SFC-18 will be sample tested (5% sample size minimum) at L00 MHz through approximately 500 MHz.

Loading of the slab material for the conical tip will be verified via insertion loss testing"

1.5 Conclusion

These quality control procedures will be strictly adhered to and will lead to top quality anechoic absorber material being used for the anechoic chamber refurbishmmt. Combined with precise installation procedures the anechoic absorber material wil be of equal or better specification than the original material used.

I.EHMAH GHANBERS

Paul E Lehman, lnc

Page # 7

Tapered Anechoic Chamber Refurbishment Mandatory REuirements

2.0 CONICAL AREA ABSORBER

As discussed later in Section tr, Technical Understanding and Approach, the conical area absorber will covered using only flat anechoic absorbing material. The flat material will be gradually transitioned to wedged material in the tapered section.

3.0 SCHEDULE

I€hman Chambers will complete all construction" induding testing, within 180 days of contract award. Furthermore, we will strive to complete the refurbishment with as aggressive a work and delivery sctredule as possible. Following is a preliminary project schedule showing the major tasks necessary to complete the project and an estimated timeline.

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Page # I

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1 . O INTRODUCTION

This section discusses principles and techniques for the establishment and proof of adequate elecbomagnetic characteristics of the chamber test environment. [r pardcular, this discussion relates to test facilities which are intended for use in measuring the free-space far-zone performance of antennas.

For purposes of consistenry in notation, it will be convenimt to assune that all antennas are tested on receiving. The criteria and techniques discussed herein are also valid for the design and evaluation of ranges used to test antennas operated on transmission.

The ideal test environment for determining far-zone performance would provide for a plane wave of uniforsr amplitude to illuminate the test aperture. Various approaches to simulation of this ideal electromagnetic environment have led to the evolution of two basic types of antenna test ranges,

(1) Free-space Ranges

(2) Reflection Ranges

Free-space ranges are those in which an attempt is made to suppress or remove the effects of all surroundings, induding the range surface or other surfaces, on the wavefront which illuminates the test anterna. This suppression is sought through one or more of such factors as (a) directivity and sidelobe suppression of the source antenna and the test antenna, (b) clearance of the line of sight from the range surface, (c) redirection or absorption of enerry reaching the range surface, and (d) special signal processing techniques such as gating of the desired signal or by use of short pulses or both.

The typical geometries associated with the free-space approach include the ELEVATED RANGE, thE SLANT RANGE, ANd thc RECTANGIILAR ANECHOIC CHAMBER.

Reflection ranges are designed to make use of €nergy which is reradiated from the range surface(s) to create constructive interference with the direct-path signal in the region about the test aperture. The geomety is controlled so that a small, essentially symmetric, amplitude taper is produced in the illuminating field. The two major types of reflection ranges in use are the GROTIND REFLECTION RANGE and the TAPERED ANECHOIC

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IEHMAil GHAMBERS Paul E Lehman, lnc

Page # 10

NSWC - Dahlgren Division Volume ll - Technical Proposal Tapered Anechoic Chamber Befurbishment Technical Understanding and Approach

2.0 ELECTROMAGNETIC DESIGN CONSIDERATIONS AND CRITERIA

For either basic type of range, the fundamental electomagnetic design criteria deal with control of:

(1) krductive or radiation coupling betrveen antennas,

(2) Phase curvatule of the illuminating wavefront,

(3) Amplitude taper of the illuminating wavefront,

(4) Spatially periodic variations in the illumirnting wavefront caused by reflections,

(5) lnterference from spurious radiating sources.

Items (1) through (a) primarily establish the dimensional requirements on the range design, and limiting values of source.antenna directivity. Item (5) must be considered in the overall design.

Effects of Couplinq between Antennas - - - At the lower microwave frequencies, the effects of inductive coupling between the source antenna and the test antenna must be considered. Such effects are usually considered negligible when the criterion

R>10x1, (1) is satisfied, where R is the separation between antennas and l, is the wavelength in the same units. This aiterion is based on the field equation for an elemental electric dipole, from which the ratio of the amplitude of the induction field to that of the radiation field is seen to be

P= ?r/2xnxR at R > 101., p <1/20 x r, and the criterion is seen to be equivalent to the requirement that

20*1og (p) < -36 dB

Mutual coupling due to scattering and reradiation of enerry by the test and source antennas is also of concern. If the source antenna produces a significant illumination taper between the center and edges of the test aperture, interaction between the antennas can cause a measurable error in the signal levels observed near the peak of the test antenna's main lobe. The effect of mutual coupling on sidelobe accuracy is usually negligible.

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Pap # 11

Tapered Anechoic Chamber Refubishment Technical Understanding and Approach

The level of the signal a-ivi.g at the test antenna due to retransmission from the source antenna will be at least 45 decibels below the original received signal if the amplitude \ '\ taper across the test region is less than 1 dB.

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, .', ,,*t'\ In add"ition to the error produced by rekansmission from the source antenna, error can * ,.',, it "t;.,{ caused if the signal source is not isolated from the source antenna. The power level and -., r;$'n) frequenry of the source may ctnnge because of the variation in loading caused by the ," o

, +fu mutual coupling effects. Error from this source can be virtually eliminated by isolation and t * stabilization of the signal source.

Effect of curvature of the incident Phase Front - - [r the absence of reflections, the phase variation of the field over the aperture of a receiving antenna of a given size and operating at a given depends almost mtirely on the separation between the source antenna and the antenna under test and not upon the beamwidth of the source antenna.

If the receiving antenna is in the far zone of the transmitting antenna, the phase front of the approaching wave deviates very little from a section of sphere centered on the transmitting antenna over the major portion of the main lobe. If the receiving antenna subtends less than a half-power beamwidth the deviation of the test aperture from the sphere centered on the transmitter is negligible. [r practices, the antenna under test will subtend considerable less than a half-power beamwidth in order to reduce error from mutual coupling and from amplitude taper of the incident field over the test aperture.

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LEHMAII GHAMBEBS

Paul E Lehman, lnc

Page # 12

Tapered Anechoic Chamber Returbishment TechnicalUnderstanding and Approach

An expression for the phase deviation over a planar test aperture can be determined from

Figure 1. Since

R2 +D2/4 = (R + aR)z , , AR = rflgn if AR2 is neglected. The corresponding phase deviation is given by

AQ = 2fu, R/?'' = rP2l(aln) radians.

A commonly employed criterion for debrmining the minimum allowable separation between the source antenna and the antenna under test is to restrict A$ to a maximum of n/8 or22.5 degrees. Under this condition, R.>2 D2 /?u.If antenna measurements are made at a range of Zf /7", there will be some departure of the nulls of the radiation pattern and the location and levels of the minor lobes from their infinite-range values. The amount of the deviation depends on the original side-lobe level and structure.

Effect of Amplitude Taper Over the Test Aperture - - For accuracy in simulated far zone measuremenb, the illuminating field mustbe sufficiently constant in amplitude both along the line-of-sight and in planes normal to the line-of-sight.

Consider an antenna under test on receiving, whidr has a maximum dimension, L, of its active region along the line.of-sight. If the separation between the source antenna and the center of the active reglon is R , then the ratiogt p of the power densi.ty at the forward extreme of the active region to that at the rear is given by

L0log (p) = 2O log (( R+L/2)/(R - L/2)) decibels

Severe axial variations of the illuminating field can cause measurement error, particularly in the minor lobe sbucture of radiation patterns. For most antenna types which have significant depth to their active regions, such error is usually negligible when the power density over the reglon is constant to within one decibel. This condition corresponds to an approximate restraint on R of

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January 34, 1998 Page # 13

V

Tapered Anechoic Chamber Befubishment Technical Understanding and Approach

The criterion, for such structures as high-gain disc-on-rod antennas, often is rnore restrictive than the greater of the previously discussed range'length criteria whic-h were based on suppression of inductive coupling and phase survature.

The effect of amplitude taper of the incident field over a plane nornral to the line of sight and adjacent to the test aperture can be considered f$h the view-point of reciprocity.

Variation of the amplitude of the field over the aperture is receiving is analogous - - within the accuracy of the aperture field approach - - to modification of the aperture illumination by the primary feed on kansmitting. For example, consider the pattern of an antenna whose feed would produce an aperlure illumination f(Q,r) on transmitting, where (Q,r) indicates position in the aperture. If illuminated on receiving by a source antenna whidr produces over the test aperture an amplitude taper g(0,r), the measured pattern would be analogous to that of a transmitting antenna illuminated by a feed which produces an illumination of f(Q,r)B(0,rLJhe*meastrred*pattesl--would*.fus*analogsuv*to*-that*"of*a.-fransmitt@tennri:ltuminate&by*a*feed*whieh*produees-an*itrl.trn:rlnatien-ef*f $'49{+&F over the aperture. If g(Q,r) is constant in amplitude and phase over the aperture, the measured pattern will be the same as the infinite-range pattern for the illuminations f(Q,r).

The greater g(0,r) deviates from constant, the greater will be deviation of the measured pattern from the infinite-range pattem. The quantitative effect of nearly constant functions g($,r) cannot be determined, however, without assumption of f(Q,r).

The decrease in measured gain caused by aperture taper is determinud by the amount of taper and by the aperture-illumination function of the antenna under test. A criterion of

0.5 dB is commonly employed for the limit in tapered chambers of the amplitude taper over the test aperture. For high accuracy measurements, the iltumination is held to within

0.25 dB with a decrease in measured gain of less than 0.L dB.

Forg@ranges,theamplitudetaperoftheilluminatingfieldalonga horizontal line ttrough the test aperhrre, normal to the line of sight, will be determined almost entirely by the source directivity, the test aperture width and the range length, just as for elevated ranges. The taper along a vertical line through the test aperture, however, is virtually independent of the directivify of the source antenna, and depends almost entirely on the height of the center of the test aperture above the range surface. This is illustrated in Figure 2. The height of the test antenna is adjust"dY that the dilect and reflected path enerry arrives in phase at the test aperture.. The result is a mufti-lobed field as the field is probe in the vertical plane. For the broadest possible illumination taper

HHMAH GHAMBERS

Paul E Lehman, lnc

January 30, 1998 Page # 14

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NSWC - Dahlgren Division Volume ll - Techniel Proposal Tapered Anechoic Chamber Returbishment TechnicalUnderstanding and Approach in the test region, the lowest lobe is used for testing. It can be shown that for a uniform field across the test region on the order of 0.25 dB, the height of the test region needs to be on the order of 4*D . The height of the transmitting antenna is found from the relation L - l"R/4h"t- r

The apparent phase center of the source is given by ht, =(1-K)/(l+K)h, The plane in which the phase and amplitude vary essentially symmetrica!$bout the center of the test aperture is seen to be inclined from the vertical by an angle c given by

0 = tan ((h, - \, )/R)

In like manner, the amplitude taper across the test region of a tapered chamber is s6t by the design of the tapered section, i"e., the included angle of the taper, the location of the source antenna within the tapered section and the reflection coefficient of the wall absorbers. Due to the tight constraints within the chamber, dhamber symmefiry is important in order to maintain symmetrical performance of the amplitude taper in the test region. Consider the geometry of Figure 3. The antenna array formed by the source antenna and its images determines the field distribution across the test region within the tapered chamber. Due to the constraints of the walls, the amplitude distribution is centered in the mouth of the tapered section of the chamber. The degree of amplitude

LEH]RAII GHAMBERS

Paul E Lehman, lnc

January 30, 1998 Pagn # l5

Figure 2

NSWC - Dahlgren Division Volume ll'TechnielProposal Tapered Anechoic Chamber Befurbishment Technical Understanding and Approach taper is set by the apparent array factor of the source and its images, the distance from the source to the test region, the reflection coefficient of the wall absorbers and the of operation. To get the broadest amplitude taper in the test region, the separation betrarem the source antenna and the chamber walls need to be less than one wavelength. Thus for optimum operation, the source antenna generally must be located at different locations along the axis of the chamhr for optimum illumination in the test reglon depending upon the frequency of operation and the physical characteristics of the antenna. The field distribution in the vertical plane will be slightly tilted due to the geometry of the chamber in this plane. The data provided on the chamber indicates that this angle is 8.1 degrees.

Experience has shown that the included angle of the tapered section must be less than 30 degrees for optimum chamber performance. The NSWC chamber information indicates that the included angle is 22 degrees. Once the c-hamber geometry is set, @U" remainder of the design is a function of the backwall absorbers, selection of the absorbers in the throat of the chamber, and the fransitions from absorber to absorber along the axis of the chamber. Another important factor is the transition from the rectangular test region into the backwall absorbers. It is important that adequate length of wedge be provided behind the test region and the backwall to minimize backscatter from the edges of the pyramidal materials used to control the exfraneous energy in the test region.

IEHMAil GHAMBERS Paul E Lehman, lnc

January 30, 1998 Page # 16

Figure 3

Tapered Anechoic Chamber Retubishment Technical Understanding and Approach

3.0 CHAMBER LAYOUT

3.1 Absorber Layout

The absorber la/out is shown on the layout drawings located in the appendix. The layout is very similar to the original layout as it presently is in the chamber. The changes Cuming

Corporation proposes are based on our technical description of the tapered chamber design and function following this section. (Additional full size layout drawings have been sent in a separate mailing) gA Receiving End Wall

The end wall absorber will be our premium performing C-RAM SFC-72. This absorber is

72 in, (5 ft,) tall and has a base dimension oI 2 ft. x 2 fl.

Cuming Corporation uses the twisted pyramid design on standard absorbers above 48 in, height, the C-RAM SFC-7zis the twisted design and hence exhibits absolutely minimum in droop due to the tapered shoulder support extending out to 40 in. from the back wall.

On the end wall, the first shift of absorber will be a half width pyramid against the 6 in.

thick Corner Block, or picture frame material (see detail on our absorber layout drawing).

This installation practice provides an ideal taper from the wedge material on the side walls, ceiling and floor. The reflectivity performance of the absorber is listed in the attached technical bulletin #390-1 where the stated reflectivity values may be taken as minimum performance levels.

3.3 Side Walls And Celling

The absorber layout on the side walls and ceiling is similar to the existing absorber layout, the specular patcheg*will be treated with C-RAM SFC-18 turned 45( in order to minimize --l -

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-dnfffri scatter, Around the specular patches will be C-RAM SFC-12 Wedge. Cuming

Coqporation proposes to use 48 in. long wedges to the extent possible; this will reduce by approximately 50% the butt joints of wedges where an offset of wedges can cause some reflection and perturbance of the field. When installing the wedges alignment will be assured; if any small misalignment appears the butt joining wedges will be spot bonded together in the proper alignment.

Cuming Corporation proposes one change in the absorber layout. The present specular patch is larger than required; we propos€ to terminate the patch 12 ft. in front of the end wall as shown on the ittached layout. The wedge treatment behind the patch helps prevent any back scatter entering the Quiet Zone.

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LEHMAH CHAMBERS

Paul E Lehman, lnc

Page # 17

NSWC - Dahlgren Division Volume ll - Tachnical Proposal

Tapered Anechoic Chamber Refurbishment Technical Understanding and Approach

3.4 Floor Absorber

The floor layout is shown on the attached absorber layout drawing. The walkway from both the double door and the personnel door is mostly 4 ft. x 4 ft. square panels for optimal stability and strength. C-RAM SFC-12 with an added 2 in. base (designation C-

RAM SFC-14 walkway) and with a total height of 1.6 Q ft. will be used as walkway and for high frequenry testing 2 ft. x 4 ft. blankets of 5 in. convoluted absorber is supplied to cover the walkway in the critical area under and in front of the Quiet Zone.

All absorber on the turntable will be mounted on hardboard and a minimum of 3 sections will be supplied for ease of handling. The hardboard will mate against the raised edge of the turntable and prevent the absorber from sliding when the table is tilted.

3.5 Ghamber Taper

The chamber taper will be treated with C-RAM SFC-12 wedge material 2 ft.'.wide x 4 ft, long, (six wedges per 2 ft.width). The wedges will run almost length wise of the chamber following the center line of the taper and the rectangular test secdon.

From our inspection of the existing chamber it appears that the chamber has a relatively short square-to-round transition section. This transition is not indicated clearly on the original drawings. A minimum t0 ft. long square-to-round transition section will be installed as part of the refurbishment; this will allow for optimal propagation through tlre tapered section.

3.6 ConlcalTaper

The Conical Taper ftom the chamber apex to the start of the round-to-square transition will be lined with 12 in. thick solid material with a lossy loading closely matching that of the wedge material in the square taper. The last 2 ft,length of the Conical Taper near thp transition will be treated with a flat to wedge transition section, thus avoiding any abrupt geomebry change. The Conical plugs for the apex section minimum 2 ft. long will be supplied as required.

3.7 General Description Ol The C-Ram SFC Type Absorber Used

Cuming Corporation employs a computerized contour cutter in the manufacture of our

. pyramidal and wedge shaped absorbers. This provides the best possible geomebry l ) consistency and accuracy of the absorbers.

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IEHMAH GHAMBERS

Paul E Lehman, lnc

Page # 18

NSWC - Dahlgren Division Volume Il - Techniel Proposal

Tapered Anechoic Chamber Befurbishment TechnicalUnderstanding and APProach

The C-RAM SFC-72 twisted absorber and the C-RAM SFC-12 wedge material are both contour cut after saturation. This will provide the ultimate in geometric stability and will grve the best possible match and alignment from wedge piece to wedge piece.

The C-RAM SFC-18 pyramidal absorber is contour cut prior to saturation for optimal processing proficiency.

Cuming Corporation proposes to supply all the absorber unpainted; i.e., with a black finish in order to optimize the performance at the higher frequencies. The absorber can be painted, if requested by the government, at no additional drarge, however, a small sacrifice in performance at the higher frequencies may occut. Further evaluation of this tradeoff will be performed after contract award.

Cuming Corporation walkway material is enclosed on the top and sides with a rigid vinyl foam, this adds to the strength and stability of the walkway material which is rated at a load bearing capacity of 200lbs.lsq. ft.

The Conical Taper section of solid foam material is laminated and custom cut to fit in the shop, thus minimizing field cuts of this critical part of the chamber.

4.0 CHAMBER DESIGN

The backwall absorbers sets the basic perfornnnce of the chamber. The twisted 72 inch absorber is recommended for this application. This absorber design is self-supporting and has very little tip droop with time, whieh the standard square pyramid experiences. It also has 1.5 % more mass which improves the low frequency response of the absorber. It is also more uniform in loading since is its laminated from a set of flat absorbers which are carefully impregnated to have a uniform loading throughout the slab. The latter is essential to achieve the 60 dB performance at the high end of the band. Around the perimeter of the backwall absorbers in a picture frame of flat material. This interfaces with the 72 :r;rch material and provides a smooth transition from the sidewall wedge material. See the chamber layout drawing for details.

IEHMAH CHAMBERS

Paul E Lehman, lnc

January 34, 1998 Page # 19

,VSIVC - Dahlgren Division Volume ll - Technial Proposal

Tapered Anechoic Chamber Refurbishment TechnicalUnderstanding and Approach

Chamber perfonnance for an 8 ft. Spherical Quiet Zone and"/or 8 fL Diameter x

15 tL long Quiet Zone:

Frequenry (GHz)

0.100 0.500 2.4 8.0 18.0 25.0 40.0

Frequency (GHz)

0.100 0.500 2.0 8.0 18.0 26.0 40.0

Design Reflectivity

-28 dB -45 dB -53 dB -60 dB -60 dB -60 dB -50 dB

Design Reflectivity

-24 dB -41 dB ,48 dB -55 dB- -55 dB -55 dB -ss dB

Guaranteed Reflectivity

-26 dB -43 dB -50 dB 57 dB -57 dB -58 dB -58 dB

Guaranteed Reflectivity

,22 dB {'9?B:= :45"d8 -52 dB -52 dB -53 dB -53 dB

The chamber Quiet Zone will largely meet or exceed the design Reflectivity Performance. It is, however, possible that a few discrete locations in the Quiet Zone will only meet the guaranteed reflectivity level. This is in part because of the inherent lack of symmetry in the chamber design resulting in an unfavorable incidence angle on the floor absorber.

It is possible to extend the Quiet Zone to a 10 ft. diameter x 20 ft. long volume. With the

1.0 ft. diameter we are exceeding the recommended ratio of Quiet Zone diameter versus chamber height and width, and the performance can be expected to drop by -5 dB, largely due to the unfavorable incidence angle with respect to the walls for enerry that reflects into this extended volume of Quiet Zone.

Chamber Performance for a 10 ft. Diameter x 20 ft.long QuietZone

As stated for the 8 ft. diameter Quiet Zonez This 10 ft. diameter Quiet Zone will largd meet the Design Reflectivity specification, but in a few discrete areas the performance can drop to the Guaranteed Reflectivity level for the same reasons as stated for the 8 ft.

diameter Quiet Zone.

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IEHMAH GIIAMBERS

Paul E Lehman, lnc

Page # 20 l l

I l

NSWC - Dahlgren Division Volume II - Technial Propsal TaPered Anechoic Chamber Befurbishment Technicat Llnderstanding and Approach

To insure that the above performance is achieved, the carbon loading in the foam is very critical. This is illustrated in Figure 4. Cuwe (a) illustrates how the performance is effected if the carbon loading is too great."This can easily occur if the loading is not carefully controlled because at the low end of the frequencyband, the conductivity of the carbon increases with lowering of the operating frequency. The high loading causes the incident €nergy to reflect rather than penetrate the olt"riul, this is an effective surface reflection. At the higher frequencies the performance peaks because of the large number of wavelength into the material. But, again, at the very high frequencies the tip scatter occrrs due to the high loading and the performance rapidly rolls off.. If the loading is optimum(curve (b)), the performance of the material closely follows the performance of a one section filter, i.e., the performance falls at a 6 dB per octave rate over the linear portion of the performance cuwe. This is the same as the dispersion loss in free-space. If the loading per unit volume is too light (curve (c)), then the entire curve shifu to the right, but has the same 6 dB per octave roll-off. The critical loading provides for the maximum possible bandwidth from the material by extmding the perforrnance at both ends of the operating frequenry band. As discussed in the quality control section, the products are

L00"/o tested in the one Ghz frequenry range for proper control of the overall perforrrance of the material.

0.1 0.3 1.0 5,0'10,0

Figure 4 - The Effect of Loading on Pyramidal Absorber i

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UHMA]I GHAMBEBS

Paul E Lehman, lnc

Page # 21

Nswc - Dahtgren Division vorume il -TechnicatPropos

Taoered Anecioic Chamber Refurbishment TechnicallJnderstanding and Approa

The &t absorbing material in the conic section is selected to provide the proper reflecti coefficient to reduce the amplitude of the source images so that a broad uniform field is set-up in the test region of the chamber. Careful selection of the flat to wedge material in the transition section provides a smooth field without any discontinuities in the properties of the wall absorbers to cause any distortions in the illuminating fietd' L2 inch rnredee material is used to cover the remainder of the taper. Uniformity of the loading materials is important in the throat of the chamber to insure that the axial ratio requirements of 0.75 dB from 100 MHz to 500 MHz and better than 0'5 dB from 500

MHz to 18H2. are met.

The lg_inch-grramidal material in the test zone patch is selected to reduce wall scattering'

It is critical that the pyramidal material at the rear of the test region be terminated as soon as possible to insure that scattering from the sides of the pyramids do not raise the extraneous energy level in the test region. The 12 inch wedge material gurdes the energy into the backwall where it is terminated'

The Eisht Ft. Test Region is located slightly forward of the center of the test region rectangular patch. The amplitude taper across the test region will be symmetrical within'a few tenths of dB from side to side and the taper will be orr the order of one dB or less'

The phase taper is a function of the operating frequency and the range length, i'e" for a

22.5 degree taper the following equation applies:

D = (R?^"/2)r/2

5.O PERFORMANCE VERIFTCATION

rrre 4q$LJg{ lert--r-eggl of the Tapered Chamber will be probed transversely and longtuJinatly *ir,S the Free Space VSWn method to measure the chamber reflectivity'

These tests will be conducted at 100 MHz, 500 MHz, 2 GHz, 8 GHz, L8Ilz,26 GtTz' and

40 GHz. The probe antenna will be stepped in 10 degree steps from 0 to 90 degrees and from 90 to 1g0 degrees. The axial ratio and cross-polarization measurements will be conducted on axis at 100 MHz, 500 MHz, 2 Gt1z,8 GHz, and 18 GHz. The latter procedure is accomplished by clocking the source antenna in 30 degree steps and than taking a fu11360 polarization pattern at the center of the test region. The axial ratio is determined by plotting the magnitude of the peaks versus an$1€ and the cross-polarization properties are determined from the depth of the Patt:m nulls' -Slegdald gan -

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t-o a0 GHa LPA 9r Yqgl 94t9nn19 w!}t I'eusg{ 3! 100 JvIHz and

50Q MHz, ffi January3o,1998

Paul E Lehman, lnc Page#22

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Taoered Anechoic Chamber Refurbishment TechnicalUnderstanding and Approach

The {0 dB Quiet Zone perforrnance testing is not a kivial matter. When testing head on, it can be exhemely difficult to have the resolution required to read the actual performance; however, as the probe antenna tums off axis to a point where the signal level drops by 20 dB it is entirely possible to read the -60 dB performance.

The height of the probe mast required in a chamber of this size can amplify vibrations to a point where the high frequenry performance data are impacted. However, with proper care the 26 and 40 G}Jz testing is possible.

6.O CONSTRUCTION PRACTICES

Lehman Chambers will provide all labor, equipment, supplies, and supervision necessary to complete the refurbishment of the tapered anechoic chamber. With over 45 years experience in the construction industry, l-ehman Chambers has the project managemmt and construction expertise necessary to successfully accomplish this complicated project.

6.1 Project Management

Upon award of project, Lehman Chambers assigns a Resident Project Manager (RPM) and sets into motion its design/engineering team. The RPM will work throughout the project with the team members to adrieve the smoothest possible project. At this point, I€hman Chambers intends to assign Mark Bellman as Resident Project Manager for this project. Mr. Beltman has extensive experience in the installation of anechoic absoibing materials, refurbishment of anechoic chambers (induding tapered chambers), and numagement of govemment projects.

I€hman Chambers understands that close coordination between l€hman Chambers, Cuming Corporation, and ATDS will be necessary for the smooth completion of the project. To further enhance our project nuuragement skills, we have implemented the use of a project procedures manual. This manual has been developed and used successfully to assist the RPM in ensuring subcontractor ferformance, quality control, and the consistent use of proper safety habits and practices on the project. A copy of l,ehman Chambers

Project Management Plan and Policies and Procedures Manual will be on site at all times and is available for review upon request.

Safety is also a primary concern throughout the construction process. All Lehman

Chambers personnel and subcontractors will conform to OSHA rules as well as to the

I.EHMAH CHAMBEBS

Paul E Lehman, lnc

Page # 23

NSWC - Dahlgren Division Volume Il - Techniel Proposal Tapered Anechoic Chamber Refurbishment Technical Understanding and Approach facilify Safety Rules. Ow RPM also serves as the safety officer for Lehman Chambers while on-site and will ensure that the procedures established in our safety manual are strictly followed. The basic safety objective of Lehman Chambers is to ensure that no job is so important that we cannot perform it in a safe manner. In order to assist our field personnel and sub-contractors in the development of a sEong safety program our PELI

Construction Site Safety Manual has been developed. Our safety manual will be on site at all times and is available for review upon request. It has been approved by major corporations such as Dupont, Xerox, and HP.

6.2 Chamber Refurbishment

The first task in the refurbishment will be the removal of the existing anechoic absorbing materials. t€hman Chambers will remove all the existing absorber within the chamber and then transfer it into waste receptacles provided by a local waste nranagement service. The receptacles will then be taken to a landfill and the material disposed. Based upon the era and manufacturer of the edsting material, it is assumed for the purposes of this proposal and pricing that the anechoic absorbing materials are non-hazardous.

After removal of the anechoic absorbing material, the shielding surfaces will be cleaned with the proper equipment and solvents to remove all traces of absorbing material so that proper adhesion will occur between the shielding panels and new anechoic absorber.

Copper shielding tape with conductive adhesive on one side will be installed on the seams between all mechanical joints on the chamber walls and ceiling as necessary to maintain the shielding integnty of the chamber.

The six existing lighting fixtures in the chamber will be replaced with four new RF shielded high-hat lighting fixtures. These fixtures will be individually filtered and feature 400 watt metal halide bulbs that can be replaced from the exterior of the chamber with minimal difficulty. The two resulting holes in the shielding will be covered with sheehnetal with all joints covered with copper shielding tape.

Lehman Chambers will also replace the RF gasketing material and fingerstock on all of the chamber doors. The proper materials and installation procedures will be followed to ensure that adequate shielding and operation of the doors is achieved.

HHMAH GilAMBEBS Paul E Lehman, lnc

Page # 24

Tapered Anechoic Chamber Refurbishment TechnicalUnderstanding and Approach

The new anechoic absorbing materials will then be installed to all surfaces of the chamber per the design specified elsewhere in this proposal. The absorber will be attached on the walls and ceiling with high strength adhesive and laid snugly on the floor. Careful attention and installation procedures will be followed to insure that no gaps between absorber pieces occnrs and that proper alignmmt is achieved. I€hman Chambers understands that the proper installation of the anechoic absorber can have a profound effect on the overall performance of the tapered chamber. An Absorber Installation Plan

6.3 Testing and Drawings

Within 30 days of completion, two copies of as-built drawings detailing the refurbishment and anechoic absorber configuration will be delivered to the government. Unless otherwise specified, all data witl be prepared using AUTOCAD Release L3 for Windows. l€hman

Chambers will store all drawings, both elechonic media and hard coPy in an environmentally controlled and protected storage area. All data wili be backed uP by electronic media and stored in a separate location. This will ensure the preservation of all drawings and enable Lehman Chambers to deliver data that is essential to support the project requirements.

4_ !-ejljlgf$gffplgl"ryl[.b.ges.b-Ftts-4 !q -tr" goJ-einment for approval .pnor.tp- the

_S$gg!gS"ge_g{ tg-gSt".d,*nrr-aguxeme-r-r.S. lehman Chambers expects a rePresentative of the govemment to be available to witness the testing and will be gve two weeks prior notice of the testing dates, +TPF* W$-_C$$gg!el$S__!gl"tt*in-4--&e"-resulte=.1pU-.be forwarded to Lehman Chambsq* Iwq _qpp-is.g qf.,the-final,test re-qultp rtll-E d-91iy-er9{t51 the golernment within 30 day.s of test completion

IEHMAH GHAMBERS

Paul E Lehman, lnc

Page # 25

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!I cuMrNG MrcRowAVE CORPORATIONf^l TECHNICAL DATA

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C-RAM Corner Block is a carbon loaded urethane foam material used to fit anechoic chamber materials, such as C-RAM SFC, into the corners and edges of the chamber.

These products meet all of the fire retardancy requirements of NRL Specification 8093.

TYPICAL PROPERTIES

C-RAM Corner Block is made of the same material as the pyramidal absorber it is being used with. lt will have the same density and impedance as the C-RAM SFC foam.

i ]he product is black throughout, and generalty is sprayed with a light blue surface coating, both for cleanliness and to provide better light reflection inside a chamber" The absorber can, be left unpainted if requested.

AVAILABILITY

C-RAM Corner Block is custom made to each anechoic chamber job, and is available in almost any dimension. Cuming Corporation engineers can help you specify materials for a chamber application, and can recommend specific configuraiions of corner blocks.

Generally, the block is as thick as the base of the absorber piece butted next to it, and as wide as the height of the absorber facing it" For example, a 3 in x 18 in x 24 in piece would be sed when blocking SFC-18's in a corner.

METHOD OF APPLICATION

C-RAM Corner Block is applied to chamber , walls just as the C-RAM SFC pyramidat

,) absorber would be. lt is generally bonded in

230 BODWELL STREET AVON, MA 02322 (800) 432-6464 (s08) 580-2660 FAX (508) 584-2309

TECHNICAL BULLETIN 390-1 1

C-RAM CORNER BLOCK

LOSSY FOAM FOR FITTING ABSORBERS TO CHAMBER CORNERS

place with a neoprene contact adhesive, or it can be supplied with Velcro pile backing to attach to strips of Velcro hook bonded to the walls.

Edges and corners of the chamber are generally blocked out first, then the C-RAM SFC pieces are fit from the floor up to the ceiling. Corner Block is readily cut with a sharp knife, or, preferable, using a bandsaw or electric carving knife.

The information in this technical bulletin, although believed to be accurate, is not to be taken as a warranty for which Cuming Corporation assumes legal responsibility, nor as permission or recommendation to practice any patented invention without license; it is offered for veriflcation by the customer, who must make the final judgement of suitability for any application.

Document Control No. N-15-000-50733-0 2127197 page 1 of 1 tI cuMtNG MtcRowAVE f ^l coRPoRATloN TECHNICAL DATA I 230 BODWELL STREET AVON, MA 02322 (800) 432-6464 (s08) 580-2660 FAX (508) 584-23

TECHNICAL BULLETIN 390.1 O

C-RAM SFC-WALKWAY

HIGH PERFORMANCE BROADBANDED WALKWAY RF ABSORBER

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C-RAM SFC-WALKWAY is a series of high performance broadbanded RF absorbers, fabricated from standard C-RAM SFC pyramidal absorber. The SFC absorber is fit with a low density polystyrene foam matching section to square the piece off, and the sides and top of the block are fit with a half inch (13 mm) thick sheeting of fire retardant polyvinyl chloride (PVC) rigid foam.

Walkway absorber provides an effective access path for servicing the transmit and receive areas of an anechoic chamber.

TYPICAL PROPERTIES

Reflectivity performance of a walkway is degraded from that of the SFC absorber comprising it, due to reflections from the PVC skin. The degradation becomes more significant with increasing frequency; below 1 GHz, the degradation is small, at 10 GHz, reflectivity is limited to about 25 dB, in mm-wave applications, other methods of accessing equipment should be considered, such as removing and replacing absorber before beginning testing.

Weight of a walkway piece is approximately 20- 25%…