2.1_NWL Technical Note TN-G-12-74.pdf

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NWL TECHNICAL

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NOTE TN.G-12174

April 1974

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L. Wilson Pearson

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NWL TECHN]CAL NOTE TN-G_12/]4

April 1974

PERFORMANCE DESCRIPTION AND MEASUREMENT ERROR ANALYSIS

OF THE NWL TAPERED ANECHOIC CHAMBER

by

L. Wilson Pearson

NWL Technical Note TN-G-12/74 April 1974

PERFORMANCE DESCRIPTION AND MEASUREMENT ERROR ANALYSIS

OF THE NWL TAPERED ANECHOIC CHAMBER

by

L. Wilson Pearson

ACKNOWLEDGEMENT

The information reported here is a technical description of the end product of a najor facility procurenent. the aulhor wishes

-to-take this opportunity to thank all of those who played a part in bringing the facility into being.

Particular thanks are in order to Mr. Fred p. Brownell, senior Engineer, Rantec Division of Enerson Electric conpany, who designed the facility. Thanks are also due to cDR. F. L. Endebrock, NWL public works Officer and to Messrs. G. o. paddy, J. G. King, s. B. Huxtable and T. E. Jones, Jr. on the Public works Departnent staff. Their assistance and cooperation was most significant in the completion of this project.

The author thanks Dr. M. F. Rose, supervisory Research physicist, under whose adninistrative supervision the facility was built.

Mr. F. E. Bray, Jr. of the Advanced systens Departrnent was an invaLuable colleague for technical colraboration throughout the developnent of this project.

FOREWORD

This work was supported by the Naval Air Systens Comand under AIRTASK No. A350-SS0F/291B /2F00-582-S01.

The facility to which it relates was procured with support through NAVMAT TASK 2F0990208.

This Technical Note has been reviewed and approved by Mr. Kenneth C. Baile, Head of the Systerns Engineering Group.

Released by:

ABSTRACT

This Report provides a technical performance description of the Naval Weapons Laboratory Tapered Anechoic Chanber. A general description of the facil-ity and the support equipment for it is included. Ttre detailed technical performance of the chanber including all perfornance data gathered to date is reported. An-tenna neasu?enent accuracy considerations are discussed, and approxirnated erlor bounds for pattern, gain, and power density neasurenents are derived.

111.

I.

II.

TABLE OF CONTENTS

INTRODUCTION

Page

T4

DESCRIPTION OF THE

A. Configuration.

FACILITY.

B. Positioners.

C. Apex Access.

IO

III.

D. Provisions for

INSTRTIMENTATION OF

Data Transfer

THE FACILITY.

A. Introduction B. ControL/Receiving Console.

C. Transrnitter Oscillator Equipment D. RF Components.

E. Antennas . .

IV. THEORY OF THE TAPERED CHAMBER GEOMETRY

A. Introduction .

B. Theory of Operation.

C. Feed Antenna Placement and

REFLECTIVITY PERFORMANCE OF THE

Gain.

V. CHAMBER. 23

VI.

A. Introduction B. Measurernent of Reflectivity.

C. Reflectivity perfornance of the NWL Tapered D. Rotational Perfornance of the Chanber. .

Chamber.

ERRORS IN PATTERN MEASUREMENTS IN TI{E CHAMBER.A. Introduction B. Approximate Pattern Error Analysis C. Interpretation of pattern ErrorsD. Pattern Errors for Large Apertures

VI I. GAIN AND RADIATED POWER MEASUREMENTS

A. Introduction B. The Gain Cornparison Method C. Errors Due to Reflections in Gain Measurement.D. Gain Errors Due to Back-Wall ReflectionsVIII. CONCLUSIONS.

A. General.

B. Reconnendations for Further Work

REFERENCES.

APPENDIX

A. DISTRIBUTION

4I

Lv

LIST OF FIGURES

Figure Page

I Isonetric View of the Anechoic Chanber and Supporting Areas . . 3

2 Yiew of the Chamber Through the Apex Opening 4 3 View of the Test Region. 5 4 Apex Region of the Chanber with Overhead Positioner. . 7 5 View with One-half of Snallest Clarnshell in Positioner 8 6 View of Apex Positioner in Extreme Forward Position. 9 7 Yiew of Control Console. L2 8 Parallel Ray Concept of the Tapered Design 16 9 Conditioning of Test Region Arnplitude Distribution by a Tapered Chanber L7 10 Quiet Zone Field Distribution

v. Source Antenna Position 19 11 Quiet Zone Field Distribution for Various Feed Horns 20 12 Quiet Zone Field Distribution for

Dipole Feed v. Horn Feed 22 13 Measured Reflectivity Performance of the

Chanber at UHF 25 14 Measured Reflectivity Performance of the

Chamber at Microwave Frequencies 26 15 Measured Worst Case Reflectivity as a Function of Frequency. 27

Sanple of Raw Data for Rotational Measurements Using the Ro11-Away Section to Supress Coupling to Shielded Roon.

Primary Error Component in Pattern Measurement Approxirnate Pattern Error as a Function of Pattern Level and Chanber Reflectivity Specinen Measured Pattern Uncertainity at 30 dB Reflectivity Pattern Cornparison Showing Cyclic Nature of Error.

Gain Cornparison Measurernent Technique.

Backlobe Error in Gain Measurement

2L

Gain Cornparison Error Due to Back Reflections for a Dipole Standard

Wa1 1

I. INTRODUCTION

This Technical Report is written to serve the user or prospective user of the Naval Weapons Laboratory Tapered Anechoic Chanber in de-ternining the accuracy of particular measurements in the chanber.

The NWL Tapered Chanber went into service in March 1973. Prior to that time a partial evaluation of the chanberrs perforrnance was made by l'fr. Fred P. Brownell of Rantec Division of Enerson Electric Company [1] under the observation of Mr. L. Wilson Pearson of NWL.

Since these original tests, NWL personnel have run additional tests in order to more ful"ly characterize and evaluate the performance of the chanber.

The NWL Tapered Chanber is a quasi-tapered, conical anechoic chanber intended for use in the frequency range 100 MHz to 40 GHz.

During Rantec testing, however, a reflectivity measurenent was nade at 77 Wlz in order to characterize the low frequency rolI-off in perfornance, This data point will allow nore intelligent use of the chamber in the 50-100 MHz range for power density tygre measure-rnents. Though no measurements have been made, to date, above 10.0 GHz, the specified perforrnance of the chamber includes frequencies through 40.0 GHz and there is good reason to expect that this per-fornance is on the par with lower frequency neasured perfornance l2l.

The NWL Tapered Chanber has been evaluated for reflectivity and rotational perfornance as of this writing. These data are adequate to characterize errors due to reflections in pattern, gain, power density, and circularity and cross-polarization neasurenents. These rneasurenent errors are discussed in this document. The charnber nay be used for radar cross-section and antenna boresiting measurenents, but additional. performance data is required before such neasurenents can be made. A nethod of establishing the effective chanber bore-site is described by Hickman and Lyon t3]. The rneasurements required to characterize background for RCS neasurenents are described in available publications, e.g. f2,3,57.

II. DESCRIPTION OF l]IE FACILITY

A. CONFIGIIRATION

Figure I gives a cutaway drawing of the NWL Tapered Chanber and associated work areas. Figures 2 and 3 give some perspective on the test volune itself.

The chanber is 98 feet Long with a rectangular test volume of dinensions 27 x 27 x 30 feet long. The cross-section of the ghamber is tapered from a 27 x 27 foot square to a two inch diameter circle

38.5 inches above the floor level. The conical apex section is twenty-two feet with a conical to pyranidal transition seventeen feet long. The included angle of the conical region is twenty-two degrees and the nechanicaL boresite slopes at 8.1 degrees to horizontal.

A transmitter roon L4 x 22 feet encloses the apex and provides a mininun of approxinately 200 square feet of floor space. An equip-rnent pit twelve feet deep and with 19 x 25 foot floor houses the test nodel positioner and provides 450 square feet of equipment and instnrmentation space. the three volunes just described - the chamber, the transnitter room, and the pit - are enclosed by a single RF shield.

B. POSITIONERS

The test antenna positioner is a Scientific Atlanta 5323 azimuth over elevation positioner with a series 5800 model tower. The positioner provides two axes of rotation for measrrrernent, namely azinuth and polarization. A third axis, elevation, is used for loaci-ing test nodels but assumes a fixed position during testing. A twelve foot diameter absorber-covered turntabLe covers the metallic portions of the positioner nechanism and tilts with the elevation axis of the positioner. The load linit of the positioner system is dictated by the rnodel tower. The limits are 450 pounds total. model weight and 750 foot pounds torque exerted by a moment arn nornal to the mounting face.

The source antenna positioner is a custon design delivered by Rantec and suppl.ied through a subcontract with Scientific AtLanta.* The design comprises a modified series 5800 Scientific Atlanta l4ode1 Tower mounted inverted on a tower cart. The cart assenbly tracks on overhead rails which are sloped at 8.1 degrees i.e. parallel to the

* Mr. Allen Wilcox of Scientific conceptual developrnent of this

Atlanta was instrumental in the design.

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the Test RegionView of chanber axis. This unique design was necessitated by the sloping boresite configUration of the chanber. The tower is attached to the cart via three adjustable jacks to facilitate alignnent of the posi-tioner head on axis and to allow alignrnent cotrpensation under heavy loading. A nanual azimrthal rotation axis is provided for the same reason. The tower polarization head is rotatable through 360 degrees.

It is equipped with electrical output of angular location so that po!.arization patterns nay be nrn with the antenna under test sta-tionary.

The cart is noved along the overhead rails by a notor-driven winch. The cart nay be located over a range of positions appropriate to nounting antennas whose rnajor dinension is as gfeat as five feet or as snali as two inches. Ttre cart assembly is shown in Figure 4.

C. APEX ACCESS

The access to the apex of this chanber is a unique configuration designed to allow the twelve foot travel of the cart necessitated by the ieed antenna size range while naintaining circular synnetry of the structure for the sake of rotational performance.* The sections are apparent in Figure 4. The structure is nade up of two renovable split i'clamshell"t' sections and a Latge rolL-away section. As nany sections as necessary are attached to provide an inside wall dianeter cornpatible with a given feed antenna. The overhead cart is positional to the rear of the snalLest section required so that the antenna rnay be mounted on it.

At the front of the ro11 away section, the interior dianeter of the charnber is approximately 40 inches. Fron this point forward to the point where the dianeter is 60 inches, a Longitudinal. slot in the top of the chanber allows nast novement.

The photograph in Figure 4 shows the exterior of this apex ar-rangenent. figUre 5 pictures the smallest clanshell with one side renoved and an x-band horn nounted, and Figure 6 shows the rnast forward in the slot so as to acconmodate a sixty inch antenna.

* The configuration was developed by Mr. Thomas Contracters and Installations and by Mr. Fred Rantec.

Nabor of Technical P. Brownell of

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D. PROVISIONS FOR DATA TRANSFER

Twelve coaxial cable rt.ns pass between the control roorn area and and the pit area. Ten of the runs are of type RG/218-U cable and two are of 7/8 inch rigid air-dielectric cab1e. The air dielectric cable is low in loss and a pressurization nanifold in the pit area facilitates filling the cable with an insulating gas for high-power applications. the cables are terminated with Type N Female butkhead connectors.

Ten Type N Fenale bulkhead feedthroughs are located in the shield wall between the control console and the apex region.

Ten coaxial cables of RG/218-U type connect bulkhead connectors in the pit to a bulkhead panel at floor leve1 outside the wal1 of the chanber opposite the pit stairs.

Three brass setup panels each with a through-ho1e one foot by one foot are available between the control roorn and the apex for special purpose filtering or penetTations. Such things as waveguide penetrations, filtered signal lines or filter water lines nay be temporarily mounted in these panels. These panels are visible in Figure 7 in the next chapter.

An additional brass setup panel with a two by eighteen inch through-hole provides a feedthrough innediately behind the chamber apex into an adjacent special pur?ose laboratory.

A11 of the positioner functions are controlled exterior to the shield and synchro data of axis locations are available to exterior recorders. A11 of the cable runs in the positioner systen are pernanently installed with filters at shielding penetration points.

III. INSTRI.JMENTATION OF THE FACILITY

A. INTRODUCTION

The NWL Tapered Anechoic Charnber is intended to be a flexible facility providing capability for naking a broad class of radiation rneasurements. Naturally, a gteat nany specialized neasurenents re-quire specialized instnrnentation tailored to requirements of the nreasurelnent. The basic instnrnentation complenent of the chamber is designed to provide fundamental antenna range instnrmentation for the caLibration of the range and of any specialized instrunentation which accompanies any given set of measurements.

B. CONTROT/RECEIVING CONSOLE

Figure 7 pictures the control/receiving console for the chanber.

In this console resides control of the positioner motors. l*{onitor-ing of all rotation axes is provided with 0.1 degree resoLution synchronous motor indicators. The position of the translating cart is nonitored visualLy through the doors from the control roon to the transnitter room.

The three instnrmentation axes have position inforrnation con-mrnicated into a dual recording system. Either a rectangular or a polar recorder drive nay be used for recording positional infornation.

The rectangular recorder provides high resolution recording capability for engineering purposes while the polar recorder is useful for pro-viding clear conceptual records. The rectangular recorder acconmodates charts on which the angular axis is either 8-L/2 inches long (note-book sized) or twenty inches long. The full-scale excursion can be set to 360, 60, or 10 degrees. The polar recorder nay be scaled to either thirteen or eight inch dianeter charts.

A dual channel Scientific Atlanta series 1740 receiver provides extrenely sensitive swept frequency RF detection capability. This phase coherent receiver provides excellent stability and linearity over a 60 dB dynamic range. Over this range the receiver alone provides .25 dB linearity and the receiver recorder systen combined provide 0.5 dB linearity. ExternaL mixers and a low frequency con-verter provide frequency coverage frorn 50 MHz through 40 GHz.

A digital logarithnic rationetet provides analog to digital conversion of receiver outputs and nay be used to nininize the signifi-cance of transnitter power or for swept frequency measurements. Trans-nitter and nixer frequency response characteristics may be subtracted out using matched detectors so that the recorded response is solely that of the device under test.

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C. TRANSMITTER OSCILLATOR EQUIPMENT

The transmitter rack conprises a Singer sweeper systen, a Wavetek solid state sweeper, a digital power meter, and an autonatic transfer oscilLator frequency counter.

The prinary sweeper system is the Singer rnodel 650. Plug-in RF heads provide coverage frorn 250 MHz through 40 GHz. A11 of the plugins except the 26.5 - 40 GHz unit are internally leveled.

This sweeper systen is generaLly frequency progranned fron the receiver systen and through a phase lock loop with the receiverts stable cavity oscillator provides an ultra stabLe transmit source.

(Measurenents of stability of the overall systen have not been nade per se. However, amplitude stability.of 0.L dB/day and frequency ffi'UT-nty in excess of onb part in 104 per day are observed routinely.)

The Wavetek 2001 sweeper provides a low-frequency transnit source from 1 Mllz to 250 MHz. The fact that it operates through 1.4 GHz a11ows it to serve as backup to the prirnary sweeper system.

A GeneraL Microwave digital power meter provides an absolute power reference for power density measurenents of a power nonitor for conpara-tive rneasurenents. RF heads are available fron 10 MHz through 40 GHz, A Systron Donner nodel 6057 automatic transfer oscillator counter allows frequency monitoring of the signal source. The counter functions autonatically and covers the range 20 Hz to 18 GHz. Absorption fre-quency meters are available in the two bands between 18 GHz and 40 GHz.

D. RF CO\,IPONENTS

A collection of RF and nicrowave components provides the necessary complement of I'plunbingtt necessary for most nicrowave measurements.

Coaxial components span the range 100 MHz to 12.4 GHz. The four waveguide bands (8.0 - L2.4 GHz), Ku (12.4 - 18.0 GHz), K (18.0 - 26,5 GHz), and Ka (26.5 - 40.0 GHz) to standard waveguide hardware is available from 8.0 - 40 GHz.

The component conpLement comprises directional couplers, hybrids, tees, filters, elbows, attenuators, matched and short circuit termina-tions, tuners, and detectors.

Slotted Lines and a VSWR meter are available for impedance measurenents and for tuner adjustment.

E. ANTENNAS

Ttre chanber is equipped with feed antennas and standard gain antennas fron 100 MHz through 40.0 GHz.

fire chanber is nost conveniently fed up to 2.0 GHz using a crossed dipole feed antenna. Above this frequency 18.0 dB horns are available. From a performance point of view, the feed antenna is an integral part of the chanber. Chapter y provides infornation about the chanber performance using these feeds and a linited anount of data about the influence of the feed on perfortnance

Gain standards in the forn of standard gain dipoles cover the frequency range 77.0 Wz to 600 MHz. Scientific Atlanta series 12 standard gain horns are provided in pairs from 500 It{Hz through 40 GHz.

The availability of pairs of gain standards nakes possible the use of the dual channel receiver and rationeter for the swept frequency neasuretnent of gain.

L4

IV. T}IEORY OF THE TAPERF,D CHAMBER GEO.,IETRY

The operation of a tapered anechoic chanber is based on exploita-tion rather than suppressiln of wal1 reflections of radiated signals' As a result of this,'there is an intinate interplay between the-chanber and the feed antenna.* It is essential that the user of the chanber understand this interplay in order to be abLe to obtain relia-ble neasurenents.

B. THEORY OF OPERATION

The first docurnented use of the tapered chanber desigrr is by

B. F. Goodrich Company for Bunker-Rano Corporation [5r-8]:

reference [8] the iefiective properties of the tapered side walls are descriUed as follows: "Energy is here launched (reflected) in such a manner that it is propagaied parallel to the walls rather than at an angle to then." Ttris ltatu*enl refl'ects the first concept that ift"-luorEtry of the chamber taper relative to the test region of the chamier is such that specular ieflection fron the tapered wal1s -is refl.ected into the baci< waL1 of the chamber at near normal incidence as shown in Figure 8. This initial concept, it turns out, serves onty as r torrgf, first approxination to tapered chamber performance.

A rnore accurate concept is discussed in subsequent paragraphs.

The width of the ray lines in Figure 8 indicates qualitatively the relative rnagnitude of the propagating waves.

One observes fron Figure 8 that this concept leads to the l"ogicaL conclusion that the side walLs of the chamber may be arbitrarily re-flective so long as the rear wall is a sufficiently good absorber as to diminish reflections to the desired 1evel in the chanber. Exploit-ing this conclusion for the sake of econorny, only the bac_k wal1 absorber neEd exhibit high performance at the lower frequencies of operation.

tests on the design is reported by Emerson a better reflectivity perfornance was than that of a rectangular chanber of work by Hickman and Lyon [3] explains this result. They showed that the side wal1 illunination in regions which contribute reflectiolrs which enter the test region directly rnay, in fact, favorably effect ffinna" used herein refers to the antenna located in the apex of the chamber whether it be a transmit or receive antenna.

A surprising result of and BrownelL [5]. That is, seen in the tapered chanber conparable size.

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the illurnination of the test region. That is, the inage sources work suitably in concert with the real source so as to produce a field aperture across the test region approaching plane wave condi-tions. Figure 9 describes this concept graphically. Figure 9a de-picts, quatitatively, the signal anpLitude taper across a test region

"fticft-one v,rould observe due to the pattern roll-off of a typical transmitting antenna. This is the distribution which one would ob-serve in "fiee-spacerr or a good rectangular anechoic chanber. Figure 9b shows the effect of placing this antenna in a tapered anechoic chamoer.

Reflected or 'tinage" signals Ei add with the directly incident signal Ed to produce a signal anpl.itude distribution of the fonn pictured- That is the inage signals add in a partially destructive fashion senring to flatten the taPer.

C. FEED ANTENNA PLACEMENT AND GAIN

.An important consequence of the image effects described above is that the plrformance of the chamber is intinately associated with feed antenna characteristics. In fact, chamber performance and feed antenna characteristics are not separable because of this image coupling.

The result of irnproper feed placement is illustrated by the set of measured data shown in Figure 10. The method of taking the data was the I'Free-Space VSWRTT nethod of transverse probing. The paraneter L in the curves is the longitudinal displacenent of the antenna from the location where it is a'rsnug fit." The greater the value of L, the further the phase center of the antenna is fron the side walls.

It is evident that the field distribution departs from that of plane wave nore for greater values of L.

Gruner and Kreutel [10] describe the same behavior in a chamber of similar dimensions and cone angle. With swept frequency plots, they show that certain positions of the feed antenna can result in a nulling of signal at certain frequencies at the center of the test region.

The pattern characteristics of the feed antenna are important to the illunination of the chanber as wel1. Figure 11 shows the illumina-tion anplitude distribution for three different feed horns. T\e 24 dB horn is a Long-flare pyramida.L horn. The 15.0 dB horn is a short flare pyranidal horn. The 18 dB horn is a long-flare exponential horn.

Though side-wall illurnination is low with the high gain horn, its aperture size forces a wide separation of the phase center from side walls. The result is an excess of destructive interference in the center of the test region. The 15.0 dB horn is smaller in aperture and hence the phase center is nearer side walls. The high sidewall i1- Lumination of the lower gain short-fl"are horn produces excessive can-cellation as we11. The 18 dB exponential horn represents a compromise

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(Rotation of the antenna and end-roading of dipoles througir wall contact are obvious limitations. ) Figr:re 12 conpares typical test region illunination for apertures and for a crosied-pair-of sleeve dipoles.

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V. REFLECTIVITY PERFORMANCE OF THE CHAMBER

A. INTRODT'CTION

The motivation in using an anechoic chamber for radiation neasure-ments is, of course, to provide a test voLune throughout which the level of incident energy due to refl.ections fron enclosing walls is snall conpared with directly incident energy, The test volume of a chamber is generally terned the 'rquiet zone.r'

The figure of nerit which is used to characterize the degree to which reflection-free performance (or more precisely apparent reflec-tion free perfonnance in light of the discussion in preceding chapter) is attained in a given chamber is termed "reflectivity."

The reflectivity characteristic to a given quiet zone is the ratio of the level of signal due to side wall reflections inpinging fron a given direction to the level of signal arriving at the quiet zone along a direct path fron the feed antenna. Symbolical.ly, the reflectivity r nay be 'defined by r(O,e)=er(0,0)/ed, (1) where ud rup""rents the total electric fieLd signal.* entering the quiet zone along a direct path fron the feed antenna and er(0r0) de4otes the totaL electric field signal of the sarne poLarization as eo enter-ing the quiet zone fron the (Qre) direction due to reflections by the side wal1s.

Since er(0re) is defined as that incident energy which is polarized in the sane way as the direct signal, coupling of one vecto" conponent into another vector conponent in the course of reflection is not ac-counted for in reflectivity. Two other chanber paramete"s, cross-poLarization ratio and axial ratio, are used to quantify cross-coupling effects.

Typically, reflectivity is expressed in decibeLs i.e.

ft = (r)ag = 20 loBr'r

The reflected signal is, in actuality, a highly complex quantity comprising a specular reflection conponent and a conglomeration of scattered and multipath signals enanating from all of the edges and tips of rf absorber naterial throughout the chamber. That sone of

The tern "signal" is used for sinplicity of notation. It is assumed that the feed and test antennas are polarized compatibly.

(2) the scattered and multipath components are not significant is not irunediately obvious. However, experienced chanber designers find that a treatment of the specular component alone by neans of ray tracing is adequate for design nodeling [6].

It is in order to comnent that the reflectivity is a function of the reflection coefficient(s) of the absorber lining the chamber, the chanber geonetry, and the path length of the reflected signal.

B. MEASUREMENT OF REFLECTIVITY

In the course of developnent of anechoic chanber technology, nany nethods of testing reflectivity perforrnance have been used. Sone nethods provide rnore conplete and more reliable infornation than others, and sone methods are more denanding in tenns of instrunentation than others.

Reference [7] is cormonly considered to be the authoritative document on the topic of reflectivity neasurenent.

The "Free Space VSWRr technique is, today, the nost comnonly used method of chamber evaluation. (This nethod is tenned the "B. F. Goodrich nethod in [7], p. 38.) The method requires only moderately conplex equipment for reasonable accuracy and the inforna-tion content of the results is quite high.

C. REFLECTIVITY PERFORMANCE OF THE NWL TAPERED CH.AMBER

In the course of acceptance testing, reflectivity dam E* gathered at six frequencies [1], by means of the free space vSWR nethod. The resul.ts of the acceptance measurements is given in Figures 13 and 14 for the six frequencies and the worst case for each frequency is summarized in Figure 15 to allow interpoLation to other frequencies.

Attention is cal,Led to the two reflectivity curves for 1.0 GHz in Figure 14. The lower val.ues of reflectivity data were taken with a probe antenna of 6 dB gain, while the higher reflectivity data were taken with a 13 dB gain test antenna. The higher curve exceeds the lower one by an arnount approximately equal to the difference of the gain of the test antenna. ltlhen the raw data from which the reflectivity curves were derived is anal.yzed a startling conclusion is drawn. The back waIl reflected energy collected by a 6 dB gain antenna pointed toward the back wall is approxinately equal to the energy coilected by a L3 dB gain antenna! firis result can effect the inierpretation of errors resulting fron back wal1 ref1ections.

Brownell [6] puts forth a possible explanation of this phenomenon.

The wavelength at I GHz frequency is sr:ch that the scattering rron the back wall is highly diffuse. The test antenna is sufficiently oouu)q c t{otrqr :tr tiD O A+J d b4xr-r +rts

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close to the back wall, that it does not operate in a normal plane wave mode and the gain is not a good predictor of the energy that the antenna gathers. It is not understood why the energy gathered is almost constant for widely differing antennas. Brownell states that measurernents in other chanbers bear out the constant energy observation. rt is unclear what the range of frequencies is over which this behavior is observed.

rn the preceding chapter it was pointed out that reflections fron the sidewalls of the taper serve to favorably condition thefield distribution at the quiet zone. For this reason, it is not proper to interpret reflectivity in the near forward directions(t so degrees or so) in a strict sense. Errors in measurement due to non-plane wave character of energy arriving in the forward direction may be more correctly accounted for in terrns of the analytical response of the antenna to the field distribution present in the chanber.

D. ROTATIONAL PERFORMANCE OF TT{E CHAMBER

The rotational perfornance of the chanber is characterized in terms of two paraneters. Axial ratio is the ratio of the ninimrm received signal to naxinum received signal when linearly polarized source and test antennas are rotated with polarizations parallel.

cross polarization ratio is the ratio of power received in an ortho-gonal poLarization to that in a parallel polarization as the antennas are rotated. These paraneters are significant in the neasurement of antenna axial ratio and cross-polarization ratio.

In practice, synchronous rotation of feed and test antennas isimpractical. consequently, both axial ratio and cross-polarizationratio are measured sinultaneousLy by rotating continuouriy one of the antennas for discrete increnents in rotation of the other antenna.Figure 16 shows typical tTaces obtained in such a neasurenent. one 360o rotation passes through two maxinurn coupling points and two cross-eoupling nulls. The depth of the shallowest nul1 is the crosspolarization ratio (c) . The axial ratio (o) is the rnaxirnun deviation between peaks. The accurate conduct of the axial ratio measurementis quite difficult. The axial ratio is less than 0.s dB at alLfrequencies, and this value is less than the error associated withthe receiver/recorder combination. However, the error of the recorder nay be eliminated by reading the peak amplitudes from the receiverdial scale rather than fron the recorded chart. cutting all of thetraces requires several minutes. As a result, long term equiprnentdrift is another source of error. It nust be moniiored and- oiten datacorrection must be made to account for it.

2B

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q) dE +r q) dF{o5 o 3.ddo&2 a L!

lrj tr (9 td lrl

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UJ

F z o trj td t!

tfi '--rod oo r<.Fl &+rEd(d+Jv)o

(8p) u3l od 3At-Lvt3u

Table L gives the values of axial ratio and cross-pol.arizationratio which have been neasured in the tapered chanber. swept axialratio frequency data has been taken betwlen 500 and 750 WIz and also between 750 and 1000 MHz. The tenns noted by brackets representworst case values across these bands. The two axial ratio valuesnoted by asterisks were taken during contractor tests. No effort was made to reduce errors any more ihan necessary to demonstrate at_tainnent of specification. consequently, it is Lelieved that theactual value of axiar ratio is sonewhat snaller than the value dis_played in the table.

ross- Frequency Polari zation

Ratio Axial Ratio

Feed Antenna

100 MHz s00

I 000

3.0 GHz

9,25

16 dB

>25

>30

0.2 dB

0. 5*

0.3

0.35

0. 16

Nurad crossed Dipoles

Nurad crossed Dipoles sA 12-0.5 Horn sA 12-.75 Horn sA r2-2.7 Horn

Narda X-band Horn

0. 13

0,28*

Measured Values of Cross Polarization and Axial Ratios

Table 1 t

It is noteltorthy that the transnitter roon is, at lower frequencies, a resonant cavity operating in a low order node. lhe coupling of dipole feeds to the cavity significantly effects rotational measurenents. (For that matter, any movement in the transnitter roon rnay ef,fect low frequency neasurenents.) In order to render this coupting insignificant, the feed must be properly backed with either a reflector or absorber.

The 100 ffiz measurenents were nade with the roll-away apex section close behind the feed for this reason. Figure 17 pictures the con-figr:ration used. the clanshell section nay be used in a sinilar fashion, provided the feed is nounted on an appropriate extension.

FIGURE I.7

Using the Roll-Away Section to Supress Coupling to Shielded Roon

VI. ERRORS IN PATTERN MEASUREMENTS IN THE CHAMBER

Error analysis of any radiation measurement is difficult. The controLLed radiation environnent afforded by an anechoic chamber may be excellently approxinated, however, for nost practical antennas.

In fact, the 'tpattern cotnparison nethod't [7] of chanber evaluation uses such an analysis to quantify chamber reflectivity.

The total field in the test region of the chamber can, in principle, be represented in terns of an angular spectrun of plane waves. An ideal test ilLumination would eomprise a single plane wave propagating in the neasurenent direction. The error analysis is treated by con-sidering as the source of error only the doninant component of the renaining spectrun - that is, the plane wave propagating along the main bean direction of the test antenna.

This analysis does not yield an error bound in a rigorous sense.

In practice, however, error signals are seldom total.ly destructive or totally constructive. Consequently, the error rrboundsil derived herein nay be practically treated as bounds.

B. APPROXIMATE PATTERN ERROR ANALYSIS

The two plane waves shown in Figure 18 are the doninant signals arriving at the test antenna provided the antenna has a single main lobe u'hich is sensibly narrow. If all. other conponents of the total field in the test region are neglected, bounds niy be set on the resulting approxinate errot.

Let p(0,0) denote the norrnaLlzed complex field pattern of the antenna under test. Further, let the coordinate system in terns of which the pattern is defined be adjusted so that the nain bean of the pattern is aligned with the 0 = 0 = 0 direction for sinplicity. Note frorn Figure 18 that the positioner coordinates are the nlgative of the antenna coordinates. When the test antenna is receiving fron the Q,0 direction, the nain bean is receiving a reflected signai fron the -0, -0 direction in chanber coordinates, i.e. the effective field illuninating the antenna is

"tot (0, o) = ed [p ( o, e) + p (-0, -o) ] (3) where "d i, the directly incident field and p(O,e) is a,,complex re-flectivity quantity introduced so that (3) is formally correlt.

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,a-

I p(0,0)l = r(0,0) (4) a4d p is used in (3) to indicate that the interfering signal phasor eo p possesses an arbi(rary phase relationship with respect to the direct signal phasor e$. In the worst case interference instances r"g "dp

- "rg "dp (s) i.e. the interference is either naxinally constructive or naxinaLly destructive, representing worst case error conditions.

In the worst case situation l";:t(6,e)l = l"dl tlp(o,o)l t r(-0,-e)I (6) leact(q,0)l = l"dl I p (6,0)l

Cl.early fron (6) and (7) the reflectivity represents an error in the pattern measurement.

The data presented in the preceding chapter shows that r is a small quantity. However, in most pattern measurenents, p is also a snall. quantity for sone angles. thus the error is nore appreciable for lower pattern levels.

Since nost antenna patterns are recorded on a logarithmic sca1e, it is nost convenient to express the emor indicated in (6) as a ratio-matric error in decibels. Forning the ratio of worst case neasured pattern to actuaL pattern from (6) and (7) one obtains fo" = ftao"

(7) f er _ lu*3. (o,e)

I- lea%tl = I p(0,0)tr(-0,-e) (8)

I p (0,e) I

Expressed as logarithnic e:c:ror

."t = [ert Jds = 2o logro tffi.rnl

Figure 19 gives plots of logarithnic error calculated via (9) as a function of pattern level below nain bean for several values of re-flectivity. It is clear from the plots that the error, particularly the destnrctive error becones nore and more significant at lower pat-tern levels. In fact, when reflectivity and pattern level are equal they can cancel corresponding to an infinite logarithrnic error (Fortunately recording systens possess lower stops.) such things as false nulls can begin to appear at pattern leveIs equal or bil.ow the reflectivity level.

C. INTERPRETATION OF PATTERN ERRORS

Figure 20 shows a hypothetical measured pattern of an antenna possessing only a front and back Lobe. The range of uncertainty is shown by the shaded region about the trace assuning a -30 dB reflectivity.

Notice that the positive uncertainty is equal to the destructive error from Figure 19. i.e. the actual pattern nay exceed the measured pattern by the anount of cancellation atiributabre to errors. similarly, the negative uncertainty is equal to the constructive error.

The data given by Figure 19 may be used with the worst case data fron Figure L5 in order to quantify errors in a given measurenent. A stricter bounding of errors may be achieved by using the data in Figures 13 and 14 and taking into account the direction of the nain bean for a given rneasutetnent direction.

In practice, it is sonetines possible to discern the sign of the error from observation of recorded patterns. Figure 21 ihows a pattern taken in the NWL chamber compared with a pattern nade by the National Bureau of standards on the sane antenna. The NBS pattern was nade on a ground reflection range and is certified accurate to ! .25 dB to * 72o and to t 1.0 dB at maxinun angle.

Notice the oscillatory character of the error on the second side Lobes. T'his oscillatory behavior is a consequence of changing phase between directly received and reflected signals. Careful scrutiny of the neasured pattern would reveal the periodic variation without Lenefitof the contparison Pattern. A smooth curve night then be drawn through the cyclic variation.

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E PTANE PAT.

3.O GHz s A t2- 2.7 HORN t U t, ij t-!

tr t, MEASURED IN CHAMBER

------ MEASURED AT N B S

ct (r ld

=o [L lrJ

F J bJ (r

AZIMUTHAL ANGLE

FIGURE 21

Pattern Comparison Showing Cyclic Nature of Error

D. PATTERN ERRORS FOR LARGE APERTURES

The foregoing discussion assumes that the only source of tneasure-ment error is that of side wal1 reflections. When the antenna aperture is large conpared with wavelength, the phase distribution of the direct incident wave may depart substantially frorn that of plane wave.

The criterion for source and test antenna separation.

s > 2D2/x is often stated, This criterion limits the naximun antenna dinension D which nay be accomodated in the fixed separation of the chanber.

The basis for the linitation is maintaining the phase constancy across the test aperture within 22.5 electrical degrees. A detailed error analysis of the effects of this curvature is complex and depends on the tfpe of antenna under test. Hollis, at.al. give a discussion which is a good beginning point for such an analysis. [13]

4o

VII. GAIN AND RADIATED POWER MEASUREMENTS

rn the tapered chamber, free field conditions at the quiet zone are conditioned through the judicious use of reflecti-ons from the tapered walls. This concept is discussed in chapter IV. An important consequence is that the feed antenna and the charnber must be considered a single unit and power density at the test region cannot be predicted via free-space spreading equations. This restrictsl5ln-ana power density measurement techniques to those of cornparison or rrgain transferrr rneasurements. Reference [11] explains the technique. For the sake of an error discussion, the technique is outline here.

B. THE GAIN COMPARISON METHOD

The Friis transmission fornula [12] nay be written in logarithrnic form as

P" = Gt + Gt + Pt + 2o 1og (\/4rr), ( 10) where Pt and Pa represent received and transrnitted power, respectively, and are referenced logarithnically to some connon reference 1evel (dBn, dBw, etc.). Gr and Ga are the gains of the receiving and transmitting antennas, respectively, referenced to an isotropic radiator; and the log term accounts for beam spreading as a function of antenna separa-tions. Figure 22 shows a configuration to which (10) applies. If the setup in Figure 22a ts constructed with the receiving antenna being one of known gain, some received power Pf, for the standard gain case may be measured and the transmission equition constructed

Pi = G; + [Gt * Pt * 20 long (I/anr) I . (11a) where the terms in brackets are unknown. If the standard antenna is replaced with an antenna of unknown gain as indicated by Figure 22b a second received power P$ nay be measured. Comespondingly, Pl = C$ + [Gt + Pt + 20 log 0,/ a.;;,:)l

If proper care is exercised the tern in parentheses (11b) is invariant and subtraction of (11b) fron (1

. ( 11b) for each (11a) and 1a) yields

4'r

C) d .11 tr ooF P c.)

F

NO

c!h ll.lul dd 5C)(J Hr&tr o ut.rl cd o, o(J

,Ft (d (9 z =oz Yz l .ct

c) E az F ci i;

r; W

G$=Gi+P$-Pf. ( 12)

Notice that the unknown gain depends on only the relative received power and the signal measuring need not be an absolute power measuring device need measure absolute power.

The preceding discussion is strictly valid only for free-space conditions because only then does the spreading tern apply. rf the environment of the antennas is other than free space uui ttre test antenna is in an essentially plane-wave field, the spreading TEn isto be nodified by the environnent. rf the environment is constant through the two measurenents, the spreading terrn will stil1 be invariant and (12) sti1l holds. such is the case in a tapered chamber.

rf radiated power is the quantity to be measured a sinilar pro-cedure is followed. In this case the test antenna is the transtnitt"r and the "feed" antenna is the receiver. A standard neasurement is made with an antenna of known gain and the power delivered to the antenna is measured. Accordingly

Pi = Pi + Gt + Gr + 2o tog (\/4rr) ( 13a)

The unknown radiatgl ir then placed in the test region and operated.

A measurement on this unknown is nade so that

P$ = Pt * Gt * Gr + 2o 1og (),/4nr)

Because of the invariance of the term in brackets (13a,b) to yield

(13b) nay be solved

Pt=P;*c;-ct+pg-p: (14)

If GF is determined through a separate gain neasurement the transnitted powei nay be extrdcted from (14). Notice that the received powers in(14) appear as a relative quantity and an absolute power meaiuring systen is not required. The transmitted power must be measured by an absolute power neasuring device.

C. ERRORS DUE TO REFTECTIONS IN GAIN MEASUREMENT

The origin of reflection related errors in the measurements des-cribed in the preceding section lies in the departure of the illumi-nation of the test antenna from that of a plane wave. The departure takes two forms.

The first departure is that the illunination of the quiet zone is not constant in arnplitude and phase as a function of position. Chapter III discusses the taper of the anplitude distribution across the quiet zone and presents data presently available on the phenomenon for NWL tapered chamber. Phase data has not, to date, been taken. Reference 13 discusses the deviation of neasured performance in the presence of non-planar illumination.

The second departure from free-space conditions is the presence of signals at the antenna feed point arriving through regions on the antenna pattern other than the point at which gain is being measured- The effects of these elrors are difficult to quantify in general. One nust consider the pattern of the antenna under test vis-a-vis the reflectivity pattern of the chanber for a particular antenna and locate and quantify prirnary error conponents accordingly. The work of Moelier [f+] for a ground reflection lange nay prove useful as a guide for the evaluation of such errors where they are judged significant.

Generally, this source of error is nore significant when relatively 1ow gain antennas are involved in the measurement either as the test or siandard antenna since their pattern can accept more side and rear wal1 reflected energy. Regretably, the occurrrence of 1ow-gain antennas is nore frequent at lower frequencies where chanber perfornance is poorest.

In order to give the reader a feel for the nagnitude of gain errors due to reflections, the following analysis is carried out accounting for back-wall reflections a1one. Under sone circumstances, the results nay be extended to give error estimates for gain measure-nents where error signals arriving through side lobes is significant.

D. GAIN ERRORS DUE TO BACK-WALL REFLECTIONS

Consider the antenna test configuration shown in Figure 23. An antenna with arbittaTy Pattern is oriented with the main bearn a1onS.

the chanber axis. ft lnight be either the standard or the subject antenna in a conparison rneasurement.

An extraneous plane wave reflected from the back wal1 of the chamber illuminates the antenna through the back 1obe. The super-position'of the direct illunination and the extraneous illumination d(, .r{ pbotrc\t t{ OOE trl Fi 4) d liF{ JA)(9 ttt Ho(d lL !O va r{ C) d

FA

must be done in terms of field or voltage quantities. Thus an t'effectiverr field is postulated in terms of the worst case interference situations, nanely e"ff =5"d1 pto,o)l t .lp[r,o) l], (ls) where *d i, the direct path illuninating fieltt, p(0,0) is the conplex field pattern of the antenna, and r is the reflectivity paraneter pre-viously defined. The two values of p in (15) are the main and back lobe levels, respectively, and the ratio f - lp(0,0) /p(n,0) I ( 16) is the field front-to-back ratio of the antenna. The gain of an antenna under test is deternined from (12) through a comparison neasure-nent as described above. Equation (12) may be written in ratio form as s$ = cfrf/nf. (r7) since the received power quantities in (17) are proportional to the square of the inpinging electric field signals, the ratio e$ = giqe$lef)2 t18) rnay be written. The field ratio measured for the evaluation of (17) is not the ideal plane-wave field, however. It is the ratio bf effective fields as given by (15) for each antenna. If p(r,0) is eliminated from (15) in terns of (16) and the resulting effective field is used for both ef and e{ in (18), the following expression resul ts u .11'*l ieg/ef)2. (1e)8r=8rtt*r7t=

This expression presumes that ed is the same for both the standard measurement and the unknown measurement. The bracketed term represents an error ratio influencing the actual rneasurenent in a way that it departs fron the ideal neasurement Tepresented by (18). The extremes of the ratio occur when the signs in the numberator and denominator are conjugate so that the limiting values

K-

I t r/fu i20)t t r/fs may be defined.

Notice that the error ratio is nearer unity for lower reflectivity and for higher front-to-back ratio for either antenna. It is reiteratedthat this analysis is not comprehensive since back wal1 reflections only are taken into account. The dependence on pattern cones to bearonry through front-to-back ratio in (19).