report no.'do0t,/faa/rd-81/52 · report no.'do0t,/faa/rd-81/52 ' (o/faa/ct-81/46 if...

Report No.'DO0T,/FAA/RD-81/52' (O/FAA/CT-81/46 If

INVESTIGATION OF WILCOX MODEL 585BVERY HIGH FREQUENCY OMNIDIRECTIONAL

RADIO RANGE (VOR) SYSTEM (PART 3)

Wayne E. Bell

FEDERAL AVIATION ADMINISTRATION TECHNICAL CE3TERAtlantic City Airport, N.J. 00405

of TR.j4

0

FINAL REPORT

OCTOBER 1981

Document is available to the U S pubho throughthe National Technical Information Service,

Sprngfield, Virginia 22161

Prepared for

U. S. DEPARTMENT OF TRANSPORTATIONFEDERAL AVIATION ADMINISTRATION

Systems Research & Development ServiceWashington, D. C. 20590

NOTICE

This document is disseminated under the sponsorship ofthe Department of Transportation in the interest ofinformation exchange. The United States Governmentassumes no liability for the contents or use thereof.

The United States Government does not endorse productsor manufacturers. Trade or manufacturer's names appearherein solely because they are considered essential to

the object of this report.

Technical Report Documentation Page1. Report No. 2.Government Accession No. 3. Recipients Catalog No.

FAA-RD-81-52 * .-

4. Title and Subtitle 5. Report Date

October 1981INVESTIGATION OF WILCOX MODEL 585B VERY HIGH FREQUENCY 6. Performing Organization Code

OMNIDIRECTIONAL RADIO RANGE (VOR) SYSTEM (PART 3) ACT-100

8. Performing Organization Report No.7. Authort s)

Wayne E. Bell FAA-CT-81-469. Performing Organization Name and Address 10 Work Unit No. (TRAIS)

Federal Aviation Administration I.Technical Center 11. Contract or Grant No.

Atlantic City Airport, New Jersey 08405 041-305-83013. Type of Report and Period Covered

12. Sponsoring Agency Name and Address

U.S. Department of Transportation Final ReportFederal Aviation Administration March 1980-February 1981Systems Research and Development Service 14. Sponsoring Agency Code

Washington, D.C. 20590 ARD-30015. Supplementury Notes

16. Abstract

A three-part investigation of the Wilcox 585B Very High Frequency OmnidirectionalRadio Range (VOR) System was conducted. In Part I, the magnitude of the grounderror was reduced by modification and suitable adjustment of antenna elementlengths of the field detector. In Part 2, investigation developed an acceptablecalibratioi procedure for system 30 hertz (Hz) modulation. Obtaining compatible30 Hz mo~dulation reading between aircraft (far-field) and ý'edge of counterpoise"(near-field) measurements was an additional requirement. In Part 3, tests resolvedany discrepancies in the tuning adjustments prescribed by the manufacturer's equipmentmanuals.

This report, which is the last in a series of three, contains an outline of the testsand procedures for setting the lengths of the adjustable field detector elements,a recommended procedure for obtaining compatible near-field and airborne 30 Hzmodulation readings, and recommended changes to the manufacturer antenna tuningprocedures in order that the system meet required operational tolerances.

This effort is in response to FAA Form 9550-1, AAF-410-078-003.

17. Key Words 18. Distribution Statement

Slotted-Cylinder AntennaI Document is available to the U.S. publicVOR through the National Technical Information

Service, Springfield, Virginia 22161

19. Security Clossif. (of this report) 20. Security Classil. (of this page) 21. No. of Pages 22. Price

Unclass if ied lnlsiid4

aorm DOT F 1700.7 (8-72) Reproduction of completed page authorized

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TABLE OF CONTENTS

Page

INTRODUCTION 1

Purpose 1Background 1

Description of Test Facility 1

TEST PROCEDURES AND RESULTS 1

General 1Field Detector Investigation 4Modulation 30-Hz Investigation 14Near- and Far-Field Equality Measurement 14Antenna Adjustments and Tolerances 16

CONCLUSIONS 22

RECOMMENDATIONS 23

REFERENCES 23

APPENDICES

A - Procedure Sequence for Space Modulation ChartB - Investigation of Wilcox 585B VOR System (Part I)

r C' C P

• V.

LIST OF ILLUSTRATIONS

Figure Page

1 Wilcox Model 585B Shelter 2

2 Wilcox Model 585B VOR Cabinets 3

3 Standard and Modified Wilcox Field Detector 5

4 Modified Wilcox Field Detector Schematic Diagram 6

5 Field Detector Antenna Element With Adjustable Sleeve 7

6 Test Setup to Determine Proper Field Detector Element Length 8

7 Maximum Absolute Bearing Error Versus Field Detector Element 9Length - Field Detector Mounted at 22.50 Intervals Aroundthe Counterpoise Edge, Frequency 116.7 MHz

8 Ground Check Error Curve, Frequency 116.7 MHz, Modified Versus 9Standard Detector/Element

9 Maximum Scalloping/Roughness Error, 1350 Radial Inbound, 11Modified Versus SLandard Detector'Element

10 Maximum Absolute Bearing Error Versus Field Detector Element 11Length - Field Detector Mounted at k?.5* Intervals Aroundthe Counterpoise Edge, Frequency 109.0 MHz

11 Maximum Absolute Bearing Error Versus Field 'etector Element 12Length - Field Detector Mounted at 22.50 inter:vals Around

the Counterpoise Edge, Frequency 112.25 MHz

12 Wilcox Detector Element Adjustments 13

13 Maximum Absolute Bearing Error Versus Field Detector Element 15Length - Field Detector Mounted at 22.50 Intervals Aroundthe Counterpoise Edge, 32-Foot Diameter Counterpoise,Calverton, New York

14 Maximum Absolute Bearing Error Versus Field Detector Element 15Length - Field Detector Mounted at 22.50 Intervals Aroundthe Counterpoise Edge, 52-Foot Diameter Counterpoise,Salisbury, Maryland

15 Sample Spectrum Recordings 17

16 Ground Check Error Curve, Station Frequency 109.0 MHz 19

17 Bearing Error Curve, Bendix FA-4165.3A Receiver, 10 and 25 nmi 19Orbit, Station Frequency 109.0 M1z

iv

4 LIST OF ILLUSTRATIONS (CONTINUED)

Figure Page


19 Bearing Error Curve, Bendix FA-4165.3A Receiver, 10 and 25 nmi 20Orbit, Station Frequency 112.25 MHz


21 Bearing Error Curve, Bendix FA-4165.3A Receiver, 10 and 25 nmi 21Orbit, Station Frequency 116.7 MHz

LIST OF TABLES

Table Page

I Measured Detector Balance Error 10

2 Tabulation of Indicated VSWR Measurements 18

Nv

INTRODUCTION DESCRIPTION OF TEST FACILITY.

The Wilcox model 585B VOR station wasPURPOSE. installed at the center of a 400-foot

diameter asphalt pad adjacent to theThe purpose of this project was to main runway at the FAA Technical Center.investigate a Wilcox model 585B Very Figure 1 shows the circular metalHigh Frequency Omnidiiectional Radio shelter used to b.use the system withRange (VOR) System to readuce the magni- the modified field detector in a brackettude of the ground error, develop an position.acceptable calibration procedure forsystem 30 hertz (Hz) modulation, and to The equipment cabinets shown in figure 2correct the nonconformity of the initial contain a complete dual transmitter/dualadjustments wherein specification limits monitor system. Included in cabinet No.were exceeded. Investigation considera- I are a monitor/VOR test generator, ations included developing adjustment local control panel, a radiofrequencyprocedures and minor equipment (RF) di.tribution unit, a goniometer, amodifications. tra 1,smitter, modulator/power supply/

keyer, a terminal board panel, andBACKGROUND. a blower. Since the cabinets are

identical, tine control unit and testThere are several VOR stations in the generator can be installed in eitherNational Airspace System operating with cabinet.waivers because the initial adjustmentsdid not conform to tbe criteria in theFederal Aviation Administration (FAA) TEST PROCEDURES AND RESULTSPandbook 6790.4A, "Maintenance of VHrOmnirange Equipment." A request, viaFAA Form 9550-1, AAF-410-078-003 was GENERAL.i.niLiated for the Technical (enter toinvestigate the manufacturer's tuning Investigation of the Wilcox 585B VORprocedures and system tolerances, System was accomplished in three parts.to investigate ground check error In part one, the magnitude of the groundcomponents, and to develop calibration error was reduced by modification andprocedures for the field detector suitable adjustment of antenna elem.ontemployed in making the space modulation lengths of the field detector. Theadjustment. Several system elements second part of the investigationthat can contribute to the error in the developed an acceptable calibrationgr.und check include the field detector, procedure for system 30 Hz modulation.antenna, goniometer, transmitting equip- Obtaining compatible 30 Hz modulationment, and counterpoise. The effort readings between aircraft (far-field)associated with the reduction of ground and "edge of counterpoise" (near-check error was previously reported in field) measurements was an additionalFAA Technical Center letter report requirement. In part three, testsNA-80-27-LR, "Investigation of Wilcox resolved any discrepancies in the585B VOR System (Part 1)" (see appendix tuning adjustments prescribed by theB). The calibration procedure was manufacturer's equipment manuals.reported in the interim report numberFAA-RD-80-124, "Investigation of Wilcox A Convair 580 aircraft, N-49, and aModel 585B VOR System (Part 2)," dated Grumman Gulfstream, N-47, were employedApril 1981, by Wayne Bell and James to obtain flight test recordings ofDong. Pertinent information from both bearing information using a Bendixof these reports is included herein. FA-4165.3A receiver. The amplified

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POWER SUPPLY! MODULATOR/KEYE-R No. I' POWER SUPPLY/

BOARD- TERMINAL

PA NANEL

81-46-2

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course deviation indicator (CDI) voltage to a monitor where comparisons are madeof the receiver was recorded with a to preset signal levels. An alarm isdual-channel Tecas Instruments recti- initiated when the comparison of thelinear recorder. detected signals are not within the

established limits of the presetTen-nautical-mile (nmi) orbits at 1,500 signal level.feet above mean sea level (m.s.l.) and25-nmi orbits at 2,500 feet above m.s.l. The standard and modified field detec-were flown to obtain a sampling of the tors investigated are shown in figure 3.course radiated by the VOR system Figure 4 is a schematic of the modifiedoperating at 109.0, 112.25, and 116.7 detectors In each detector tested,megahertz (MHz). Course sensitivity care was taken to keep coil Li center-(space modulation), polarization effect, tap balanced with the coil being com-roughness, and scalloping were recorded pressed or expanded to allow capacitorson the 90* and 135' radials at an C2A and C2B to be adjusted for maximumaltitude of 1,500 feet above m.s.l. and signal. Detector accuracy (balance) wasa distance of 20 nmi. measured by the differeince between the

monitor resolver dial reading with theReference track for all orbits was field detector mounted normally and whenprovided by the Extended Area Instru- reversed 180'. A difference of *0.2'mentation Radar (EAIR). Dlistance is the maximum allowable as defined inguidance and 10' ezimuth marks from the the Wilcox instruction manual.EAIR were used by the aircraft fororbital flights; 2-nmi distance marks The standard Wilcox detector had theand azimuth guidance were provided by center tap of Li grourded while theEAIR to the aircraft cn all radial modified detector had an Ohmite Z-144,flights. 1.8 microhenry RF choke connected

between Li center tap and ground. TheFIELD DETECTOR INVESTIGATION. 1.8 microhenry RF choke was selected

because it has been used successfully inTests were conducted to determine the other types of VOR field detectors. Themagnitude of the ground check error antenna elements for the standardcomponent caused by the field detector/ detector were 12 7/8 inches long, whileelement. The design factors considered those of the modified detector werewere raising the inductor-capacitor adjustable in length. The design of the(L-C) tank circuit above RF ground to adjustable antenna element for theimprove detectnr balance and optimizing modified field detector is shown inthe length of the detector antenna figure 5.elements to reduce reradiation.

The test setup shown in figure 6 wasThe monitor/field detector is used configured to determine the properwhen making ground checks to determine/ element length for the field detectorestablish bearing accuracy of the VOR when mounted 10.5 feet from the \ORsystem. The monitor consists of four antenna tuned to operate at 116.7 MHz.chstuels, each of which evaluates a The amplified detected signal from thedifferent parameter of the VOR statioa yagi antenna on the 135' radial was fedoutput signal. The four channels are: to monitor No. 1 and the Hewlett-Packard(1) identification (keyed 1020 Hz), (2) model 7402 recorder. The yagi signalssubcarrier level (9960 Hz), (3) variable were used to balance monitor No. 2 withlevel (30 Hz), and (4) phase (compares the field detector disconnected andthe phase of the 30-dz variable to that removed from the counterpoise. Monitorof the 30-Hz reference). The field No. 2 connected to the field detectordetector supplies a demodulated signal with adjustable elements was used to

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81-46-6

FIGURE 6. TEST SETUP TO DETERMINE PROPER FIELD DETECTOR ELEMENT LENGTH

assure that sufficient signal was from the detector antenna elements.available when minimum interference was Once the minimum element length wasobserved on monitor No. 1. When the established, a crandard ground checkdetector elements were extended greater error curve was taken from the edgethan 12 7/8 inches, monitor No. 2 of the counterpoise to compare the

operation was unstable due to the indicated course using the standardincrease in signal level. Therefore, detector with that of the modifiedno data were collected for element detector. Results of these testslengths greater than 12 7/8 inches. In accomplished at 116.7 MHz are shown inaddition, data collection ceased when figures 7 and 8. Figure 7 shows themonitor No. 2 indicated a minimum yagi detected bearing error caused byoperationally acceptable level of 150 parasitic reradiation from the fieldmicroamps of 30 Hz variable signal level detector mounted on the counterpoiseon monitor meter M2 required for satis- edge. The bearing error was measuredfactory operation of all monitoring by the yagi antenna located at 135°functions, azimuth, 500 feet from the facility, and

40 feet in height. Minimum elementFor each selected element length, the length was determined to be 3 7/8 inchesfield detector with adjustable elements for this frequency. The detectorwas mounted successively at 22.5' antenna element length could have beenintervals around the counterpoise edge, decreased further, as indicated in

•! while the recorder processed the yagi figure 7, resulting in an additionaldetected course distortion (bearing reduction of bearing error. However,error) caused by parasitic reradiation the minimum operational acceptable

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ELEMENT LE! 1TH IN INCHES81-46-7

FIGURE 7. MAXIMUM ABSOLUTE BEARING ERROR VERSUS FIELD DETECTOR ELEMENT LENGTHFIELD DETECTOR MOUNTED AT 22.50 INTERVALS AROUND THE COUNTERPOISEEDGE, FREQUENCY 116.7 MHz

- STANDARD WILCOX DETECTOR/ELEMENT

--- MODIFIED WILCOX DETECTOR/ELEMENT

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AZIMUTH IN DEGREES81-46-8

FIGURE 8. GROUND CHECK ERROR CURVE, FREQUENCY 116.7 MHz, MODIFIED VERSUS STANDARD

DETECTOR/ELEMENT

9

monitor No. 2 signal level was reached tion on the optimum element length forat the 3 7/8 inch element length pre- various frequencies. The bearingcluding further reduction. Figure 8 errors for 109.0 MHz are shown in figureshows the ground check error curve using 10. Figure 11 contains the results ofa standard and modified field detector. 112.25 MHz. It was determined that the

The peak-to-peak error measured with the optimum element length was 6 7/8 inchesunmodified field detector was 1.4, for a station frequency of 109.0 MHz;whereas, it was 1.1 with the modified the optimum element length was 5 inchesfield detector. for 112.25 MHz.

Table 1 tabulates the improvement in With the optimum element lengthdetector balance obtained with modifi- established at 109.0 and 116.7 MHz, thecations and adjustments compared to the optimum length was predicted for 112.25standard detector. Thr'ee standard MHz, assuming that th• optimum lengthWilcox field detectors werc. used in the varied linearly with frequency, as showninvestigation, by the dotted line in figure 12.

However, the measured value at 112.25

Radial flights were accomplished at MHz for the optimum element length didi,500 feet m.s.l. with guidance provided not agree with the predicted value. Theby EAIR. The results of the radial difference between the measured andflights shown in figure 9 indicate the predicted value was caused by thereduction of scalloping/roughness using optimum element length established forthe modified detector with proper length 116.7 MHz. The optimum length at thiselements. frequency was determined by reaching

minimum signal level required for

Ground measurements were accomplished at proper monitoring, not minimum bearing

109.0 and 112.25 MHz to provide informa-

TABLE 1. MEASURED DETECTOR BALANCE ERROR

Detector

Field Detector Balance ErrorSample Condition (degrees)

No. 1 Standard Detector/Element 40.6RF Choke Added ±0.15Modified Detector/Element ±0.1

No. 2 Standard Det•t.or/Element ±0.4RF Choke Added 10.15Modified Detector/Element ±0.15

No. 3 Standard Detector/Element ±0.2RF Choke Added ±0.15Modified Detector/Element ±0.15

10

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0.9 MODIFIED WILCOX DETECTOR/ELEMENT

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DISTANCE FROM STATION (NAUTICAL MILES)814 -

FIGURE 9. MAXIMUM SCALLOPING/ROUGHNESS ERROR, 1350 RADIAL INBOUND, MODIFIED

VERSUS STANDARD DETECTOR/ELEMENT

2.0 STANDARD ELEMENT

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81-46-10

FIGURE 10. MAXIMUM ABSOLUTE BEARING ERROR VERSUS FIELD DETECTOR ELEMENT LENGTHFIELD DETECTOR MOUNTED AT 22.5S INTERVALS AROUND THE COUNTERPOISE

EDGE, FREQUENCY 109.0 MHz

1.11

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81-46-11

FIGURE 11. MAXIMUM ABSOLUTE BEARING ERROR VERSUS FIELD DETECTOR ELEMENT LENGTHFIELD DETECTOR MOUNTED AT 22.50 INTERVALS AROUND THE COUNTERPOISEEDGE, FREQUENCY 112.25 MHz

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error. Taking minimum monitor signal calibration procedure include "Mainte-requirements into consideration, the nance of VHF Omnirange Equipment" andproper element lengths for other "VHF Omnidirectional Radio Range (VOR)frequencies can be determined by the Electromagnetic Spectrum Measurements"solid line of figure 12. (references I and 2). These procedures

calibrate the nonlinear field detectorZGround measurements were accomplished at diode and assure that ground measure-

Calverton, Lotig Island, Very High ments made with the calibrated field9; Frequency Omnidirectional Radio Range detector to obtain the space modulation

and Tactical Air Navigational (VORTAC) chart did not introduce adjustmentfacility (117.2 MHz) and Salisbury, errors to the airborne measurements.Maryland, VORTAC (114.5 MHz) to provide The Wilcox model 585B VOR System hasinformation on optimum detector element a solid-state goniometer instead of alength at field sites with 32- and mechanically rotatable unit contained in

S52- foot diam eter counterpoise , earlier VOR models. Consequently, therespectively. Results of the bearing field detector was moved successively to

k error for the 32-foot counterpoise are the several counterpoise ground-checkshown in figure 13; results for the bracket locations to obtain intermediate52-foot counterpoise are shown in values between the minimum and maximum.figure 14, Results indicate that the The measured values of field intensitybearings error can be reduced by modi- correspond to radial position measure-fying and reducing the field detector ments made by keeping the field detectorelement length. A simple procedure for fixed and rotating the mechanicaladjusting the element length is to goniometer. The basic procedures in theadjust the field detector antenna FAA Handbook AF-6780.4A, "Maintenance ofelement for a monitor 30-Hz amplitude VHF Omnirange Equipment," were applied.modulation (AM) signal level reading of150 on monitor meter M2. The recommended procedure for obtaining

the space modulation chart for use withMODULATION 30-Hz INVESTIGATION, solid-state VOR equipment is presented

in appendix A.The Wilcox 585B VOR installation rwanualdoes not include a calibration procedure NEAR- AND FAR-FIELD EQUALITY MEASUREMENT.for the field detector diodes. Th especified 20' course sensitivity of the Tests were conducted to determine if theVOR is dependent upon the space modila- calibration procedure foi a space modu-tion being 30 peiý'ent. Space modulation lation chart detailed in appendix A(variable signal level) is adjusted and would permit comparable percent modula-monitored by the Lield detector. A tion readings (30-Hz AM) betweenmethod of calibrating the field detector aircraft and edge of counterpoisediodes to obtain a space modulation indicators.chart was required. The tests wereaccomplished with the Wilcox test site With the modified field detect-r at theoperating at a frequency of 109.0 MHz. 135° bracket position and using theInstrumentation employed in the testing space modulation chart, the modulationinclude a Fluke model 8600K digital was adjusted to 30 percent (using themultimeter, Hewlett-Packard model 7402A standard oscilloscope technique employedrecorder, and a Hewlett-Packard spectrum at VOR field sites for measuring theanalyzer comprised of a model 141T minimum and maximum 30 Hz AM). Thedisplay unit, a model 8553B RF unit, spectrum analyzer was connected toand a model 8552B intermediate freqeuncy a dipole antenna at the 135° bracket(IF) unit. Documents used as guidance position (edge of counterpoise)in formulating the VOR field detector ind recordings made. Spectrums were

14

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ELEMENT LENGTH (INCHES)81-46-13

FIGURE 13. MAXIMUM ABSOLUTE BEARING ERROR VERSUS FIELD DETECTOR ELEMENT LENGTHFIELD DETECTOR MOUNTED AT 22.50 INTERVALS AROUND THE COUNTERPOISEEDGE, 32-FOOT DIAMETER COUNTERPOISE, CALVERTON, NEW YORK

2.0STANDARD ELEMENT

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ELEAENT LENGI•H (INCHES)81-46-14

FIGURE 14. MAXIMUM ABSOLUTE BEARING ERROR VERSUS FIELD DETECTOR ELEMENT LENGTHFIELD DETECTOR MOUNTED AT 22.50 INTERVALS AROUND ThE COUNTERPOISEEDGE, 52-FOOT DIAMETER COUNTERPOISE, SALISBURY, MARYLAND

15

then recorded using the yagi antenna achievable. The initial toleranceslocated 500 feet from the counterpoise. investigated were: ±0.750 ground checkFinally, airborne spectrums were error, ±2.50 flight check error, andrecorded with the spectrum analyzer antenna transmission line voltageaboard the N-47 (Grumman Gulfstream) standing wave ratios (VSWR) of 1.02 oraircraft flying inbound at an altitude less for the carrier and sideband feedof 1,500 feet m.s.l. at 140 knots at 90° lines, with a goniometer input VSWR ofand 1350 radials. During these tests no 1.33 or less. Polarization effect waschanges were made to the VOR system also measured to determine ma'oimumadjustments. deviation of course caused by vertical

polarization (VP). The tolerance for VPUsing the spectrum analyzer technique is ±2° deviation.(reference 2), the modulation ratio is

Sdescribed as the ratio of sideband Field Facilities use a built-in RF(ES) to carrier (EC), i.e., Es/EC. wattmeter in measuring VSWR of systemThe values for this ratio were obtained components. A Hewlett-Packard modelfrom the recorded spectrums. Samples of 8405 vector voltmeter and model 778Dthe spectrums are shown in figure 15 dual directional coupler were used aswith their average percent modulation another technique (reference 4) tofor each test condition, confirm VSWR measurement accuracy.

Average values were the same for the The slotted-cylinder antenna was tunedairborne spectrum data from the 90° and to 109.0, 112.25, and 116.7 MHz by1350 radial flights inbound from 20- to adjusting its controls to prescribed7-nmi range to the VOR site. The settings obtained from the manu-modulation value measured from the yagi facturer's tuning chart. Additionalantenna, located 500 feet from the fine tuning to reduce VSWR at eachcounterpoise, was 0.3 of a percent frequency was required. The antennagreater than the airborne value, and sideband feed lines and carrier feed1.6 percentage points greater than the line were checked separately using thevalue measured at the edge of the built-in, thru-line wattmeter utilizingcounterpoise. If the 500-foot ground the carrier from the transmitter as themeasurement is considered far-field and signal source. The manufacturer sug-averaged with the airborne measurements, gests adjusting the carrier power levelthe difference between near- and far- to 20 watts for VSWR measurements. Evenfield is a modulation percent difference at this power level it was difficult toof 1.45. Without any compensation for get accurate VSWR measurements. There-this difference, the procedure used to fore, these VSWR tests were conductedset the 30-percent optimum adjustment with the carrier power adjusted to 100resulted in an airborne measurement well watts. The carrier feed line VSWR, whenwithin the tolerance of 25 to 35 percent operating at 109.0 MHz, could not belevels listed in reference 3. reduced lower than 1.2. Carrier line

adjustments (upper and lower plungerANTENNA ADJUSTMENTb AND TOLERANCES. capacitors C2 and C4) would not reduce

the reflected power. A "T" connectorThe latest model slotted-cylinder VOR was installed in the carrier line at theantenna manufactured by Wilcox is antenna, and a stub was installed andclaimed (by Wilcox) to meet all adjusted to obtain minimum VSWR.tolerances specified in chapter 4 of the Minimum VSWR was achieved when the stubFAA Handbook 6790.4A and Flight Inspec- was adjusted to 4 inches. Wilcoxtion Manual OA P 8200.1. Ground and factory engineering confirmed thisairborne measurements were made to condition and they agreed that the stubdetermine if specified tolerances were was required to achieve a 1.02 VSWR when

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operating at the low end of the VOR Results of the ground and airborne testsfrequency band. were within the established tolerances

and are shown as follows. Figures 16After the antenna was tuned to each and 17 are presentations of datafrequency, a ground check error curve obtained while the station was tuned towas taken and goniometer adjustments 109.0 MHz. The maximum ground checkwere made to minimize station error, error was ±0.550 and the maxim,,m flightPrior to flight testing, the VSWR was check error was ±1.7° for the 10- andrecorded using both the RF wattmeter 25-nmi orbit. Figures 18 and 19 showmethod and the vector voltmeter method. the maximum ground check error curve of

±0.650 and the maximum flight checkA comparison of the VSWR readings using error of ±1.5* for the 10-nmi orbitthe RF wattmeter and vector voltmeter and ±1.46° for the 25-nmi orbit whenmethod is presented in table 2. The operating at a station frequency oftable shows that the VSWR commissioning 112.25 MHz. Figures 20 and 21 show thetolerance of 1.02 was achieved only maximum ground check and maximum flighttwice using the wattmeter, and never check error while the station wasachieved using the vector voltmeter operating at 116.7 MHz. The groundmethod. The VSWR measured with the check error was ±0.60 and the flightvector voltmeter averaged 0.035 greater check error was ±1.450 and ±1.5° forthan the wattmeter readings. Also, 10 and 25 nmi, respectively.the VSWR of 1.02 required for allthree feedlines at each frequency Two vertical polarization checkswas never achieved. The reflected including 30° wing rock and headingpower measured at the goniometer input effect were conducted. These tests werewas less than 300 milliwatts with 15 made approximately 20 nmi from the VORwatts forward. This is within the at an altitude of 1,500 feet abovespecified VSWR tolerance of 1.33. m.s.l. A value of ±0.250 of VP error

TABLE 2. TABULATION OF INDICATED VSWR MEASUREMENTS

Frequency

MHz MHz MHz Commissioning VSWRSystem Component 109.0 112.25 116.7 Tolerance

Carrier Feedline VSWR

RF Wattmeter 1.02 1.04 1.04 1.02Vector Voltmeter 1.078 1.072 1.075

No. I Sideband Feedline VSWR


No. 2 Sideband Feedline VSWR


18

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21

was measured using the 30° wing rock 4. Minimal improvement was obtained inmethod; a ±0.5° heading effect was scalloping/roughness error.recorded. Both of these results arewithin the established tclerance. 5. A procedure employed for calibra-

tion of the diodes on the fieldDuring a rainstorm, an unstable moni- detector in a solid-state Very Hightoring condition occurred resulting in a Frequency Omnidirectional Radio Rangefacility shutdown. The cause of this (VOR) System using field detector atcondition was moisture in the plug/ ground check brackets can be used whenconnector (PI/JI) on the field defector. there is no rotatable goniometer.Making the plug/connector waterproof byusing Devcon rubber (rubber in a 6. A spectrum analyzer techniquesemipaste form) and electrical tape used to measure percent modulationeliminated the problem. provided the accuracy to observe

differences in near- and far-fieldComponent failures which occurred during measurements.2 years of intermitL-nt operation of thedual 585B VOR system are: 7. Using 100 watts of carrier power

during the voltage standing wave1. K-3 relay, component of modulator/ ratios (VSWR) measurement resulted inpower supply/keyer. more accurate VSWR measurements.

2. U-I transistor, two each, modulator/ 8. A "T" connector and stub must bepower supply/keyer. added to the carrier feedline to achieve

minimum VSWR.3. Power switch, component of monitor.

9. The vector voltmeter VSWR4. HP82-2800 diodes, two each, measurements were greater than thecomponent of field detector. RF wattmeter readings, but not

significantly.

CONCLUSIONS 10. The initial VSWR tolerances listedin the "Maintenance of VHF OmnirangeEquipment," FAA 6790.4A, cannot be

Based on the test results, it can be achieved.concluded:

11. The initial ground check tolerance1. System improvement was obtained with listed in FAA 6790.4A can be achieved.the modification of adding a radio-frequency (RF) choke and keeping the 12. Flight check tolerances listed inind.ctor capacitor (L-C) circuit in the Handbook OA P 8200.1, "United Statesdetector about RF ground. Standard Flight Inspection Manual," were

achieved.2. Reduced course offset and bearingerror were obtained by adjusting field 13. The vertical polarization error wasdetector elements to the proper length negligible.corresponding to frequency.

14. Fi'ld detector connector P1 /Jl is3. The ground check error was reduced subject to moisture contamination

field detector elements to proper monitoring.

length.

22

RECOMMENDATIONS 5. Procedures should be modified toreflect the use of 100 watts of carrierpower during VSWR measurements.

Based on the investigation results, itis recommended that: 6. Field detector connector (Pl/JI)

should be waterproofed to eliminate1. The modifications of a radio- unstable facility monitoring caused byfrequency (RF) choke and the use of moisture contamination.adjustable elements on the fielddetectors be employed at field siteswith a Wilcox 585 Very High Frequency REFERENCESOmnidirectional Radio Range (VOR)System.

1. Maintenance of VHF Omnirange Equip-2. The calibration procedure included ment, FAA New England Region supplementherein for obtaining a space modulation 6790.4A, AFI, March 16, 19'9.chart be used for solid-state VORsystems. 2. Kanen, Garth M., VHF Omnidirectioial

Radio Range (VOR) Electromagnetic3. The Wilcox installation handbook be Spectrum Measurements, Technical Note,updated to include the addition of a "T" Project 213-060-78, October 1978.connector and stub in the carrier feedline. 3. United States Standard Flight

Inspection Manual, FAA Handbook OA P4. The initial voltage standing wave 8200.1, Section 201.5, Chg. 27, Mayratio (VSWR) tolerances listed in FAA 1977.6790.4A be changed to a maximum valueof 1.08. 4. Measurement of Complex Impedance,

Hewlett Packard Application Note 73-3.

23

APPENDIX A E and EM were measured from tne

detected waveforms jack (07) on thePROCEDURE SEQUENCE FOR SPACE Wilcox model 585B monitor panel.

MODULATION CHART EE. E1 0 0 = 100 x

Emax

The transmitter carrier power, adjusted (proportions the scale to 100)to 100 watts, is fed to the northwest(NW) and southeast (SE) antenna slots. F. s p t4T -5 scale proportion to 40 for

This is sideband (SB) No. 1 (the sine 2.5sideband). To accomplish this, discon- convenient use on the oscilloscope.

nect the carrier out cable and terminateit into a dummy load. Connect the SB To develop column E of table A-1, the

No. 1 output cable to the jack where the following sequence is used:

carrier output cable was connected.Connect the SB No. 2 output cable to a Place the field detector at the 45°

dummy load. Turn the goniometer power azimuth bracket and record the digital

off. voltmeter reading at P7 on the monitorpanel. Continue recording the voltmeter

In preparation of a space modulation readings for each azimuth bracket

chart, measured values for the different location between 450 and 1350. Figure

parameters for various field detector A-i is a space modulation chart plottedangles are listed in table A-1. An from the data in table A-i.

additional counterpoise ground checkbracket is installed at an azimuth of78.75°, providing another field detector TABLE A-i. VALUES FOR SPACE MODULATIONcalibration position. CHART

The following terms and sequence were A B C

used:Modulation

A. p= field detector angle (used in Degrees Degrees Percent

lieu of goniometer angle).0 45.0 100

B. 0 = p + 45 = azimuth of field 22.5 67.5 44.6

detector location. 33.75 78.75 28.5745.0 90.0 17.16

C. 100 = percent modulation. 67.5 112.5 3.9690.0 135.0 0

I - sinpI + sinp

D E F

(Notice that values for parameters inlines A, B, and C are related by theirphysical geometry. These values can be Volts 100 100/2.5

determined and entered in the tableprior to any electrical measurements.) 0.01 0 0

6.85 44.63 17.85

D. E = Field Intensity (measured by 9.40 61.24 24.5

digital voltmeter). 11.59 75.51 30.214.35 93.49 37.4

EM Maximum field intensity at 15.35 100.00 40.0P = 90.

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APPENDIX B

INVESTIGATION OF WILCOX 585r5 VOR SYSTEM (PART I)

A

NAFEC TECHNICALLETTER REPORT

NA-80-27-LR

INVESTIGATION OF WILCOX

585B VOR SYSTEM

(PART I)

byWAYNE BELL

FEDERAL AVIATION A[M1INISTRATIOrNATIONAL AVIATION FACILITIES EXPERIMENTAL CENITER

Atlaidic City, New Jersey O8t§5

faal

NOTICE

This document is disseminated to provide for quickreactio:i release of technical information to selectedusers. The contents may not conform to FAA formalpublication standards and the technical informatiornis subject to change and/or incorporation into aformal publication at a later date.

Approved by t

S, Ntionl Aiation Facilities

ýkxperimental CenterFederal Aviation AdministrationDepartment of Transportation

ABSTRACT

Tests were accomplished on three standard and three modified fielddetectors with adjustable length elements to investigate reductionof ground check error curve and scalloping at VOR sites with theWilcox equipment. Orbital and radial flights were accomplished at116.7 MHz which verified improvement with modified field detectors.A drawing of the field detector element with adjustable sleeve isincluded to facilitate fabrication. A figure is included to provideguidance for setting detector elements to optimum length at WilcoxVOR facilities operating with a 21-foot diameter counterpoise.

PURPOSE

The purpose of this effort was to test a Wilcox 585B VOR systemto determine the magnitude of the ground check error componentcaused by the field detector/element. Raising the field detectorabove radio frequency ground and optimizing the length of thedetector antenna elements were the only design factors includedin this initial test phase.

BACKGROUND

There are several VOR stations in the National Airspace Systemoperating with waivers because the initial adjustment did notconform to the criteria in FAA Handbook 6790.4A. A request byFAA Form 9550-1, AAF-410-078-003 was initiated for NAFEC toinvestigate ground check error components. Several systemelements that can contribute to error include monitor/fielddetector, antenna, goniometer, transmitting equipment, andcounterpoise.

DESCRIPTION OF TEST FACILITY

The Wilcox VOR system operating initially at a frequency of116.7 MHz was installed in a 21-foot diameter metal drum typeshelter. Location of the shelter was at the cer.ter of a 400-footdiameter asphalt pad (formerly NAFEC MOPTAR Site). Figure 1 showsthe types of field detectors employed in this test phase.

TEST PROCEDURE AND RESULTS

The monitor/field detector was used when making ground checks todetermine bearing accuracy of the VOR system. The detectorsupplies a demodulated signal to the monitor and a comparisonwas made to a preset signal level. If the detected signal wasnot within the preset limit, an alarm was initiated.

Three standard Wilcox field detectors were used in the investiga-tion. In each detector tested (figure 2), care was taken to keepcoil Ll center-tap balanced with the coil being compressed orexpanded to allow capacitor C2A and C2B to be adjusted formaximum signal. Detector balance was measured by the differencebetween the monitor resolver dial readings with the fielddetector mounted normally and when the field detector wasreversed 180 degrees. Tests were made with a standard detector/element which has 12 7/8-inch elements and a grounded center-tapto coil Ll. These tests were repeated with each sample modifiedwith on Ohmite Z144 1.8 microhenry RF-choke added as shown infigure 2.

2

Providing adjustable elements or. the field detector was anadditional modification that was accomplished (figure 3). Teststo determine proper element length for 116.7 MHz and a radialdistance of 10.5 feet utilized a test setup shown in figure 4.A Yagi antenna was used with monitor 41 and a Hewlett PackardModel 7402 recorder was used to record error for various detectorelement lengths. Monitor #2 connected to the detector withadjustable elements was used to assure sufficient signal wasavailable when minimum interference was observed on Monitor #I.

Results of these tests accomplished at 116.7 MHz are shown asfollows:

Figure 5, the course offset error obtained with variouselement lengths; figure 6, the bearing error obtained with variouselement lengths; figure 7, the ground check error with modifica-tions and optimum element length; and table 1, tabulates theimprovement with modifications and adjustments compared tostandard detector.

Orbital and radial flights were accomplished at 1,500 feet meansea level (MSL) with guidance provided by extended area instrumenta-tion radar (EAIR). Orbital flights at 5 and 20 nmi demonstratedthat the VOR system was within tolerance. The results of theradial flights shown in figure 8 indicate the reduction ofscalloping using the modified detector with proper length elements.Optimum element length could be decreased as indicated by thecu-rves to reduce course and bearing error, but the loss of signalrasulting from shortening the elements at this frequency restrictedfurther reduction in element length.

Ground measurements were accomplished at 109 MHz and 112.24 MHzto provide information on the optimum detector element length forvarious frequencies. Results of the course and bearing errors for109 MHz are shown in figures 9 and 10 respectively, and similarlyfigures 11 and 12 are the results for 112.25 MHz. With theoptimum element length established at 109 MHz and 116.7 MHz, anoptimum length was predetermined for 112.25 MHz assuming thatoptimum element length varied linearly with frequency as shownin figure 13. However, the measured value at 112.25 MHz for theoptimum element length did not agree with the assumed value.Conseqnently, the proper element lengths for other frequenciesshould be determined by the solid line in figure 13 at fieldsites with a Wilcox 585 VOR system and a 21-foot counterpoise.

CONCLUS IONS

Based on the test results, it can be concluded:

1. System improvement was obtained with the modification inadding a RF choke and keeping the L-C circuit in the detectorabove RF ground.

3

2. Additional improvement was obtained by adjusting field

detector elements to the proper length corresponding to frequency.

RECOMMENDATIONS

1. The modifications of the additional RF choke and the useof adjustable elements on the field detectors be employed at fieldsites with a Wilcox 585 VOR system.

2. Measurements be accomplished at sites with a Wilcox 585VOR system employing counterpoise of 32-foot and 52-foot diameterto determine system performance with appropriate detector elementlengths.

This project was accomplished under NPD 04-309, Subprogram 041-305-830. For further information, contact Wayne Bell or Edward M.Sawtelle, NAFEC Program Manager, ANA-100B.1, telephone FTS 8-346-3911, commercial (609) 641-8200, extension 3911. Results of thisreport will be included in a final report covering additionalsystem elements on the Wilcox VOR system.

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