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. ... GEORGIA INSTITUTE OF TECHNOLOGY ' l ::_.,..1 '.• • .··· - ·. ~ Engineering Exper~ment Station '-'~- :~-~~-}(: . _, -, ·;-: I - . PROJECT INITIATION . ~:: ··-~.:'~.~ ~-~,:··· '. ~ - . . ::. ··~- . Date: Project Title: ~ f· i~ea on Site for Telephone Swi.tehing Equipment Facility
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I~A-73 GEORGIA INSTITUTE OF TECHNOLOGY Engineering Experiment Station
PROJECT TER,\HNATION
26~ Date ______February 1974__
PROJECT Switching. Equipment Facility··
PROJECT
. ~ .. PROJECT l-fr• J. C.. Toler ":""' :ri 1 > •~-;--;' ,,., • •• ~ ' ' . -·~ . Centra1 .Bell Telepbmlet:. Company; -· SPONSOR:
:,. · . ..)·: :.. ~ · "" . CHARGES SHOULD CLEAR ACCOUNTING BY: J~tsa.Jy_: 31,. 1974
- , - - " "11 .. ' ~ ~ CON'rBACT CLOSEOUT ITEMS. R.EMAINillG: Subm:l.t:~ Pinal Invoice for: eos ts up. to contra;et amount. unless additional. costs authorized by sponsor. -~ · · · ' ·~~ >~ · ~;:. ~ :.. , .
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COMMUNICATIONS DIVISION
COPIES. TO:
Project Director General Office Services Director Photographic Laboratory Associate Director Purchasing Assistant Direc.tors Report Section Divisiol"l, Chief Library Branch Head vSecurity Accounting Rich Electronic Computer Center Engineering Design Services
'"o rrn EES 402 (R 10-52)
SURVEY OF ELECTROMAG0lliTIC ENVIRONMENT NEAR JACKSON NISSISSIPPI
By
J. C. Toler, B. M. Jenkins, and B. J. Wilson
December 1973
For
SOUTH CENTRAL BELL TELEPHONE COMPANY P. 0. BOX 811 JACKSON, MISSISSIPPI 39205
Communications Division ENGINEERING EXPERIMENT STATION Georgia Institute of Technology Atlanta, Georgia 30332 FOREWORD
This report was prepared by the Systems and Techniques Department of the Engineering Experiment Station at the Georgia Institute of Technology. The work was performed under the general supervision of Mr. D. H. Robertson, Chief, Communications Division, and Mr. J. C. Toler, Project Director. The report presents the technical activities and results of a program con cerned with measuring the radiated electromagnetic environment at a proposed Data Processing Center near Jackson, Mississippi. The authors gratefully acknowledge the assistance provided by Mr. Ed Mooneyham, South Central Bell Telephone Company, Jackson, Mississippi. TABLE OF CONTENTS
PAGE
I. INTRODUCTION • 1 I I. SITE LOCATION 2
III. ~lliASURE~lliNT PROCEDURES AND CONFIGURATIONS 2 1. Discrete Frequency Measurements 5 2. Scan Measurements 7 IV. MEASUREMENT RESULTS 9 V. CONCWSIONS 14 VI. REFERENCES 16 APPENDIX A - PHOTOGRAPHS OF TEST CONFIGURATION AND ANTENNAS . 17 APPENDIX B - MATHEMATICAL PREDICTIONS OF FIELD INTENSITY 20 SURVEY OF ELECTRO:t-1AGNETIC ENVIRONMENT NEAR JACKSON, MISSISSIPPI
I. INTRODUCTION
During the past few decades, utilization of the electromagnetic spec trum has undergone an unprecedented gro\vth. The areas within which this growth has occurred literally run the gamut from electronic toys for children to highly sophisticated communication and radar systems for national defense. Generally, utilization of the spectrum for these purposes is desirable, and in cases such as national defense, probably mandatory. As might be expected, hmvever, this rapid increase in spectrum utilization has not been without certain problems. Many of these problems are discussed in terms of electro magnetic interference* considerations. They commonly manifest themselves as electromagnetic nuisancesl such as congested communication channels and signal cross-talk between adjacent cables; however, electromagnetic inter ference is also capable of causing equipment malfunctions2 that may jeopar dize human life. One of the areas within which electromagnetic interference is receiving increasing attention is concerned with electronic equipment susceptibility or vulnerability. This attention is being directed to the fact that major electronic equipments have been shown to be incapable of functioning pro perly during exposure to ambient electromagnetic environments. As a result, it has become necessary in many instances to design facilities which house these equiprnents in such a manner that electromagnetic environments are re duced to tolerable levels. The technical efforts undertaken on this project were directed to de
termining the magnitude of the electromagn ~ etic environment at a site pro posed for a telephone data processing facility. This magnitude was of · concern because of the suspected sensitive nature of electronic equipments
*Electromagnetic interference is generally defined as either malfunctioning of or performance degradation in electronic devices caused by undesired signals transmitted through space or conducted along wires. The undesired signals may occupy either narrow or broad portions of the frequency spectrum.
1 to be used for data processing. Of particular concern were discrete fre quency signals being transmitted from radio and television broadcast towers located near the proposed site. Subsequent portions of this report identify the site location, describe the test procedures and equipments used, and present the field intensity magnitudes measured.
II. SITE LOCATION
The location at which these field intensity mappings vJere made \vas a proposed construction site for a South Central Bell Telephone Company Data Processing Center near Jackson, Mississippi. The site was southwest of Jackson, as shown in Figure 1, and was bounded on two sides by State High\vay 18 and County TV Road, as shown in Figure 2. Also shown in Figure 1 are the approximate locations of various broadcast towers relative to the site.
These~ towers were used by AN radio, FM radio, and television stations and were the primary source of intense fields at the proposed site. The site was approximately 40 acres in size, and was comprised largely of a gently rolling hill. The knoll of this hill was the proposed site for the Data Processing Center, and field intensity mappings were made at three different locations on this knoll. These locations and their approxi mate position relative to each other and to State Highway 18 are indicated in Figure 2.
III. MEASUREMENT PROCEDURES AND CONFIGURATIONS
The measurements of field intensity at the proposed site were made on December 18 and 19, 1973. Two different measurement approaches were used for these field intensity mappings: (1) signal levels at discrete frequen cies corresponding to known local broadcast stations were measured, and (2) a frequer:.cy scan from 14 kHz to 1.3 GHz was made to determine whether other signals of appreciable magnitude existed. During every mapping, the antenna height above ground was ten feet to correspond approximately with the second floor level of the proposed Center. Weather conditions on December 18 were clear with an approximate temperature of 50 degrees
2 ~~------~------~------~ I County TV Road I
0 0 ~ N Figure 2. Test Site with Approximate Measurement Locations. 4 Fahrenheit, but changed on December 19 to overcast skies, light rain, wind, and an approximate temperature of 45 degrees Fahrenheit. During all efforts to detemine the field intensity at the proposed site, operating procedures and calibration factors provided for the measure ment antennas by the manufacturer were used. As a further verification that the recorded field magnitudes were accurate, a known electromagnetic environment \vas generated in a shielded enclosure3 and the test antenna was used to map the resulting field intensities. The measured intensities com pletely qualified both the operating procedures and calibration factors provided by the manufacturer, thereby providing assurance that field inten sities measured at the proposed site were correct. Specific procedures and configurations for the discrete frequency and frequency scan approaches used for field intensity mappings are presented in the following paragraphs. 1. Discrete Frequency Measurements Two measurement series, each involving different test instrUJI1ents, were used to map the discrete frequency electromagnetic environment. The basic test configuration was as shown in Figure 3 and the antennas used are listed in Table IA. During one series, the spectrum analyzer was operated in its Receive Mode and used to measure the intensity of signals correspond-· ing to frequencies of local AM, FM, and TV stations. The second series in volved using the tunable receiver to verify the measurements made at FM and TV station frequencies by the spectrum ana ~ lyzer. During every measurement, the measure~men t procedures cons is ted of first positioning the appropriate antenna at the~ ten-foot height on the tripod and providing a connection to the test instrurr1ent. The test instrument was then tuned to the frequency of the broadcast station and the radiated signal mag nitude was observed. The antenna was then rotated in both azimuth and eleva- tion positions until the direction yielding maximum signal level and optimum polarization was established. The magnitude of this signal \vas determined from the test instrument display and used in conjunction \vith known antenna characteristics to provide field intensity in units of volts per meter. 5 Receiving Antenna HEWLETT PACKARD r--- MODEL 8640B I SIGNAL GENERATOR I I FAIRCHILD MODEL EMC-25 RECEIVER I I HEWLETT PACKARD I -- --~ MODEL 8552 SPECTRUM ANALYZER Figure 3. Test Configuration for Discrete Frequency Measurements. TABLE IA RECEIVING ANTENNAS USED DURING COMMERCIAL BROADCAST S~~TION MEASUREMENTS Frequency Band Antenna 535-1605 kHz Fairchild Model ALR-25 Loop Antenna (A1'1 Band) 88-108 MHz Scientific-Atlanta Model 15-77 (FM Band) Standard Gain Dipole 60-66 IviHz Scientific-Atlanta Model 15-77 (Channel 3) Standard Gain Dipole 204-210 MHz Scientific-Atlanta Model 15-200 ( Channe 1 12) Standard Gain Dipole 482-488 :MHz Scientific-Atlanta Model 15-350 (Channel 16) Standard Gain Dipole 560-566 MHz Scientific-Atlanta Model 15-350 (Channel 29) Standard Gain Dipole After each measurement, a signal generato·r with a calibrated frequency scale was used to precisely identify the frequency at which the measurement was made. 2. Scan Measurements The spectrum analyzer offered the capability of visually display• ing all signals within a selectable band of frequencies. This capability was used during scan measurements to view the 14 kHz to 1.3 GHz portion of the frequency spectrum for the purpose of determining all signals of signifi cant magnitude. Specifically, these me.asurements were made to determine whether high intensity signals other than those associated with the known broadcast stations were present. The configuration of test instruments and the antennas used for each frequency ra.ngE~ within the 14 kHz to 1.3 GHz band are presented in Figure 4 and Table IB, respectively. After positioning the appropriate antenna on the tripod, the test procedure consisted of initiating the electronic scan on the spectrum analyzer. This provided a scan of each 7 Receiving Antenna HEWLETT PACKARD MODEL 7402A CHART RECORDER 9.< '"",t"'t ...... 0.. ~ m ~ ·- t"'t 0 HEWLETT PACKARD -- MODEL 8552 SPECTRUM ANALYZER Figure 4. Test Configuration for Frequency Scan Measurements. TABLE IB RECEIVING ANTENNA USED DURING FREQUENCY SCANNING TEST Frequency Band Antenna 14 kHz - 25 MHz Fairchild Model RVR-25 Vertical Monopole 25 - 200 MHz Fairchild Model BlA-25 Bi-Conical 200 - 1000 MHz Fairchild Model LCA-25 Log Conical frequency range at a speed slow enough to permit the analyzer's video output signal to be recorded on the strip chart recorder. IV. MEASUREi:1ENT RESULTS Field intensity magnitudes are commonly measured in units of volts per meter while susceptibility limits for electronic devices are typically specified in units of decibels above one microvolt per meter, consequently, field intensities measured at the proposed site are presented in both units for convenience. Further, conversions between the two units are provided in Table II as an aid in efforts to determine the need for shielding in any facility that may be constructed on the proposed site. Discrete frequency field intensity mappings were made during the first day of testing at locations #1 and #2 on Figure 2. On the second day of testing, mappings were made at location #3 on Figure 2. These mappings involved use of both the spectrum analyzer and tunable receiver to measure the magnitude of known broadcast signals. Resulting field intensity magnitudes are pre sented in Tables III, IV and V for AM, FM, and TV stations, respectively. As the test results indicate, the most intense AM broadcast signal was 71 millivolts per meter and occurred at locations #1 and #3. This signal intensity was measured at both 620 kHz, corresponding to Station WJDX, and at 1590 kHz, corresponding to Station WWUN. The most intense F~r broadcast signal occurred at 95.5 MHz and had a magnitude of 462 millivolts per meter. This frequency corresponded to Station WLIN and the 462 millivolt signal was 9 TABLE II CONVERSION TABLE Volts/Meter dB/'tJ.V/m Volts/Meter dB/tt-V /m 3.160 130 0.178 105 2.820 129 0.159 104 2.510 128 0.141 103 2.240 127 0.126 102 1.995 126 0.112 101 1.778 125 0.100 100 1.585 124 0.089 99 1.413 123 0.079 98 1.259 122 0.070 97 1.122 121 0.063 96 1.000 120 0.056 95 0.891 119 0.050 94 o. 794 118 0.044 93 0.708 117 0.039 92 0. 631· 116 0.035 91 0.562 115 0.031 90 0.501 114 0.028 89 0.447 113 0.025 88 0.398 112 0.022 87 0.355 111 0.020 86 0.316 110 0.017 85 0.282 109 0.015 84 0.251 108 0.014 83 0.224 107 0.012 82 0.200 106 0.011 81 10 TAB~ III FIELD INTENSITY AT PROPOSED SITE--AM COMMERCIAL STATIONS Measured RF Field Intensity Station Frequency Location #1 Location 113 Volts/meter dB/',.LV/m Volts /meter dB/~V/m \.,lJDX 620 kHz 0.071 97.0 0. 071 97.0 HSLI 930 kHz 0.046 93.3 0.052 94.3 \~ RBC 1300 kHz 0.052 94.3 0.052 94.3 t~J JQS 1400 kHz 0.032 90.0 0.028 89.0 \AJ JXN 1450 kHz 0.02 86.0 0. 02 86.0 1-' 1-' tvOKJ 1550 kHz 0.04 92.0 0.04 92 .o \AJtv UN 1590 kHz 0.063 96.0 0.071 97.0 TABLE IV FIELD INTENSITY AT PROPOSED SITE--FM COMMERCIAL BROADCAST STATIONS Measured RF Field Intensity Station Freguency _Location iff 1 Locatio':l #3 Volts/meter dB/J,J.V/m Volts/meter dB/1-J.V/m WKXI 94.7 MHz 0.104 100.3 0.082 98.3 WLIN 95.5 MHz 0.462 113.3 0.462 113.3 HSLI 96.3 :MH.z 0.029 89.3 0.026 88.3 WJMI 99.7 MHz 0.0103 80.3 0.0082 78.3 HJDX 102.9 :MHz 0.065 96.3 0.046 93.3 TABLE V FIELD INTENSITY AT PROPOSED SITE--TV COMMERCIA~ BROADCAST STATIONS Measured RF Field Intensity Channel Function Frequency Location if: 1 Location #2 Location #3 Volts/meter dB/~V/m Volts/meter dB/~V/m Volts/meter dB/~V/m 3 /HLBT Video 61.25 MHz 0.02 86.0 0.03 89.8 Audio 65.75 IviHz 0.016 84.0 0.016 84.0 12/WJTV Video 205.25 MHz 0.158 104.0 0.15 103.6 Audio 209.75 MHz 0.076 97.6 0.067 96.6 16/WAPT Video 483.25 MHz 0.76 117.6 . 0.51 114.1 1.14 121.1 Audio 487.75 MHz 0.45 113.1 0.32 110.1 0 .s 1 114.1 29/WMAA Video 561.25 MHz 0.174 104.8 0.174 104.8 0.174 104.8 C' C. C' -, C' .. KTT- A "I-,/. "I A/. 0 f'\ "1"1 "lf'\A n f'\ "1'"1/ ., A"'' () JUJ • I J .L•Ul:G Vel./'+ l.V'+oO v.l.l. l.VV.O v. l.L.'+ l.Ul..O measured at both locations #1 and #3. An intense signal from this station was expected since it was located approximately one mile from the proposed site. Regarding television stations, the most intense signal was measured at location #3 and the 483.25 Wlz frequency corresponded to the video signal of Channel 16. Again, an intense signal from this television station was ex pected because its transmitting antenna was mounted on the same tower used by WLIN. This tower location relative to the proposed site is shown as position 9 on Figure 1. The magnitude of the Channel 16 video signal was 1.14 volts per meter. Measured magnitudes of all discrete frequency signals, relative to each other, varied as anticipated when the Figure 1 locations of transmitting towers and specified power ratings were considered. For example, signals from position 9 on Figure 1 WE~re much more intense than those from positions 1, 2, 3, 4, 5, 7 or 8 on Figure 1. ConcuLrently, positions 10, 11, and 12 intensities were reasonably high since their main-beam antenna patterns illu minated the proposed site when directed toward Jackson. the fcequency scan test conducted .over the 14 kHz to 1.3 GHz range indi cated that the only significant signal magnitudes at the proposed site were those contributed by the known AM, FM, and TV broadcast stations. All other signals recorded were lower in amplitude by 10 dB or more. V. CONCLUSIONS Based on these results, the follmving conclusions can be drawn: (a) If the proposed site is selected as the location for the Data Processing Center, electronic equipments within the Center will have to be able to function properly in an average environment of at least 47 millivolts over the 600 to 1600 kHz AM band, 130 millivolts over the 90 to 105 M~z FM band, 258 millivolts over the 60 to 570 MHz television band. (b) The highest signal level measured was the 1.14 volts per meter video signal at 483.25 MHz from the Channel 16 television station. This signal therefore represents the worst-case environment which electronic equiprnents in the Data Processing Center would have to tolerate. 14 It is noted that the fie ld intensity levels measured are applicable for the specific conditions under 1.vhich the measurements 'i.vere made. Under different conditions, i.e., propagation variations, soil moisture content, new construction in area, etc., these intensities could vary considerably. Any shielding offered by materials used in construction will reduce by a corresponding level the field intensities to 'i.vhich equipments in the Center will be exposed. The shielding required is that necessary to reduce the field intensities to a level below the susceptibility threshold of the most sensitive electronic equipment. In fact, an environmental level reduced as much as 10 dB below this susceptibility threshold is recommended. 15 VI • REFERENCES 1. E.E. Donaldson, Jr. and B.M. Jenkins, 11 Equipment Evaluation ·Test Re port, Vol. II - Television Receiver Susceptibility Investigations", Georgia Tech Report No. 3, Project A-1191, Contract No. DAHC-69-C-0119, December 1973. 2. J.C. Toler, F.R. Williamson, D.R. Sentz, and G.R. Taylor, "Cardiac Pacemaker Susceptibility to Selected Electromagnetic Environments", Georgia Tech Final Report on Project A-1416-001, Standard Industrial Agreement with Cordis Corporation, Miami, Florida, September 1973. 3. F. Haber, "Generation of Standard fields in Shielded Enclosures", Proceedings of the I.R.E., November, 1954, p. 1693. 4. Reference Data For Radio Engineers, 4th Edition, p. 714. 5. 1971 Broadcasting Yearbook, Published by Broadcasting Publications, Inc., Washington, D.C. 6. Television Factbook, 1970-1971 Edition, No. 40, Published by Television Digest, Washington, D.C. 16 APPENDIX A - PHOTOGRAPHS OF TEST CONFIGURATION AND ANTENNAS 17 Loop Antenna Used for Field Intensity Napping '·. ; .-. . ~ \ ~ - Biconical Antenna Used for Field Intensity Mapping 18 Conical Log-Helix Antenna Used for Field Intensity ~"lapping 19 APPENDIX B - MATHEMATICAL PREDICTIONS OF FIELD INTENSITY 20 The electromagnetic field intensity at a specified location due to a remote transmitter is a function of many variables. Hmvever, if sufficient knowledge of these variables can be obtained, this field intensity can be statistically predicted using procedures and data given in the Federal Com munications Cormnission (FCC) Rules and Regulations, Volume III, Part 73. If such predictions could be accurately made, the time and expense of site measurements could be greatly reduced if not completely eliminated. There fore, a limited effort was undertaken to determine the extent to which field intensities at the proposed site for the Data Processing Center could be pre dicted. The primary variables capable of influencing field intensity and there fore of concern were as follows: (a) Topology - hills, valleys, and terrain characteristics that de scribe the general lay of the land at the test site, (b) Ground electrical characteri:3tics - Conductivity and dielectric constant of the earth at the test site, ~c) Local siting effects - trees, building structures, power lines, etc., (d) Transmitting antenna parameters - Height and directional chara teristics, (e) Transmitter radiated power and frequency, (f) Separation distance between transmitting antenna and test site, and (g) Height above the test site average terrain at which field in tensity values are desired for frequencies in the FM band and above. At the outset of the prediction effort, it was recognized that detailed in formation defining some of these variables was incomplete. It was also re cognized that, even if knowledge for all of these variables was readily available, predicted field intensities at the site could still vary considerably as a function of climatic conditions. For example, moisture content of the soil at any point in time appreciably affects ground conductivity which, in turn, influe nces field intensity. In fact, v2rying climatic conditions often 21 cause major differences in field intensities measured at the same location but at two different points in time. Information known and/or assumed for the above variables was as follows: 4 4 (a) ground conductivity of 10 MMHOS/:H, (b) ground dielectric constant of 5 6 15 esu, (c) transmitter radiated powers a.nd frequencies ' , (d) transmitter 5 6 antenna heights ' , (e) separation distances, and (f) the test site was free of trees, buildings, power lines, etc. Since the available data in the FCC Rules and Regulations was applicable only for a 30 foot antenna height, at the test site the predicted field intensities were valid o~ly at this height. No information was available regarding directional characteristics of transmitter antennas. Using this information, field intensities at the proposed site were pre- - dieted for each local AM, FM, and TV station. These predictions are presented in Tables VI, VII, and VIII. Over the A~l broadcast band, Table VI shows that the measured and predicted values of field intensity differ at most by 6 dB (a factor of 2). This difference is generally considered an acceptable correlation for the two methods of obtaining field intensity at a test site. This "good" correlation is attributed to the fact that propagation in this frequency range is primarily via ground waves. Therefore, factors which significantly influence propagation through space are of little consequence. In Tables VII and VIII, the measured and predicted field intensity values differ by as much as 13 dB (a factor of 4.5). These difference can be attributed to numerous things, including the fact that directional charac teristics of the transmitting antennas were not considered and the assumed values for ground conductivity and dielectric constant were probably in accurate. Also, the predicted field intensity values are applicable to a 30 foot height above the average test site terrain while the measured intensities were obtained at a height of 10 feet. This difference in heights is not thought to be a major source of error since the measured values were obtained at a 10 foot antenna height above a knoll at the proposed site. Therefore, the 10 foot antenna height for the measured intensities should compare reasonably well with the 30 foot height above average terrain used for the predicted in- tensities. Based on the results obtained, it is evident that (1) the predicted field 22 TABLE VI COMPARISON OF CALCULATED AND MEASURED FIELD INTENSITIES - AM BROADCAST BAND Measured Field Intensities Calculated Field Intensities Station Location if 1 Location #3 FCC Method dB/J..LV/m dB/J..LV/m dB/J..LV/m WJDX 97.0 97.0 93.0 WSLI 93.3 94.3 89.3 WRBC 94.3 94-.3 93.2 WJQS 90.0 89.0 89.0 WJXN 86.0 86.0 87.7 N w n 1 c 1,!0KJ J J. • .) \Vl;\TUN 96.0 97.0 91.0 TABLE VII COMPARISON OF CALCULATED AND MEASURED FIELD INTENSITIES - FM BROADCAST BAND Measured Field Intensities Calculated Field Intensities Broadcast FCC LOS Station Location iFl Location #3 Method Method dB/1-LV/m . dB/1-LV /m dB/1-LV/m dB/1-LV/m WKXI 100.3 98.3 * ·k HLIN 113.3 113.3 119 118.0 WSLI 89.3 88.3 99 10L,8 WJMI 80.3 78o3 82 100.8 iV +' WJDX 96.3 93.3 100 102.8 *Transmitting antenna height unknown - unable to calculate. TABLE VIII COMPARISON OF CALCULATED AND MEASURED FIELD INTENSITIES - TELEVISION BROADCAST BAND Measured Field Intensity Calculated Field Intensities Broadcast FCC LOS Station Location {fo 1 Location #3 Method Method,_ dB/1-LV/m dB/j.LV/m dB/j.LV/m dBfl.l.V /m Chn. 3/WLBT Video 86.0 89.8 99 100.8 Audio 84 .. 0 84.0 92 93.8 Chn. 12/WJTV Video 104.0 103.6 107 107.8 Audio 97.6 96.6 100 100.8 i....:. V'1 ., r IT''\".&~..- Chn. 1.0/ WB..t'l. Video 117.6 121.1 130 129 .o Audio 113.1 114.1 123 122 .o Ch. 2 9 /\-JMAA Video 104.8 104.8 106 107.8 Audio 104.8 101.8 99 100.8 intensity values compared reasonably well only over the AM broadcast range of frequencies and (2) unless considerably more information regarding variables capable of influencing field intensity at a g ive n site can be provided, test site measurements will still be required. A second method for mathematically determining field intensity at a site \vas briefly investigated to determine :its feasibility as a prediction tool. This method, usually referred to as the Line of Sight (LOS) method, yields worst-case levels of field intensity because it assumes main-beam illumination. It is calculated using the following formula: ~PG F.I. = V ~ t ~ 4n d 2 where: F.I. Field Intensity in volts per meter, Pt Transmitted power in watts, Gt Transmitting antenna gain, and d Separate distance between the transmitter and test site in meters. Calculations using this method were made for the FM and TV frequency bands and the results are shown in Tables VII and VIII, respectively. \\Tith the ex ception of FM station WJMI, the field intensities calculated by the LOS method compared favorably with those predicted using FCC procedures. For FM station WJMI, the LOS method yielded a field intensity nearly 20 dB (a factor of 10) greater at the proposed site. As in the case of predictions using the FCC procedures, it appears that field intensity levels predicted by the LOS method are not sufficiently accurate to preclude test site measurements. However, the predicted field intensities will provide an upper bound on the environment possible at a test site of in terest. • •'