Tell Hot Tower Climbing FCC Project PB92125186

8/2/2019 Tell Hot Tower Climbing FCC Project PB92125186

1/94

R IC H A R D TEL LA s SOC I ATE S , I_="_C. ~ , PB921251B6I 1111111111111111111111111111111111111,Induced Body Currents and Hot AM Tower Climbing:

Assessing Human Exposure in Relation to the ANSIRadiofrequency Protection Guide

October 7,1991

Prepared fo rFederal Communications Commission .Office of Engineering and Technology2025 M Street, NW,Washington, DC 20554

byRichard A. TellRichard TeUAssoclates, Inc.6141 W. Racel StreetLas Veaas. NV 891311912

REPRODUCED BYU.S. DEPARTMENTOF COMMERCENAnONAL T e C H N I C , ~INFORMATION SERVJCESPRINGFIElD. VA 22161 _. (702) 645 - 3 3::S 8 61 4 1 W. RAe E L ST. LAS V E GAS. :\ E \ ' A D A S 9131 -


2/94


3/94

..

L l leI*t Del()"tober 7 1991

'0272 - 1111REPORT DOCUMENTATION 11. RlPO,", MO.PAGE - FCC/OET RIA 91-01Co Tltl. .net Subtltl. Induced Body Currents and Hot

Assessing Human Exposure in Relation toquency Protection GuiqeAM Tower Climbing:the ANSI Radiofre-

L PS92-125186

Richard 4. Tell . . I"wrf1lrntlrw Orpnlat lon IteIIL .....FCC/DET RIA 91-01I . "-rformll'C Orpnlutlon N. . . . . .nd AcId_Richard Tell Associa tes, Inc .6141 W. Race1 StreetLas Vegas, NV 89131-191212. SlIOftwri". O. . . "lut lon N.",nd AcId_

Federal Communications CommissionOffice of Engineering and Technology1919 MStreet, N.W.T.r:' "'h; """ .. "',, DC 2055415. Suppl.",.nt. ') ' Nat..

10. f'nIfKtlTaU/WorIl Unit No.I I . ContrKt(C) Of Qrant(Q) No.(C )(Q)

I&. AIlstrKt (Umlt: 200 _rd l )This report documents the results of a study of the radio frequency (RF) currentsinduced in individuals who climb energized AM broadcast towers fo r such purposesas tower maintenance. Information is needed on such currents in order toquantify the absorption of RF energy by tower climbers and to compare thisabsorption to levels allowed by such RF safety standards as the AmericanNational Standards Institute 1982 radiation protection guide. In this sutdydata on induced currents were obtained using two different AM towers, withelectrical heights of 0.23 and 0.53 wavelengths, respectively. A theoreticalanalysis of the electric fields near the towers showed that induced bodycurrent could be correlated with the radial component of the e lect ri c f ie ld .These results were consistent with those of ~ earlier study performed by theFCC and the EPA. Based on this study a number of conclusions were reachedre1ative"to the question of RF absorption on live AM towers.

17. Docu....PIt "".""1. .0 Deacrtpte. .Human BodyAMAmplitude ModulationRadiofrequencyElectromagnetic Radiationtl- t a i i i i l f i . " / O ~ . ~ T. . . . .INon-IonizingRadiation

Co COSATl fleld/G"'1III.. Av.nlbillty St.te. . . .";

ULTDcs- ANSI-Z39.1I)

Safety, StandardsExposure

I " I e e u ~ c,-. . mol. R.1lOft)TTNrr. A. c: c:. . l e c u ~ c,- mol......>

.... '"etrvctloM OIl It. . . . . . .

21. No. of ......'69 -12. Prlc.

OPTIONAL fOil . . Z7Z (4,Fo. . . . . rly N T I ~ 3 5 )Dell.""'."' 01 co",,,,.rc


4/94


5/94

Induced Body Currents and Hot AM Tower Climbing: Assessing HumanExposure in Relation to the ANSI Radiofrequency Protection Guide

Table of Contents

Summary,. . 1Background. . 6Technica l Approach . . . . . . . . , . . 9Measurement of RF Current in the Wrist . . . . . . . . 10

SAR and Cur rents '............................ 10SAR and Current D en s i t y . . . . . . . . . . . . . . . . . . . . . . . . . . 13Other Mechanisms of Curren t Induct ion .......... 17Induced Current Instrumentation . . . . . . . . . . . . 20The AM s ta t ions Used in the study . . . . . . . . . . . 24Measurement Resu1 ts . . . . . . . . . . . . . . 25spec i f ic Absorption Rate Estimates . . . . . 30Discussion of R esults (Insights) .............. 46Conclusions .................. e ,o o. 51Re f erences 'w 55Tab l es ....................... 59Figures ...................................... 67

J I


6/94


7/94

I II J

DisclaimerThe mention of commercial products , procedures, opinionso r recommened pol ic ies in t h i s repor t in no way cons t i t u t e anendorsement by the Federal Communications commission o r theu.s. Government.

AcknowlegmentThe conduct of t h i s study required the cooperat ion ofsevera l individuals . The ass i s t ance of Dr. Robert F.Cleveland in the O ffice of Engineering and Technology, FederalCommunications Commission, Washington, DC, was inst rumenta l inident i fy ing appropriate AM radio s ta t ions fo r inclus ion in thestudy and h is e f fo r t s in in te rac t ing with s t a t ion personnel

p r i o r to the study as well as on-s i t e ass i s t ance in datarecord ing i s ~ t f u l l y a c k n o ~ e d g e d . Also, th e cooperat ionof the e n g i n ~ a t the radio s t a t ions was absolutelyimperative fo r a successful study; th e ass i s t ance andcooperat ive and support ive a t t i tude of Mr. Bob Turner a t radios ta t ion KWAC in Bakersf ie ld , Cal i fo rn ia and Mr. John Pat tersona t radio s ta t ion KDIF in Riverside, Cal i fornia are herebyacknowleged. Their wil l ingness and tha t of s t a t ion managementto par t i c ipa te in the study was invaluable and represen ts as ign i f i can t con t r ibu t ion to be t te r understanding the re le van tfactors associa ted with exposure assessment in ho t .towerclimbing.


8/94


9/94

Ust of Tables

Table 1. Calib ration data for interpret ing the meterindicat ion on the Simpson Model 39-05330 RFmilliammeter determined a t 1 MHz . . . . . . . . 59Table 2. Calibrat ion data fo r interpret ing the meterindicat ion on the Simpson Model 39-05330 RFmilliammeter determined a t 60 Hz 60Table 3. Measured body current obtained dur ing ear l i e rEPA study in Spokane, washington (EPA, 1988),

a t radio s ta t ion KKPL, 630 kHz, with 1 kWpower. Tower was 0.25 A t a l l 61Table 4. Measured body current obtained While climbingthe KWAC tower in Bakersfield, California ,1490 kHz, with 1 kW power. Tower was 0.23 A

t a l l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Table 5. Measured body current obtained whil e c limb ingthe KDIF tower in Riverside, California , 1440kHz, with 1 kW power. Tower was 0.53 A t a l l 63Table 6. Summary of data on wris t breadth, wris tcircumference and derived gross, crosssect ional areas of wrists of\3859 u.S. AirForce male personnel ..................... 64Table 7. Summary of calculated radial elect r ic f ie ldstrengths a t di fferen t heights , 1 m adjacent

to a 100 m t a l l cyl indr ica l tower, driven with1 kW. Fields were computed with MININEC andthe tower was modeled as 50 segments.Different frequencies were assumed to simulatedi fferen t e lec t r ica l heights for the tower .... 65

Table 8. Projected power l imits for AM radio s ta t ionsoperating with towe rs r anging between 0.25Aand 0.625A t a l l to maintain wris t currentsSUfficiently low to control wris t SAR to nomore than 8 or 20 W/kg for workers wearingconventional work gloves 66

v


10/94


11/94

Ust Of FiguresFigure 1. Relationship between body current inducedwith whole-body exposure to RF f ie lds andfrequency.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... 67Figure 2. I l lustrat ion of the technique for measurementof induced body current on the KDIF towerusing a non-metallic l i fe - l ine ........... 68Figure 3. Photograph of the body current measurementdevice as viewed by the climber. The meter

i s a RF thermocouple type milliammeter.Copper strapping material forms goodelectr ical contact with the hand and thetower.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... 69

Figure 4. Photograph of the side view of the bodycurrent measurement device showing the shapefor convenient attachment to tower membersand a protective fuse........................ 70Figure 5. Relation between RF milliammeter indicationand actual RF current flowing in meter forthe Simpson Model 39-05330 meter movement;

l inear- l inear display 71Figure 6. Relation between RF milliammeter indicationand actual RF current flowing in meter forthe Simpson Model 39-05330 meter movement:l og - log display ............................ 72Figure 7. Photograph of the non-uniform cross-sectiontower a t KWAC, Bakersfield, California ...... 73Figure 8. Close-up photograph of la t t ice work l ikearrangement of tower construction a t KWAC,Bakersfield, California 74Figure 9. Photograph of the uniform cross-section tower

a t KDIF, Riverside, California . . . . . . . . . . 75Figure 10. Measured body current vs. relat ive radial

electr ic f ie ld strength on 0.25A t a l l towera t AM radio stat ion KKPL, Spokane,Washington. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Figure 11. Measured body current vs. relat ive radialelectr ic f ie ld strength on 0.23A t a l l towera t AM radio stat ion KWAC, Bakersfield,California. Tower modeled as uniform cross-section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77


12/94

,

VI

Figure 12 . Measu red body cur ren t vs . re la t ive r ad ia le l e c t r i c f ie ld s t r eng th on O.23A tall towera t AM radio s t a t ion KWAC, Bakersf ie ld ,Cal i fo rn ia . Tower modeled as a t apereds t ruc tu re ......... ................ 78F ig ure 13. Measu red body cur r en t vs . r e l a t i v e r ad i a le l e c t r i c f ie ld s t r eng th on O.25A tall towera t AM radio s t a t ion KDIF, Rivers ide ,

Cal i f o rn i a .................. 79Figure 14. Dis t r ibu t ion of gross cros s - sec t iona l area

o f w ris t in 3859 male members o f u.s. AirForce. Cross - se c ti onal a re a s derived fromdata co l la ted by NASA (1978) ............... 80Figure 15. Ef fec t of tower heigh t on r a t i o of maximumtower cur r en t to base cur r en t fo r towers ofd i f f e r en t e l e c t r i c a l height 81


13/94

Induced Body Currents and Hot AM Tower Climbing: Assessing HumanExposure in Relation to the ANSI Radiofrequency Protection Guide

SummaryThe common pract ice of conducting maintenance work on

energized (hot) AM radio broadcast antenna towers has corneunder question as to the poss ib i l i ty of hazards produced bythe extremely in tense e lec t r ic and magnetic f ie lds inherent tothe surface energized towers . This question has becomemore important s ince the FCC began requir ing broadcasters tocomply with the recommendations of the American NationalStandards Ins t i tu te (ANSI) radio frequency protect ion guide(RFPG) in January, 1986. Complex theoret ica l work i sunnecessary to recognize t ha t the surface f i e lds on AMbroadcast towers are very strong and can eas i ly exceed thef ie ld s t rength l imi ts of the ANSI RFPG. This being the case,only one p rac tica l a l tern ative ex is ts for more thoroughlyevaluating exposure of tower climbers, the assessment of thespec i f ic absorption ra te (SAR) in the body of the c l imber.This repor t documents a study of the RF currents which canflow between the tower and the c l imber I s body and the SARst ha t wil l resu l t from the flow of these currents . There la t ionship of these currents to e lec t r ic and magnetic f ie ldsproduced by AM radio s ta t ion towers i s also examined.

Measurements of induced body currents were made on twod i f fe ren t AM radio towers selected for the study inBakersf ie ld and Riverside, Cali fornia . Currents weredetermined via a thermocouple type RF milliammeter arranged ina conf igura t ion allowing the measurement of the currentflowing in to the arm of the cl imber. Induced curren t datawere obtained on towers tha t had e lec t r i ca l heights of 0.23and 0.53 wavelengths. Both s ta t ions were operated with 1


14/94

AM Tower Induced Body Currents, page 2ki lowatt (kW) and had fre qu en cies of 1440 kHz and 1490 kHz.In both cases, the maximum, measured body cur rent was near 250mill iamperes (IDA). A theore t ica l analys is of the e lec t r i cf ie lds near the tower showed t ha t the induced body currentswere correlated with the r ad ia l component of th e f ie ld ; byusing two, s igni f icant ly d i f f e r en t e lec t r i ca l height towers,having very d i f f e r en t e lec t r i c f ie ld d is tr ib ut io ns , th i sr e la t ionship could be c lear ly observed. The data obta ined int h i s study were cons i s t en t with an ea r l i e r study conducted bythe Environmental Protect ion Agency (EPA) in 1987 in Spokane,Washington on a quar ter -wavelength t a l l tower in which amaximum body cur rent of 110 rnA was measured a t a frequency of630 kHz.

SAR in the wris t was est imated by computing the loca lcur rent densi ty in the wris t and taking into account theconduct ivi ty of the t i s sues . The r e su l t s of the analys isshowed t h a t th e wris t SAR may range between 95.4 W/kg and 153W/kg, depending on w ris t s ize for the maximum cur rentmeasured a t the two Californ ia s ta t ions . s ince SAR i sdi rec t ly propor t ional to the frequency of the current , therange of wr i s t SAR for the Spokane s ta t ion was determined tobe 18.2 to 29.1 W/kg. The above values o f SAR are based onbare-handed contac t with the tow er; use of protec t ive clo th ingsuch as insu la t ive gloves can s igni f icant ly reduce the cur rentm agnitude and, h enc e, the r esu l t ing SAR. Unfortunately, thereis a wide range in the e lec t r i ca l insulat ion performancecha rac t e r i s t i c s of d i f f e r en t types of glove mate r ia l s and thedegree to .which sweat soaked gloves can impede the flow ofcur rent i s highly quest ionable . Hence it i s not poss ib le Ibased on m easurem ents obtained in t h i s study, to offerspec i f ic guidance on the e ff ec ti ve ne ss o f protect ive clo th ingfor mitigat ing exposures.


15/94

AM Tower Induced Body Currents, page 3Strong magnet ic f i e l d s c i rc le about AM towers and must

a lso be cons idered when assessing exposure of tower cl imbers .Magnetic f i e lds wi l l lead to two forms of induced curren ts ,eddy curren ts c i r cu l a t i ng w ith in the body cross -sec t ion andloop-currents formed wi th in the loop formed by the body andthe tower as the individual climbs the tower.

An analys i s of the SAR within th e body fo r a 1 kW AMs ta t ion opera t ing a t 1 MHz ind ica tes t h a t th e loop curren tcould be responsible fo r about 0.006 W/kg, an i n s i gn i f i can tvalue compared to the ANSI RFPG value of 0.4 W/kg averagedover th e en t i re body o r 8 W/kg averaged over anyone gram oft i s sue . On th e o ther hand, a 50 kW AM s t a t ion opera t ing a t1 .6 MHz would be expected to produce l oop-cur ren t genera tedSARs up to 0.764 W/kg averaged over the whole body and as muchas 445 W/kg in the wris t , assuming t h a t both the f ee t andhands were in good e l ec t r i c a l contac t with the tower . It i squest ionable whether such e l ec t r i c a l con tac t condi t ions , v iathe fee t , can be a cc omplish ed und er prac t i ca l ci rcumstancesand hence, it i s unl ike ly , though not ye t proven, t h a tmagnetic f ie ld induced loop-current SAR wi l l ac tua l ly r e su l tin s ign i f i can t SARs.

Eddy curren ts t h a t wil l be genera ted within the body, anda re not amenable to d i r e c t measurement, wi l l lead to SARsabout the periphery of th e body of up to about 0.004 W/kg froma 1 MHz, 1 kW AM s t a t ion tower . The loca l ized eddy cu r ren tSAR t h a t could be produced by a 50 kW s t a t ion tower , opera t inga t 1. 6 MHZ, however, i s about 0.54 W/kg. The whole-bodyaveraged SAR associa ted with th es e c ondi tio ns would be about0.3 W/kg.


16/94

AMTower Induced Body Currents, page 4Simple mathematical r e la t ionships are provided to as s i s t

the reader in carrying out an evalua t ion of SARs, depending onfrequency and s ta t ion power l eve l .

Based on these analyses , a number of ins ights weredeveloped:

(1) Hot AM tower work sUbjects the climber to very s t ronge lec t r i c and magnetic f i e ld s . These f ie lds r e su l t in inducedbody currents tha t cari be s ign i f i can t in the context of RFskin burns and development of excessive SAR.

(2 ) Body currents flowing through the wris t can be eas i lyand accura te ly measured using simple devices to assessexposure.

(3)di rec t lye lec t r i c

Induced body curren t in the arm of a cl imber i scorrelated with the s treng th of the su rface rad ia lf ie ld component.

(4 ) While the loca tion on the tower where the bodycurrent is a maximum i s a fun ctio n o f the e lec t r i ca l height ofthe tower, the maximum value of body cur rent appears to bere la t ive ly independent of tower height , permit t ing a mores impl i s t ic approach to applying the measured data to a rangeof tower heights used by broadcas ters .

(5) Induced cur ren t appears to be re la te d to , among o the rfactors , the tower cross- sec t ional s ize ; o the r factorsremaining the same, such as tower current , the surfacee lec t r i c f ie lds appear to be l ess for l a rger cross-sect ionsand appear to r e su l t in l esser values of body cur rent . Work,therefore, ins ide large cross-sect ion towers equipped with


17/94

AM Tower Induced Body Currents, page 5

ladders may r esu l t in subs tant ia l ly lower exposures butfur ther evaluat ion i s needed.

(6 ) Induced body current i s d i r e c t l i proport ional to thefrequency of th e s ta t ion ; the range of frequencies within theAM s ta nda rd b roadca st band can account fo r a 3 fold di f fe rencein th e body cur ren t and a 9 fold .d i f ference in the resul t ingwr i s t SAR, o th er f ac to rs remaining th e same.

(7 ) wr i s t SAR depends s t rongly on wri s t s i z e . (cross-s ec tio na l a re a) . Data obtained on U. S. Air Force personnelshow t h a t th e wris t SAR can vary by a fac to r of 2 t imes j u s tdue to v ar ia tio n in wr i s t s ize in the popula t ion.

(8 ) Data c olle cte d in t h i s stUdy show t h a t considerablepower reduct ions are required to insure t h a t the peak SARl imi t of the ANSI RFPG i s not exceeded during hot towerclimbing. Depending on frequency, rad ia ted powers as low as afew tens of watts may be necessary to comply with the ANSIrecommendations. Use of protect ive gloves, not yet adequatelycharac ter ized , w ill l ik e ly allow higher, bu t still grea t lyreduced powers fo r broadcast ing during tower work.

(9 ) RF burns can eas i ly occur while working on hottowers, even a t the 1 kW power l eve l , espec ia l ly i fin ad ve rte nt c on ta ct with guy wires i s made. Paints withsuperior e lec t r i ca l in s ul ati on p ro p er ti es may prove to be auseful mitigat ion mater ia l to reduce the chance of RF skinburns.

(10) A recent ly promulgated standard in Canada has se t amaximum contact current l imi t of only 40 rnA; Canadianbroadcasters wil l have more d i f f i cu l ty in complying with t h i s


18/94

AM Tower Induced Body Currents, page 6l imi t than even u.s. broadcasters wil l with r espect to theANSI RFPG.

(11) pending the d e v ~ l o p m e n t o f a dd it io na l ins ight to thei ssue of body currents and exposure mitigat ion fo r hot AMtower work, broadcas ters s ho ul d p ro ce ed in a caut ious mannerwith respect to author iz ing rout ine tower work while .the toweri s energized. This same caut ionary note appl ie s toce r t i f i c a t ion of compliance with FCC administered regula t ionson s ta t ion l i cense renewals and appl ica t ions for modificat ionof f a c i l i t i e s where hot tower work may occur.Background

Anyone famil iar with AM radio broadcast ing i s aware of theprac t i ce of conduct in g ant enna t ower mai nt en ance on energizedtowers. The replacement of tower lamps ( for 1 ighting) andpa in ting are rout inely accomplished by climbing AM towersw hile they are "hot" (while the tower i s being driven by thet ransmi t te r ) ; considering the length of t ime requi red tor epain t a tower, the of f -a i r t ime would genera l ly beprohibi t ive for most broadcasters to te rm in ate b ro ad ca stserv ices dur ing tower work. Consider ing the f ac t tha t thereare no known, documented cases o f a dv erse heal th e f f ec t s fromsuch ac t iv i t i e s over the many years tha t t h i s prac t i ce hasbeen observed, aside from RF burns, hot tower clim bing has notbeen ser ious ly questioned as a potent i a l hazard un t i lrecent ly. In January of 1986, the FCC began requir ingbroadcasters to cer t i fy during l icense renewals , orappl ica t ions fo r new l i censes or f ac i l i t y modif ica t ions , t ha texposure of workers a t th e i r s ta t ions i s cons i s t en t with therecommendations of the ANSI (FCC, 1985a) . In the case of FMand t e lev is ion (TV) s ta t ions , engineers more c lear ly


19/94

AM Tower Induced Body Currents, page 7recognized the poten t i a l fo r exceeding the ANSI spec i f iedf ie ld s t rength l imi t s fo r workers climbing in to the aper tureof high-powered antennas. After a l l , FM radio , for example,l i e s di rec t ly within the most s t r ingen t ly control led pa r t ofthe frequency spectrum where ANSI l imi t s RF f ie lds to thelowest values (the equiva lent of 1 mil l iwat t per squarecentimeter (mW/cm 2 ) or 4000 vol t s squared per meter squared(v 2/m 2 ) e lec t r i c f ie ld s t rength and 0.025 amperes squared permeter squared (A2/m2 ) magnetic f ie ld s tr en g th ).

The i ssue of seriously examining hot AM tower cl imbing,however, has been slower to occur, part ly because of the lackof apparent prac t ica l indicat ion of a hazard and par t lybecause of the much higher, more permissive, f ie ld s t rengthl imi t s recommended by ANSI (100 mw/cm2 , 400,000 v2/rn2 and 2.5A2/m 2 ) . The FCC, however, issued guidance to broadcasters(FCC, 1985b) providing information on th e c le ara nc e dis tancesaround AM towers within which the e lec t r i c and magnetic f ie ldscould exceed the ANSI l imi t s ; t h i s being pr inc ipa l ly for usein conducting environmental analyses of possible publicexposure. These clearance dis tances vary between 3 meters (m)and 12 m, depending on power leve l o f the s ta t ion , frequencyand antenna height . While these dis tances have beenin te rp re ted , general ly , as overestimates of the ac tua ldis tances needed fo r control l ing exposure (Tel l , 1989),c lear ly , the tower i t s e l f must be assumed asa potent ia lsource of overexposure fo r workers in di rec t contact . Veryin te ns e s urfa ce f ie lds do ex i s t on AM towers, even a t qui temodest power levels , and AM tower work should be consideredju s t as seriously as any other type of RF work a t a broadcas ts ta t ion when it comes to the issue of potent ia l hazards andlegal compliance with FCC regulat ions . FCC ru les require t ha tbroadcasters comply with the ANSI standard (ANSI, 1982) and,


20/94

AM Tower Induced Body Currents, page 8re la t ive to indiv iduals climbing hot AM towers it would seemv i r tua l ly impossible to warrant compliance with the ANSIstandard shor t of shut t ing down o pe ra tio ns fo r the length oftime needed to carry out the required work on th e tower. Insome l imited cases , commonly those where auxi l i a ryt ransmit t ing f a c i l i t i e s al ready exis t , broadcasters a re facingthe RF exposure i ssue by, in f ac t , turning off the power. Butin most cases , tower climbing continues, leaving theb ro ad ca ste r in the predicament of determining how to ce r t i f yth a t exposures do not exceed the ANSI l imi t s .

The stUdy documented here was based on the concept tha t ,v ia an a l te rna t ive measure of exposure, based on the currentstha t would be induced to flow betw een the tower and the bodyof a cl imber produced by e lec t r i c f ie ld coupl ing, thelocal ized value of spec i f ic absorpt ion ra te (SAR) in the wris tcould be determined and t h i s SAR compared to the underlyingSAR l imi t of the ANSI standard of 8 W/kg averaged over anyonegram of t i s sue . In addi t ion , considera t ion of the currents(and resu l t ing SARs) produced by the st rong magnetic f ie lds onthe tower would be analyzed in terms of so-cal led loopcur rent s and eddy cur rent s . Loop-currents are those currentswhich are produced by magnetic f ie lds flu xin g th ro ug h. th eaper ture of a loop formed by the body and the tower. Eddycurrents are those currents produced a s c irCU la tin g currentswithin the body cross-sect ion caused when the magnetic f ie ldin in cid en t normally to the cross-sect ion having the l a rges te f fec t ive rad ius .

Based on an induced cur rent approach, the i n t en t of thestudy was to e st ab li sh l ik e ly values of SAR, both whole-bodyaverage and loca l , peak values, tha t might o ccur d urin g hottow er C limbing . A secondary objec t ive was to assess the


21/94

AM Tower Induced Body Currents, page 9condi t ions under which hot tower climbing might beaccomplished without exceeding the SAR exposure l imi t s of theANSI standard.Technical Approach

The ANSI standard fo r l imi t ing e l e c t r o m a g n e ~ i c f i e ldexposure i s based on th e concept o f l im i tin g th e r a te a t whichenergy i s absorbed by th e t i s sues of th e body. The spec i f icabsorpt ion r a te (SAR) , i s expressed in uni t s of watts perkilogram (Wjkg) of t i s sue : the maximum value of the SAR, whenaveraged over the en t i r e body mass i s 1 imi ted to a4 Wjkg.Taking in to account the fac t t h a t electromagnetic energy i snot absorbed in a perfec t ly uniform manner, the ANSI standardpermits up to 8 Wjkg SAR when averaged over anyone gram oft i s sue . Both of these SAR values are designated as the t imeaveraged values when averaged over any s ix-minute period ofexposure. Hence, fo r con ti nuous exposure, the SAR l imi t s a reas spec i f i ed above but fo r exposure durat ions shor te r than s ixminutes, higher values are allowed, as long as the averagedoes not exceed e i t he r 0.4 Wjkg o r 8 Wjkg fo r whole-body orspa t ia l peak SAR, respect ively . The standard also indicatestha t the f ie ld st rength l imi t s may be exceeded i f , using anappropriate method, the SAR values do not exceed these values.Hence, even i f the e lec t r i c and magnetic f i e lds on an AM towermight exceed th e f ie ld s t reng th l imi t s , t h i s fac t does notnecessar i ly imply t h a t an over-exposure wi l l occur whenclimbing the tower. The r ea l crux of the i ssue i s , then,whether a det erm inat ion o f the SAR can be made and, i f so, howthe r e su l t o f the determination compares to th e ANSI speci f iedvalues .


22/94

AM Tower Induced Body Currents, page 10Measurement of RF Current in the WristThe ap proach used here followed a methodology i n i t i a l l y

performed by the Environmental Protec t ion Agency (EPA, 1988).The method involves , in prac t i ca l terms, inser t ing an RFmilliammeter in se r i e s between the hand of the cl imber and thetower. This was accomplished by using a standard,thermocouple type RF milliammeter meter movement arranged in aj ig which support s the meter , a fuse fo r p ro tec ting th e metera ga in st i na dv er te nt, excess ive cur ren t flow and elec trodessu i t ab le fo r holding by the hand and making e lec t r i c a l contac twith the tower s t r uc tu r e . When placed in con tac t with thetower, th e RF cur ren t flowing in to the arm of the cl imber i sdi rec t ly read from the RF milliammeter . In prac t i ce , a t anygiven posi t ion on the tower, the cl imber uses a non-conductivel i f e - l i n e to suspend himself from the tower such t ha t nophysica l contac t need be made to support the body, other thanthe fe et re st in g on a cross-member of the tower; the cl imberleans out away from the tower and, holding the curren tmeasurement device in one hand, makes contac t between thedevice and pa r t of the tower s t r uc tu r e . Using th i s method,the RF cur ren t flowing into the arm of the cl imber caused bycapaci t ive coupling between the e lec t r i c f ie ld and th e body i sdi rec t ly determined.

SAR and CurrentsThe SAR i s expressed in uni t s of wat ts per kilogram of

body mass (Wjkg) and i s r e la ted to the in te rna l e lec t r i c f ie ldst rength in the t i s sue by the r e la t ionship :

where [1]


23/94


SAR = spec i f ic absorpt ion ra t e (W/kg);o = t i s sue conduct iv i ty (S/m):E = e lec t r i c f ie ld st rength in t i s sue (Vim);p = mass densi ty of t i s sue (kg/m 3 ) .In p rac t i ce , most experimental s tud ies of SAR in exposed

objec ts re ly on e i t he r a measurement of th e f ie ld st rength inthe exposed t i s sue o r the increase in tempera ture caused byabsorpt ion of the RF energy. I f temperature i s the measuredparameter, th e SAR i s obta ined by us ing the re la t ionsh ip :

[2 ]

Cs = th e spec i f ic hea t of th e t i s sue or t i s sueequivalent mater ia l (approximately = 0.84) ( the spec i f ic heatof water i s equal to 1 ) :

Tf = the f ina l temperature of the t i s sue or m ateria l(Co) ;

= th e i n i t i a l temperature of the t i s sue o r m ateria l= durat ion of exposure to RF f i e lds (seconds);the spec i f ic heat o f wa te r e xp re ss ed in J/kg-Co .

Ti(Co) ;

t4185 =Through a c r i t i ca l examination o f the techn ica l

I i t e ra tu re , ANSI e lec ted to use the SAR, determined as anaverage over th e mass of the en t i re body, as a fundamentalbasis fo r the RFPG. But because di rec t measurements of wholebody averaged SAR in in div id ua ls i s not prac t ica l , ANSI choseto es tabl i sh guides on maximum values fo r the e lec t r i c andmagnetic f i e ld s t reng ths or plane-wave equiva len t powerdens i ty . In ac tua l i t y , th e e xte rn al f ie ld l im its are in termsof the squares of th e e lec t r i c and magnetic f ie ld s t reng ths(E2 ~ n H2 ) . For a plane wave in free space, the power


24/94

AM Tower Induced BOdy Currents, page 12densi ty can be r e l a t e d t o t h e squares o f t h e e l e c t r i c andmagnetic f i e l d s as follows:

where [3]

S plane wave equivalent power d e n s i t y (mw/cm2 ) ;E = e l e c t r i c f i e l d s t r e n g t h (Vim);H magnetic f i e l d s t r e n g t h (Aim);

and t h e f a c t o r s 3770 and 37.7 are f a c t o r s a s s o c i a t e d with t h eimpedance of f r e e space (377 ohms) and conversion t oappropria te u n i t s o f m i l l i w a tt s per square cent imeter . Forconvenience, ANSI chose t o round the value fo r t h e impedanceof f r e e space from 377 ohms t o 400 ohms. So, t h e a c t u a ll i m i t s as given by ANSI a r e expressed i n s l i g h t l y d i f f e r e n tform from equation [3 ] as follow s:

[ 4 ]

The observation t h a t SAR i s not n e c e s s a r i l y d i r e c t l yr e l a t e d t o the s t r e n g t h of RF f i e l d s in n e a r - f i e l d exposureenvironments has l e d t o the i nv e st i ga ti o n o f o t h e r dosimetr icparameters which may have more relevance i n evalua t ingexposure t o h o t - s p o t type f i e l d s (Tel l , 1990). Aside fromconducting d e t a i l e d , labora tory SAR s t u d i e s using phantommodels o f t h e human body i n which s l i g h t e l e v a t i o n s of t i s s u etemperature are r e l a t e d t o t h e SAR, measurements of inducedc u r r e n t s have been i n v e s t i g a t e d as a surrogate measure of SAR.For example, s t u d i e s by T e l l e t a l . (1979), GUy and Chou(1982), H i l l and Walsh (1985), Deno (1977) and Gandhi e t a l .(1986) have demonstrated t h e r e l a t i o n s h i p between induced body

c u r r e n t s , as measured a t the i n t e r f a c e between t h e foot andt h e ground. Figure 1 i l l u s t r a t e s t h e r e s u l t s from some of


25/94


these data in which th e induced body cu r ren t i s seen to bed i rec t ly propor t iona l to frequency. The exposure condi t ionsused in these s tud ies were t h a t th e body i s immersed in auniform RF e lec t r i c f i e ld while s tanding on th e ground. Thisi s analogous to a monopole antenn a o ver a ground plane . Thecurren t induced in the body by th e inc iden t f i e ld flows toground through the f ee t , provid ing a reasonable poin t a t whichto perform th e curren t measurement. An approximate empir icalre la t ionship (developed by Tel l e t a l . , 1979) fo r th emagnitude of the induced body curren t in a s tanding adu l t i s :

I sc = 0.3 E f where [6 ]I sc = th e induced shor t c i r cu i t cu r ren t to ground (rnA):E = the incident e lec t r i c f ie ld s t reng th (Vim):f = the frequency of th e f i e ld in MHz.This re la t ionship shows t h a t the induced cu r ren t i nc reases

with frequency and f ie ld s t reng th . This general re la t ionshiphas been s tud ied more recen t ly by Dimbylow (1988), Allen e ta l . (1988) and Gandhi e t a l . (1986). The genera l t rend ofincreased induced curren t with increased frequency has beenver i f ied but Allen e t a l . (1988) have repor ted a tendency fo rth e cu rren t to become somewhat nonl inear above about 30 MHz.Allen (persona l communication, 1989) sugges ts t h a t theapparent non li ne ar i nc re as e beyond 30 MHz to approximately 40MHz a t which th e body resonates , observed by others (Gandhi e ta l . , 1986), may be due to extraneous pickup by th e associatedmeasuring ins t rumenta t ion.

SAR and current density - The SAR in the body can also beexpressed in terms of th e lo ca l c urre nt densi ty according toth e foi lowing re la t ionship:


26/94

AM Tower Induced Body Currents, page 14where [7J

SAR = spec i f ic absorpt ion ra te (W/kg);J cur ren t densi ty in the t i s sue (A/m2 ) ;a = t i s sue conduct iv i ty (S/m);p = t i s sue mass densi ty (kg/m3 ) .

Tissue densi ty has been assumed to be 1000 kg/m3 fo rc a lc u la ti on s i n t h i s analys i s .

Equation [7J, when prac t ica l ly applied in the f requ ency r angeof AM radio broadcast ing (0.54 to 1.6 MHz) for muscle t i s sue ,can be s impl i f ied to :

SAR = O.0025J 2 (W/kg) [8 ]The prac t i ca l s ignif icance of t h i s express ion i s t ha t i f

the induced currents can be determined in the body, the SARcan be est imated . This becomes especia l ly re levant whenconsidering current s t ha t flow through the legs , ankles andfee t o r through th e hands and wris t s when touching obj ec t swhich have RF cur ren ts flowing on them. In pa r t i cu l a r , thearm current s which re su l t when climbing hot AM towers can beused to assess th e local izedSAR in the wri s t s . I t i s in th i scontext t ha t equation [7 ] wil l be found to provide ameaningful way to i n t e rp re t the s ign i f icance of human exposurefor AM tower climbers. The wri s t i s used as the region ofconcern fo r evaluating loca l SAR s ince it rep resen ts theanatomical region of small es t c ro ss sec t ional area and, hence,the a rea sub jec t to the grea t e s t c ur re nt d en si ty .

A n a tu ra l a p pl ic at io n of re la t ionships [6 ] and [7] i s tot he d et erm in at io n of the maximum body cur ren ts which would be


27/94


allowed by th e ANsi RFPGfor cases in which spa t i a l peak SARi s the l imi t ing fac tor : i . e . , the induced currents which areassocia ted with peak SARs of 8 W/kg. It i s noted t h a t thee lec t r i c f i e ld s tre ng th p ola riz at io n component which i simportant fo r current induction which would flow through thefee t in an exposed, standing individual i s the ver t i ca lcomponent, or t h a t component which i s para l l e l with the longaxis of the body. I f t h i s f i e ld al ignment condi t ion i sassumed, then th e maximum expected e lec t r i c f ie ld inducedcur ren t may be est imated by se t t ing the value fo r E inre la t ion [6] equal to the l imi t in the ANSI RFPG fo r any givenfrequency. More exact ly , re la t ion [6] holds s t r i c t ly only forfrequencies up to about 40 MHz a t which th e maximum inducedcurrent wil l occur: a t h igher f re quenci es , th e induced currentwil l decrease . For example, in an inc iden t e lec t r i c f ie ldst rength of 63.2 V/m a t 30 MHz, an induced curren t o f 569 mAwould be expected to flow between the body and ground. Thiscurrent i s then dis t r ibuted between the two fee t andapproximately 285 rnA flows through each ankle and foot toground. At AM b ro adca st f re quencie s, only 1/30th as muchcurrent wil l flow through th e body, resul t ing in about 19 mAoThe rea l concern, in t h i s pro jec t , however, i s an assessmentof individuals who are in physica l contact with energizedradio towers , not those who may be standing on the ground nearthe tower.

Since the loca l SAR i s direct ly re la ted to the cur ren tdensi ty , determining the effec t ive cross-sec t iona l areasthrough which the current flows becomes the c r i t i ca l i ssue.In general , th e two prime areas of potent ia l ly high SAR arethe ankle and th e wris t , those two par t s of the anatomy havingthe smal les t cross-sec t iona l areas . But even within the ankleand wris t , considerable var ia t ion in conduct iv i ty ex is t s s ince


28/94

AM Tower Induced BodyCurrents, page 16there i s a var ie ty of t i s sue types involved inc luding not onlymuscle t i s sue but l a rge amounts of bone and tendon which arere la t ive ly nonconductive. Thus, a major concern becomes thedeterminat ion of how much high conductivi ty t i s su e e xis ts inthese regions . One approach to t h i s problem i s examining thec ross - sec t iona l anatomy of the s t ruc ture through anatomy t ex tbooks and ass igning physica l areas to each of the major t i s suetypes . Another fac tor i s tha t , while the ef fec t ive conduct ivecross sec t ion can be represented as a f rac t ion of the grosscross sec t iona l area of the w ris t , the s i ze of th e wris tvar ies considerab ly , depending on th e in div id ua l. Hence,assess ing the SAR fo r a given wris t cur ren t can yie ldconsiderab ly d i f f e ren t values from one person to another sincethe SAR i s a function of the square of the loca l currentdensi ty .

Gandhi e t a l . (1986) have derived an expression fo r theef fec t ive conductive, cross-sec t ional area of the wris t .The ir expr es s ion i s :

[9 ]

where Ac ' Al and Am are th e physica l areas o f high watercontent an d low water conten t t i s sues , an d of the regioncontaining. red marrow (medium conductivi ty t i s sue ) , ofconduct ivi t ies ac ' a l and am' respect ive ly . Each of th es e a re as ,Ac ' Al and Am, may b e d ete rm in ed by re ference to an a t l a s ofanatomical areas and expressed in terms of a p erc en ta ge o f thegross c ross - sec t iona l area o f the wris t . Chen and Gandhi(1988) r epor t t ha t es t imates of these percentages , taken fromanatomical diagrams of the wris t cross sec t ion (obtained fromEycleshymer and Shoemaker, 1911) , are 31 .2 , 54.8 and 14 . 0percent of the gross c ross - sec t iona l area of the high , low and


29/94


medium-conduct ivi ty t i s sues , respec t ive ly . Based on theseda ta , the e f fec t ive conductive cross sec t ion of th e wr i s t maybe es t imated by determining th e gross c ro s s - s ec t ion . Foranalyses presented in t h i s repo r t , the wr i s t was assumed to berepresented by an e l l ipse , the area o f which i s given by wab, aand b being the semi-major and semi-minor axes of the e l l i p s e .

Chen and Gandhi (1988) have t abu la ted va lues fo r t i s sueconduct iv i t i es a t 1 MHz, taken from the l i t e r a t u re as:

ac = 0 .4 S/m;"'1 = 0.03 S/m;am = 0.22 S/m;

Other Mechanisms of Current InductionIn add i t ion to th e c ap ac it iv ely coupled curren ts in the

arm due to contac t with the tower, cu rren ts a re a lso developedin the body via ac t ion of the magnetic f i e lds which c i r c l eabout the tower. These magnetic f ie ld s w il l tend to develop acurren t w ith in the loop conf igura t ion formed by the body andthe tower as shown in Figure 2. Because shoes and gloves area t l ea s t p a rt ia l l y ef fec t ive in "breaking the c i r cu i t l l betweenthe body and the tower, th e loop i s no t necessar i ly completeunder most cl imbing condi t ions . For purposes of t h i sanalys i s , however, an est imate of the maximum po ss ib le c u rr en tt h a t could be induced in the body-tower loop c i r cu i t was made.using Faraday 's law, th e loop curren t can be computed as:

I = [ 2 W f A ~ o H ] / Z twhere f = frequency (Hz);

[10]


30/94


lAo = permeabi l i ty of t i s sue , taken to be the same asa i r , 12.56 X 10-7 h e n r y / m ~

A = cross- sec t ional area of the loop (m2 ) ;H = magnet ic f ie ld s t reng th (Aim);Zt impedance of body (0);

The impedance of the body was taken from Gandhi e t a l .(1985) and Chatter jee e t a l . (1986) in which meas ured bodyimpedance was determined fo r 197 male and 170 female sUbjects.For a subset of 70 males in the age range of 18-35 years , thebody impedance magnitude was found to be 371 ohms (0) with as tandard devia t ion of 39 0; in a group of 59 females in thesame age range, the mean value of body impedance was 459 n.These values were fo r a frequency of 1 MHz and for thec on dit io n o f grasping a 1.5 cm diameter, 14 cm long, brass rodwhile stand ing barefooted in contac t with a ground pla tee lec t rode . Hence, these values can be taken to represent thein te rna l body impedance for the c on ditio n o f good e lec t r i c a lcon tac t o f a cl imber with the tower; in r ea l i ty , t h i s i s aworst case assumption s ince shoes wi l l s igni f icant ly increasethe impedance such tha t the curren t wil l be reduced. Theef fec t ive area of the body-tower loop was est imated to beapproximately 0.65 m2 determined by sketching a cl imber on atower . The magnetic f ie ld s t rength was est imated a t adis tance of 30 cm from th e tower sur face based on th e c urren tf lowing in the tower .

Magnetic f ie ld s w il l also induce c i rcu la t ing eddy currentsin the body which are a funct ion of the radius o f the c ross sec t ional area through which the f ie ld l in es f lux . Theseeddy currents wil l also lead to energy absorpt ion and theloca l cur ren t densi ty can be used to c alc ula te the SAR. For as inusoidal ly varying f ie ld , it can be shown t ha t th e curren t


31/94

AM Tower Induced Body Currents, page 19den sity in t i s sue i s given by:

J = CTlJo1foHf [11]where J

CT1J00

flow (m) ;Hf

= cur ren t densi ty (A/m2 ) ;= conduct iv i ty o f t i s sue (S/m) ;= permeabi l i ty of a i r ;= effec t ive radius of body through which curren ts= magnetic f ie ld s t reng th (Aim);= frequency (Hz).

At 1 MHz, th e conductivity of muscle t i s sue i s approximately0 .4 Sim and r e la t ive ly insens i t ive to changes in frequencyover th e AM s ta ndar d b ro adca st band (Chen and Gandhi, 1988).

I t can be seen t h a t the magnitude of the eddy curren t i sd i rec t ly proport ional to th e radius of th e t i s sue area .Magnetically induced eddy curren ts produce a maximum curren tdensi ty in th e periphery of the body. The effec t ive radius ofthe body through which curren ts c i rcu la t e was taken to be theradius o f a c i rc l e having th e same area as an e l l i p t i c a lrepresen ta t ion of the human body as modeled by Durney e t a l .(1986) . Durney e t a l . (1986) represented the body as ane l l ipso id with s em i-ma jo r axe s of 0.875 (corresponding to oneha l f th e h eig ht) , 0.195 (corresponding to one-half the width)

. and 0.098 m ete rs ( co rr es po nd in g to one-half the body depth,ches t to back) . The effec t ive radius was then computed as0.293 m.

Using the above approaches, SARs were est imated bydetermining curren t dens i t ies , to the ex ten t poss ib le , t h a twould r e su l t from climbing hot AM towers . One f ina l approach


32/94

AM Tower Induced Body Currents, page 20ta ke n in clu de d r e f e r r a l to the r e su l t s on RF dosimetry workconta ined in the Radiofreguency Radiat ion Dosimetry Handbookby Durney e t ale (19B6). The handbook con ta ins the r e su l t s ofmany d i f f e r en t numerical methods fo r computing the whole-bodySAR in prola te spheroida l models of. the human body. The twomethods most commonly used in the medium frequ ency ran geinc lude the l ong-wave length approx ima t ion and th e extendedboundary-condit ion method. Graphical i l l u s t r a t i on s presen tthe SAR fo r an inc iden t plane wave power densi ty of 1 mW/cm2as a func t ion of frequency. While the r a t e a t which energywil l be absorbed from a near f i e ld exposure s i t ua t ion , such asa cl imber on a ho t AM tower, wil l be l ess than for a planewave with the same e lec t r i c or magnetic f ie ld s t reng ths , t h i smethod o f est imat ing the whole-body SAR should r e su l t in aconservat ive value .

Induced Current InstrumentationTo measure the wr i s t current in a cl imber, a very sim ple

device was const ructed , s im ilar to tha t assembled fo r th ee a r l i e r study by EPA (1988). An assembly, fabr ica ted fromplywood, was made which support s a thermocouple type RFmilliammeter . Through the use of copper s tr ap pi ng ma te ri al ,e lec trode con tac t areas were formed a t one end of the assemblydesigned to c on ta ct t he tower and a t the handle end which thecl imber would hold during the measurement process . Figures 3and 4 show the body current measurement device . By hold ingonto the device and hooking the fa r end to a hor izon ta l towermember, th e RF cur ren t f lowing between the body and the towercan be di rec t ly measured by reading th e meter movement. Notethe copper s t rapping mater ia l which l i ne s th e ins ide ando uts ide of the handle opening and the hook-shaped end of theassembly. A fuse holder mounted on the s ide of the assembly


33/94


held e i t h e r a 125 mA or 250 mA fuse to p ro t ec t the metermovement, depending on th e use of a shunt re s i s to r , descr ibedbelow.

The meter movement employed in t h i s s tudy was th e SimpsonElec t r i c Company Model 39-05330 RF mill iammeter 1 This devicehas a sca le cal ibra ted from 0 to 100 on the meter. Ful l sca lei s nominally equivalent to a cur rent of 115 rnA and th eresponse i s logar i thmic , Le . , nonl inear . . Hence, it i s notposs ib le to d i rec t ly read the actua l cu rren t value from theind ica t ion on th e meter but r a the r a char t o r some other meansmust be consul ted to a r r iv e a t th e cur rent . This par t i cu la rmeter movement i s d i f f e ren t from the un i t used by EPA andprovides fo r be t t e r reso lu t ion of low values of cur rent . Themeter ' s impedance a t 60 Hz i s 5 .5 O.

The meter was cal ibra ted by flowing a known RF cur rentthrough it and recording the meter ind ica t ion . This procedurewas followed a t 1 MHz and a lso a t 60 Hz. A funct ion generatorwas connected to the me te r th ro ugh a known value of res is tance(10.30) and the vol tage drop measured a cr os s t he r e s i s t o r wasused to derive the cur rent flowing through the meter movement.A t rue rms v oltm eter (B alla ntin e In strume nts, In c. Model 323)capable of accura te response to beyond 20 MHz was used fo r thevol tage drop measurement; the u nc erta in ty of the Ballant inemeter was independently determined to not exceed 1.9% a t 1MHz. These ca l ib ra t i on data are l i s t ed in Tables 1 and 2 fo r1 MHz and 60 Hz respec t ive ly . The data in these t ab les showt h a t the meter i s in se n sit i ve to frequency in t h i s range. The1 MHz ca l ib ra t i on data are p lo t ted in Figures 5 and 6 where

Simps9n ,E lec t r ic Company, 853 Dundee Avenue Avenue,E l g ~ n , I l l ~ n o ~ s , U.S.A., 6 0 1 2 " O - 3 0 9 0 ~ Phone: 3 1 2 - 6 9 7 - 2 2 6 0 ~Telex: 7 2 - 2 6 1 6 ~ FAX: 312 -697 -2272 .


34/94

AM Tower Induced Body Currents, page 22th e rIDS RF cu r r en t i s r e l a t ed to th e meter ind ica t ion . Figure5 shows th e data in a l i nea r fashion whi le Figure 6i l l u s t r a t e s th e response based on a logar i thmic d isp lay o f thecur r en t and meter i nd ica t ion . The cur r en t ca l i b r a t i on da tawere f i t t ed to a l e a s t s qu a re s expr es si on o f the form

Log(I) = A + B.log(reading)

where I rm s cu r r en t (mA);A = 0.9917B = 0.5364

[12]

Express ion [12] was used to conver t a l l meter readings toac tua l cur r en t s in mill iamperes .

In an t ic ipa t ion o f th e poss i b i l i t y t h a t RF cur ren ts mightbe g rea t e r than th e fu l l sca le va lue of 115 mA t h a t th e bas icmeter movement could i nd ica te , a meter shun t re s i s tance wascons t ruc ted cons is t ing o f two 10 0 carbon t ype r e s i s t o r s inpa ra l l e l , forming a shunt re s i s tance o f approximate ly 5 O.This shun t was placed d i r ec t l y across the terminals o f themeter movement and was found to provide fo r a 2.17 fo ldincrease in meter measurement range. Hence, with th e shun t inp lace , th e maximum cur r en t t h a t could be measured . with th edevice was about 250 mAo This value was accep ted as areasonable maximum s ince RF burns are assoc ia t ed with RFcur r en t s l e s s than even 250 rnA (Rogers, 19B1). Thus, it wasf e l t t h a t i f , dur ing th e course o f th e measurements on th etowers , th e body cur r en t exceeded f u l l sca le on th e meter wi thth e shun t in p lace , then da ta co l lec t ion would be s topped .Also, th e recen t rev is ion o f th e ANSI s tandard , IEEE C95.1-1991 (IEEE, 1991) , con ta ins a maximum c o ~ t a c t cu r r en t l im i t o f100 rnA app l i cab le fo r f requencies above 100 kHz.


35/94

AM Tower Induced Body Currents, page 23Current measurements were made by climbing the towers and

a t approximately uniformly spaced points , connect ing the l i f e l ine to the tower, leaning back away from the tower as shownin Figure 2, removing the glove on the hand holding the meter( the r i gh t hand), grasping the handle of the cu rren tmeasurement assembly , hooking the device over a hor izon ta ltower member and observing the meter movement. In prac t ice ,it was often found t ha t the copper s t rapping mater ia l on themeasurement device needed to - be scraped aga in st the towermember to remove pain t in the immediate region of contac t withthe tower to insure good e lec t r i ca l contac t . Theeffec t iveness of the pain t in res i s t ing the flow of bodycur ren t was noted as the meter f luc tua ted from t ime-to-t irne asa good contac t was being made. The readings recorded were thehighest r ea di ng s obta in ed a t any given height on the tower,a f t e r a s tab le up sc al e r ea ding was evident.

posi t ions on the tower a t which cur ren t measurements weretaken was determined in one case (a t the s ta t ion inBakersfie ld) by di rec t ly reading the heigh t above the baseinsu la to r with a f iberglass measuring tape t i ed to thecl imber who pul led it up as the tower was sca led. At thes ta t ion in R iverside , because of i t s height exceeding thelength of the tape measure (300 fee t ) , the approach used wasto attempt reaching posi t ions a t known points on the towerre la t ive to painted tower sec t ions . This was accomplished byan observer on the ground watching from a long dis tance andre lay ing in stru ctio ns to the cl imber via a portable , VHFhandi - ta lk ie . Tower sec t ion heights were l a t e r measured sot ha t exac t loca t ions on the tower could be establ i shed on thebas is of the re la t ive posi t ions previously determined.


36/94

AM Tower Induced Body Currents, page 24The AM Stations Used in the Study

RF induced currents were measured on two AM radio towersin C alifo rn ia . Additional data , obtained in Spokane,washington and previous ly reported by EPA (1988), have beenanalyzed and are also repor ted here fo r helping providecontext for the Cal i fornia measurements. The AM radios ta t ions in Cal i fornia par t i c ipa t ing in the pro jec t were:

KWAC, Bakersf ie ld . ca l i forn iaFrequency 1490 kHz:Power 1 kW:Tower height = 150 fee t :Tower type = nonuniform cross-sec t ion , guyed; 8inch square a t top and bottom and 3 fee t square in c e n t e r ~Elec t r i ca l height = 0.23 AROlF. Rivers ide, Cal i forniaFrequency 1440 kHz:Power 1 kW:Tower height 365 fee tTower type = uniform cross-sec t ion , guyed;t r i angu la r , 17 inch face;Elec t r i ca l height = 0.53 AThe KWAC antenna tower shown in Figure 7/ being nonuniform

i n c ro s s- se c ti on , was very d i f f i c u l t to climb s ince very fewof the tower cross members were hor izonta l and near the topand bottom of the tower the cross members were very close lyspaced. Figure 8 shows the ty pica l la tt ic e-w ork l ikest ructure of the tower. The RDlF tower, shown in Figure 9,was of uniform cross-sect ion with hor izonta l membersapproximately every 15 inches.

The s ta t ion in Spokane, for which p re vi ou sl y o bta in ed d atawere used in t h i s study, was:


37/94

AM Tower Induced Body Currents, page 25KKPL, Spokane . Washing tonFrequency = 630 kHz:Power = 1 kW:Tower height = 390 fee t :Tower type = uniform cross-sec t ion , 17 inch face:Elect r ica l height = 0,25 A .Each of the above s ta t ions had kindly agreed to

par t i c ipa te in the s tudy , providing access to the antennatower and will ingness to momentarily turn o ff t ransmi t te rpower to prevent the poss ib i l i ty of severe RF burns wheni n i t i a l l y contact ing the tower a t ground l eve l . The s ta t ionengineers provided much app re ci at ed coope ra ti ve s uppo rt duringeach of the tower cl imbs.

The par t i cu la r select ion of the Cal i fornia s ta t ions wasmade on the bas is of ident ifying AM. s ta t ions using a s ing lemonopole antenna tow er, operating a t 1 kW (to reduce the1 ike l ihood of exc essiv ely high induced RF curren ts in theclimber) and without any other broadcast antennas mounted onthe tower, such as an FM s ta t ion antenna which could introducemajor dis tor t ion in the resul t ing data . Dr. Robert F.Cleveland in the FCC's Office of Engineering and Technology ,Washington, DC, made use of the FCe's broadcas t s ta t iondatabase in f inal ly ident i fy ing two sui table s ta t ions for thestUdy.

Measurement ResultsBody cur ren t data ob ta ined du ri ng the ea r l i e r EPA stUdy in

Spokane, Washington, revealed t h a t the induced cur ren t wascor re la ted with the radia l ly directed component of theeleCtric f i e ld on the surface of the tower. Using theseea r l ie r data , Cleveland e t ale (1990) discussed elements of amodel fo r p red ict in g induced body current in tower climbers.


38/94


The Spokane data were reanalyzed for inclusion here bycomputing the e lec t r i c f ie lds near the tower using a method ofmoments mathematical technique implemented in a softwareprogram cal l ed MININEC. This program, designed fo r use on apersonal computer, i s a avai lable commercially (Rockway e ta l . , 1988) and permits the computation of e lec t r i c andmagnetic f ie lds o f w ir e- ty pe a nte nn as , i . e . , l i nea r arrays ofconductors arranged to form an antenna.

The MININEC program i s extremely powerful but does have aninherent l imi ta t ion in tha t it cannot accura te ly computef ie lds a t points immediately near the tower Rockway e t a l .(1988) ind ica te tha t the code can accura te ly ca lcu la te nearf ie lds a t points located a t leas t one segment distance fromthe antenna surface. Each monopole analyzed was broken into35 s e g m e n t s ~ the one segment cr i t e r ia would imply tha tcomputations not be considered accurate a t p oin ts c lo se r thanabout one meter for the KWAC tower or about two meters for theKDIF towe r. A cc ord in gly , e lec t r i c f ie lds were calculated a t adis tance of two meters from the tower model; the assumptionmade was tha t the f ie lds a t two meters from the tower aregood re la t ive indicators of the magnitude of the e lec t r i cf ie lds on or very near the tower surface. The towers weremodeled as cyl inders having c irc um fe re nc es e qu al to theperimeter around the actual tower. In the case of thenonuniform tower cross-sect ion a t KWAC, the mean value of thetower cross- sec t ion was computed based on dimensional data onthe tower.

Induced body current data obtained in the ea r l i e r EPASpokane s tUdy are l i s ted in Table 3 and plo t ted in Figure 10as a function of height on the tower. Also plo t t ed on thef igure i s the re la t ive rad ia l e lec t r i c f ie ld st rength


39/94

AM Tower Induced Body Currents, page 27determined with MININEC. The radia l component of e lec t r i cf i e ld has been a rb i t ra r i ly adjusted to exam ine any funct ionalre la t ionship between it and the measured induced body current .Figure 10 i l l u s t r a t e s t h a t the induced cur ren t tends to t rackthe r e la t ive value of the radia l e lec t r i c f ie ld st rength .This i s consi s t en t with the observat ions given in the EPArepor t (EPA, 1988) and als o c on sis te nt with the t heore t i ca lconcept t ha t currents induced in the arm of th e cl imber wouldtend to be driven by any e lec t r i c f i e ld component in the samegenera l di rec t ion as the arm. Also, it i s the radia lcomponent of the f ie ld which would tend to be s ign i f i can tre la t ive to consider ing th e body of the cl imber as acapac i t ive ly coupled objec t to the tower f i e lds . In thenearf ie ld of the AM tower, Le . , within a tower ' s height o fthe tower, th e e lec t r i c f ie ld is composed of two components,one t h a t i s ver t i ca l and para l l e l with th e tower and anotherone t ha t i s pointed in a r ad ia l d ir ec tio n , outward, away fromth e tower. At o r very near the surface of the tower, ther ad ia l component can grea t ly exceed the magnitude of thepa r a l l e l component. In the f a r f i e ld , th e rad ia l componentdisappears and only th e ver t i ca l component remains.

Measu red body current data obtained a t KWAC are l i s t ed inTable 4 and plo t t ed in Figure 11 along with the computedr e la t ive radia l e lec t r i c f ie ld s t reng th . Again, the genera lcorre la t ion between th e induced current and th e e lec t r ic f ie ldi s seen, however, with some r e la t ive ly maj or dev iations a td i f fe ren t locat ions along the tower. A maj or observat ion,during the measurements was tha t the induced cur ren t did notincrease in a l inea r fashion beginning. a t th e base of thetower as had been seen in the Spokane data . One possibleexplanation for the nonlin ear nature of the current i s thenonuniform cross-sec t iona l area of the tower; the tower i s a t


40/94

AM Tower Induced Body Currents, page 28i t s l a rges t s ize a t the mid-point height . Physical i n tu i t i onargued tha t th e su rf ac e e lec t r i c f ie lds would be somewhat l essa t the po in t of grea tes t cross- sec t ional area , s ince in t h i sregion th e charge on the tower would be more widelydis t r ibu ted over a gr ea t e r surface area , leading to a l e s se rsurface f ie ld s t rength than a t the surface of the tower, thanif the tower had been uniform in cross- sec t ion . The actua lmeasurement data appear to co r re la te with t h i s in tu i t ivenotion in tha t the ra te of increase in measured cur rent t endedto dec re as e w it h increas ing height on the tower near i t s m i d ~sect ion (th e area of l a rges t cross-sect ion) and to in cre as e a sthe top of the tower was approached, where t he c ro ss -s ec ti onwas diminishing in s ize (presumably leading to an enhancedsurface f ie ld due to a sm aller area over which e lec t r i ca lcharge can d i s t r i bu te i t s e l f ) .

As can be seen, the computed value of the re la t ive r ad i a le lec t r i c f ie ld s t rength was found to increase l inear ly overmost of the tower height , when modeled as a uniform crosssect ion tower. An addi t ional exercise in modeling of thetower was performed by using a non-uniform cross- sec t ion . Inth i s case, the physical s t ruc ture of the tower was broken into20 segments of equal length but e ac h h av in g r ad i a l dimensionsapproximating the s ize of a cyl inder which has the samecircumference as the circumference of t ha t sect ion of thefour-s ided tower. The resu l t s of th i s a na ly sis are shown inFigure 12. unfor tunate ly , the computed resu l ts seem to o ffe rlittle in s igh t to p rec ise ly why the measured body cu::r;rentdevia tes from a l i nea r r e la t ionship near the cen te r o f thetower. Although guy wires, fo r simpl ici ty sake in themodeling process , were not included in the computer modeling,the on e major difference between th e Bake rs fie ld data and the


41/94

AM Tower Induced Body Currents, page 29Spokane data i s the f ac t t h a t one tower i s not uniform incross -sec t ion .

Measured body cur ren t s and calcula ted re l a t ive rad ia le l ec t r i c f ie ld s t reng ths fo r the KDIF tower in R iv erside areshown in Figure 13. The body cur ren t data are given in Table5. The KDIF tower i s very close to a half-wavelength inheight , e lec t r i ca l ly . Theory would suggest a maximum invol tage on the tower near the base and a t the top, with aminimum near the cen ter of the tower . In f ac t , the computede lec t r i c f i e ld s t reng th i s seen to follow j u s t in th i s manner.Also, the measured body currents appear to match, in a verys imi la r way, to the rad ia l f ie ld values , except fo r a fewdevia t ions which were noted near various guy wires . It wasnoted t ha t , on occasion, the measurement points happened tocorrespond to where guy wires were at tached to the tower.Near these loca t ions , the measured cur ren t s tended to becomel ess wel l behaved in terms of t h e i r normal var ia t ion withhe igh t on the tower. I t was found, fo r in stan ce , t ha t whenthe curren t measuring device was momentarily brought incontac t with a guy wire , within reach of the tower but on theouts ide of the s t r a in insu la to r at tached to the tower, themeasured cur ren t was very s ign i f i can t ly g rea t e r than a t thesame height when in contac t with the tower. This f indings t rongly suggests t ha t the guy wires , as used on th i s tower,are s t rong sources of induced cu rren t in an individual whomight contac t the wires and it i s highly l ike ly t h a t thee lec t r i c f i e lds between the tower and the nearby end of theinsula ted guy wire are very s t rong, probably much s t rongerthan even the surface f i e lds on the tower i t s e l f a t loca t ionsnot near a guy wire.


42/94

AM Tower Induced Body Currents, page 30The c o r r e l a t i o n between t h e body c u r r e n t an d p r o j e c t e d

r a d i a l e l e c t r i c f i e l d i s r a t h e r compelling an d suggests t h a t ,d es p it e t h e i n a b i l i t y t o model t h e s u r f a c e e l e c t r i c f i e l d s i nan e x a c t l y a c c u r a t e way with MININEC, indeed, e l e c t r i c f i e l d sare t h e p r i n c i p a l source f o r t h e measured c u r r e n t s and t h i s i sa general conclusion t h a t seems t o em er ge fro m t h e t h r e e s e t sof d a t a obtained during h o t AM tower climbing. The lack ofconfidence i n numerical values of e l e c t r i c f i e l d s t r e n g t h s ont h e tower s u r f a c e does not d e t r a c t from t h e b a s i c i n s i g h t t h a tt h e r e l a t i v e magnitude of induced arm c u r r e n t s experienced onhot AM towers can be p r e d i c t e d on t h e b a s i s of t h e vol taged i s t r i b u t i o n on t h e tower s i n c e e l e c t r i c f i e l d s t r e n g t h w i l lbe d i r e c t l y r e la te d t o t h e p o t e n t i a l on the tower with r e s p e c tt o ground.

Specific Absorption Rate Estimatesusing t h e mathematical methodology o u t l i n e d above, t h e

measured body c u r r e n t s were used t o e s t i m a t e t h e SAR i n t h ew r i s t of a climber . Inspect ion of r e l a t i o n [7] permits t h ec a l c u l a t i o n of t h e c ur r e n t t h a t would be expected t o r e s u l t i na SAR i n t h e w r i s t e q u i v a l e n t t o any s p e c i f i c value. I t i sof p a r t i c u l a r relevance t o examine a l o c a l i z e d value of SARequal t o 8 W/kg s i n c e t h i s value i s c a l l e d out as a l i m i t i n gvalue f o r s p a t i a l peak SARs i n t h e body i n the ANSI standard(ANSI, 1982). The r e c e n t l y approved r e v i s i o n of t h e ANSIstandard (IEEE C95.1-1991) contains a r e l a x a t i o n i n therecommended maximum s p a t i a l peak SAR f o r t h e e x t r e m i t i e s of 20W/kg as averaged over an y 0.1 kg (100 g) of t i s s u e . This i ss i m i l a r t o I a l though not e x a c t l y t h e same a s , t h erecommendation of t h e I n t e r n a t i o n a l Radiat ion P r o t e c t i o nAssociat ion (IRPA, 1988) i n which 2 W i s permitted as averagedover any 0.1 kg of t i s s u e ( t h i s i s e q u i v a l e n t t o 20 W/kg).


43/94

AM Tower Induced Body Currents, page 31Canada has very recen t ly adopted new l imi t s fo r exposure toelectromagnetic f i e lds for workers which ca l l fo r a loca l izedSAR l imi t of 25 W/kg fo r the body surface and th e l imbs whenave raged ove r 10 g of t i s sue (Canada, 1991) .

Rela t ion [7] requ i res t h a t the c u rr en t d ens it y be known inthe t i s sue area of in te res t , in t h i s case the wris t . Thec urre nt d en sity , in tu rn , i s s t rongly influenced by theeffec t ive conductive cross-sec t iona l area w ith in th e wris t .Chen and Gandhi (1988) provide the percentage of t h e grosscross-sec t iona l area of the wr i s t t ha t i s associated withthree major t i s sue types fo r the purpose of computing theeffec t ive conductive area of the wris t . A major compendium ofdata obta ined on wr i s t breadth (width as viewed from the topof the w rist) and wr i s t circumference was consul ted (NASA,1978) to explore the var ia t ion in the human, male w ris t c ro ss sect ion . Based on a 1965 study of 3859 u.S. Air Forcepersonnel , wris t data were compiled according to percent i les .For example, th e 50 percen t i l e wr i s t breadth was determined inth e s tudy popUlation to be 5.7 cm with a correspondingcircumference of 17.0 cm.

These data were used to ca lcu la te the area of an e l l ipsehaving the same wr i s t circumference. The assumption was madet h a t the human wris t i s approximated by an e l l i p t i c a l crosssec t ion . The area of an e l l ipse i s given by nab, a and b beingthe semi-major and sem i-m inor ax es of th e e l l ip se . Theapproximate circumference P of an e l l ipse i s given by:

[13]

Equation 13 (Weast and Selby, 1967) was used to compute avalue fo r b based on using one-ha l f of the breadth value for a


44/94

AM Tower Induced BOdy Currents, page 32and the circumference P. Thence, the area of the equivalente l l ip se was computed. Table 6 l i s t s th e data obtained fromthe Air Force s tudy and the e l l i p t i c a l area of the wris t asout l ined above. Figure 14 i l l u s t r a t es th e var ia t ion in grosscross- sec t ional area of the wr i s t in the s tudy p o p u l ~ t i o n .

Table 6 shows t h a t the wris t gross cross- sec t ional areavar ies from 12.3 cm 2 for the 1 percent i l e to 38.5 cm 2 for the99 percent i l e ; the 50 percent i l e area i s 22.9 cm 2 Forexample, 50% of the study populat ion had wris t s with areasequal to or l ess than 22.9 cm2 while the remaining 50% hadwrists with areas grea ter than 22.9 cm2 . These data , then,permit the ca lcula t ion of the ef fec t ive conductive crosssec tion of the wris t of males in the populat ion.In te res t ing ly , the r a t io of wris t areas between the 1 and 99percent i l es represents a fac tor of 1.5B t imes and s ince SAR i sd ir ec tl y r el at ed to the square of the c urr en t d en sity , th i simplies tha t the 8AR wi l l range over a factor of 2.5 tim es,depending on the s ize of t he c limbe r's wris t .

Applying re la t ion [9] , the ef fec t ive conductive crosssec t ional area of the wris t was determined as 8 .00 , 9.98 and12 .7 cm 2 fo r the 1, 50 and 99 percent i l es of wris t s izesrespect ively. When these areas are used in conjunction withre la t ion [7J, the fol lowing currents are projec ted as beingassocia ted with loca l ized SARs in the wris t of 8 and 20 Wjkg.


45/94


Projected Currents to Produce SARs of 8 and 20 W/kg in the WristCurrent to produce wris t SAR of

Wrist s ize percent i le 8 W/kg 20 W/kg1 45 7250 56 8999 72 114

It i s worthwhile to note here t h a t the revised ANSIstandard (IEEE C95.1-1991) spec i f ies a maximum con tac t currentof 100 rnA fo r the purpose of reducing the poss ib i l i ty of RFburns. The above r esu l t s suggest t h a t . a l imi t ing currentvalue of 100 rnA could r esu l t , however, in exceeding a localSAR of 20 W/kg in the wris t . Canada r ec en tl y e sta bli sh ed amaximum contact cur ren t fo r occupat ional exposure of 40 rnA(Canada, 1991).

The above data immediately show tha t , based on themeasured body curren ts obtained from hot AM tower climbing(up to 250 rnA), tha t loca l SAR l imi t s of 8 or 20 W/kg in thewris t would I ikely be exceeded on 1 kW AM towers , assumingt h a t protect ive gloves were not used. I t should also be notedt h a t the hands are not the only par t of th e body surface t h a tcomes into contact with a tower dur ing c limb ing; the arms andlegs may be in contact from t ime-to- t ime and, in someins tances or environments, d i rec t skin con tac t may be madewhen minimal clo th ing i s worn (for example, the wearing ofshor t s and no s h i r t in hot environments).

The data reported here on body currents suggests tha t ,depending on frequency and locat ion on the tower, a 1 kW AMtower can eas i ly r esu l t in body curren ts of up to 250 rnAduring climbing. Clearly , a 250 rnA curren t w il l resu l t in


46/94

AM Tower Induced Body Currents, page 34excessive SAR in the wris t , ranging between 96.9 and 244 Wjkgfo r the 99 and 1 percent i l es of human wris t siz:e. The SARassocia ted with the 50 percent i l e w ris t s ize would be 157Wjkg. These values are obviously s ign i f ican t ly grea ter thanan y of the local ized SAR l imi t s contained in various RFprotect ion guides.

From a prac t ica l a pp li ca ti on p er sp ec ti ve , it would beuseful i f the maximum induced body cur rent t h a t would exis tdur ing climbing any height tower , operated a t any frequency inthe AM broadcast band and a t any authorized power l eve l couldbe known. The data p re se nt ed h er e must be cons idered l imitedbu t cer ta in ins ight s do emerge tha t may be helpfu l inpredict ing induced body currents fo r o the r towers. The datatend to support the content ion t ha t the maximum induced armcurrent i s f requ ency dependent . For example, the maximumvalue of induced cur rent measured on two almost ident i ca le lec t r i c a l height towers, operat ing a t the same 1 kW powerl eve l , but a t di f fe ren t frequencies appears to be close lyr e la ted to simply the d iff er en ce in frequency a s su gge sted byequation [6] . The ra t io of the maximum induced cur rent a t1490 kHz to th e maximum cur ren t a t 630 kHz was found to be(252 mAjllO mA) equal to 2.29. This compares to the r a t io ofth e two frequencies of 2.37 (1490 kHzj630 kHZ) within 3%.

Also of in te r e s t from the cur rent measurements i s t h a t themaximum current found on two very d i f f e r en t e lec t r i c a l heighttowers (O.23A vs. O.S3A) operated a t the same power andv i r tua l ly the same frequency was essent ia l ly the same, 252 rnA.This suggests t ha t towers of othe r e lec t r i c a l heights withint h i s height range may produce maximum body currents of aboutthe same value . Since the ins ight i s t ha t t h e . induced armcur rent i s c orre la te d to the s trength of the rad ia l component


47/94


of the e l ec t r i c f i e ld on the tower, one poss ib le means ofexamining the above c on te nti on o f a ce i l ing value fo r inducedcur ren t , independent of tower height (within l imi t s ) , would beto compute th e rad ia l e lec t r i c f ie ld st rength fo r di f fe ren te lec t r i ca l height towers . A s imi l a r i ty in th e values obtainedfo r th e maximum e lec t r i c f ie lds on d i f fe ren t towers wouldsuppor t the concept of a single maximum cur ren t value . Ananalys i s of th e rad ia l e lec t r i c f ie ld st rength component wascar r i ed out using MININEC fo r a 100 m t a l l tower, modeledas a cy l inder with a diameter of 0.4 m, dr iven with 1 kW. Thetower was assumed to be composed o f 50 segments. Elec t r icf ie ld st rengths were computed a t 10 m i n t e rva l s in height a t adis tance of 1 m from the tower. The r esu l t s of t h i s analysisare given in Table 7.

Table 7 revea ls t h a t the maximum calculated e l ec t r i c f ie ldst rengths fo r towers in the e lec t r i ca l height range of 0.25 Ato 0.625 A are somewhat l e ss than fo r the 0.25 A tower buttha t the maximum values are not very diss imi la r . Hence, i fthe computed f i e lds are representat ive of the a ctu al su rfa cef i e lds on the towers , the above hypothesis of a ce i l ing valueof induced curren t i s general ly supported. However, ani ~ p o r t a n t f inding revealed in Table 7 i s t h a t towers tha t aree lec t r i ca l ly very shor t can produce very high e lec t r i c f ie ldst rengths a l l along the tower tha t might be as much as 5 t imesgrea te r . Also, the e lec t r i c f ie ld i s es sen t ia l ly uniformalong the tower, fo r the 0.1 A t a l l tower, suggest ing t h a t thevol tage d is t r ibu t ion i s nearly f l a t from bottom to top. TheTable 7 r esu l t s , while useful only as an ind ica to r ofpoten t i a l r e la t ive e lec t r i c f ie ld st rength values , poin t outt h a t fo r towers l e ss than 0.2 A in e lec t r i ca l h eig ht, su rfa cef i e lds may increase sharply in magnitud e and , hence, produceSUbstant ia l ly grea te r induced arm curren ts assuming frequency


48/94

AM Tower Induced Body Currents, page 36and power remain constant . This would sugges t t h a t spec i a lcare i s needed in at tempting to ex t r apo la t e th e data, co l lec tedin th i s s tudy to pro jec t induced cur r en t s t h a t would ex i s t onvery shor t towers .

For towers in the range o f 0.25 to 0.625A, the re appears tobe r ea so na bl e s up po rt to assume t h a t the maximum induced armcur r en t s wi l l be s im i l a r i th e locat ion on the tower where th i smaximum value wi l l occur , however, can be very d i f f e r en t ,depending on the par t i cu l a r phys ical desc r ip t ion o f th e tower.I f th e maximum arm cur r en t i s taken to be approximate ly 250 rnAa t a frequency o f 1490 kHz, and i f th e induced c urre nts a reassumed to be d i r ec t l y propor t iona l to frequency ( f in MHz),,then th e maximum wr i s t cur r en t can be expressed as :

I (wr i s t max) = 168 f (MHz)Jp(kW) ; (IDA) [14]In a s imi la r way, th e maximum wr i s t SAR can be expressed as:

SAR(wrist max) 70.7f(MHZ)2 p ; (Wjkg) [15]

I f the frequency of th e s t a t i on i s given in megahertz and th eantenna input power P i s given in k i lowat t s , th e abovere la t ionsh ip wi l l express th e maximum poss ib le SAR in th ewr i s t o f a cl imber , fo r towers in th e h eigh t range of 0.25 to0 . 6 2 5 ~ , acros s the AM radio frequency band, withinapproximately 10%. This express ion i s , then , usefu l fo rexamining the cond i t ions of s t a t i on f requency and p ~ w e r l eve lst h a t would be expected to have th e po t en t i a l fo r producingwr i s t SARs of spec i f ic va lues . For example, a s t a t i onopera t ing a t 1500 kHz would have to reduce power to 50 W tol im i t the wr i s t SAR to 8 W/kg, assuming th e c l imber has goode l e c t r i c a l con tac t with the tower . A s t a t i on opera t ing a t 540


49/94

AM Tower Induced Body Currents, page 37kHz would be able to use 388 W s ince th e SAR wil l be reducedbecause of th e lower operat ing frequency. Expression [15]shows, therefore , unfortunately , t h a t most AM radio s ta t ionswould have to operate a t very major power ~ e d u c t i o n s to insurecompliance with th e SAR l imi t of the ANSIRF protect ion guideas adopted by the FCC (C95.1-1982) (FCC, 1985a).

The induced arm current wil l tend to d is t r ibu te i t s e l faccording to conduct iv i ty throughout the body and,consequently, wil l r esu l t in only ra ther low, lo ca l cu rren tdens i t ies due to the s ign i f i can t ly grea te r body crosssec t iona l area . Dimensions fo r an e l l ipso ida l model of askinny man, taken from Durney e t a l . (1986), were lised tocompute a body cross-sec t iona l area of 402 cm 2 a t the midsect ion of the body. At the mid-point , then, the SAR would beapproximately equal to 0.0442F2p. Were a l l of the arm cur ren tto flow through the body cross-sec t ion , the resul t ing SARwould be about O. 0442 Wjkg due to arm currents on a 1 kWtower a t 1 MHz. The arm curren ts wil l not necessar i ly a l l gothrough th e body, however, because of leakage o ff th e body tothe surrounding environment due to s tray capaci tance of thebody (Stuchly e t a l . , 1991). Although t h i s may be r e la t ive lyi n s ign i f i can t a t AM b roadca s t f requenci es , even a 10% decreasein RF currents flowing in other par t s of the body, fa r the rfrom the arm, can have a s tr ong i nf lu en ce on th e resul t ing SARs ince the SAR i s proport ional to the square of the localcurrent densi ty .

The ex ten t to which protect ive gloves can be effec t ive inreducing the wris t SAR has not been extens ively evaluated andwas not a pa r t of the protocol fo r t h i s study. Nonetheless , asingle measurement on th e KWAC tower was performed with aglove when th e induced current meter , without the glove on,


50/94

AM Tower Induced Body Currents, page 38i n d i c a t e d f u l l s c a l e , o r the equivalent of 115 mAo When t h eglove, a common worker type, made of a c a n v a s - l i k e f a b r i c , wasplaced on t h e hand holding t h e ins t rumentat ion , t h e measuredc u r r e n t reduced t o a value of 58.6 mA, i n d i c a t i n g a reduct iono f almost one-hal f t h e un-gloved reading. Data on t h ee f f e c t i v e n e s s of any kind of gloves i n reducing c o n t a c tc u r r e n t s f o r g ra sp in g ty pe of c o n t a c t s i s not a v a il a b l e i n thel i t e r a t u r e . C h a t t e r j e e e t a l . (1986), however, did e v a l u a t et h e e f f e c t of Type 1, ANSI/ASTM D120 l ineman 's rubbere l e c t r i c a l s a f e t y gloves on measured body imped an ce when theSUbject touched j u s t t h e f r o n t of t h e f i n g e r t o a 144 mm 2e l e c t r o d e while s tanding barefoot on a meta l ground p l a t e .Their comments were t h a t " E l e c t r i c a l s a f e t y shoes ande l e c t r i c a l s a f e t y g lo ve s p ro vid e adequate p r o t e c t i o n only a tf requencies l e s s than about 1 MHz and 3 MHz, r e s p e c t i v e l y . "This comment i s apparent ly based on data contained i n Gandhie t ale (1985) which cont ai ned i nf ormat io n showing a measuredvalue o f body impedance of 950 0 with no glove compared t o animpedance of 11,800 0 with t h e glove. This , touching type ofc o n t a c t s i t u a t i o n cannot be used, however, t o surmise t h ee ff e ct i v e n e s s o f t h e glove i n reducing c o n t a c t c u r r e n t f o r t h ecase o f grasping c o n t a c t , anc important c o n f i g u r a t i o n of t h ehand dur ing tower climbing. The s i n g l e measurement obtaineddur ing t h i s s tudy i s more r e p r e s e n t a t i v e of t h e order ofmagnitude o f c u r r e n t reduction t h a t might be afforded bygloves but can i n no way be considered adequate t o t h eo ri z eabout the e f f e c t i v e n e s s of gloves g e n e r a l l y o t h e r than t o sayt h a t gloves may be h e l p f u l i n reducing body c u r r e n t s . Also,v i r t u a l l y any tower work w i l l include the use of some kind o fgloves simply from an abras ion r e d u c t i o n p o i n t of view. Thef a c t t h a t tower p a i n t e r s commonly use p a i n t -s a tu r at e d m i t t s t os l i d e over t h e tower s t r u c t u r e a l s o r a i s e s a question as t ot h e c u r r e n t r e s i s t a n c e a s s o c i a t e d with such p r a c t i c e s .


51/94


One observat ion made dur ing the course of these towermeasurements was t h a t sweat ing can in f luence th e qua l i t y ofe l ec t r i c a l contac t between the cl imber and the tower . Forexample, the p erc ep tio n o f RF burning at loca l ized poin t s onth e sur face of the sk in seemed to be a f fec ted by the amount ofpersp i ra t i on on the sk in ; the sUbject ive percep t ion was t h a tcu r r en t flow (perceived as sur f ace heat ing) was more ev iden twhen, in a brush ing contac t with th e tower , or guy wires , thearms o r body was heavi ly sa tu ra ted with sweat and the bodycu r r en t was su f f i c i en t l y high ( typ ica l ly above 75 mA). Theupshot of t h i s comment i s t h a t any thorough evalua t ion ofglove e f fec t iveness in inc reas ing the body impedance tocu r r en t flow must t ake in to account the p o ss ib il i ty t ha t thegloves may become sa tu ra ted w ith sweat, making them l essr e s i s t i v e and decreas ing t h e i r ab i l i t y to reduce cu r r en t f low.

The SAR effec t iveness of the magnetic f i e l d s on the towerswas assessed by, f i r s t , apply ing equat ion [10] and us ing ana ss umed body impedance of 371 0 as determined by Gandhi e t a l .(1985) and discussed e a r l i e r . Equation [10] provides anes t ima te of th e maximum poss ib le cu r r en t t h a t could flow inth e apparen t loop formed by the c l imber ' s body and the tower .For the ca l cu l a t i on , a value of the sur face magnet ic f i e l ds t reng th i s needed and t h i s was a r r ived a t by applying Biot -Sava r t ' s law:

H = I I (271"a)in which H = magnetic f i e ld s t reng th (Aim);

I = cu r r en t f lowing in conductor (A);a = di s t ance from the conductor (m);

[16]


52/94

AM Tower Induced Body Currents, page 40While t h i s r e la t ionship i s simple and appl ies to s t a t i cf ie lds , it is also found to apply with good accuracy to RFf ie lds in the AM broadcast band produced by tower cur rent s .EPA (1991) found t ha t the quas i - s t a t i c calcula t ion of magneticf ie lds given by [15) were almost ident ica l to the f ie ldcalculated by the Numerical Electromagnetic Code method ofmoments code ( the l a rge computer vers ion of MININEC) andgenera l ly compared well with measured magnetic f ie lds n ear thebase of AM towers. This i l l u s t r a t es a case where more complexcalcula t ional procedures are not necessar i ly any be t te r thansimple techniques , a t l e a s t for the near vic in i ty of thetower .

To develop in s igh t to magnetic f ie ld induced loopcur rent s , a general approach was taken to es t imate themagnetic f ie lds of AM radio towers. Fi r s t , the base cur ren tflowing in a quarter-wavelength t a l l monopole tower wascomputed for the case of an ideal . monopole r ad ia t ing 1 kW.This was arr ived a t by taking the rea l pa r t of the impedanceof the base of th i s ideal ized tower to be 36 n. The resu l t ingbase cur rent was determined to be 5.3 A. An analysis wasconducted to determine i f using a base cur rent of 5.3 A wouldbe conservat ive in es t imat ing magnetic f ie lds for other he ighttowers , L e . would not underest imate the resu l t ing f ie lds .This analysis was accomplished by applying another commercialvers ion of MININEC cal l ed ELNEC (ELNEC, 1991) which i ssuper io r in terms of the use r in ter face for computing thecur rent dis t r ibu t ion and many o the r pe rfo rmance ,a spect s on anantenna. ELNEC, however, does n ot p ro vid e fo r computation ofn ea rf ie ld s o f an antenna. ELNEC was used to find the peakcur rent on simulate

Tell Hot Tower Climbing FCC Project PB92125186

Documents

Transcript of Tell Hot Tower Climbing FCC Project PB92125186