Antennas Lightining Protection

7/29/2019 Antennas Lightining Protection

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Southern Africa has thehighest incidence oflightning in the world.In this article, Jonas Aleksa andChristopher Stockman, examinethe nature of lightning anddiscuss a range of measures thathave been developed tominimalize orpreventlightning damage in radio commu-

nication installations.

L ightni ng i s a transient electri cal currentth at equal iz es a dif ferenceofpotentialand creates an expl osionofli ght and sound

in t he process. About 75% of all l i ghtning

activi ty occurs w it hin cloud or betw een

clouds. The remaini ng 25% is cloud- to-

ground di scharges, whi ch are more easily

studied and more practi cally impor tant .

Wind wo r k son the moi sture in th e

clouds. I ttears droplets apart physically ,separating them into positive and negative

ions. The force of the w ind driv es the l i ght er

negative i ons back int o the cloud, but t he

posit iv e ions, whi ch are heavier, fall to

eart h. As t he process cont inu es, the cloud

becomes increasingly negative and the

ground acquir es a stron g positi ve charge,

held i nconcentrat ion by att raction of the

cloud ov erhead. The posit iv e area on earth

tracks t he movement of the cloud above like

a shadow.

M ore charge is generat ed as t hestorm

progresses, and when the force of att racti on

betw een ground and cloud i sstrong enough,

a chain reacti on electri cal breakdown causes

the airt o i on i ze and become a bett er

conductor. Ionizat ion starts at the bott om of

a cloud in narrow paths that branch as the

process mov e s t o t h e g r o u n d . T h i s

negati vely charged step leadev r apid ly

branches its w ay t o earth and is met by a

streamer of positively charged electrical

part icles mov ing up from the ground. The

path of i onized air connecti ng the cloud and

ground completes th e electri cal cir cuit , and

the resulti ng i ntense current flow, called the

f i r s t r e t u rn s t r oke , l i gh t s up a l l t he

bran ches. Because of branchi ng, where th e

leader seeks a pat h of l east r esistan ce to t he

ground, li ghtning can tr avel up to 12 mil es

from it s parent storm cloud before stri ki ng.

Lightning strikes are typi cally t w o to t hree

miles apart.

Afterthe firstreturn str oke, dart l eaders

w ill quickly travel the samepathto earth t o

be met by subsequent return stro kes, and t he

l i ghtning appears to fl i cker. The cor r e-sponding heat bui ldup expands in al l di rec-

ti ons at supersonic speed. As it cools, it

moves back to cancel t he parti al v acuum it

created at an audi ble rat e and generat es

thund er t hat can be heard up t o 10 miles

from the lightning.

Dow nw ard movi ng negatively charged

leaders account f or about 90% of t hecloud-

to- ground di scharges.Di fferentcondit ions

can make t he flash lookdi fferent.

This pi cturesque cloud-t o-ground li ght-

ning is also know n as v erti call i gh t n ing .

Worl dw ide, there are about 100 cloud-to-

groundf lashesper second. The number of

cloud-t o-ground l ashes per km * per year is

a maximum of 30 to 50, and a typi caloverland value is tw oto fi ve.

Tw o adjacent negati vely charged clouds

al so have a pot enti al di fference betw een

them as w ell as wi th t he earth. I f t h e

atm osphere betw een t he clouds breaks down

fi rst, the discharge remains overhead and i s

horizontal lightning, either intracloud or

cloud to cloud. The lightning that finds it s

w ay t o ear th t ransfers energy t hat i s

typical ly measured a t upw ard s o f

125 000 000 vol ts an d t ens to hundreds of

tho usands of amperes.

The fl ashthat you see is a single display

of lightni ng that is made up of indivi dual

strokes, typical ly tw oorthr ee; the total

electri cal di scharge from cloud to ground has

a durati on of one-thi rd t o one-half second.

A strok e-of l ightni ng is, in fact, three of

f o u r high current pul ses; each str oke la sti ng

about a mil l i second. Lightni ng may appear

to fl ickerbecause the eye is capable of j ust

resolving the individual pulses in each

stroke.

rounding, bonding, shielding, andsurge suppression are the fourcommonly applied methods toprotect from the indirect effects of

hghtning. The first step is to identify whatneeds to be protected and the paths thatlightning currents and voltages can use toreach equipment. Grounding, bonding,and shielding reduce surges by providingadditional paths for hghtning currents toflow to earth. Surge suppression is impor-tant for critical sensitive equipment andequipment connected by cables over long

distances.Protecting RF equipment investment

and revenue from lightning damagerequires proper transmission hne ground-ing. An unprotected transmission line is aconduit to radio equipment for lightningtransients. Installing both grounding kitsand surge arrestors will help manage light-ning effectively to safeguard equipment.

GROUNGING KITS

The first lme of defense against th edamaging effects of lightning is thegrounding kit that i s attached to transmis-sion hnes at various points along the cablesupport structure (Fig. 3). In essence,

groundmg funnels lightning transients offthe outer conductor. Each transmissionline, at minimum, should be grounded atthe top and bottom of the vertical run andat the entrance to the equipment shelter.For longer vertical runs, additionalgrounding kits should be installed at 60-m(200-ft) intervals.

DURABLE DESIGN

Ground strap connections mustwithstand extreme outdoor conditionsand still maintain solid mechanicalconnections to maximize protection. Thekey is to maintain low electrical contactresistance. A well designed grounding kit

should conform to mdustry standards suchas IEC 1024-1, Protection of Structures

Against Lightning, a n d MIL-STD-188-124A, Military Standard forGrounding, Bonding and Shielding.Some specific requirements of thesestandards include:

SOUTHERN AFRICAN WIRELESS COMMUNICATIONS AUGUST/SEPTEMBER 1997


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Fig 4: The map represents

lightning flash detection during a

month of the most intense storm

activity in South Africa.The map

information comes from the

Optical Transient Detector

(OTD), which detects, locates,

and measures the intensity of

lightning flashes, cloud-to-ground,

cloud-to-cloud, and intracloud,

around the globe.OTD was

designed and built at the U.S.

National Aeronautics and Space

Admin istra tion s Marshall SpaceFlight Centre. This space-based

sensoris capable of detecting and

locating lightningevents both day

and night.

Orbits 424Areas 17444

Flashes 105994Groups 534872Events 1079825

(Created : 06/02/97)

I 1 2 3 4 5 >5 >lO >l5 >25 >50 >

Flash scale

1997 March

A 16mm2

cross sectional areafor copper bonding conductorsGalvanically compatible bondsCleaned bonding surfacesResistance of less than1 milliohm per bondEncapsulated bonded surfaces

The requirements for encapsulatedbonds is critical to both kit design andinstallation. Moisture infiltration will spe-edcorrosion, increase resistance, and lessenthe effectiveness of the grounding kit. Thepoint mos t vulnera ble to corrosion iswhere the grounding kit is attached to thetransmission line. A good grounding kit

will supply adequate sealing materials,such as butyl rubber and plastic tapes, toseal this connection. Grounding kits withbraided wire conductors are likely to have

Fig 2: Distributed groundcurrents. The voltage drop,measured between two grounded points (left),results

from current through the resistance posed by interven-ing earth. The magnetic field encircling the current

path (right) exists in and extends above the earth andinduces voltage in any conductor that cuts the magnetic

field.The field will cut both conductors of a pairequally in magnitude and phase.This longitudinal effect causes ground current rises inoverhead and buried power lines.

corrosion pro-blems as thec a p i l l a r yaction of theconductor wi-cks moistureinto the con-nection.

according to manufacturers specifica-tions. Overtighte-ning must be avoided toprevent cable damage that may result ingreater system return loss. Good ground-ing kit design uses calibrated straps, expan-sion joints, or spring clips to help ensureproper tightness. The grounding kit mustbe sized for the cable being installed.

When attaching to the tower or around

INSTALLATION bar the down conductor should-be as

Proper installation of grounding kits is short as possible and oriented as straightessential to establish a reliable, low imped- downward as possible. Long and/or

ance path for lightning current. After curved leads increase impedance and

establishing the ground kit location, a reduce grounding kit effectiveness.

small section of cable jacketing must be As mentioned, grounding kit connec-removed to accept the cable attachment tions must be kept dry to prevent corro-

strap. In wet conditions, the exposed outer sion which reduces effectiveness.

conductor should be dried immed iately Weather-proofing materials should be part

before attaching the strap. When attaching of each grounding kit and must be

the strap, all hardware must be tightened properly applied. Butyl rubber and plastic


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NASA /MSFC

tapes have been used for many years andhave been proven effective in thousands ofinstallations. Recently, rubber boots andplastic shells have been introduced onsome manufacturers grounding kits toprovide this essential weather protection.These kits will add a measure of conve-nience for the installer.

The final requirement in the groundkit installation is attachment of the termi-

nation lug. A small section of jacketingmust be carefully removed from thecopper conductor to avoid nicking, thenthe lug must be screwed or crimped inplace. Crimp lugs should be sealed to theconductor with heat shrink tubing. Theground wire terminal can be attached to atower member, bus bar, angle adapterattached to a tower member, or towerdownconductor. The use of hose clampsto attach ground terminals is not recom-mended. Aviation colour paint, but notthe zinc plating underneath it on thetower, or oxidation fi-om the surface ofthe bus bar should be removed in the areawhere the termination lug will be bolted.A layer of conductive grease applied tothis area will ensure good electricalcontact when the lug is bolted mto place.

SURGE ARRESTORS

Sensitive microwave equipment oftenrequires an additional measure of protec-tion against lightning transients. In thesecases, surge arrestors (Fig. 5) are installedto remove transient current that may beinduced onto the inner conductor ofcoaxial cable before it reaches the equip-

ment.Quarterwave shorting stub (QWS) and

gas tube (spark gap) surge technologieshave been proven very effective in RFapplications. Each has unique characteris-tics that lend themselves to certain appli-cations better than others. The propersurge arrestor for a given application addsthe final level of lightning protection to atransmission line system.

The QWS surge arrestor operatessimilar to a bandpass filter, where a tunedstub shorts the centre conductor of the

device to its body, allowing only a speci-fied frequency range to pass. This shortingstub is effectively one-quarter wavelengthin size at the center of the desiredfrequency band. When a lightning surgewith frequencies lower than 160 kHzenters a QWS arrestor, it sees the quarter-wave stub as a direct path to the outerconductor and to earth.

Since QWS arrestors are frequencyspecific, they exhibit low VSWR and lowinsertion loss, but they must be matchedto the systems frequency of operation.They react instantly to a lightning surgebut allow a small level of energy propor-tional to the magnitude of the lightning

strike to pass (throughput energy). Theheavy duty construction of the shortingstub provides high operating powerhandling capability and is known for itsability to survive multiple lightningstrokes, making it ideal for high powerand/or lightning prone sites. The QWSarrestor, however, cannot be used inapplications that require a dc bias.

Gas tube surge arrestors are anotherchoice for divertmg lightning current offthe centre conductor of a coaxial cable.This technology differs in that it allowspassage of a wide frequency band,typically 0-2500 MHz, makmg it idealfor applications that use tower top devices

that require a dc bias fed through thecenter conductor of the coax.

Where the QWS arrestor is frequencysensitive, the gas tube arrestor is voltagesensitive. The gas tube device appears asopen until a specified voltage threshold isreached at which time it changes to a lowimpedance surge arresting state. Again, asmall amount of throughput energy willpass as a finite amount of time is requiredfor the fast rising current of a lightningsurge to activate the gas tube.

Because the gas tube arrestor is voltagesensitive, average operating power is influ-enced by gas tube selection. It is impor-tant to follow manufacturer recommen-dations on gas tube selection to achievethe proper power rating for a givensystem.

Gas tube surge arrestors are typicallyrated to protect against lightning impulsesup to 20 kA. Since there is no way ofknowing whether or not a gas tube maybe compromised a high level light-ning surge, periodic maintenance isrequired to replace surge arrestors or thegas tubes (arrestors with replaceable gastubes) at locations of a suspect lightningstrike.

Surge arrestors are available withcommon interfaces such as 7-16 DIN orType N for easy integration into anytransmission line system and should beinstalled on all transmission lines as closeas possible to the equipment they areintended to protect. Systems that usetower top-mounted amplifiers shouldhave a surge arrestor installed at the topand bottom of the coax run.

Prior to installation, all groundingsurfaces should be cleaned and free ofoxidation. Most surge arrestors ground

through their body. Some use a keyedbulkhead, while others depend on aflange or stud that must be fastened withprovided hardware to the sites groundingsystem. A popular practice is to fasten thegrounding mechanism on all surgearrestors to a single copper bulkhead ormultiple copper bus bars, both of whichare bonded to a master bus bar and earthground. Proper torque must be applied tothe hardware on the arrestor-to-ground-ing bar connection to ensure a low resis-tance connection. A special washer isoften included with bulkhead mountingarrestors to help achieve this. A newwasher should be installed whenever the

arrestor is reinstalled after removal forsystem maintenance.

PERFORMANCE TESTING

Surge arrestors can be readily tested formicrowave performance by using anynetwork analyzer. For QWS arrestors,VSWR (return loss) and insertion loss arespecified for the operating bandwidthonly. The shorting stub characteristics onsystems with QWS arrestors require thatfault location testing be performed byusing a vector network analyzer with timedomain capability. Some test equipmentmanufacturers refer to this type of fault

location testing as Frequency DomainReflectometry (FDR)

The lightning management practicesdescribed above for RF transmission linesare just a part of a complete site ground-

Fig3: The grounding kitprovides electrical contactbetween the outer conductor of the cable and a towermember or separate conductor to provide lightningcurrent a low impedance to earth.


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mg plan. Towers, guy anchors, fences,telco and ac power lures, and other equip-ment within the site must also be tied intoan earth grounding system. Manymanufacturers of radio equipment outlinespecific requirements on the use of buriedconductors, placement of ground rod,and bonding of connections. Followingtheir requirements to achieve a low resis-tance earth grounding system is vital tothe overall effectiveness of lightningmanagement.

CONCLUSION

Lightningwill strike twice, and it willfrequently choose communications sitesbecause of geographic location and towerheight. A single regional system in SouthAfrica may experience over 100 strikesper year. Consequently, the need forprotecting expensive radio equipmentagainst the destructive power of lightningis crucial. Through good groundingpractices, this threat can be managedeffectively. Properly grounded RF trans-mission li nes combined with surgearrestors tied into a well designed earthgrounding system will help ensure thatlightning goes to ground before it goesthrough RF equipment.

References:

FreemanRoger L ., Reference Manual

forTelecommunications Engineering,

second edition .

New York: JohnWiley&Sons, 1994.

Guthrie, A.K., " Learningto Live

With Lightning, Communications,

May 1977.

GlobalH y d ro lo g y andClimateCenter,

National Aeronautics and Space

Administration Marshall Space Flight

Center,July1997.

Uman, Marti n A., Natural

Lightning,IEEE Tr ansactions on

Industry Applications, Vol 30, 1994.

About the Authors:

JonasAleska isSeniorEngineer and

ChristopherStockmanis product line

managerforH E L I A X Accessories at

Andrew Corporation headquartersin

Orland Park, Illi nois, USA. Formove

inf ormati on on Andrew products in

South Africa, contact AndrewSatcom

Afr ica, P.O. Box 786117, Sandton

2146, South A fri ca, on:

(Tel)+27 (0) 11-444-5041.

Surge Protectionand GroundingAndrew Corporation, represented hereby Andrew Satcom in Sandton, designsand manufactures lightning surge protec-tion for PCS, cellular, and wireless

communications base stations.G round ing and surge arres tor

products offered by the company deliverhigh levels of protection and optimisesystem RF performance. SureGroundgrounding kits, for example, protectagainst lightning strikes as powerful as

125 kA. Easy to install, no special toolsare required as the kits pre-tensionedsolid copper chp snaps around the cableand ensures proper surface contactpres sure . SureGround kits include aheavy duty 16mm2 ground lead toreduce dc resistance and a weatherproof-ing kit to ensure long service life.

Arrestor Plus surge arrestors are used

in conjunction w i t h SureGroundgrounding kits to eliminate lightningbefore it can damage sensitive radioequipment. Arrestor Plus surge protec-tors are said to provide dependableprotection against lightning, lowVS WR, low inser t ion loss , and

complete weather and vibration resis-tance.

Many models are available withstandard 7-16 DIN and Type N inter-faces to simplify system planning. The T-Series Arrestor Plus surge arrestors offercompact size and installation versatilitywith bulkhead or 3/8 hardware mount-ing options. The Integrated Series

arrestors attach directly to Heliax coaxialcable, eliminating the need for a separateconnector. The Gas Tube Series featuredc pass capabilities, making these idealfor applications that use coaxial cable fordc power.

Contact:+27 (0)11 444 5041 .

Andrew Corporation10500 W. 153rd Street

Orland Park, IL 60462 USA

Contact us in North America at: Contact us from Anywhere at:Tel: 1-800-255-l 479 Tel: 708-873-2307

Fax: l-800-349-5444 Fax: 708-349-5444

Fax-on-Demand: l-800-861 -1700 Fax-on-Demand: 708-873-3614

Internet: http:// www.andrew.com

Bulletin 10217 (2/98)

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Antennas Lightining Protection