coaxial cable lightning protectors white paper

7/29/2019 Coaxial Cable Lightning Protectors White Paper

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Coaxial Cable Lightning ProtectorsWhite Paper


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Coaxial Cable Lightning Protectors | Surge Protection Solutions for PTC

Coaxial Cable Lightning Protectors

Antenna manuacturers utilize shunt-ed dc grounded antennas as a means o impedance matching andproviding some orm o lightning protection to their customers. It has been proven that these antennasdo work and should be used as a means o diverting a portion o the direct strike energy to the tower

and its ground system. Unortunately this protection is designed to help the antenna survive and not theequipment. A direct hit, or even a near hit, can ring an antenna whether it is grounded or not since it is atuned (resonant) circuit. The ringing waveorm will contain all resonances that are present in the antennaand its coax phasing lines. This means both on requency ringing and other requencies present willbe propagating down the transmission line towards the equipment. The on requency energy will notbe attenuated by a high Q duplex lter or a 1/4 wave grounded stub being used as a protector. In bothinstances, the on requency energy will pass right through. Also, i we look at a typical dc grounded/shunt-ed antenna at the top o our 150-oot tower example, both the center conductor and shield will beat the same 243kV potential above ground at the antenna eed. Although the grounded antenna will helpprevent arc over o the transmission line, it will have a 6kA peak current traversing its length. The sameparallel tower segment will have 12kA. The shared strike current, between the tower and the coax, willcontain mostly low requency components.

The lack o high requency components is due to both the grounding o the antenna and the inductanceo the tower/coax, which acts as a lter. The antenna ringing voltages, with much higher requencies,will ride on top o these lower requencies towards the equipment. A no grounded antenna will arcover between center pin and shield, creating major high requency components that will traverse thetransmission line to the equipment.

I the coax line were let exterminated as it reaches the master ground bar, the coax could arc over thecenter conductor to shield even i a grounded antenna were used. This is due to the dierence in seriesimpedance at lightning requencies between the shield and center conductor and the additive ringingvoltage. It is important to eliminate or stop this energy rom being delivered to the equipment. Since coax

lines are rarely let unused, (especially connected to an antenna) these voltages will be converted tocurrent either by a dc continuity coaxial cable arrestor, a shunt ed cavity, or by arcing over dc blockingcapacitors inside the equipment.

Contrary to popular belie, lightning energy does not disappear in the arrestor/protector box. Simplyconnecting a protector in series with the coax line and expecting protection rom a strike is wishul thinking.Only a properly installed and grounded coax center pin protector can oer any measure o equipmentinput protection.

The Need For Protection

Skin eect is a physical phenomenon that relates to the limited penetration into a conductor o an RF

signal, according to its requency. This eect is present in coax cable, keeping the RF signal insideand any coupled outside intererence on the shields outer surace. The eect begins to all apart asthe requency is lowered and the penetration, which is a gradient, begins to mix the shields outsideintererence energy with the desired inside energy. A ground loop, which imparts 60 Hz onto a desiredsignal, is causing ac current fow between ends on the coax shield due to dissimilar ground potentials andis low enough in requency to couple energy through to the center conductor.


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With lightning, the main requency range isdc to about 1 MHz (-3dB). This is in the rangethat aects the coax transer impedance. Thethicker the shield material, the less the eect othese low-requency currents.

A test was perormed on 50 eet o LMR1200(7/8) coaxial cable typically used as a eeder.The center conductor and shield on the surgeside were shorted to simulate a shunt-edantenna. The current rom the resulting voltagedrop across two 0.001 Ohm current viewingresistors at the ar end o the cable was viewedusing an HP-54522C Oscilloscope. The coaxialeeder assembly was pulsed with a HaeelyPsurge 6.1 surge generator with PHV 30.2combinational waveorm plug-in module. Thesurge generator was set or a combinational

waveorm output o 1.2 x 50 Sec at 6kV opencircuit voltage and 8 x 20 Sec at 3kAmps shortcircuit current (in accordance with IEC 1000-4-5 and IEEE C62.41 specications). The peakoutput voltage and current indicated on theHaeely were 4300 volts and 1750 amps. (SeeFigure 1.) The resulting peak currents on theshield were 1531 Amperes positive and 688

Amperes negative. The currents on the centerconductor were 234 Amperes positive and 63

Amperes negative. Both the shield and centerconductor returned to pre-surge levels within

2 oscillations. A slight propagation delay wasnoted on the center conductors peak currentreerenced to the shield peak current

The same test was perormed on 6 eet oLMR600 (1/2) coaxial cable typically used asa jumper. The jumper assembly was pulsedwith the same combinational wave shape. TheHaeely indicated peak voltage and currentoutputs were 1020V and 2940A respectively.(See Figure 2.) The resulting current on thecoax shield was 1875 Amperes positive and 563

Amperes negative. The current on the centerconductor was 969 Amperes positive and 156

Amperes negative. Both the shield and centerconductor returned to pre-surge levels ater1 oscillation. A slight propagation delay wasnoted on the center conductors peak currentreerenced to the shield peak current.


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We should not be surprised by the aboveresults. Ater all, even the manuacturer callscoax unbalanced cable! The current rise timeat the top o a eeder coax attached to a towerwould be much aster, perhaps 1 or 2 S duringa lightning strike. The dierentials between

shield and center conductor with a asterpulse rise time would be much higher. Sincelightning requency pulses travel through boththe dierent impedances o shield and centerconductor, the larger circumerence shield willhave lower inductance, thereore a aster currentrise time than the center conductor. Since thepulses arrive through dierent impedances, adierential voltage would occur across the shieldand exterminated center conductor.

In the rst example, using a 50-oot length

o eeder coaxial cable, the positive peakdierential between the center conductor andshield currents was 1297 Amperes, and thenegative peak dierential was 625 Amperes. Iterminated to a capacitively coupled circuit (highimpedance at lightning requencies), the centerconductor voltage would quickly rise and arc through the equipment input back to shield potential. Iterminated in an inductively coupled circuit (low impedance at lightning requencies), current fow on thecenter conductor would continue through the inductive coupling loop back to shield potential. High peakcurrent fow through the input circuit could destroy the input connector, the coupling loop, and continuethrough to the next stage(s). Obviously, this pulse dierential must be equalized and prevented romentering the equipment!

A coax cable center pin protector could be considered a very ast voltage sensitive (gas tube) or requencydiscriminate (lter) switch. When a given threshold voltage is exceeded or a gas tube type protector,the protector switches the energy rom the center conductor to the shield (ground). When a lter typeprotector sees the lower lightning requencies (out o its pass band), it directs them to the shield (ground).In both cases equalization occurs between the center conductor and the shield.


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DC Continuity Arrestors Share Lightning Surge With Equipment

Lightning arrestors with dc continuity, such as an air gap, simple gas tubes, and 1/4 wave shorted stubs,cannot divert this strike voltage dierential without sharing some o it with the equipment. This sharingor dc continuity coaxial gas tube arrestors occurs during the time period between zero volts and whenthe threshold or turn-on has been achieved. Expect a short, high-voltage spike to occur at the output

beore the gas in the tube has time to ionize and become conductive (a short duration 700 to 1kV peakoccurs with a 3kA, 8/20s waveorm test pulse, and the arrestor output connected to a 50 ohm load. SeeFigure 3). This high peak voltage goes to the equipment causing arcing, degrading capacitive inputs, orcreating damaging current fow in shunted inputs.

For 1/4 wave shorted stubs, rom 2GHz and down,the inductance o the stub will still allow considerablevoltage to be presented to the equipment input.(+6Vpeak, -2Vpeak ringing or the entire test pulsewaveorm measured or a 1900MHz 1/4 wave stubwith a 3kA 8/20s test waveorm and the stub outputterminated to a 50 Ohm load. See Figure 4.) This

is due to its inherent L di/dt inductive voltage drop,along with perhaps making the on-requency antennaringing voltages greater, because o its own high Qringing. A higher peak voltage will be present i theequipment has internal capacitive coupling to thecenter conductor o the coax line. I it doesnt, (e.g.,a shunt-ed repeater duplexer) the lower requencyvoltages are immediately converted to a current.In this case, dc continuity type arrestors would berelatively useless in stopping the surge current sincethe gas tube arrestor would not turn on in time andthe 1/4 wave stub would share surge current with the

equipment.

DC Blocking Is The Answer

PolyPhasers dc blocked lter type arrestors (seeFigure 5), when tested with the same pulse in thesame conguration as described above, will typicallylet through less than 500 milli-volts peak or lessthan 10 nanoseconds!

Insert photo 53.

Surges Damage Duplexers And Isolators

Not all duplexers have shunt eeds, but those thatdo can handle some o the lower requency lightningsurge current i properly grounded. It depends onthe length o the cavity (requency band), the size othe shunt-ed loop and its rigidity. (It is really best to


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prevent the lightning energy rom ever entering the equipment building, let alone the equipment itsel.)Large magnetic elds can be generated in the duplexer that can bend the loop, de-tune the cavity, andallow even stronger magnetic elds to exist in subsequent strikes. The strike can also weld the cavityinput connectors together so the coax line cannot be removed. The on-requency antenna ringing cancreate large voltages inside the cavities and cause internal arcing. I the rst piece o equipment seen bythe incoming low-requency coax surge is an isolator, with each strike (i it survives) a gradual increase in

insertion loss will occur due to the surge currents magnetic eld re-orienting the isolators magnetic eld,and/or changing the magnets fux density.

The Best Protector

The most eective type o lightning arrestor is dc blocked. There is no center conductor continuityrom connector pin to pin. This internal capacitive coupling prevents the sharing o low-requency surgecurrent with equipment and limits the throughput energy to an amount that can be coupled only by theelectrostatic eld in the capacitor. This allows the dc blocked gas tube type Impulse Suppressor to reas the voltage reaches the turn-on threshold.

PolyPhaser has given considerable attention to the gas tube design to insure that, when transmitting, the

RF power will not keep alive the gas in the tube ater a strike. Many other protectors, even those licensedby our patent, use a type o gas tube that will not extinguish properly. The transmitted energy continuesto excite the tube which becomes a broadband noise generator and will burn up unless transmit powerceases.

Some arrestors use an internal grounding coil designed to drain any coax voltage build-up. (There wouldnot be any, i a dc grounded antenna were used.) The coil is in parallel with the gas tube and does nothelp lter higher requency components like antenna ringing, etc. This type o design uses a simple gastube and has the gas tube extinguishing problem.

An additional problem o this design is the coil, which has added insertion losses, resonances and iswound on a errite torrodial core. When a hit occurs, the coils magnetic eld orients the domains o theerrite core and degrades the inductance value o the coil, causing urther RF losses with each successivehit. (Over 90% o the strikes are o the same polarity, so the eects o repeated hits are cumulative to theerrite core.) PolyPhaser uses only air core coils where they are required. The coils carry a very smallinductance and create a low L di/dt voltage drop.

I a grounded antenna cant be used and voltage does build up, it will not get to the equipment. As theprotector reaches threshold or turn-on in a dc blocked circuit, it will go into a momentary sot turn-on asthe gas barely ionizes and bleeds the static charge to ground. This does not create noise since it will notget to the arc mode and lasts only a short time.

Have it both ways!A dc blocked r path, with isolated and protected low-voltage injector, pick-o, or

pass-through ac/dc or tower top powered devices. A series o protectors designed or receive only rom50 MHz up and power handling transmit/ receive protectors rom 800 MHz up are available. This couldbe a bias T replacement that includes r and dc protection.

Filter Type Protectors.A laser-cut spiral inductor on the surge side eectively grounds the dc and low-requency lightning components, while allowing the desired requency range to pass through a fat plateseries capacitor to the equipment side (dc blocked, low Q, wide band pass). Product requency ranges(at this writing) are rom 800 MHz to 6 GHz, with ranges and bandwidths designated by model number.


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Intermediation.Careul attention should be paid to intermodulation distortion specications on all coaxialproducts due to increased equipment densities and closer requency assignments at Cellular and PCSsites. Peak power requirements are considered to assure adequate headroom or digital modulationtechniques.

Intermodulation problems due to non-linearity have always been a problem. With increasing demand or

mobile communications, the need or greater channel capacity and more sensitive receivers has madePassive Intermodulation Distortion (PIM) more o a problem than ever. There are many causes o PIMin a communications system. One that directly aects coaxial protector products are connectors andconnections to them. The ollowing list was compiled rom several articles on PIM:

Restrict connector materials to copper and copper alloys.

Connector body plating of silver or white bronze with a minimum plating thickness of 6m m (0.0002).

Avoid use of stainless, nickel, or ferrite in signal path. Use gold center pins.

Quality machining - minimum nish of .4mm.

Properly designed interface at connection panel, and contact surfaces.

Avoid crimp connections - all connections should be soldered. Clamp and solder outer contacts for beststatic and dynamic perormance.

DIN connectors are less susceptible to Intermodulation than N connectors.

Avoid hermetic seals containing Kovar.

Lightning Surge Current Ratings. Surge current ratings on coax lightning protectors are like horsepowerratings or cars. Is more better? Some manuacturers point to a 50kA rating and say the protector will take50+ strikes at 50 kA beore ailure. Although this is interesting, you might also ask how much energy (witha 50 kA strike) does the protector let through to your equipment? The standard test or any coax protectoris a 3kA 8/20 microsecond waveorm pulse (other standard pulses are being introduced such as a 10/350or 10/1000), with the output connector terminated in to a 50 Ohm resistive load. The let through energyis calculated rom the integrated peak voltage and pulse width.

Since the purpose o any coax protector is to equalize the center conductor potential with the shieldpotential minimizing current fow through the equipment input, how much lightning current will actually beon the center conductor o your coax line? To answer this oten-asked question, we need to determinetwo things:

How much current is available?

Into how many paths will the current divide as it travels toward earth ground?

The total current available rom a direct strike is a given amount. Typically, the current will be below 65kA,a 50% occurrence strike will have 18kA, and only 10% will have more than 65kA. We will use the 65kAgure or this discussion.


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For a tower with one antenna and coax line, the amount o current delivered to the master ground bar(MGB) or bulkhead is a unction o where the line leaves the tower and the length o the run to the MGB.The higher the grounding kit is on the tower and the closer the MGB and cable entrance is to the tower,the more current will travel toward the equipment. It is all a matter o inductance. I more than one coaxline is on the tower, the inductance path between the tower and the equipment will be less (inductancesin parallel divide) and thereore, the strike current to the equipment will increase. Even though the total

strike current to all the equipment is increased, the amount on each coax line will be less (divided). Ingeneral:

The more coax lines there are, the more the current is divided and the less there will be on any givenline.

The lower the coax is grounded to the tower, the less shield current there will be on each coax line.

The lower the inductance path to ground from the MGB or bulkhead, the less shield current will enterthe building.

The farther the tower is spaced from the MGB/ building entrance, the more inductance the coax line(s)

will have, and the less current will be on the line(s). We do not recommend adding loops to increaseinductance. They can couple more energy like a transormer (depending on orientation to the tower)instead o reducing it.

The total strike current will rst be divided between tower (lowest inductance) and all coax cables. Currenton each coax will be divided between the shield and the center conductor. The shield has a much largersurace area, thereore less inductance, so the higher requency components o the strike will easilytravel on it. This means the shield will have a higher peak current with a shorter duration while the smallerand more inductive center conductor will have less current but longer duration.

Typically, the center conductor will have less than hal the total peak current. This means that whencalculated, the typical center conductor surge on a coax cable is not 40kA, not 20kA, and not even10kA. For only one coax cable and a 65kA strike (10% occurrence hit), a worst-case center conductorpeak current value would be less than 7.5kA. For a cell site with nine (9) same-size coax cables, thecenter conductor peak current would be less than 850A each! The amount o strike current on the centerconductor will have a slower rise time and lower peak current. This is important to know since 1/4 wavestubs or other dc coupled protectors, with a dc path on the center pin, will share this strike current withthe equipment input.

A throughput energy rating, in Joules with a standard wave shape, is a much better way o evaluatingthe perormance o a lightning arrestor than knowing how many tens o thousands o amps is required toblow it up!

High Frequency Ringing

Antenna and 1/4 Wave Stub. I your antenna is hit or i a strike is close to the tower, the voltage risetimes at the strike attachment point can be on the order o 20 to 50ns. This can cause antenna ringingsince the antenna is a tuned circuit. Once this happens, the ringing will propagate down the coax line ontop o the low-requency energy going toward the protector.

A 1/4 wave stub will not reduce this on-requency ringing and could increase the voltage since it too is a


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narrow band tuned circuit. In order to see this ringing, one needs a scope with a bandwidth large enoughto cover the operating requency. Many 1/4 wave stub manuacturers use 100 mega sample/ secondscopes while looking at a 900MHz device and dont show the whole picture. Observations made, usinga 4 gigasample/second scope, with a 1.1GHz bandwidth, show the ringing eects o cellular antennaswhen they are hit and have produced the same ringing eects in 1/4 wave stubs.

The PolyPhaser lter series protectors are a low Q wide band pass tuned circuit and not as likely to ringas lters with a narrow band pass high Q tuned circuit.

Secondary Effects. Since the shield also has a dc path to your equipment, the arther away the coaxialprotector is rom the equipment, the more likely it is to re-introduce the dierential on the coax line.The coupling o the shield and center pin is what caused the dierential initially. I the distance romthe protector (on your single point ground) and the input on the equipment rack exceeds approximately20 eet, another center pin protector should be mounted to the single point ground connection at theequipment rack.

coaxial cable lightning protectors white paper

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