Lightning Loops?

[RolandSWL]

 Ameritron suggests looping the coax before it enters their remote coax switch. This is supposed to reduce damage from a lightning strike/near miss.
 What is the theory behind this?

Thanks, Roland...............

[kb3ouk]

I don't know about that, but I've always heard to loop the coax like that to form a sort of common mode choke.

[W7TFO]

Even better if you loop it thru a toroid core or two as Nautel recommends.

73DG

[KA2DZT]

A loop creates inductance just like a coil.  Lightning doesn't like inductance.  But, don't bet the farm that a loop in your coax in going to protect you from lightning strikes.

Always disconnect your antenna cables from your rigs when not in use.  The disconnected cables should then be grounded.

Fred

[Jim, W5JO]

Also don't make the loops the wrong size.

[k4kyv]

Here is an interesting tutorial on feeding base-fed monopoles, beginning at page 10.  The writer claims that lightning loops are unnecessary (p.14).

http://www.nxtbook.com/nxtbooks/newbay/rwee_20120613/#/10

When Chris came down last week with his impedance bridge and we measured my antenna impedances, we found that adding even a few inches of wire to the horizontal lead  running from the matching network/measuring instrument to the base of the tower at the insulator made a noticeable change to the impedance reading.

[WA1GFZ]

good chance a toroid will explode if placed as a common mode choke on coax.
Lightning currents will saturate it very quickly. Air core choke is better, but a good long duration strike will go right through the choke. Best thing to do is have a good ground on the shield before it goes into the building

[Bill, KD0HG]

Lightning is NOT DC, but a wave form at about 1 Mhz. It's RF, not DC.

Being that, a bit of inductance in series to the antenna is useful for keeping lightning out of the shack.

Almost every AM broadcast station feeds its tower(s) with a lightning loop from the antenna tuning units, usually 2 or three turns of 1/2" copper pipe in a 2 or 3 foot diameter loop. Or you can use toroid cores, but they tend to get hot and unreliable with lots of RF flowing through them. And they will likely explode during a direct lightning hit.

A ham station could use several large turns of coax, taped together for the same effect. Just a few turns maybe a foot in diameter. Smaller than that, you lose usefulness as the stroke will just arc right across the inductor.

Bill

[KA3EKH]

On the two 5 kW AM broadcast stations I take care of we have one turn loops between the base of the tower and the ATU, big coils in the ATU between the output of the ATU to ground and a spark gap between the base insulator and ground. Also have three inch steel balls between the base of the tower and the ground system. Take lightning strikes all the time and other then the transmitter cycling not much else happens. The FM transmitter sites we ground the transmission line where it connects to the antenna, where it departs the tower and before it enters the building. Three of my FM sites are the largest structures in the county so they always get struck without many issues. Every engineer I have talked with has a theory on how and what lightning will do, if it goes from cloud to ground or ground to cloud, weather it’s the difference of potential or the electro magnetic pulse that dose the damage, if a thousand little discharge points dissipate lightning before it strikes and everything else you can imagine. I just believe in lots of ground strap, and a loop or two cant hurt.

[Bill, KD0HG]

The key thing during a lightning strike is to ensure everything is at an equal potential inside the transmitter/equipment building/residence.

It doesn't matter if everything in the shack is elevated to 100,000V during a strike, you have to make sure that everything is also elevated to equal voltage.

For example, an AM or FM broadcast site. You put a ring of copper strap around the building. Mr. Faraday will say everything within that ring is all at the same voltage, therefore no harm will result.

So you put Johnny Balls at the base of the tower, separated enough so they don't arc over during normal operation. But lightning strike *will* arc across them, taking most of the energy down to earth at the base of the tower.

You put an inductor between the tower and the transmitter building. If a ham setup, a few turns of coax, about a foot in diameter, taped together. No more than that, or it will arc over during a strike.

You put a grounding ring around the house or ham shack. Heavy gauge copper braid or strap.Everything is connected to it. Transmitter, linear, receiver, audio gear. Everything within that ring is completely protected from the strike. See Mr. Faraday Cage, again.

Even better is to put the coax loop choke at the base of the tower, then bury the cable underground the rest of the way to the ham shack.

And disconnect during storms.

You do need a good ground at the base of the tower or mast. I have a 4'x4'x4' hole that my tower base is buried in concrete. The hole is lined with 6" wide copper strap (flashing copper) that the tower is bonded to with #4 copper wire.

You need to go *around* the concrete with the grounding strap. You do *not* want the bolt to travel; through the concrete, or it could explode.

In spite of a number of strikes, I have never had a single problem with lightning strikes. And here in Colorado, it's worse than anywhere else, except for Florida.

Bill

[W7TFO]

I've mentioned this on the forum several times before.

Lightning hits are really hard to withstand.  In this analogy, you have two choices:

Say you have a really mean bully in your neighborhood.  He is coming to beat you up.

You:

A...Suit up with padding and such to hope to avoid any broken bones or lacerations, and let him have at you .

B...Direct him somewhere else, and he leaves you alone . 

I have installed static dissipators on many buildings and towers, all of which sustained damage from previous hits.  These items are relatively inexpensive and durable.

After installation, not one of the protected sites has been hit.

http://www.nottltd.com/index.html

I don't have any pecuniary interest in Nott Ltd, but I sure do believe in their products' ability to prevent lightning strikes and the resultant damage. 

I always leave any grounding systems in place, they can't hurt.

73DG

[WA1GFZ]

lightning is a complex waveform. Take a look at RTCA DO160 ane check out waveform 5A 40usX120us. Pretty low frequency component and lots of energy.
Common mode chokes only help in low level high frequency induced strokes like waveform 2 or 3 where there isn't a lot of energy. you are dreaming if you think a bead will do anything for high energy. The only thing you can do it try to divert the energy to ground so no voltage is sent back into the building. E=IR so do the math a good lightning hit can be 200,000 amps. Say you are .01 ohms to ground your coax shield is sitting at 2000 volts during the stroke.

[KA3EKH]

Yea, I kind of buy into the grand elevation thing on how everything gets elevated when you take a strike and as long as everything is tied together real well it all goes up and down together with no damage, kind of figure that why sometimes have issues with phone lines and phone couplers burning out on the remote system being the phone company is at a different potential from the site. The other question is the EMP thing, the idea that when lightning strikes the tower and travels down the tower and lines to ground it produces a huge magnetic pulse and that pulse is what's doing the damage. Sounds like a load of crap but I do know from first hand experience at television transmitter sites back in the days of analog tube displays that you would come out to the site and have to de-gauze  all the color monitors after a storm because they would get magnetized  and know that once I left a laptop connected to a service port on a transmitter with a cheap non shielded serial cable  and after a storm came back to find both the serial port on the laptop and in the transmitter whacked  so want to think that was a EMP thing. Has nothing to do with the subject but always thought the whole nuclear bomb EMP threat was a load crap also being that at radio and television sites we have been experiencing huge EMP events for years and those worlds never came to an end so don’t buy into the whole thing about nuclear EMP destroying everything for miles and miles. Don’t remember much that’s useful from physics but recall something about magnetic fields decrees by the square of their strength so it would take some huge fields like the local field generated by close proximity to a lightning strike to do any damage and the strongest fields can be stopped by the smallest amount of shielding. But what do I know, not even cretin on how to pronounce the word nuclear. Torks off the liberals I work with when I use the Bush pronunciation.

[W4NEQ]

The premise of adding cable or feed impedance to sensitive equipment attached to a tower is that lightning energy has alternate paths to ground which are less damaging, like a grounded tower, and / or close-spaced spark gaps.   More current will flow through lower-impedance paths.

Even a shunt-fed folded unipole (grounded tower) needs a feed spark-gap due to induced current.

There is much anecdotal evidence of success using toroidal common mode chokes on AM tower feeds - which are connected 24/7 to a solid state transmitter.

Chris

[W7TFO]

The ferrites shown in the previous photo by me are supplied by Nautel, and they have detailed instructions provided for installing them.

Not only are they used on the output coax, but every cable in or out gets them.

Running one of their new transmitters without them will make any warranty very hard to secure in the event of lightning damage...

73DG

[W4NEQ]

More anecdotal evidence:

In a former life where I had a fleet of tower sites fed with rural power, lightning energy entering via AC mains initially caused much havoc - several fails per site per summer.  I modified the aerial utility AC mains feed with a three turn coil of all conductors, about 3-4 feet in diameter.  
It took some persuasive effort to get the utility to so this, but it was done at low cost.

It made a huge improvement in storm-related reliability - lighning-induced problems fell dramatically.  I also added some MOVs, but they rarely failed.  I believe the biggest improvement was the added inductance, allowing more of the current to flow through the utility's arrestors.

Chris

[N0WEK]

Direct strikes are hard to beat in the typical ham installation; but the, much more common, close strikes should be helped by some of these methods.

[W7TFO]

Put a Stati-cat or two on your tower and you won't get hit.

73DG

[KA3EKH]

Have a six fifty tower in Delaware (WDPB-TV) that we installed the whole Stati Cat song and dance ten or fifteen years ago. Still gets hit, just not as much. The tips of the nails melt on a direct strike. Not saying it don’t work, just nothing is 100%

[k4kyv]

Direct strikes are hard to beat in the typical ham installation; but the, much more common, close strikes should be helped by some of these methods.

I lost my best 0-10 amp thermocouple RF ammeter in a lightning strike. Not sure if it was direct or a near-hit, since I was away from home at the time.  The antenna feedline was disconnected at the tower and everything was grounded, but the buried coax line I was using at the time between shack and tower picked up enough induction surge to turn the meter to charcoal.  There were even carbon traces all over the front of the meter scale.  Nothing else was damaged, except for a few of the a.c. light switches in the house that were welded open or closed.  Fortunately, they are all the old buzzard type switches with ceramic body and nothing more that a piece of heavy paper serving as a front cover under the switch plate (most of those having long since rotted away with age), so all it took was removing the switch plate and a small thin screw driver and a shot of WD-40 to fully restore them to operation again. The slider-type a.c. power switch on a nuvistor rf pre-amp was also welded in position.  I replaced it, and nothing else in the pre-amp was damaged.  I still have it but don't use it any more.

[w5rkl]

I found this thread very interesting. I recently moved my shack from a front corner bedroom to a 10 x 20 shack. Power for the new shack in the
back yard is obtained from the houses' breaker, fed through 8-3/G cable fed through 3/4" schedule 40 PVC pipe 24 inches below the ground to the
shack.

The shacks' internal safety ground and neutral wires are connected together at the shacks' breaker box. A #8 solid copper wire connects
from the safety ground and neutral wires to a ground rod driven into the ground at the shacks' power entrance. The #10 solid copper wire, part
of the 8-3/G wire, is also connected to the shacks' breaker box ground then runs in the same schedule 40 3/4" PVC pipe back to the  house and
connects to the house breaker box ground. This places the shacks' ground and the house ground at the same ground potential with the shack
having a ground rod at the shacks' power entrance.

The shacks' coax runs out through a hole in the shacks' floor to a connection to the left of the shacks' front steps, I haven't finished the
installation yet. The remaining coax runs underground through 1/2" PVC pipe to an Ameritron RCS4 switch that's mounted on the back yard
chain link fence vertical support pole.

If I suspect a T'Storm will occur, one is forecast, or I leave the house for any length of time when a T'storm may occur, I disconnect the coax
at the RCS4 switch and also at the connection next to the shacks' steps. I then temporarily locate the shacks' coax on hooks, above ground on
the side of the shack.

My biggest concern is whether I have taken all the steps necessary to ensure the power grounds in the house and the shack are correct.
The other question is, I assume each tower leg (I'm installing a 30 foot self supporting tower) must be grounded and the tower ground must
be connected to the shack and house ground through underground solid copper wiring so the tower, shack, and house grounds are all on the
same ground potential. Also how many Poly-Phaser lightning devices are needed for a tower, rotor, and wire coax connections? I assume one
for each or do I need one at each end of each coax feed and rotor wire?

73s
Mike
W5RKL

[K5UJ]

Lighning can get anything.   This myth some hams nurture is that broadcast stations can take a hit and drive on.   Yes and no.  You can do the perimeter ground thing and cadweld strap and the rest of the nine yards and turn your QTH into a commercial broadcast site but you won't be immune from damage.  One of the local 1-As a few years ago took a direct hit on their tall stick and it wiped out most of the tuning house and they had to spend a nice chunk of money on a new DX50.    Some hits are worse than others (positive strikes are hell).

As a ham I just unplug and disconnect everything.  This doesn't mean all the professional methods are pointless or no good--they are worth doing if you want to do them because what they give you is a higher probability of avoiding damage, or extreme damage.  Just don't think they guarantee immunity. 

Rob

[Bill, KD0HG]

Lighning can get anything.   This myth some hams nurture is that broadcast stations can take a hit and drive on.   Yes and no.  You can do the perimeter ground thing and cadweld strap and the rest of the nine yards and turn your QTH into a commercial broadcast site but you won't be immune from damage.  One of the local 1-As a few years ago took a direct hit on their tall stick and it wiped out most of the tuning house and they had to spend a nice chunk of money on a new DX50.    Some hits are worse than others (positive strikes are hell).

As a ham I just unplug and disconnect everything.  This doesn't mean all the professional methods are pointless or no good--they are worth doing if you want to do them because what they give you is a higher probability of avoiding damage, or extreme damage.  Just don't think they guarantee immunity.  

Rob

Rob, there is a lot of truth to what you say. Bear in mind that a broadcast station with a 300-700 foot tower is guaranteed to be hit many times a year. And the majority of the time, they escape any damage. One of our AM sticks is 667' tall and a lightning counter (yes, they make one) records dozens of direct hits every year with no damage at all. The transmitter pops off the air fora few seconds due to high VSWR, then it comes back.

These days, I see a lot of neglect in AM broadcast sites. Broken or stolen ground straps, missing or corroded apart ground systems and radials, even johnny balls spaced so far apart as to be nearly worthless. As long as the place in nominally on the air, no one pays attention to these things. Until the damage is done.

Contrary to intuition, usually, the damage done to an AM BC transmitter from a strike is done via or with respect to the AC power lines, not the antenna system. Hence the advice to unplug the whole ham shack during a storm is a wise one.

Bill

[k4kyv]

The shacks' internal safety ground and neutral wires are connected together at the shacks' breaker box. A #8 solid copper wire connects from the safety ground and neutral wires to a ground rod driven into the ground at the shacks' power entrance.

I'm not sure that's a good idea. I believe there should be one and only one point in the entire system where ground and neutral are connected together, and that's at the entrance to the main panel, right at the meter.  Beyond that, the neutral should be treated exactly the same as a hot. I believe some local codes state that it is ok (but not required) to connect ground and neutral together at the entrance to a separate exterior building, but I can't see that it would serve any purpose, and could introduce ground loops, since the safety ground would be forced to carry current whenever the neutral has any potential ≠ absolute zero volts. A separate shack is electrically no different from an addition physically attached to the house, and at every locality, I'm almost positive that the code prohibits connecting ground and neutral together at any point in the house beyond the main panel.

But there should be a good heavy safety ground, complete with rod, right at the shack, and better still, a few ground radials bonded to where the rod is driven into the earth, and a heavy copper wire, #4 or #6 or larger, firmly connected between the shack ground rod to the main entrance ground rod at the meter.  In mine, I didn't run the separate safety ground wire through the same underground conduit that carries the 3-conductor power cable between the buildings, but buried the ground wire about 4" under the surface of the soil, much like a ground radial, running it in a perfectly straight line between the two rods.

I have noticed that I can temporarily short the safety ground to the neutral at the fuse box in the shack using a clip lead, and sometimes see a small but visible spark just as I make the connection. Unless everything in the shack is turned completely off, there is nearly always a small voltage difference between ground and neutral, and if something heavily loads down the line, such as an electric heater, air conditioner or the transmitter plate supply  running full strap, this potential difference can jump up to several volts.  Inevitably, if any audio equipment in the shack is running at the time, as soon as I touch the tip of the lead to the conductor, I hear a click in the speaker and the hum level goes up.

In addition, a few years ago I added an isolation transformer between the 115v power mains and the receiver/audio/speech equipment and other station peripherals, so that one side of the 115v mains voltage is not near zero with the other at full voltage, but instead use a mid-tapped choke across the output of the isolation transformer to simulate a mid-tapped secondary, with the mid-tap connected to the safety ground, which means that all the peripheral 115v equipment associated with the station is running with balanced power line voltage, each side at nominal 57.5 volts above ground. This made no detectable difference with some equipment, but in others, it audibly affected the quality of the hum and/or reduced it a few more dB.

[Bill, KD0HG]

I'm not sure that's a good idea. I believe there should be one and only one point in the entire system where ground and neutral are connected together, and that's at the entrance to the main panel, right at the meter.

That is required by the NEC.

Bill