Anyone had lightning strike coax and start a fire? - AVS Forum
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Old 08-09-2014, 09:53 AM - Thread Starter
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Anyone had lightning strike coax and start a fire?

I've noticed from just about all the pictures, diagrams and guides on the net that coax, plus ground wires are usually tacked directly to the roof, and then to siding. Has anyone taken a direct hit from lightning and not had a fire occur?

Back in the 90's, and early 2000's I've had two direct hits to my antenna at my old house. Both times, my siding caught on fire from the energized coax. Also the 6 gauge ground wire which ran from the gable mounted mast, following the end roof rafter had scorched the paint pretty badly.

Now, I've been considering ota once again since my new location (parents old house) doesn't have a history of lightning strikes for at least the past 40 yrs (and there were a lot of tall antenna masts in this area up til the mid 80's). What I am considering is a roof, tripod mounted 30 foot telescoping mast, but I am leery of running the ground or coax against anything that could catch on fire. Thinking about running the coax away from the mast about halfway up, gently sloping to the back of the house, and the ground wire from the bottom of the mast, which I believe should be approximately 2 feet from the peak of the roof, making no house contact en-route down to the ground rod.

The roof pitch is about 20 - 22.5 degrees, and the ground wire would run this pitch for about 15 feet before gently angling down to the ground rod. With that shallow an angle should I worry about lightning flashover? If I attach the ground wire higher up the mast, would that increase the chance the tripod feet possibly flashing over if lighting were to strike?

I thank you all in advance for your opinions!
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Old 08-09-2014, 10:58 AM
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You had some seriously bad luck. I have installed antennas professionally for about 35 years and have never seen siding scorching. I have had a slew of antennas hit by lightning, but they were mostly on hi-rise buildings where the downleads ran along and into cinder block and cement walls. When an antenna is hit by lightning, the balun, where there is one, usually gets blown apart like someone had put a fire cracker in it.

Back when downleads were nearly all 300 ohm twinleads, the the National Electric Code (NEC) required the downleads to in standoffs that held them 3" from the siding. The plastic inserts in those standoffs eventually came to be designed to hold coax as well as twin lead, and you can surely find them if you search for them. I don't remember the code ever requiring the use of standoffs for the groundwire. but I doubt you'd be in violation of the code if you used separate downlead standoffs to keep your groundwire separated from the siding.

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Old 08-09-2014, 11:52 AM
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In an OUTDOOR Balun, the Coax Ground interconnects to the Twinlead, which connects (ONLY) to the ACTIVE, UNGROUNDED Antenna Elements (e.g. Bowties, One Element in a Yagi, MANY Elements in an LPDA). And in many Antennas, the PASSIVE Elements are Grounded to the Mast (e.g. Bowtie Reflector, Reflector/Directors in Yagi). Hence Balun end of Coax typically doesn't have ANY NEARBY interconnection to Ground.

Adding an ADDITIONAL Spark Arresting "Coax Ground Block" near the base of the Mast should provide an alternative high current path to discharge most of the lightning strike into the Mast Ground.

Since you seem to be in a high risk area, you also might want to keep the Mast Ground Wire away from the building structure, running it well ABOVE the roof to a free standing metal post anchored into concrete several feet away from the house. I think that if properly installed (see NEC Guidelines) the metal post should fulfill the NEC Grounding Requirements without any additional ground stakes.

Although unlikely, if this "double coax ground" [actually triple ground, counting the one inside the housd] arrangement results in ground-loop problems (such as Hum in Audio...a problem with some CATV Coax connections), you might also need to insert a Coax Ground Isolator between the existing Coax and the Tuner (or the RF Splitter that feeds multiple Tuners)....it also provides another layer of static electricity buildup protection for your equipment:
http://www.amazon.com/s/?ie=UTF8&key...l_68f8p1g194_b

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Old 08-09-2014, 12:22 PM - Thread Starter
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Thanks for the reply, Mike. I had bad luck for sure (3 total lighting strikes), and that wasn't the end of it either. Many years after, also had a tree blow over out back, knocking down the high voltage power lines onto the coax, which caught a few houses on fire.

I know the standoff's you mentioned. I bought some from Radio Shack after the 2nd strike, but never put the antenna system back up after that. Both times, the coax burned all the way to the ground rod connection, but not after that point. Funny thing was, there was no damage to the electronics in the house, which had no grounded outlets. Later, I had purchased a ups (grounded to the two ground rods, and original water pipe ground which were all bonded together), plus a surge protector to protect my electronics, but everything connected to that was destroyed when a third lightning strike hit a half dead tree in front of my house. Glad to have moved out of that area.

My question about the shallowness of the roof angle; When I had first put up an antenna at my parents house (where I live now), I had the ground and coax lying across the roof. Our neighbor, a ham radio operator said I should never run the coax or ground at less than 45 degree's, and best to run both straight down to ground with no angles at all, but everything I see with a roof mounted tripod has less than 45 degrees, and even sometimes, right angles for a long distance over the roof peaks. Aren't these installs at extremely high risk?
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Old 08-09-2014, 02:39 PM
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Topic title made more specific.

Walking the fine line between jaw-dropping and a plain ol' yawn.
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Old 08-09-2014, 03:07 PM - Thread Starter
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Thanks, DrDon!

holl_ands, didn't see your post until now, and thanks for your input.

How high above the roofline should I go for the 6 gauge ground attachment point? What size, and type of material would you recommend for the nut/bolt?

Now as long as I have an absolutely low impedance ground, what are the chances of an arc from the tripod feet, and guy wires? (lol, I know I'm paranoid).
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Old 08-09-2014, 03:12 PM - Thread Starter
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Btw, I will have an uvsj (chassis ground screw) securing it to the mast. Will that be a bad idea?
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Old 08-09-2014, 06:45 PM
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Quote:
Originally Posted by holl_ands View Post
In an OUTDOOR Balun, the Coax Ground interconnects to the Twinlead, which connects (ONLY) to the ACTIVE, UNGROUNDED Antenna Elements (e.g. Bowties, One Element in a Yagi, MANY Elements in an LPDA). And in many Antennas, the PASSIVE Elements are Grounded to the Mast (e.g. Bowtie Reflector, Reflector/Directors in Yagi). Hence Balun end of Coax typically doesn't have ANY NEARBY interconnection to Ground.

Adding an ADDITIONAL Spark Arresting "Coax Ground Block" near the base of the Mast should provide an alternative high current path to discharge most of the lightning strike into the Mast Ground.
I'm not sure I understand your terminology. The coax downlead circuit is an unbalanced circuit and its outer conductor is grounded. Many common antennas develop a source impedance of 300 ohms and discharge or transmit the signal they develop most efficiently when connected to 300 ohm load. The device to interface the 300 ohm balanced antenna output to the 75 ohm unbalanced coax downlead is called a balun, which is short for bal(anced)-un(balanced).

The baluns that are commonly marketed as "outdoor" baluns are typically 3/4" in diameter and have two separate solid wire leads to connect to the 300 ohm antenna dipole terminals. The baluns that are commonly called indoor baluns are often 1/2" in diameter and on the input side have a short, flat, 300 ohm cable with stranded wire leads and fork lugs. Some other indoor baluns have a push-on F connector and screw terminals for the flat 300 ohm wire connection. I have observed that all of the push-on indoor connectors I tested, there was not a conductive electrical path between the input contacts and the output contacts in any of them, whereas on nearly all the Tootsie Roll Midgie, 1/2" indoor baluns I checked, there was a conductive path. I checked them over a decade ago, when I was more often called upon to jury-rig a ground isolator while working on-site.

It is my recollection that the so-called outdoor baluns I checked in fact isolated the 75 ohm coax outer connector ground from the 300 balanced input terminals, but I had no cause to seek out a larger sample of those baluns for testing at that time. If an outdoor balun consisted of a simple primary and secondary and had a solid conductive path to one side or the other of the balanced winding, I'd expect it to lose 3dB in signal strength. If there is some other internal balun construction that would enable the coax ground to connect to a center tap on the 300 ohm input winding while using capacitors to steer the high frequency signal away from that ground path, I will leave it up to the engineers and hobbyists here to so enlighten me.

I don't know what you mean by a spark arresting coax groundblock that you recommend be attached to the mast. The most common coax ground blocks only ground the coax outer conductor and as far as I know that is all the code presently requires. Back when antenna leads were commonly 300 ohm twinleads, there was a spark arrestor that clipped onto the bottom of the mast and that used serrated washers (if someone comes up with the proper term for them, I'll start using it) that penetrate the twinlead and create a path to some narrow spark gaps between the outer conductors and ground that allow static electricity to be bled off without actually sinking their electrical potential down to ground, but those were dropped from the code back in the 1980s. And unfortunately, in one code revision, I think it was 2005, they seemingly unwittingly reinserted the term anti static or static arresting device without defining it, only to specify in the next successive revision that the good ,old outer conductor ground block still satisfied the NEC downlead grounding requirement.

There is a splitter-sized thingie that combines the function of an outer conductor ground block with a center conductor discharge unit, but unless they slipped another one by me in the 2014 revision, it is permitted but not required.

Grounding of the mast supposedly makes it a less inviting target for lightning by drawing off any static charges that develop from wind blowing on the elements. The science upon which that mandate is predicated is disputed by many. Nevertheless, it just doesn't take much wire conductivity to drain off static electricity, so you can ground a mast with 8 gauge aluminum, 10 gauge copper or 17 gauge copper clad steel wire. I suspect that the only concern in mandating different gauges is to assure survivability in ordinary, non destructive exposure.

If an antenna gets hit directly by lightning. there is enough electromotive force there to split a tree, so if there is one downward path along nice, juicy 6 gauge copper wire and a somewhat less conducive path because of a tiny spark gap, regrettably, the lightning is likely going to overwhelm both.

The most important ground from a safety standpoint is the one on the outer conductor of the coax that is as near as possible to the point at which it enters the house. If a power line breaks loose in a storm and contacts your coax, that outer conductor ground will usually be adequate to draw away that harmful current until the coax melts, thereby breaking that hazardous electrical path.

As far as the 45 degree angle stuff is concerned, you are not courting disaster when you fail to adhere to it; it just means that your ground path is somewhat less optimal

Last edited by AntAltMike; 08-09-2014 at 08:51 PM.
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Old 08-09-2014, 08:13 PM
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Spark Arresting "Coax Ground Block" would be like the fol. example device, which have fairly high arc over voltages (100's of volts) so they are primarily intended to minimize damage from a direct strike...the subject of this thread:
http://www.nsccom.com/productpdfs/picomacom/grb-hrl.pdf
FWIW, I've read that breakdown voltage for air is about 30 kV/cm, which is 3 kV/mm....so don't expect a Coax Ground Block to provide hardly ANY Static Electricity Buildup protection for your expensive electronic equipment.

FYI: A quick search for "RG6 breakdown voltage" found a range of 2.7 to 3.0 kV although recommended max voltage was in ballpark of 277-300 VAC. Although commercial coax is rarely built to it, MIL-C-17/180B Mil-Spec has several max voltage specs: 2 kV max working, 2.7 kV "Corona Extinction Voltage" and some that are even higher. So I would expect that a Ground Block "SHOULD" arc over at something LESS than 2.7 kV....how much less we don't know, because Ground Block mfrs DON'T PROVIDE SPECS. [And I don't think it's in the NEC Guidelines.]

In an earlier post I described additional devices (with much lower protection voltages) that can be used in ADDITION to the NEC Required Ground Block to improve protection against static electricity buildup killing attached equipments:
The Official AVS Antenna and Related Hardware Topic!

Nearly ALL of the OUTDOOR Baluns I just NOW re-measured (C-M, Philips, RCA & Philmore....but NOT R-S Gold Indoor/Outdoor Balun from about 5-years ago) have DC continuity between the Twinlead wires and the Coax shield, which provides a path for static electricity discharge from buildup on the ACTIVE (ONLY) Element(s). And ALL of the various INDOOR Baluns I have do NOT have this continuity....Of course, YMMV....

In all of the above OUTDOOR Baluns that had DC continuity, the resistance was in the ballpark of about 5-ohms, which would be consistent with going THRU some portion of the Transformer windings. Since a conventional (Input winding to Output winding) "True Transformer" doesn't have sufficient bandwidth for UHF TV Band, most commercial Balun designs use 2 or more sets of Coils that act as different length DELAY LINES to perform the Balun functionality, mimicking what is done in Coax Baluns....or Delay Lines built into the etch pattern in PCB Baluns. Hence they inherently have DC continuity between the Input and the Output (see Section 6), which actually REQUIRES the addition of a Capacitor(s) if the designer desires to BLOCK the DC path (e.g. apparently standard for Indoor CATV applications):
http://www.datasheetarchive.com/inde...SA00365764.pdf

OTOH, a so-called "Half-Wave 4:1 Coax Balun" does NOT have DC continuity between the Active Antenna Elements and the Coax Shield:
http://n-lemma.com/calcs/dipole/balun.htm

More of my postings re Delay Line & Coax Balun Info:
http://www.digitalhome.ca/forum/show....php?p=1069670
http://www.digitalhome.ca/forum/show....php?p=1308414
http://www.digitalhome.ca/forum/show....php?p=1939738

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Old 08-09-2014, 08:40 PM - Thread Starter
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Damn, Mike!

Now I'm even more paranoid, lol, jk. At my parents old house (which is where I'm living at now), never seems to get hit, but I still want to take all the precautions I can, including a gas discharge unit for the coax entry point to the house. Hope that also reduces the risk.

Static charges on ungrounded, or poorly grounded conductive parts such as the mast, and antenna do build up very high voltages, slowly discharging due to conduction via humidity, but extremely high voltages can also discharge via a spark gap (close proximity to grounded parts). I've seen this happen in extremely dry winter weather mostly. The ungrounded parts act as capacitors, storing thousands of volts then discharging when the spark gap is overcome. Think of when the hairs on your head rise during a lightning storm (you're a human capacitor). Isolated elements on antennas act this way as well. My old area must have had soil that did not yield good ground conductivity, which made almost everything above the deep down conductive soil act as a capacitor. It's the only thing that makes sense to me.

Lightning strikes are scary stuff. Like you said, it can split a tree. The last strike that occurred before we moved, blew up the dead parts of the tree, which was about half of it. It was scattered everywhere.

Hmmm. Now I'm thinking more about moving the mast several feet or more away from the house, like Holl_ands suggested for the mast ground point. Would 10' foot of 2" rigid pipe set 4 foot in cement support my 30' Channelmaster mast w/out guy wires? I would only have an MXU59 4' down from the top, and a YA10713 about 6' below that.

Also, driving an 8' ground rod to 12' deep, then connecting 6 gauge copper wire to the bottom of the mast would make for a better ground, no? I would also bond that ground to the service ground.
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Old 08-09-2014, 08:44 PM - Thread Starter
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Heh, Holl_ands posting when I'm slowly typing again!

Yeah, my TV-2900, and CM-9444 baluns have dc shorted primary and secondaries
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Old 08-09-2014, 08:56 PM - Thread Starter
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I see a 40 or 45 foot tower on CL for $50. I'd have to take it down tho, which I would not look forward to, but it still is tempting.
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Old 08-09-2014, 09:02 PM
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Quote:
Originally Posted by Don_H View Post
Damn, Mike!

Now I'm even more paranoid...
Paranoia will destroia!

I don't ground lots of antennas, and when I do ground, I often ground to cold water pipes in non-compliance with the code, because cold water pipes make excellent grounds. They are prohibited as ground points simply because if someone repairs a cold water pipe in the future with a plastic section, that disrupts the ground.

Quote:
Would 10' foot of 2" rigid pipe set 4 foot in cement support my 30' Channelmaster mast w/out guy wires? I would only have an MXU59 4' down from the top, and a YA10713 about 6' below that.
You'd be asking for trouble because those masts are flimsy. If you have to go un-guywired, I think Channel Master makes two different gauges of telescoping mast so if they do, at least buy the heavier one.

Quote:
Also, driving an 8' ground rod to 12' deep, then connecting 6 gauge copper wire to the bottom of the mast would make for a better ground, no? I would also bond that ground to the service ground.
You have to bond it with 6 gauge wire, but I see no benefit to driving it to 12' deep. You might ask a local inspector about your local ground rod specs. Some places with poor soil require two ground rods, and some require conductivity tests for the main ground electrode conductivity, which probably does not apply to your supplementary ground rod, but if you are paranoid, you might follow the inspector's advice for enhancing conductivity, like mixing salt into the soil or something.

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Old 08-09-2014, 09:23 PM - Thread Starter
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Thanks Mike

I bought two of those masts back in the 90's, and one is still unused. Think you're right about the flimsiness unguyed, at least with the third topmost section extended.

The first one I had up, I guyed at only the top of the second 20 foot section, which didn't seem to sway at all even in 70 mph wind gusts. I only had one very large low vhf-fm-ufh combo antenna up then.

Is the 2" rigid pipe strong enough, say with a welded assembly of three five foot steel tubing arms welded to the top, extending outward to make guy wire anchor points?
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Old 08-09-2014, 10:25 PM - Thread Starter
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Ooops, forgot to answer my thinking on going 12' deep with the ground rod. The conductivity should be greater at that depth due to the higher moisture content of the soil, but it isn't always a sure thing due to different types of soil or sand at that depth. About 75 back into that yard is an asphalt road at 3' - 4' deep, and sand below that. Run into it a few times when burying deceased pets in the past.
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Old 08-09-2014, 10:41 PM
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Yeah....I've been adding some details into my above post....

One more Reference on Lightning Protection, targeted for Amateur Radio Operators:
http://www.bwcelectronics.com/articles/WP30A190.pdf

I don't recall if this idea was included in the various other references I cited....but one other idea I ran across was to consider adding several Lightning Rods on Poles (or tall trees) to reduce the likelihood of a direct hit on the Antenna. One manufacturer used multiple "spikes" on the top of the pole to presumably increase the chance of attracting a strike.

Last edited by holl_ands; 08-09-2014 at 10:48 PM.
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Old 08-09-2014, 11:19 PM - Thread Starter
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Ty for that link, Holl. I've quickly skimmed about 1/3rd of it, and I can see it's more informative than any other article I've read.

Funny that you mentioned multiple spikes on the top of the pole, or trees to attract lightning to them instead of the antenna mast. After the first strike, the electrician who came out to analyze the damage, advised me to lower my antenna down on the mast, leaving 3 feet or so at least, or install a lighting rod (I used a 5 foot long copper pipe with a ground attached to the top of the mast, running a 6 ga wire to the bottom of the mast ground point, connecting to the 6 ga ground wire at that point all the way to the ground rod) to keep the antenna itself from conducting. From what I could tell after the second strike it did save the antenna itself. I didn't have any elements missing/vaporized, nor signs of high current running thru it. The mast was damaged, along with the coax after it left the mast.

Btw, very nice work on on your antenna simulation/modeling. I've run across that in the past when searching on info regarding certain antennas for other people.
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Old 08-09-2014, 11:21 PM - Thread Starter
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damn, now we're having a serious lightning storm here
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Old 08-09-2014, 11:53 PM - Thread Starter
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From the article - "Burying the top of the ground rod improves the ground electrode performance, but requires the use of direct-bury clamps or welded connection between the ground rod and the ground lead."
It may not be to code, but thoroughly cleaning and polishing the contact area of the rod, wire, and clamp, then coating the surfaces with ox-gard (a highly conductive anti-oxidant), and sealing it all up with self amalgamating tape, then driven to at least two feet below the surface works well. Cut away the tape several years later, and it will still look as good as when you installed it. I usually clamp at two points on the rod to increase contact area.

Btw, two ground rods in parallel, vertical or horizontal may measure lower dc resistance thru earth conductivity, but if they are too close together, and in a high current discharge situation, the magnetic fields will overlap and create high impedance to ground.
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Old 08-10-2014, 12:42 PM
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Quote:
Originally Posted by Don_H View Post
Btw, two ground rods in parallel, vertical or horizontal may measure lower dc resistance thru earth conductivity, but if they are too close together, and in a high current discharge situation, the magnetic fields will overlap and create high impedance to ground.
Really??? I would expect resistance to be reduced by a factor of two...PLUS you increase the chance of at least one ground rod connection surviving a direct strike. Can you cite a credible source for this effect????
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Old 08-10-2014, 04:06 PM - Thread Starter
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Wow, to do that I would have to go back in time (mid 70's) to quote that from my neighbors one of many ham radio/antenna books, but it explained why the NEC has a minimum spacing requirement for ground rods which was 6 feet minimum last I recall, and double spacing recommended (16 feet spacing for an eight foot ground rod).

Had to do with Faradays law, magnetic field/flux density, induction, varying resistance, and currents for the rods, plus emp from the lightning. This all interacted in unpredictable ways with impedance, from which I recall, they stated the impedance could reach a level where it was like no ground at all.
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Old 08-10-2014, 04:11 PM - Thread Starter
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I just did a quick search, and while it's not what I was looking for, this article is a little more optimistic. Go down to page 40, NEC ground rules. http://ecmweb.com/site-files/ecmweb....groundrods.pdf
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Old 08-10-2014, 04:14 PM - Thread Starter
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That book I had read went into mathematical formulas (way over my head tho) to help explain how the interaction would create extremely high impedance, and what the proper relationship was for ground rod spacing.
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Old 08-10-2014, 04:31 PM - Thread Starter
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That article doesn't even account for how rods with interacting magnetic fields can oppose each other (think two magnets situated so their magnetic fields oppose each other, and repel). The rate of current flow can, and most likely will vary for each of the rods, causing an opposition (repelling magnetic field). The longer the rods, and the closer they are together, the greater the effect will become, even with bonded rods. I imagine if the rods were bonded in several spots along the length, that would eliminate the effect, but then you would have to bury it like copper strap, not drive it in.

Also coming to mind is what I have read about the proper grounding of tower legs. Driving them in at diagonal angle away from the tower increases the effectiveness of grounding, by lessening the interaction of the magnetic fields between them.

Last edited by Don_H; 08-10-2014 at 04:37 PM.
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Old 08-10-2014, 04:41 PM - Thread Starter
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Heh, also, look at the interaction of parallel transmission lines that you model. You know about the effect spacing and size have on the impedance, regardless of ac, or pulses of dc.
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Old 08-15-2014, 11:56 PM
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Have recently purchased an OTA tuner. Connected it to an existing medium size outdoor antenna that is mounted in my attic. Noticed there is no ground wire on the cable or the antenna. Should there be a ground wire someplace?
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Old 08-16-2014, 03:32 PM - Thread Starter
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Normally for an outdoor type antenna, the boom is grounded via the mounting to the grounded mast, but in an attic installation the antenna isn't likely to have high static charges build up, so no, separate grounding isn't necessary.

Your coax shield will easily drain off any minute charges in the driven element thru the shield/connector/chassis of the tuner, which should have a 3 prong power plug-in, and plugged into a properly wired 3 prong wall receptacle.

If you like, to discharge any minor accumulation of voltages, you can also ground the boom by shorting the coax shield at the end of the boom. To do this, you can use a coax ground block, or shorting a coupler to the boom.
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