Scanner Antenna Lighting Strike???

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prcguy

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I think the best advise for the original poster is to seek advise from an outside expert, as there is a lot of incorrect and misleading grounding info being tossed around here.
Whats worse, not having any lightning protection at all or cobbeling something together based on info from unknown sources and thinking you are protected?
prcguy
 

jmtyra

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prcguy said:
Lightning doesn't take the path of least resistance

That's a rathar large can of worms that I'd rathar not open by proving that statement incorrect. Mainly for respect of the OP, but also because I don't like debates. =)
 

hydrolocked

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From the little bit of his webpage i looked over, that appears to be a fairly good reference.

The idea of a single point ground it a bit... well... it's not WRONG, but as time has gone on, practices change.

For transient control, and EMI design and control, a SINGLE POTENTIAL multi point ground is more what industry shoots for (this is mostly applicable to design of EQUIPMENT, but even MIL-HDBK-419A suggests driving MULTIPLE ground rods (all bonded together, of course), and tying to that grounding system in 1 or 2 different points - I know some people who have gone to this extreme for their own homes!!)

For the purposes of MOST people, the idea of a single point ground works just fine, though.
 

prcguy

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That statement leads you to believe that if a ground wire from an antenna to a ground rod has less resistance than the coax to the radio, that all the lightning will be diverted t the ground rod. That's far from true, the lightning current will divide and dissipate between all paths to ground based on resistance, skin effect, cable routing, proximity to other conductors, etc. If you have a mediocre ground system and the strike has 75,000 amps of current, 50,000 amps could be dissipated to ground via the path you want but 25,000 amps could be diverted through your radio equipment to ground. Or at least for a few microseconds until the path is vaporized. There are also huge problems caused by just grounding the antenna and not properly bonding the entire building to the same ground. If you had a perfect ground on your antenna with multiple ground rods and massive conductors but you had 30ft of #12 wire connecting your new ground to your house ground there will be a very destructive potential between the radio equipment and the rest of the stuff in your house caused by the insufficient ground wire to the new ground rod. 30ft of # 12 wire doesn't have that much resistance, (.00187ohm/ft) but with 50,000 amps of current flowing through it there will be almost 3,000v potential introduced into your delicate home electronics. I work in a facility that has been designed to survive direct lightning hits to protect the hundred and something million dollars of equipment inside and I see what is required to get there. I also have mountain top repeater sites that were designed from the ground up for lightning and have taken direct hits over the years (as evidenced by exploded and burnt fiberglass antennas on the ground) and no damage has ever occurred to the equipment inside. I'm not trying to give advise on how to ground for lightning, I'm just pointing out some often overlooked problems and how complicated the subject is to give absolute answers on a scanner forum. Is there anyone here that is trained in designing lightning protection that will assume all liability in the event someone’s equipment or house is destroyed from bad design or advise? If not, the best advise is to get proper advise from an expert.
prcguy

jmtyra said:
That's a rathar large can of worms that I'd rathar not open by proving that statement incorrect. Mainly for respect of the OP, but also because I don't like debates. =)
 

hydrolocked

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a big part of lightning protection is also CHARGE DISSIPATION. That is the ONLY advantage of the #12 wire i mentioned. (when compared to other possibilities)

resistance, (.00187ohm/ft)

ohms per foot of wire is almost neglegable when we're looking at lightning. Sure, the DC resistance ADDS to the inductance of other wire, but you really can't compare just the DC resistance.

the DC resistance of 50' of #12 wire probably isn't all that different from 50' of 3" wide flat copper strap, but guess what? the copper strap is the BETTER (not best, mind you - the best would be unattainable by even some of the richest people we can all think of, given the price of metals nowadays!) as lightning protection for a near or direct strike because it has lower INDUCTANCE per foot, which even more greatly reduces it's "ac resistance" (reactance or IMPEDANCE).

also consider that my little #12 wire runs directly to ground. The coax runs into the house, to the radio, down the 3rd wire, to the AC service panel, THEN to the ground system, which may be all the way across the house. If done PROPERLY, the direct-to-ground wire should be SHORTER, and even neglecting complex impedance, should have less DC resistance to ground.

There are also huge problems caused by just grounding the antenna and not properly bonding the entire building to the same ground.

agreed...never said otherwise, i don't think.

I'm just pointing out some often overlooked problems and how complicated the subject is to give absolute answers on a scanner forum.

correct, this could be an entirely dedicated forum for that subject, and it STILL woudl never be completely covered. it's a big topic, and there are TOO MANY MISNOMERS out there - my biggest pet peeve are the poeple who drive a second ground rod for their station, and never bond it to the building ground.


Is there anyone here that is trained in designing lightning protection that will assume all liability in the event someone’s equipment or house is destroyed from bad design or advise? If not, the best advise is to get proper advise from an expert.

I think the biggest problem many people have is where they can find an expert. Any forum has the "This has always worked for me" guy who has installed something so unsafe, his insurance company cringes everytime a drop of rain hits the ground near his place.

I work in a facility that has been designed to survive direct lightning hits to protect the hundred and something million dollars of equipment inside and I see what is required to get there. I also have mountain top repeater sites that were designed from the ground up for lightning and have taken direct hits over the years (as evidenced by exploded and burnt fiberglass antennas on the ground) and no damage has ever occurred to the equipment inside.

I'm an EMI engineer who works quite closely with projects trying to design their equipment to handle the effects of indirect effects (see DO-160's requirements - some lightning transients are on the order of several kV and several thousand amps!) - I may not be an expert, but i don't want to see someone put something in (such as a ground rod separate from the buildign ground) that may create more problems than they're worth.

(Like i said, i'm interested in discussing - I like a little debate on occasion!)
 

blueangel-eric

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Yeah like where i used to live the house next door took a direct hit to the electric meter and burned the wall of the house while my 3 or 4 antennas went untouched and the scary thing was i just got done unplugging my coax from the radios when that happened!!!!!!!!!! Either the radios or me could've got zapped!!!

Eric Burris kc0ldt

n5usr said:
Having an antenna on your chimney is only slightly going to raise the risk of being hit. Odds are, if you get hit with the antenna there, you would have gotten hit anyway. Just a couple of weeks ago, my house - with four TALL masts / antennas well above the house - was just fine, while the neighbor about 1/2 block down - with absolutely nothing on their house - got struck and the house caught fire.

Here are a couple of lightning-related links I have bookmarked:
http://members.cox.net/pc-usa/station/ground0.htm
http://www.polyphaser.com/technical_notes.aspx (This is rather technical!)

Also, it is not necessarily required to get above the hedgerow. Just being outside, away from the noise inside the house, makes a HUGE difference. Only the very weakest of signals is going to require clearing the trees. If you can hear them somewhat inside, you'll be just fine once you get outside no matter the height.
 

prcguy

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Hydrolocked, I didn't specifically pick on you with the #12 wire comment, but there have been past descriptions of an "adequate" antenna ground using a 4ft ground rod with #12 wire and no mention of bonding that to the house electrical ground. Thats a recipe for disaster in just the shock hazard, never mind the lightning. I believe the NEC states minimum #10 copper wire for connecting an antenna to the house ground, not to exceed (about) 25ft, and any additional ground rods must bond to the house ground with at least #6 copper wire. This is what the satellite dish and TV antenna guys are taught and all that is just for basic electrical safety and does little to nothing in a direct lightning hit. Again, I can't tell someone how to ground and bypass for direct lightning hits and its dangerous to take advice from amateurs on this kind of subject.
prcguy
 

dpslusser

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Ok...so...going back and forth through the info that everyone has been giving me...my cheapest way to go would to be just to suspend an antenna in the attic. The best way to go would be outside, probably on the chimney. But, that inloves, weatherproof coax, mount, pole, tap-con screws/hammer drill, ground rod, ground wire, and bonding ground wire to the meter rod. Right?

Hmmm...well...im not looking for something super elaborate. And im kinda on a budget, and working with what I got. So maybe I should just stick with the attic antenna. What do you think?
 

Ghost4658

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A 4ft. ground rod is like trying to trick god in to thanking that it is grounded!
The rod must be 8ft in the ground.
Ask someone at your power company how long of a ground rod they use.
And #10 copper is a must. And weld the wire to the rod.
I have been grounding towers for many years.
 

Airdorn

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The way I look at it is, for one fraction of a second, a certain path to ground might be the one with least resistance, but since a lightning strike seems to persist for a period of time, that path can get saturated and it will follow the next least resistant path.

There's some technical terminology for that and I don't know what it is, but it's why lightning isn't just a perfectly straight flash to ground, but instead, a complex, forked path with all sorts of twists and turns.


Anyway, my question is this:

I built an antenna mast with some of that thick gas pipe they sell at Home Depot. I dug a hold about 3ft deep, dumped some Quick-Crete (tm) in there, and placed the thick metal pipe in it. It's sturdy as hell, but do y'all think that's a good enough ground? It's about 15ft. from my electrical ground for my house. Y'all think it needs to be bonded too?

Thanks

Airdorn
 

zz0468

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Put your antenna on the roof. Buy a book like the ARRL antenna book, and see what it recommends, and build it as closely to best practices as you can afford. It's a scanner antenna, for cripesakes. It's not cost effective to build it to public safety critical facility standards. The chances that it will increase the likelihood of a strike are very slight. But like prcguy states, if you don't ground it right, you'll have essentially no protection at all.

There's nothing you can do to protect your house from that 75,000 amp strike. You simply can't afford to put that much copper in the ground. But following a few simple, well published techniques, you can at least keep your antenna and it's ground system from causing more harm than good. Find a more authoritative source, like the one I suggested, instead of randomly choosing which post here to follow. You might pick the wrong one.
 

zz0468

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Airdorn said:
It's sturdy as hell, but do y'all think that's a good enough ground? It's about 15ft. from my electrical ground for my house. Y'all think it needs to be bonded too?

No, and yes, in that order. It's not good enough, and it should be bonded to the house ground.
 

hydrolocked

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prcguy said:
Hydrolocked, I didn't specifically pick on you with the #12 wire comment, but there have been past descriptions of an "adequate" antenna ground using a 4ft ground rod with #12 wire and no mention of bonding that to the house electrical ground. Thats a recipe for disaster in just the shock hazard, never mind the lightning. I believe the NEC states minimum #10 copper wire for connecting an antenna to the house ground, not to exceed (about) 25ft, and any additional ground rods must bond to the house ground with at least #6 copper wire.

OK, i was just looking in terms of practicality for dissipation, i don't have NEC in front of me (I really need a copy, but theyr'e so dang expensive!) - #10 it is.

I do know that all must be tied together - i usedd 2/0 because i had it available. I've improved the ground, but haven't put my antennas up... we're up to 56" of snow this year, and it's finally all going away!! :)

This is what the satellite dish and TV antenna guys are taught and all that is just for basic electrical safety and does little to nothing in a direct lightning hit.

I remember living in Chicago... we had a ground rod driven for the cable TV, and it was NEVER bonded to the building ground... (shudder) - if only i knew then what i know now :)

Again, I can't tell someone how to ground and bypass for direct lightning hits and its dangerous to take advice from amateurs on this kind of subject.
prcguy

maybe - but you can give advice (which i do more often than i should sometimes)... and besides... i said i wanted a discussion! :)
 

hydrolocked

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Ghost4658 said:
And #10 copper is a must. And weld the wire to the rod.

Isn't this about like using duct tape?

I hope you're using a mechanical clamp as well.

Granted, for the POWER (voltage and current) delivered by a lightning strike, a number 10 wire is pretty much a FUSE, but your "welding" is not going to be much better than solder - an INDIRECT EFFECTS test used 2000 amps and several kV (2 or 3, i think).
 

dpslusser

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Ok guys, im gunna take some advice and see what ARRL has in their handbook. I also have a copy of NEC (2000ish), which im also going to check for grounding rules. It seems my best bet is to mount it outside. So...i'll go the extra effort and get it done. I did ask a few times about amplifiers, how much they would help. Since I work for Radioshack, I picked up some returned asseccories and parts. I was wondering which one of the raidioshack amps is the BEST one to use. (If none of them are any good, I just wont use one then. Since I can get them pretty cheap). The links on the bottom has the amplifiers radioshack carries.

1. 30db VHF/UHF, Does not state what frequency range. Also has a box that you attach on the pole, near the Antenna, then an amplifier you plug in by the device, in the house, to the wall for power.
http://www.radioshack.com/product/i...+Signal+Amplifier&support=support&tab=summary

2. (This is the one that I have) 10db VHF/UHF, also has a "Pre-amp" labeled 50-2200MHZ, then an amp you plug in by the device, which plugs in the wall.
http://www.radioshack.com/product/i...radioshack+signal+amplifier&parentPage=search
 
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hydrolocked

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either should do well, assuming you're not attempting to listen to VHF low band (below 50 MHz) - though they may have SOME gain characteristics below 50 MHz.

The mast mount is probably your best bet if you're on the fringe area, but either might work.
 

zz0468

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I would forget the use of any type of preamp, but especially the TV grade preamps from radio shack. If you must have a preamp, there are far better options that a TV preamp.

It's all about noise figure. The preamp MUST have a lower noise figure than the receiver that follows it or it doesn't enhance the ability to receive weak signals at all. Those tv preamps are designed to go in front of tv's with horrible oise figures, and drive long lossy feedlines, so in that application, they help. But the noise figure of a scanner is frequently better than those preamps, so the preamp actually can make things worse.

Buy the best antenna and coax you can afford, and save the money on the preamp. You won't need it.
 

prcguy

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When choosing a wide band preamp for scanner use where there will be no preselection, I think having a very high 1dB compression point or IP3 is probably more important for most people than noise figure. I've only found one wide band preamp out of dozens tried that will not overload and cause Intermod with a Discone at my location and its got around +23 or +27dBm IP1 spec and the noise figure is around 3dB. You can't have very high signal handling characteristics, wide bandwidth (30-1000MHZ or more) and super low noise figure in the same amplifier, choose two options and compromise on the third. And you will pay dearly for high performance in the two that you choose. Another problem I have encountered is the preamp going into oscillation when the input match gets too crazy, that’s why its good to have a broad band antenna like the Discone in front of a wide band preamp, as opposed to say a R/S Sputnik, which will only have a good match to the preamp at a few frequencies. I've seen some noise figure specs on common RF input modules used in scanners and they seem to have a 5dB or worse noise figure, so most anything is an improvement. For narrow band, high performance applications like a UHF repeater front end we use preamps (from Angle Linear) with around .4 or .5dB noise figure but there is also a bunch of preselection to keep the preamp from getting blitzed by nearby transmitters. Every preamp has its purpose and downfalls and scanner users need the most difficult and expensive combination to work under all conditions. Some people who live in the sticks can get away with a R/S TV antenna amp but that will not work for most of us.
prcguy
 
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