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Author Topic: Station ground question.  (Read 6161 times)
N1RND
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« on: November 13, 2012, 04:50:02 PM »

Before I get into any detail,

Who DOES NOT have their station/antenna ground and service entrance ground bonded together?
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LA1BRA
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« Reply #1 on: November 13, 2012, 06:32:41 PM »

You ask a real loaded question, be perpared for an onslaught of pro's and con's.

My station in Texas is grounded at the top of the tower, at the base of the tower and at the entrance to the metal building, all of this grounding is bonded to a ring of 6awg copper around the building, the common point is the ground round where my utility drop is....

On some web sites, I have seen where Ham's have stated they have no ground and are quite happy without....

However, I don't suppose they have ever been hit by a direct strike.

I was once, my ground setup was marginal at best, however all I lost was a fiberglass antenna on top of the tower.

Now, at my location where I am today in Mexico, the only ground I have is the ground lug on on 120 volt wall outlet.....

My radio room is on the second floor of this rent house......

I am not experiencing any issues with not having a permanent station ground....however I wish I had one, just to be on the safe side, I un-plug everything when I am not on the air....

I saw a video in a high voltage safety school a few weeks ago, they had a video of a test with about 50kv into a ground rod...wow is all I can say, the ground round literally jumped out of the ground.

now imagine that hitting your antenna....the whole idea of a station ground is safety, you want the strike to return to earth by the path of least resistance, if your path is your home and or your radio equipment, be prepared to argue with you insurance company on the replacement. If you follow good engineering practice, you can shunt the strike to ground and minimize the collateral damage..Nothing short of a full blown commercial type grounding scheme will keep the strike out of your equipment. There are tons of info out there, do find the Motorola handbook on there recommendation they follow for cell tower sites.

Ground your station is the bottom line.

regards
tom
XE3 / KB5VWZ   aka LA1BRA  old Mexico today...
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N1RND
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« Reply #2 on: November 14, 2012, 06:14:23 AM »

Hi Tom
Thanks  for all  that  info.  I agree also that  you  cannot  have too much ground.  I am aware that everything requiring a ground in a dwelling should  be at the same  potential.
My problem  is the service and station grounds are at opposite ends of the house.
My station ground consists of 6 ga copper from the roof mounted vertical to a ground rod, an AlphaDelta transitrap for the coax on another ground rod, short heavy braid from equipment to another rod, and then all rods bonded together with 6 GA.
As of now this is the most I can do.  Everything gets unplugged when I'm not  on the air
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W2WDX
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Posts: 188




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« Reply #3 on: November 14, 2012, 02:34:26 PM »

Yeah ... real loaded question.

Regarding lightning protection? No such thing. It's gonna go where it must regardless. It can only be mitigated (or even problematically increased) by grounding. I had a huge (and I mean huge) knife switched I used for disconnecting my ladder-line from my station which grounded the ladder-line. I got hit by lightning and it took out the switch, the 1" copper sheet strap going to the  two ground rods 2' below it, and still it jumped across the switch and zorched the tuner. The ladder line itself turned to vapor. I last used gold binding posts and spades (corrosion resistance) on my ladder-line outside and disconnected the ladder line and tossed it away from the house into the yard when not using the station. Grounding for lightning protection? No such thing.

The real issue is a matter of the two most relevant ground types for our operational uses. DC ground and RF ground. The latter being the most important for performance, the former being more important for noise and AC line protection.

I always use a ground designed for RF, no matter what. Mainly to improve antenna performance and radiation efficiency. I also use a DC shunt of some type for static noise abatement.

There is a school of thought that says "single point grounding". But this thinking is derived from the "star type" or similar grounding schemes used in the audio world. In those circumstances a single point is best, however in the amateur application you have a need for low impedance grounding necessary for a good RF ground as well as protective grounding. Sometimes one may not be compatible to the other. Or better put, protection grounding while good for safety may not be a good path for RF and be problematic for station performance.

One should not go binding grounds willy-nilly. There should be a reason. You should measure whether there is a potential across the grounds. If there isn't one, binding may not be needed and in some cases doing so can actually create problems with RF. Sometimes ground is not always ground. You can take one ground rod, clip a test lead to it and take another ground rod and clip another lead to it and actually measure a potential difference (voltage) between the two. So by not binding the two together you may actually have a voltage floating between whatever is connected to one and whatever is connected to the other. You will have all sorts of currents flowing around in this situation. However, in some cases where you have bonded, having all this wire going all over the place may actually create impedance issues that make for a poor RF ground and actually causes the various RFI issues ground is supposed to mitigate. The point is, the situation dictates the methodology.

As far as binding, I do this if there is a substantial potential between my AC safety ground and my RF ground. But also sometimes this can lead to station ground loop issues, especially in audio circuits or conversely it can help eliminate them. I use a ground bar at the station and ground all of the hardware, mainly for AC protection, which is bound to the earth ground of my AC. I do sometimes lift the ground on the edison jacks going into the outlets if I have a ground loop issue, but ONLY if the chassis is grounded to the bar. I also had a separate grounding system for my antennas, designed with RF in mind. The two were not bonded, but there was little or no potential between the two ground points since I had very high ground conductivity in that location. There was no reason to bond them, so I could use separate grounding designs optimized for each ones application - safety vs. RF.

So what I say is ... it depends on the situation.

John, W2WDX
« Last Edit: November 14, 2012, 03:32:55 PM by W2WDX » Logged

LA9XSA
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Posts: 376




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« Reply #4 on: November 14, 2012, 03:33:40 PM »

When talking about lightning protection, it's a lot easier to protect coax than ladder line. If you have a ladder line fed antenna, consider putting the balanced tuner outside and only bringing coax (with ground kits and protectors) inside.
My problem  is the service and station grounds are at opposite ends of the house.
While the ideal situation is to have a common entry point, if you bind the grounds together with an outdoor buried ground wire it can be close to what you get with a common entry point.

The electrical code usually demands that all grounds are bonded together, so it might be illegal or against insurance terms to not do it. What you have to decide is how and where you bond RF ground with safety ground and lightning ground - when using unbalanced antennas the exact "tuning" of the ground system matters. I think the good way to do this would be to attend to safety and lightning ground first, and then tune the RF ground by adding radials, counterpoises and "artificial ground". For example, a tower may have three or four buried radials made of different lengths of heavy copper wire for lightning protection; on top of the soil there might be a large number of thin RF radials. Not only would putting on the lightning radials last mean digging up the RF radials - they could be detuned.

If you look at the Motorola grounding guideline, they indeed recommend a common entry point, but to bond the entry points together through the outdoor ground ring if there are more of them.
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WD8KNI
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« Reply #5 on: November 14, 2012, 05:30:06 PM »

Ladder line is very easy to protect.  Two spark plugs to create a spark gap and a tapped copper plate and you are in business 4/6 bucks if you buy new ones.

Not so hard, coax costs more and you can not look at the protectors to see if they are still good... Fred
 
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W2WDX
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« Reply #6 on: November 14, 2012, 07:57:23 PM »

Like I said. Grounding for lightning protection? No such thing. The lightning will always take the path of least resistance. If that means taking a route through your gear instead of an air gap on a spark plug, if that has the least resistance that's were it will go first. And most likely both paths will do. A spark gap is great for high level static discharge, although a shunt inductor (choke) to ground on each leg is a more effective static drain.

But ground for lightning protection? Do not depend on it.

The NEC does state that any antenna masting on a building be bonded directly to the electrical service ground to prevent omnipotential ground situations that can cause shock injury or fire. However, do not confuse this with RF grounding, which can be a separate thing. The NEC only concerns itself within the scope of safety in AC and ground potential and not the function of RF. Furthermore, Section 810.21(J) clarifies that the bonding requirement applies only to electrodes at the same building or structure. This means a free standing tower, a separate structure, is not required to be bonded to the house ground. This also includes a dipole strung between two trees for instance. It is also a separate structure. However any lead-in wires to the house, should they happen to be grounded in some way before entering the house (a good idea) that grounding should be bonded to the house AC ground. So if you use a gas discharge or a DC shunt system on your feedline its ground should bonded to you house AC ground for NEC compliance. An air gap discharge device (spark-plug or spark gap), which is not electrically connected to the feedline in any way does not need this bonding for NEC compliance. In fact, not only is it not a good way to bleed of static (if it makes a spark static has already built up and not been bled prior, and its noisy), it is also not NEC compliant with regard to balanced feeders.

If you want to be really compliant, you must use a DC static drain on any open wire feeder, like twin-lead, window-line and ladder-line. That means a spark gap is not to code. You must use some type of shunt effectively grounding the lead-in wire to ground at DC. The only exceptions to this are using coax where the shield is grounded, or if the antenna is directly grounded. So a dipole with ladder-line must have high impedance inductors shunting to ground on each leg of the lead-in wires to truly be compliant, according to Section 810.57 of the NEC.

But again, if you have a ground at your tower and one in your building that are at different potentials you should bond them just as good practice, and if the potential is the same it is not required by NEC.

An RF ground can be separate, like the radial system on your ground mounted vertical. Basically signal grounds for electronic circuits do not have to be bonded to the AC grounds nor required by the NEC. Electronic signal grounds are sometimes not related to or even referenced by AC ground and may be at an inherently different state. A balanced antenna system is a good example of this. A loudspeaker is another. Its also one of the reason digital grounds and analog grounds are isolated from each other in some circuits.

BTW, if you are concerned about NEC then you cannot also use anything smaller gauge than 14ga copper-clad steel wire for things like HF dipoles (12ga if over 150' span) and 14ga CC steel for ladder-line or window-line. That's in the code too.

So you can be code totally compliant, but that shouldn't stop you from thinking beyond the code. After all, it only deals with safety at DC or ULF and does not take into the account the affects of ground on circuit function and efficiency in RF.

John
« Last Edit: November 14, 2012, 09:00:31 PM by W2WDX » Logged

W2WDX
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« Reply #7 on: November 14, 2012, 09:32:48 PM »

One other point. True lightning protection is physically disconnecting your lead-in wires and moving them some distance from your shack. That's it.

A Polyphaser discharge unit (or any other similar unit) will turn into a molten mass of metal within seconds of a direct hit. Your lead-in wires, whether coax or ladder-line, will turn to vapor, that vapor then providing a path of charged particles to ground BTW for the lightning. So do not depend on devices to protect from an arc that can travel hundreds or thousands of feet across air in the first place. The potential is so astronomically high, no device will impede, deflect or stop it. (Well in all honesty, there are some but no amateur station has them nor could we afford them).

Gas discharge units are good (for a limited time BTW) for discharging near hit impulses. They are technically EMP protection, not lightning protection. However they are good for this application. Spark gaps for ladder-line should be thought of in the same way. Good impulse protection. Neither bleeds off static to ground directly though, except for the grounded shield on the coax. (Except for the I.C.E. brand devices, which do have a DC shunt in addition to a gas discharge tube.) And on balanced lines spark gaps do nothing but make noise in your receiver, especially once the static builds up enough to discharge across the gap. SNAP!!! POP!!!

I use large chokes on my ladder-line to bleed of all static and impulses on my ladder-line. If you do use them they have to be able to choke below your lowest operating frequency, but will still make a DC path to ground. I was amazed how much noise that I thought was atmospheric or man-made noise was actually static from things like wind, rain and snow and distant lightning EMP, which completely disappeared in my receiver once I began using DC shunts to ground on my ladder-line.

However, always physically remove your feed-lines a good distance from the house or shack. That's the only truly effective lightning protection from a direct hit.

I remember one circumstance of a friend who had his feed-line tied to his house at a corner, then down the side to his ground devices then into the house. He had a direct hit on his dipole which then ran down the feed-line, jumped to his aluminium siding on the corner where he had fastened the feed-line and burned a hole straight down to his foundation through his siding. It also arced the two feet from his disconnected feed-line, up to the connectors on his house and through his station inside, running across the chassis and through his station grounding bar and the AC ground. The point is get the feed-line off the house so you can remove it and move it away into the yard. A few million volts doesn't see wire insulation, ignores tiny electronic circuits like gas discharge devices and just roars toward ground which ever way it chooses.

John
« Last Edit: November 14, 2012, 10:07:09 PM by W2WDX » Logged

LA9XSA
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Posts: 376




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« Reply #8 on: November 15, 2012, 03:12:31 AM »

For commercial sites that can be operated safely during a thunderstorm, we're basically talking installing the radios themselves in a special equipment room/building with heavy duty protection, but that is also electrically separate from the shelter that the operators sit in, with fiber optic cable being the only connection, right? Even if the strike turns out to be too much for the equipment room to handle, none of the strike energy makes it into the shelter. Such an installation would be out of reach for most hams.

If you're happy to sacrifice your TS-2000, I guess you could install it in a box at the bottom of the tower with its own battery, and use your HT and skycommand to operate it remotely.

For N1RND though: Bury a ground ring of heavy copper wire around your house, bonded to the ground rods on both sides. You already said you unplug when not on the air - but I'd consider grounding the coax outdoors too. If you leave the coax "floating" and a nearby strike induces current in the vertical, it could jump to the electrical system through the wall or something?
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K1CJS
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« Reply #9 on: November 15, 2012, 04:05:27 AM »

One of the big differences between commercial sites and most ham radio shack installations is this--at commercial sites, top of the line equipment and cabling is used.  Not so at most ham stations.  Then those hams wonder why their stations are hard hit by any lightning strike nearby....

To the first question--Anybody (in the US, anyway) who does not care what the NEC states about grounding.  People like that ought not to have any sort of ham radio station--or ANY radio receiver with an external (outside the building) antenna.
« Last Edit: November 15, 2012, 04:09:12 AM by K1CJS » Logged
M6GOM
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« Reply #10 on: November 15, 2012, 05:21:57 AM »

Before I get into any detail,

Who DOES NOT have their station/antenna ground and service entrance ground bonded together?

Me. Living in the UK there is absolutely no point whatsoever.
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LA9XSA
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« Reply #11 on: November 15, 2012, 08:50:51 AM »

The Norwegian electrical standard - NEK 400 - requires all protective ground leads, metallic pipes (even non-metallic drain pipes in some cases), and coax shields to be connected to a common ground terminal.

What does BS 7430 and EN 62305 say?
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WX7G
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« Reply #12 on: November 15, 2012, 10:51:48 AM »

Hi Tom
Thanks  for all  that  info.  I agree also that  you  cannot  have too much ground.  I am aware that everything requiring a ground in a dwelling should  be at the same  potential.
My problem  is the service and station grounds are at opposite ends of the house.
My station ground consists of 6 ga copper from the roof mounted vertical to a ground rod, an AlphaDelta transitrap for the coax on another ground rod, short heavy braid from equipment to another rod, and then all rods bonded together with 6 GA.
As of now this is the most I can do.  Everything gets unplugged when I'm not  on the air

You have the worse situation one can have; the AC service ground at one end of the building and the station at the other end.

Let's take the example of a 30 kA lightning strike. You have three ground rods at the station and one at the AC service. Assuming zero impedance between grounds the current will divide with 1/4 of the 30 kA taking the path to the AC service ground. That is 7500 amps through the house AC wiring. The AC wiring (three #12 wires for GND, Line, and Neutral is about equal to a #6 wire. Add a #6 wire from the station ground to the AC service ground and the current divides equally between the house wiring and the #6 wire. Now the current is 3750 amps through the house. Note that 2/3 of this must find its way from the station ground to the Line and Neutral wires.

But you don't have the added wire around the building and that is one place where things can be improved. And additional station ground rods spaced twice their length will help. But, there is the situation where the power pole can take a lighting strike. From that direction most of the current will take the path through the house wiring to the station ground rather than the AC service ground.

My station is 10' from the AC service ground. I route the coax from the antenna ground to the AC service ground. Then it's routed to the station. There is now no lightning path through the wiring or the coax to the station then to ground. The lightning current though the house wiring problem is solved.

What is left is differential voltage induced in the coax by lightning current X the coax shield transfer impedance. At lighting frequencies the transfer impedance is about 0.01 per meter. I have 20 meter of coax giving a total transfer impedance of 0.2 ohms. A 30 kA strike, giving my antenna grounding will send about  5 kA along the coax. That will induce 5 kA X 0.2 ohms = 1 kV into the coax. That is in addition to the lightning voltage directly into the antenna. I think a direct strike will not burn down my house but the ham equipment will be destroyed. If I thought I'd ever have a lightning strike I would add Polyphasers.
« Last Edit: November 15, 2012, 10:59:27 AM by WX7G » Logged
LA9XSA
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« Reply #13 on: November 15, 2012, 04:24:35 PM »

WX7G, I still think your lightning calculations look suspiciously simplistic. They do not take into account the cone of influence of a ground rod, physical distance between rods, the area of influence around the buried ground wire itself, bends in the wire, the local ground resistance, etc.

It's easy for me to understand the rule of thumb that the ground conductor should be at least the same gauge as the sum of the other conductors (they limit the incoming current by their limited current carrying capacity, though there is the added complication of skin effect and and their continued conductivity after transition to vapor form though). But I still can't understand how you figure that a cluster of three ground rods will receive exactly 3/4 of the energy while the fourth ground rod at the other end of the house gets 1/4 of the energy no matter what the impedance between them is.

Are you still claiming that you could hang the bonding conductor on the wall and it would be as good as burying it? (I apologize if I misunderstood last time.)

Have you reconsidered the possibility for arching through walls? (As in, I think it's not a good idea to hang the coax on the exterior wall when you route it to the AC entry point. I think the coax should approach the house at a right angle to prevent flashing through the wall or inducing current in the wiring inside the house, while you said it was fine to hang the coax on the exterior wall all along the house if you want?)
« Last Edit: November 15, 2012, 04:33:22 PM by LA9XSA » Logged
WX7G
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« Reply #14 on: November 15, 2012, 05:09:12 PM »

First we must understand the simplified case before developing a more complex model. Then SPICE can be used.

When modeling lightning in SPICE I build the ground as a grid of resistors. Wires are represented as inductors with series resistance. The lightning is modeled as a current source having a defined rise/fall time and amplitude. SPICE allows us to probe points to see potential differences.

For now let's do a paper calculation to see what happens when connecting the station ground rods and the AC service ground together.

Let's say the station ground is not connected to the AC service ground. Ignore the earth resistance between the station and AC ground. A lighting strike of 30 kA occurs and has a rise time of 20 kA/us. The three ground rods at the station present 33 ohms. In a little over 1 us the ground rods are at a potential of 1 MV. We now connect a 10 meter wire between the station and AC ground. The inductance of this wire is 5 uH.

di/dt = V/L = 1MV/5us = 200kA/us.

From my previous post the AC service current was 7.5 kA. In 25 ns the current would be achieved. The 10 meter wire does allow the current divide as predicted by the simplified method.

Another check is to look at this in the frequency domain. The 1 us rise time is equivalent to 350 kHz. What is the impedance of the 5 uH wire at 350 kHz? It is 11 ohms. The 100 ohm impedance of the AC service ground dominates the current through this path and so again the simplified method yields a good answer. If the wire connecting the ground rods were long enough it would dominate and there would not be (at least not during the high di/dt portion of the lightning) the 3/1 division of current.

An exact paper model uses differential equations. I prefer to put it in SPICE and let it do the work.

Conclusion: A simple model yields numbers good enough to design a lightning ground system. But to do a design we need a number to design to. All of this hinges on how much current we will allow though the house. Once that is decided the number of ground rods and their placement is simple. The potential difference between ground rods can be calculated to determine if there could be arcing from a rod/wire outside the house to a wire inside the house.







« Last Edit: November 15, 2012, 05:12:58 PM by WX7G » Logged
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