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Coax Cable and Connectors For general discussion of all things pertaining to coax cable, wave guide or any other medium for passing RF energy. This includes connectors, weather proofing and grounding.

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Old 09-23-2009, 7:52 AM
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Default To ground coax or not to ground coax

I am getting ready to mount a Yagi Y8066 800 MHz antenna on my roof. This is a receive only antenna used for scanning. The antenna mast itself is grounded properly to a ground rod via a #6 copper wire. I have no questions about this portion of the ground connection.

My question is; Is there any need to ground the coax itself (LMR-400) coming from the antenna to the scanner. I always have the option of disconnecting the coax during thunder storms and when not in use. However, I am a little nervous about having that coax running directly from the antenna into my home-office.

What do the rest of you guys do ? If there is a need for some type of grounding block in the coax path ? If so, can you provide a link to a recommended grounding block ?

Here is a diagram from the National Electric Code that shows a grounding block in the coax path. This is what is making me question if a grounding block is needed.

http://www.hdtvprimer.com/ANTENNAS/grounding.gif

However, when I look for LMR-400 coax on the scanner master page I see multiple cables with an N-connector on one end and a BNC-male connector on the other end. This would lead me to believe that running a cable directly to the scanner is acceptable.

Thanks,

Dennis
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Old 09-23-2009, 9:03 AM
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Grounding the coax would not give you any more protection that you already have since you antenna mast is grounded. If lightning hits you directly, no amount of grounding will save you anyway. Just to be safe, during strong storms, just unplug the coax. Thats what I do.
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Old 09-23-2009, 9:05 AM
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At the very least, I would add a grounded lightening or surge supressor in line prefferably as the coax enteres the building.

I would also conisder 1 or more grounding kits to gorund the coax braid. Both items can be purchased from multiple sources like TESSCO-Wireless Network and Mobile Communications Products: Network Infrastructure, Mobile Devices & Accessories, Tools & Test Equipment
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Old 09-23-2009, 9:57 AM
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Dennis,

There is no pro/con argument about proper bonding and grounding of antenna masts and coax - you either follow the safety rules and regulations or you ignore them. Your choices should be limited to "HOW MUCH ADDITIONAL PROTECTION, BEYOND THE NEC REQUIREMENTS".

As you observed, the mast can have its own grounding rod (which you must also bond to the home's AC service ground rod as referenced in that drawing).

You may then add a coax arrestor at the mast ground rod. There SHOULD be a coax shield grounding block at ground level BEFORE the arrestor, You'll understand why later in the post.

The drawing that was adapted from NEC shows the fire-safety requirements, and assumes you can replace your electronic equipment after the center conductor fries them from even a near-by strike. If you want to actually protect the connected equipment, you must ALSO add coax surge protection, often called an arrestor, for the center-conductor of each coax line. Be clear on this, the only thing following NEC requirements will do is limit the chances that a strike to the antenna will burn down your home. If you want to actually protect inside equipment, there are detailed guidelines published as the NFPA 780 Standard for the Installation of Lightning Protection Systems. Those standards are not the same as NEC rules which have the force of law regarding inspected electrical systems. The 780 standards although referenced in the NEC. are ignored by perhaps 90% of residential electricians, most never even heard of them.

If you add a coax arrestor. you should know what they can and cannot do. What an arrestor cannot do, is absorb the full energy of a strike that can be carried on both parts of the coax (center conductor and outer shield). As long as the coax outer shield is properly bonded first at the mast, then to a ground rod, then bonded to the AC entrance ground, anything that the center conductor (alone) can deliver will be safely diverted to earth via an arrestor. Anywhere in the overall design that you cut corners, such as omitting a coax shield ground before the arrestor, is allowing small weaknesses that lighning could someday exploit. Once you comply with NEC requirements, how much protection you add for interior equipment is a cost/benefit decision only the individual property owner can make.

Impulse Suppresors | Arrestors

Other parts of their website have grounding accessories.

R/
Jack
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Old 09-23-2009, 11:01 AM
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If the ground rod is isolated at the antenna you should bond it to the house electrical ground, see the NEC.
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Old 09-23-2009, 11:01 AM
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Jack,

First and foremost, thank you very much for taking the time to write up that excellent response.

Secondly, My goal is not necessarily to protect my equipment. Equipment can be replaced and I do not have that much $ connected. It's only one scanner in my home-office. This scanner is very seldom left running when I am not sitting right there listening to it. It is mostly just used for background noise in my office. Really, my goal is not to burn down my house or to cause harm to myself or anyone that may possibly be near the equipment during a strike. Keep in mind, I always have the option to disconnect the coax during lightening events. However, that still leaves a (potentially) live conductor laying on my desk.

I need some clarification on one of points you made.

Quote:
Originally Posted by OceanaRadio View Post
As long as the coax outer shield is properly bonded first at the mast, then to a ground rod, then bonded to the AC entrance ground, anything that the center conductor (alone) can deliver will be safely diverted to earth via an arrestor.

Jack
I am not sure I understand your statement about the coax being properly bonded to the mast. This is basically just a 3 foot metal antenna with a female N-connector built into the end of the antenna where the coax is connected with a male N-connector. Is simply using these N-connectors "proper bonding" or am I missing some sort of grounding connector on the roof / antenna ? I do indeed have the antenna mast grounded to the AC entrance ground and to a ground rod.

Thanks,

Dennis
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Old 09-23-2009, 11:34 AM
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N4YEK, it is obvious that you haven't had much time to look into lightning protection, study grounding,
consult with you home owners insurance company, look through the NEC on grounding and had to
replace any equipment damaged by poor practices in grounding. You can withstand a direct hit
from lightning, but you have to have done your homework and put a number of things into place. I
might even say that you may not have even learned the basics of electronics from you comment.
Being a ham radio operator, you at least had to memorize a few things about radios and electronics
to get your license. Working at water treatment plants, you should have seen some sort of
protection used at those facilities.

With that said, how do you think the normal TV and radio station survive? You haven't been
watching a local TV station during a storm before. See the signal go away and come right back on.
Yup their tower took a hit. The measures they have taken, protected them from any significant
damage.

Look at your cell phone carrier. Do you think they could survive long if their towers didn't survive
taking hit? I use to design and build cell sites for a living. We took careful steps to insure that
the facilities could take a hit and continue working. There was no one thing we did that was the
HOLY GRAIL. There were a number of things like a good ground system around the site that
both the tower and shelter were connected to. The coax cables were grounded at the top of the
tower and at the bottom of the tower. There was a right angle bend in the cable before it went to
the shelter. There was additional grounding on the coax cable as it entered the shelter. Inside
the shelter, there was a surge protector on each cable that went to a common ground bar that
was connected to the ground system at the site. The power panel for the shelter had a surge
protector on it. The telco lines entering the shelter had surge protectors on each pair.

As you can see, a number of steps are taken to ensure the ability to have a chance to survive
a lightning strike. I have been in a vehicle near a tower that took a direct strike as I was watching.
The tower steel actually steamed in the rain from the heat of the high current going through it
to ground. I went inside and everything was playing just fine.

You can do as little as you feel you should or you can take the advice of a number of people
on the chat group here. This topic comes up fairly often. it's not my house or my radio
equipment, so I don't care what you do or feel.

By the way, I too am a ham and have been one since the early 60's. At least I don't go around
giving poor advice to those that are asking for help or need information.

Jim




Quote:
Originally Posted by n4yek View Post
Grounding the coax would not give you any more protection that you already have since you antenna mast is grounded. If lightning hits you directly, no amount of grounding will save you anyway. Just to be safe, during strong storms, just unplug the coax. Thats what I do.
Have a nice Day.
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Old 09-23-2009, 1:10 PM
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Dennis,

Based on your statements, the mast grounding and bonding.is satisfactory. If that's the end of your plan, be careful around the unplugged coax inside the home. Some of us stuck them inside a mason-jar at one point or another of our flawed-lives ;-)

As far as adding additional protection v. unplugging, the maximum voltage potential that properly shield-grounded coax could bring into your home is less than about 3KV, given coax that has a center conductor of less than 0.25 inches in diameter. That's enough to provide a wallop to anyone or thing it touches, or comes very close to, but has little ignition potential due to the very short duration. In most cases the briefly VERY high current will simply arc-across from the center conductor to the outer shield, and go right back outside to ground via your nearest shield-ground. A $40 coax arrestor could have handled that easily, but there is another side of the equation if you intend to leave equipment connected. Properly applied surge protection on the power supply side of the equipment is what helps prevent transient voltages coming in via utility lines from blasting through and exiting equipment on the antenna connections. Statistically, the overwhelming majority of equipment damage to residential installations comes from lightning on the power company lines. House wiring cannot even handle 3KV so you can see that protection at the AC service entrance is also an easy task for surge arrestors added right at the panel or electric meter. Surge protection on power strips at the equipment is a measure of last resort. Better to have them than not, but the maximum voltages the home's hot and neutral wires can carry FAR EXCEED what the third-wire grounds can handle in return. As I explain to many, the home's internal ground system was never designed to handle surge energy from lightning. It is there only for taking fault-current from less than 120vAC back to the circuit breaker.

When/if you start adding radios and antennas and unplugging becomes a risky chore, you have several options. Glad to hear you did the initial groundwork, and good luck.

Cheers,
Jack


Quote:
Originally Posted by dennisjamesnc View Post
Jack,

First and foremost, thank you very much for taking the time to write up that excellent response.

Secondly, My goal is not necessarily to protect my equipment. Equipment can be replaced and I do not have that much $ connected. It's only one scanner in my home-office. This scanner is very seldom left running when I am not sitting right there listening to it. It is mostly just used for background noise in my office. Really, my goal is not to burn down my house or to cause harm to myself or anyone that may possibly be near the equipment during a strike. Keep in mind, I always have the option to disconnect the coax during lightening events. However, that still leaves a (potentially) live conductor laying on my desk.

I need some clarification on one of points you made.



I am not sure I understand your statement about the coax being properly bonded to the mast. This is basically just a 3 foot metal antenna with a female N-connector built into the end of the antenna where the coax is connected with a male N-connector. Is simply using these N-connectors "proper bonding" or am I missing some sort of grounding connector on the roof / antenna ? I do indeed have the antenna mast grounded to the AC entrance ground and to a ground rod.

Thanks,

Dennis
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Old 09-23-2009, 2:12 PM
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Thanks again for sharing your years of technical knowledge and experience. You guys have convinced me to add these two pieces of equipment to my plan.

1. Coax surge suppressor / arrestor
2. Coax shield grounding block.

Now, I need some help with choosing exactly which products to buy. Again, this is a receive-only 800 MHz antenna system for my scanner. I intend to buy my cables from a standard distributor such as ScannerMaster.com with the connectors already attached to the cables as opposed to trying to make custom cables.

Let's start with the surge suppressor. Thanks to Jack's earlier recommendation, I think this is the surge suppressor I would like to go with. It appears to have Female N-connectors on both sides and a frequency range well past the 800 MHz I need.

Impulse Suppresors | Arrestors

Model: 306/N
RF Power: 1KWPEP / 500W CSS
Frequency Range: 100-1000 MHz
Connectors: N
Price: $54.00

Does this appear to be a product that will work for me ? Does anyone see where Im off-base with this selection ?

Now for the Coax shield grounding block. I need one with Female N-Connectors on both sides since that seems to be the most common type of coax jumper cable (3 feet long) I can buy.

What do you guys thing about this one ?

N-Female to N-Female Bulkhead 0-6 GHz 90V Lightning Protector - AL6-NFNFB-9

It seems to fit the bill. Simple, straight forward construction, frequency range up to 6 GHz and Female N-Connectors on both ends.

Dennis
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