Does a roof antenna need to be grounded?

The NEC requires that outside antennas and coaxial cables be grounded for SAFETY purposes, NOT for lightning protection nor for performance purposes.


This requirement goes back many years to the beginning of radio broadcasting in the 1920's. This provision was added to the Code in response to an increasing number of electrocutions that happened when antennas came in contact with overhead power lines and killed people who came in touch with their radios after such an event. Telephone lines have the same requirements for the same reason. Cable TV also came under the same safety standards as did satellite TV antennas.


Some people confuse the need to provide a counterpoise for a low frequency antenna with the NEC grounding requirement. The former is for improved performance, the latter is for safety.


Technically grounding any TV or satellite antenna will NOT improve performance. Grounding an antenna in accordance with published NEC requirements will NOT protect equipment from direct lightning strikes. NEC grounding may not even protect you from injury from a direct lightning strike on your TV antenna.


However NEC grounding will reduce the chance of electrocution if the antenna comes in contact with an overhead power line, which is why the NEC requires safety grounding of antennas and external wiring.


One person's opinion.


Bill

This topic is sorely laden with mis-information. It comes up regarding grounding antennas and surge protectors. Let me start by stating a "fact" up front:


Nothing, no ground, no surge protector, no nothing is going to protect your equipment from a DIRECT lightning strike. That bolt of electricity has already jumped across the best know insulator (an air gap) of a mile or two. All of your ground blocks, surge protectors, surge supressors, etc. are worthless against 25,000 volts of electricity.


That said, I asked my installer about grounding the dish and antenna. He said it was a good idea because it would allow energy from nearby strikes to bleed off without getting into your electronics.


Personally, all my stuff is grounded because my installer did it that way and I trust him. He's been installing antennas of one type or another for 35 years.


Charles

Information about grounding:


National Electrical Code - Search for "dish"
http://forums.nfpa.org:8081/necfaq/necsrch.htm


Preventing Damage Due to Ground Potential Difference
http://www.cinergy.com/surge/ttip08.htm


PSIHQ - Grounding Requirements
http://www.psihq.com/iread/strpgrnd.htm


PolyPhaser Technical Information
http://www.polyphaser.com/ppc_technical.asp


Tower Page - see N1LO's GUYED TOWER TOPIC SUMMARY
http://www.qsl.net/n1lo/tower.htm

Do I need to ground my outdoor antenna?



Offering advice to NOT ground outside antennas is irresponsible, contrary to all known laws of physics on the subject, and civil laws and codes pertaining to static charge buildup, and, down right criminal!


There are three types of grounds. In order of difficulty to achieve, they are from most to least: static; RF; and electrical. The methods and practices used to achieve each of these is different. Static grounding is the most difficult to do with guaranteed results, while electrical grounding is easy and can be considered to work perfectly in most cases. RF and electrical grounding is not important nor considered in this discussion for static discharge (lightning) safety.


The general rule on outside antennas is that they need to be grounded. There is no known government agency that recommends antennas not be grounded for safety. Indeed, the opposite is what is recommended and in most cases required by local codes. And, if you don't do this and don't care, maybe knowing that violation of these codes may be grounds for rendering your home owners insurance null and void in case of a fire caused by lightning strike.

Direct and secondary static electricity (lightning) strike has strike probability increased by a buildup of static charge at points of conductivity such as a metal mast or pole of an outdoor antenna. Static electricity is built up during a thunderstorm with wind blowing over the metal structures. This static charge builds and becomes an attractor to the opposite charge of static build up in the storm clouds. By draining off the static charge continuously, you reduce the probability of strike because the potential difference is reduced. It was shown in the studies conducted on the Empire State building that probability of a strike was a direct relation between the quantity of static buildup on conductive structures. Conductive structures with no ground path were at the highest risk while structures that were intensely grounded over several contact points were the least risk. This is because it is actually rather difficult to completely eliminate all corona point ( sharp pointed shaped geometry in the metal structure) static charge buildup even with "good" grounding on multiple contacts of the metal structure.


In an antenna you can ground the mast, the boom, the dish, the director and reflectors of the antenna by contact metal bonding to a ground wire run directly into the earth via a deep ground rod, but you cannot directly ground the driven element or active element of the antenna. All you can do is make a reasonable attempt at grounding it via a special coax grounding block to reduce static charge buildup and reduce probability of a hit. In a Log Periodic yagi most of the elements are driven so that most of the antenna structure can not be grounded directly. In this case it is wise to use a coax grounding block. This block is a device designed to bleed off high voltage spikes that reach dangerous levels that would damage your receiving equipment. They don't directly short out the center conductor to the ground because this would kill the signal but rather allow a small gap that will on a continuous basis bleed off the building static charge before it reaches dangerous damaging levels to your equipment. Using one of these grounding blocks located just before the coax feed wire enters your building is what is recommended to effectively reduce the probability of small static electricity damage to your receiver. It will NOT protect against a direct lightning hit. Both the grounding block on the coax and a direct ground wire to the mast of the antenna should be used.


What are the damage risks? In a simple static charge buildup the minor hits you will get will be silent killers. These tiny hits will be damaging to your receivers RF front end. It will most likely short out sensitive IC's and diodes in the receivers rendering them useless. In the next worse case you take a secondary hit where the direct hit struck a tree or utility pole near by. Now you may see some signs of obvious visible damage such as the house wiring in your house catching on fire or your TV set getting fried right before your eyes. This happens far less than the hidden damage hit. Finally we have the rarest type of hit which is a primary direct lightning bolt strike to your antenna and or house. In this case your antenna and house was struck with the main bolt. Usually this will cause major fire damage to your dwelling and contents. Fortunately these hits are rare except in places like the open farm lands where the house structure is the only corona point sticking up out of the flat ground for miles around and it looks like a big static attractor to the thunder cloud. In areas where you are surrounded by trees and other structures your odds of a direct hit are much reduced but you are still at risk for the secondary hit and the silent static killer.


One very important thing to know about grounding. Having it present will NOT protect you if you take a direct or even a secondary hit. What it does, is reduce the probability of getting hit in the first place and it provides much better protection against the damaged caused by the silent hits which is the most common of all static electricity damage. Many misunderstand this important point and I cannot repeat this point too often: The grounding also helps eliminate the small silent killer hits. But if you have conditions that are very severe and your grounding is less than ideal (most systems are) then when you take that direct hit, having the antenna grounded or not won't matter much at all. That little 12 Ga. copper or aluminum ground wire to your 10 ft. ground rod won't drain a 50 billion volt lightning bolt where the spark diameter may be as big as a foot across with multiple branches that encage your whole house for 2-3 seconds. Again, the idea of the grounding is to reduce the probability of getting hit in the first place and to continuously bleed off small static charges to prevent the silent killer to your equipment.


This is a good timely discussion as many of us are returning to outdoor VHF and UHF antennas this year to receive the local HDTV terrestrial broadcasts and we are beginning the Thunderstorm season throughout the country. It is important to understand the hazards of outside antenna structures, especially during a thunderstorm.

You should have seen the mess on our street when an insulator blew off the utility pole in front of our house. Seems some construction workers about a 1/4 mile away were doing something, maybe it was an illegal hookup - we dont quite know, but what's the deal, they work for the county. I woke up (hey this was a few days after September 11) to the sound of an explosion followed by what at first could just as well have been automatic weapons fire. Right.


The other things that I noticed were a whole bunch of bright flashes outside my window, about as bright in my room as the noon day sun with the curtains open, except that the curtains were closed; and I heard conspicuous beeping from the UPS that feeds my comuter hardware. OK, a power failure. Maybe a bomb? Or what? I remembered that my cousin had told me that we had been having mysterious brownouts all week and that our friendly neighborhood, bankrupt utility had been out several times trying to figure it out. So I didnt totally freak out - and I figured it would be safe to look outside.


The fire was spreading quickly through the shrubs, and was threatening nearby trees that would have easily led to the total combustion of several houses. I screamed "FIRE" loud enough to wake everyone in the house, grabbed my CORDLESS phone and called 911. I managed to tell them something or another before the line went dead, grabbed a garden hose (be VERY careful doing this, touch only the rubber part of the hose, dont get yourself wet and watch where you stand, at your own risk! You have been warned) and started soaking the trees and shrubs where the fire was threatening to spread - NOT so much where the sparks were jumping from the dancing wires on the ground and in the shrubs.


Well as it turned out, one of the power lines (obviously good for powering a number of houses and probably good for at least a thousand amps) had dropped onto the ground wires on the same pole, vaporizing them in the process, and so also taking out the cable TV distrubution point for our street. This meant that neither our house, nor any of the neighbors had a utility ground anymore - even though we were being feed a lot of voltage though the cable and phone lines. Actually I suppose maybe we were getting high voltage through the ground at one point, who knows? The fire department showed up, eventually.


Fortunately the cable TV line happens to have a ground to the utility ground where it enters the house, about two feet from the electric meter, and of course they're both bonded to a ground stake that goes several feet into the ground. Gee, who would have thought of that? The cable splitters and wiring at the service entrance exploded, and my cousins DVD player didnt work after that. Also our outdoor BigUglyDish wouldnt change polarities. The BigUglyDish apparently fried the ground wire to the polarotor, but suffered no other damage. Some of our neighbors as I understand werent as lucky.


Our friendly cable company had to replace the drop to the house, and a bunch of stuff on the poles, and we had the first clear picture on all channels in twenty years. Then we said to hell with cable, and put a DirecTV receiver in the living room+every bedroom. . Put in HDTV OTA antennas around Christmas ... and yes, EVERYTHING has a proper ground - meaning actually grounded to the proper grounds - in the ground. Without it, well this situation might have been very easily lethal for the unlucky soul pouring coffee or some such thing at the wrong moment ... even with grounded outlets, or ground fault interrupters in bathroom outlets, because it might not do you much good if your "gounded appliance" is at 120 volts, or 480, or 1320 or some such thing while you are turning the faucet, or whatever. Think about it.