Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: Station Grounding / Lighting Protection  (Read 8214 times)
KG6OJO
Member

Posts: 17




Ignore
« Reply #15 on: February 05, 2012, 10:47:07 PM »

Ok, so using a neumatic hammer, I drove one 8' ground rod at the antenna mast base, and another 8' ground rod near where the coax will enter the shack.  About 80' separate the two, so do I need to put in more every 16' or so, or just connect these two rods to the service ground coming up through the slab using #6 wire?   
Logged
K1CJS
Member

Posts: 5828




Ignore
« Reply #16 on: February 06, 2012, 06:35:15 AM »

You can put in additional ground rods if you wish to, it may improve the ground system--then again, it may not.  Connecting or 'bonding' all the ground rods you have in now is the minimum you need to do, and it has to be with at least #6 cable. 

If I were you, I would use the ground cable that is used for grounding electrical services.  It has no insulation and would make a fine continuous ground if you were to bury it--execept at the ends where you clamp it to the ground rods.  Of course, if you were to cadweld the cable to the ground rods, you can bury those connections too.

Good luck, and 73!
Logged
K9KJM
Member

Posts: 2416




Ignore
« Reply #17 on: February 06, 2012, 10:35:59 PM »

Ok, so using a neumatic hammer, I drove one 8' ground rod at the antenna mast base, and another 8' ground rod near where the coax will enter the shack.  About 80' separate the two, so do I need to put in more every 16' or so, or just connect these two rods to the service ground coming up through the slab using #6 wire?   

If this were mine, I would add another ground rod about 20 feet from each end, And then one more in the middle.   A total of three more rods.  Total of 5 rods for the entire run.
Logged
WX7G
Member

Posts: 5908




Ignore
« Reply #18 on: February 07, 2012, 08:14:11 AM »

The rule for multiple ground spacing is two times the ground rod depth.
Logged
K9KJM
Member

Posts: 2416




Ignore
« Reply #19 on: February 07, 2012, 09:12:25 PM »

The rule for multiple ground spacing is two times the ground rod depth.

That is a "rule" of thumb for "average" soil types, If there is such a thing.  The more rods the better, But when you start installing them too close to each other, You are "wasting" effectivness, So is poor economy.
Logged
K1CJS
Member

Posts: 5828




Ignore
« Reply #20 on: February 08, 2012, 05:23:14 AM »

Oh, come on now boyze--stop fighting.  There are no requirements about distance apart from each other as far as ground rods go.  As I said, the way I stated is the minimum requirement. 

And it is true that putting the ground rods (8 footers) closer than 16 feet apart is redundant, but it doesn't 'waste' effectiveness.  It simply overlaps the recommended discharge area of installation.  The only thing it actually 'wastes' is the money you would spend for the extra ground rods that you're using.
Logged
W6RMK
Member

Posts: 648




Ignore
« Reply #21 on: February 08, 2012, 08:03:19 AM »


If this were mine, I would add another ground rod about 20 feet from each end, And then one more in the middle.   A total of three more rods.  Total of 5 rods for the entire run.

What engineering basis do you have for recommending more rods?  As you point out "installing them too close... is poor economy", and I think the same would apply to number as well as spacing.

Not that more rods isn't better (it is, in general), but where is the cost/benefit tradeoff? 

Let's look at the physics.  There's three reasons to connect to the soil:
1) RF ground for an antenna where the soil is part of the radiating system
2) Electrical safety, in case a power line or other live conductor comes in contact with the antenna
3) Lightning energy dissipation

Ground rods in the middle of the bonding run do nothing for #1, because the AC resistance of the wire connecting them is sufficiently high at amateur HF frequencies that it's like hooking a big resistor between the "ground" and the antenna.

Ground rods in the middle of the bonding run do nothing for #2, because the DC/60Hz resistance of AWG6 (<0.5 ohms in 100 ft) is so low that the voltage drop along the wire is small in the event of a fault, so that all the equipment stays close to "bare feet" potential. (one of the reasons for NEC grounding)

Ground rods in the middle of a bonding run don't do much for #3, because the inductance of the 20 feet of wire to that middle rod is about 6-7 microhenries.  At the nominal 1 MHz for lightning, that's an impedance of 18 ohms, and at, say, 1 kA, that's a voltage drop of 18 kV.  Buzzing along to the next rod, the current is less, but the inductance is more... The net result is that the added rods don't add much to the effectiveness of a lightning ground.    If you're worried about reducing the impedance of your lightning ground, drive more rods near the antenna, or even better, bury some radials, or, if the antenna has a concrete base, make sure it's well connected to the grounding system.

Take home message:

The bonding rules aren't for lightning or RF.  They're for line frequency faults and shock hazard.  From that standpoint, it turns out that the bonding really doesn't need to be connected to "earth ground" very well at all. (and in fact, the NEC code makers are gradually changing the wording from grounding to bonding)

Long wires (unless buried) make terrible additions to RF grounding system.

Lightning is all about keeping the strike energy somewhere other than delicate stuff,  and making sure that all equipment goes up and down together.  The currents are so high and the rise time so fast that trying to keep the voltage "low" is impossible.

Ground rods worked great for Ben Franklin, because they replaced *nothing*.  But there's a lot of better ways to do it now, and we know a LOT more about the underlying physics and applications.

Logged
K9KJM
Member

Posts: 2416




Ignore
« Reply #22 on: February 08, 2012, 09:11:45 PM »

You are correct, Ground rods do close to nothing for RF grounding. (But that interconnecting wire DOES add to RF ground)
I agree, Just a single rod is plenty for a DC/safety ground.
However,
The extra rods DO MAKE A  BIG difference to dissipate a direct lightning strike, And are effective at least out to 75 or so feet away from the strike location.
And the original post question is about lightning protection.

The cost benefit tradeoff?   As long as you are running that ground wire anyhow, for a mere 30 bucks more you can add a lot to the potential of surviving a direct lightning strike.  

The bonding IS for good lightning protection!   In fact, Good bonding is the main part of a good lightning protection system.

Read Motorola R56 if you want to better understand lightning protection systems, And read the Polyphaser notes about installing ground rods, Along with spacing.
« Last Edit: February 08, 2012, 09:14:01 PM by K9KJM » Logged
KF4WXD
Member

Posts: 52




Ignore
« Reply #23 on: February 10, 2012, 12:04:00 PM »

I would urge you all to find a copy of the May/June 1998 edition of "Power Quality Assurance" magazine.  Perhaps a note to the publisher will yield a copy or a .pdf of the article "Principles of Lightning Protection for Telecommunications Facilities" by A. J. Surtees.  My copy is on the bookshelf in my shack next to the "Handbook" and the "operating Manual".

This article covers, in excrutiating detail, how telecom professionals and companies plan and execute lightning protection and grounding in outside plant facilities, equipment huts and central-office type buildings.  Having worked for a couple of different telephone companies, Mr. Surtees' article agrees completely with what I saw in the field as "best suggested practices".

That being said, as a telecom engineer, I am a AMATEUR radio operator and there is NO need for me to be operating when there is electricity in the air.  My station antennas and grounding are in compliance with sections 250 and 800 of the NEC and the coax is connected firmly to ground and the equipment is disconnected from power when I am not operating.  If lightnng stikes your antenna, tower or house, there will be loss, possibly catastophic.  We can only minimize it and comply with the code so that insurance investigators will have a reduced ability to deny your claim.  I have this argument with auto hobbiests as well.  Modifying your car with aftermarket wheels and tires can cause your insurer to deny a claim.

If you have doubts about implementing grounding and coax entrance, hire a pro, pull permits and do it the right way.

Russ - kf4wxd
Logged
WX7G
Member

Posts: 5908




Ignore
« Reply #24 on: February 10, 2012, 12:19:04 PM »

See these MIL documents:

MIL-STD-188-124B:       http://webbooks.net/freestuff/MILSTD188.pdf 

MIL-HDBK-419:             http://webbooks.net/freestuff/MILSTD188.pdf




Logged
W7ETA
Member

Posts: 2528




Ignore
« Reply #25 on: February 10, 2012, 03:48:27 PM »

http://www.w8ji.com/ground_systems.htm

Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!