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: [1]   Go Down
  Print  
Author Topic: Ground radials for Lightning protection  (Read 4114 times)
N5VSB
Member

Posts: 36




Ignore
« on: July 17, 2013, 12:30:08 PM »

I am putting up a small tower and want to put in some ground radials to help dissipate a lightning strike if I get that unlucky.  I also want to put in radial for a 160 meter vertical in the future.  I am planning on building some type of plow to put in the radials.  Is there a preferred depth for putting in radials?  I need to put that in the design.
Jimmy N5VSB
Logged
WB6BYU
Member

Posts: 13484




Ignore
« Reply #1 on: July 17, 2013, 12:56:24 PM »

For RF the efficiency drops as you put the wires deeper, because the signal has to
pass through more lossy dirt.  (Skin depth also becomes an issue.)

For lightning you would want bare wire buried more deeply, where ground conductivity
doesn't change as much with seasonal moisture levels.
Logged
W5DXP
Member

Posts: 3639


WWW

Ignore
« Reply #2 on: July 17, 2013, 12:58:32 PM »

I am putting up a small tower and want to put in some ground radials to help dissipate a lightning strike if I get that unlucky.

I wouldn't rely on radials alone to provide a ground protection from lightning. A properly installed anode ground rod would probably be better.
Logged
N5VSB
Member

Posts: 36




Ignore
« Reply #3 on: July 17, 2013, 01:11:57 PM »

I will have plenty of ground rods.  They are easy.  The house is thoroughly bonded.  I had a lightning problem a few years ago (not radio related) and don't want to repeat it.
Look like I will need two plows for different radials.  I will see what I can come up with in the scrap pile.
Logged
WB6BYU
Member

Posts: 13484




Ignore
« Reply #4 on: July 17, 2013, 01:42:31 PM »

Optimum depth for lightning protection is likely to be several feet down, depending on your
soil characteristics.  That will require a pretty strong tractor to pull it.

A better solution is to drive ground rods to get the depth, then bond them together at
ground level. 
Logged
KB4QAA
Member

Posts: 2450




Ignore
« Reply #5 on: July 17, 2013, 03:15:38 PM »

RF radials should be shallow, no more than a foot.  They are not intended to dissipate lightening or static buildup, but nonetheless, using bare wire would improve that.

Lightening ground can  be improved by running grounds in a line away from the antenna every 16 feet and bonding them with typical techniques.  The wire and grounding rod bond points should be at least 2 feet below surface.

  The Motorola document R56 has excellent diagrams and explanation.
Logged
N5VSB
Member

Posts: 36




Ignore
« Reply #6 on: July 17, 2013, 05:28:38 PM »

Thanks.  I downloaded it.  Looks like I have a lot of reading.
The soil here is shallow.  There is a sheet of limestone about 8 ft down.  To make it worse, the soil conductivity is poor.  I want to tie a fairly large area together.
Logged
AA4HA
Member

Posts: 1586




Ignore
« Reply #7 on: July 17, 2013, 06:17:24 PM »

Long radials, just like a really long ground rod, appears as series inductance to high transient currents (lightning). Inductance in series tends to stretch out the transient current pulse, allowing it to find more convenient paths to equipment.

You want the inductance to ground to be as low as possible and if you can introduce a little bit of it into your feedline even air coil baluns) you make it more likely that the largest proportion of current will find a place to go other than through your equipment.

I had a project in Florida where they had specified that they were going to use a single ground point, a 200 foot deep well casing. It made much more sense to put out an array of 10' long ground rods to spread the current out over a larger area.

I have some stuff around here somewhere that explains all of the math behind it but just think of it like a distributed network with values of R, L and C for every part and spacing, frequency (rise times, ringing) and currents are running all over the place.
Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KC4MOP
Member

Posts: 759




Ignore
« Reply #8 on: July 19, 2013, 03:44:31 AM »

N5VSB,
I think you are already where you need to be for lightning. You mentioned several ground rods around the QTH and Tower? It gets expensive to bond them together, but worth it. I used #8 stranded, buried a little to avoid the lawn mower.
Last August we had lightning damage from UN-grounded cable TV and Satellite cables. Knocked out anything connected to the Internet. Computers, flat screen, my Flex radio.
 
You mention plows for the radials. There are slick ways to do that and make small furrows for the radials. Again just deep enough to avoid the lawn mower. There is no formula or secrets. The radials on the ground are not resonant! Lay as many as you can, as long as you can (afford). 30-40 is enough.
Fred
Logged
K1ZJH
Member

Posts: 1150




Ignore
« Reply #9 on: July 22, 2013, 08:03:10 AM »

Long radials, just like a really long ground rod, appears as series inductance to high transient currents (lightning). Inductance in series tends to stretch out the transient current pulse, allowing it to find more convenient paths to equipment.

 

I'd agree for one long ground radial... but, when you have a large number of ground radials their combined inductance values are in parallel, essentially reducing the ESR.

Pete
Logged
K1CJS
Member

Posts: 6055




Ignore
« Reply #10 on: July 24, 2013, 05:54:34 AM »

I am putting up a small tower and want to put in some ground radials to help dissipate a lightning strike if I get that unlucky.  I also want to put in radial for a 160 meter vertical in the future.  I am planning on building some type of plow to put in the radials.  Is there a preferred depth for putting in radials?  I need to put that in the design.

I can see we've got to go back to the grounding tutorials--the basics, so to speak.  OK.  There are three types of grounds.  1) electrical safety grounds,  2) lightning or static discharge grounds, and  3) RF grounds

The first two are somewhat compatible, but the third isn't, not at all.  The first two have to have a good 'connection' to moist, conductive earth.  The third should be either above the surface of the earth or just below it, and have to be (most of the time) a laid out pattern of very conductive material.

Now, to give good protection for electrical safety, static discharge or lightning, the grounding device should be long enough to get down to the water table--or at least to the point where the earth is moist and conductive.  That is one reason that the rule of thumb 'deeper is better' for those grounds exist.  It is also why an eight or ten foot ground rod is ineffective in dry areas.  You've simply got to get down to the level where the earth is conductive.  

A surface ground plane installation (such as you propose) will not--can not--do that.  Even bare wires laid out in a large ground plane pattern will not get that effect, unless those wires are laid out in ground that is perpetually wet--but not even then.  The ground may be wet above--but a layer of clay will effectively stop the seepage down, and the water will run off to the area where it can seep down.

That is the reason why both a ground rod AND a ground plane pattern of wires are required on some antennas.  The wired ground plane is for RF purposes while the ground rod is for lightning/static discharge purposes.  Usually a tower has as many ground rods as there are legs of that tower, but that is another subject!  Added--Also, usually a tower doesn't have or require a ground plane--unless the tower itself were insulated from the ground and used as the antenna.

Hope this helps you out, and 73!
« Last Edit: July 24, 2013, 05:58:23 AM by K1CJS » Logged
W6RMK
Member

Posts: 662




Ignore
« Reply #11 on: July 26, 2013, 09:44:40 PM »



I can see we've got to go back to the grounding tutorials--the basics, so to speak.  OK.  There are three types of grounds.  1) electrical safety grounds,  2) lightning or static discharge grounds, and  3) RF grounds

The first two are somewhat compatible, but the third isn't, not at all.  The first two have to have a good 'connection' to moist, conductive earth.  The third should be either above the surface of the earth or just below it, and have to be (most of the time) a laid out pattern of very conductive material.

Now, to give good protection for electrical safety, static discharge or lightning, the grounding device should be long enough to get down to the water table--or at least to the point where the earth is moist and conductive.


<snip>


Actually, moist isn't a very big issue with lightning dissipation. Surface area is, hence the Concrete Encased Grounding Electrode or Ufer ground. 20 feet of wire in the slab and foundation footings.  Invented by Herb Ufer for lightning protection in desert climates. Required by the electrical code just about everywhere.

All you care about for lightning protection is that the current density is low enough that the step potential (voltage gradient on surface of the soil) isn't too high, and that the local heat dissipation is low enough that damage doesn't occur.


The electrical safety ground (your #1 in the list) shouldn't be called ground. It's really electrical safety bonding (and there is serious discussion about changing the code to that effect).  It is there primarily to ensure that the metal cases of things are at the same potential as your bare feet standing on the floor. And, to provide a low enough impedance path to cause the overcurrent proteciton to trip/blow if there is a short from line to case.

Transient protection is another area.. but that's more about making sure you don't have loops so that everything that is connected together tends to go up and down in voltage at the same time.  Airplanes don't have ground rods, but are protected against lightning transients, for instance.
Logged
KC4MOP
Member

Posts: 759




Ignore
« Reply #12 on: July 27, 2013, 05:02:27 AM »

W6RMK and K1CJS very nice easy-easy-to-understand info on the difference between electrical/lightning and RF ground.
Fred
Logged
K1CJS
Member

Posts: 6055




Ignore
« Reply #13 on: July 27, 2013, 05:24:13 AM »

'RMK,  You can have all the surface area (BTW, let's not mistake the surface of the earth as 'surface area') in the world, yet if none of that surface area is very conductive (which moisture plays a part of) you're going to have a poor 'ground' as far as lightning charge dissipation.  That's why driving ground rods into sandy, dry soil never gets you a good ground point. 

However, it's granted that moisture isn't the only thing that makes a good ground--and I never said it was.  One thing that I didn't say is that more than one ground rod is better as far as dissipation of a charge is concerned--and I should have.

Added--
Quote
...Now, to give good protection for electrical safety, static discharge or lightning, the grounding device should be long enough to get down....at least to the point where the earth is moist and conductive.....
 
« Last Edit: July 27, 2013, 05:34:06 AM by K1CJS » Logged
Pages: [1]   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!