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Author Topic: Ground buss (station rf ground) oddity - or is it?  (Read 2847 times)
N8NSN
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« on: April 26, 2013, 08:26:18 PM »

This is something I never bothered to test. Perhaps it is a normal thing that I observed. Perhaps not normal at all. Can't recall ever reading anything about this...

First some background. We'll begin stating this oddity is occurring in the station ground path. Here is the nuts and bolts og the rf ground system.

*8 ft copper clad rod in flowerbed, driven in where downspout keeps wet earth
*5 ft of #4 solid copper brazed and acorn nutted to rod
*into shack lugged copper to 2"x 10" quarter inch thick buss
*2" wide copper strap, up 1foot from bar and 3 feet across
  rear of upper equipment shelf.
*all connections brazed or soldered and mechanically bolt or lug
  fastened, as well.
*each piece of equipment has appropriate length pig tail of 1/4"
  tinned copper braid from chassis to ground buss bar or the upper
  shelf strap
*** took resistance readings with an analog meter, and a Tenma LCR
  meter at every chassis, pigtail, strap to buss, buss to #4 wire where
  it exits the shack - back to the main lug on the buss bar. All the readings
  were acceptable between .001 to .012 ohms - nearly no resistance at all.
*mainly was looking for as close to zero resistance from each chassis to the
  Buss Bar that was possible... Seems good so far.

HERE'S WHERE IT BECAME ODD
 
So, I hook up the transmitters, one at a time to the old trusty Cantenna...
Check all those points again while transmitting one transmitter after another.
Noticed a slight increase to .05 to .1 ohm at some points. OK, now even more
oddly: perform same regiment of test points (chassis(s) to bus bar, again, mainly
looking for close to zero resistance - only this time through hooked up to a resonant
antenna. Now noticing readings as high as 46 ohms on
a couple of chassis - back to buss bar paths.

Sigh. Now, my question is this:
Is this odd?

I'm not experiencing any ground loop type RFI issues.
Just never thought about testing chassis to buss resist-
ance under transmitting loads. Maybe these readings suggest
Some type of bypass or bleedoff circuitry is faulty in one of the
rigs OR maybe suggestive that all is as it should be?

Help?
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WB6BYU
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Posts: 13353




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« Reply #1 on: April 26, 2013, 09:14:45 PM »

Multimeters are often affected by the presence of RF fields.  I've gotten some
really screwy current readings before that went away with a capacitor across
the test leads.  Most likely your meter is picking up enough RF to throw off
the reading.

Don't sweat it:  a tenth of an ohm in the lead from the shack buss bar to
the ground rod won't make any difference, because there shouldn't be
any significant current flowing through it.  (AC return currents should go
through the third pin on the AC plugs, while lightning is best kept outside
the house in the first place.  And the RF effectiveness of such a system
will be degraded by resonant effects in the conductor leads (you have
about 1/4 wave of conductor on 10m grounded at the far end:  that
presents a high impedance to the rig on that band rather than serving
as an effective ground.)
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N8NSN
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« Reply #2 on: April 27, 2013, 04:57:27 AM »

Multimeters are often affected by the presence of RF fields.  I've gotten some
really screwy current readings before that went away with a capacitor across...


*** took resistance readings with an analog meter, and a Tenma LCR
 
I forgot to mention all tests were on 40 and 30 meters... The entire linear footage of all conductors from any of the the chassis, to the point where the ground rod enters the earth
Is between 11 and 12 feet. I can see where this could be difficult on 15 and maybe 17 meters.
Also the only "round" conductor in the system is the 5 feet of #4. All else is flat stock or tinned braid.

Yes RF affects digital meters  -  this was taken into account.  The Tenma was mentioned because it's fairly stable in RF environments. Actually the LCR meter is my best pal in most antenna related ckt from tank to tuner to ... However the Fluke DVM goes bonkers in an RF environment...

Since there are no RFI issues "not broke - don't fix" would be applicable. I guess since our 70 degree temps have returned to 40 degree temps, here in SW Ohio, I got bored and thought the mind needed exercise :-)

Now, as it's cool outside again today; think I'll run through the entire test procedure again, with a VTVM. I know, it's an obsession... Not brain exercise.
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N8NSN
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« Reply #3 on: April 27, 2013, 06:02:02 AM »

Oh yes, ...thanks Dale.

So far my "worry" was that some type of leaky or shorted cap in one or more rigs may have been allowing RF voltage to be dump riding on the chassis thus riding on all pieces connected.

And the shack is never "done". Obsession 101.
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KE3WD
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« Reply #4 on: April 27, 2013, 06:34:36 AM »

**Is your shack ground rod system bonded to your house electrical ground rod system?**


It should be. 


73
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N8NSN
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Posts: 283


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« Reply #5 on: April 27, 2013, 08:07:06 AM »

**Is your shack ground rod system bonded to your house electrical ground rod system?**


It should be. 


73

Yes and yes. Thank you. Everything bonded to the hilt: telephone, cable, shack, antennas, electric service... As per NEC "...rise and fall at same rate..."

The VTVM results are in...

I'm much more comfortable now. Numbers look great. Guess the boredom paid dividens of of ease of mind. No regrets.

Thanks all for being patient with my hyper obsessions and curiosity.

73
Jimmie
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K1PJR
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Posts: 138




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« Reply #6 on: April 28, 2013, 09:22:47 AM »

Jimmie,

Why such an elaborate grounding system?  My approach is the opposite.  I do not ground my equipment.  As far as I can reason there is no need too.  Outlets are grounded and the lightening and RF grounding takes place outside.  That's all there is too grounding.  I've never had any issues or encountered any problems.  If someone thinks I'm missing some type of grounding concept then by all means say your peace.  I'm always willing to listen.

73
Phil
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K1PJR
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« Reply #7 on: April 28, 2013, 12:07:15 PM »

 Good article about grounding:

http://www.eham.net/articles/21383
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K8AC
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Posts: 1477




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« Reply #8 on: April 28, 2013, 01:16:57 PM »

Quote
Why such an elaborate grounding system?  My approach is the opposite.  I do not ground my equipment.  As far as I can reason there is no need too.  Outlets are grounded and the lightening and RF grounding takes place outside.  That's all there is too grounding.  I've never had any issues or encountered any problems.  If someone thinks I'm missing some type of grounding concept then by all means say your peace.  I'm always willing to listen.

And here's a very important reason.  Before that, just because someone has operated for some number of decades and never had a lightning problem means absolutely nothing.  I operated for 40 years and pretty much disregarded grounding issues and then one day, a tree in my backyard was struck.  Damage to the station gear 150 feet away was $3,500.  Three years later, the same tree was struck again and this time the damage was about $200, and I attribute the difference to the attention I paid to grounding after the first strike. 

Lightning doesn't have to strike your antennas or your house to do extensive damage to your equipment.  I think of a lightning bolt being similar to a freight train - there's little you can hope to do to change its direction and to be sure if a bolt hits your station directly, it's all over.  But, tremendous damage can be done by a nearby strike that doesn't hit your antennas at all.  When a bolt hits a tree, your tower, etc., extremely high currents flow through the earth in all directions and will induce voltages in any metal in the path of the currents.  That might be a water pipe, underground telephone line, underground cable TV coax, etc.  In the case of my second strike, a significant voltage was induced in the underground telephone line and the wires were blown off the telco termination on the outside of the house.  The damage to the station that time was limited to things that were connected to the phone line.  Everything was connected to a ground buss on the rear of the operating table, but there was no protection installed on the phone line inside the station.  The telco box was properly grounded to the electrical panel ground.  The purpose of the ground buss is to equalize the voltages that appear on the cabinets of the gear.  If it weren't there, various pieces of gear might be at different potentials depending upon how they're connected to conductors coming in from outside.  That potential difference (many thousands of volts) will be resolved by an arc and a high current flow (enough to blow the traces off of a circuit board).  It's interesting to think that the ground on electrical outlets and a single point ground outside will result in no potential inside at the equipment, but that's not the case.

If you're single point ground is 50 feet away from the station equipment, you may still see a significant voltage at the  equipment depending on how your cables are routed.  For example, in this part of the world, almost all houses have a dirt crawlspace and cables are often left lying on the ground there.  A good opportunity to have a voltage induced in the cables with a high current and steep wavefront traveling through the soil. 

Unless your single point ground is right there at the electrical panel (and I'll bet most aren't), then you WILL have a potential difference between the ground on your antenna or control cables and the electrical outlet ground.  Of course, you might be one of the lucky ones who never has a strike nearby (and I hope you are).  I was successful in ignoring the problem for 40 years, but I wouldn't risk doing that again. 
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K1PJR
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« Reply #9 on: April 29, 2013, 12:37:13 PM »

I don't disagree.  I believe I'm being misunderstood (which is not unusual). I agree grounding takes place outside the shack.  In addition I disconnect my coax when not in use and it's far from the rig.  Station components are also plugged into a Tripp-Lite surge suppressor.  If your rig is disconnected then the damage from lightening is literally removed.  Elaborate grounding inside the shack is no guarantee of damage to your equipment.  I just feel its simpler and safer to disconnect when not in use.

The original question referred to an RF ground.  That takes place outside along with lightening protection.  I get the ground loop thing and differences in potential.  Why make it complicated when you can just disconnect when not in use.  Sure it can be a nuisance but its foolproof...

I think Grin

Phil
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K4SAV
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« Reply #10 on: April 29, 2013, 05:59:56 PM »

There is nothing wrong with disconnecting your equipment during a storm but that is not a replacement for good grounding, which should all be outside your house.  If you don't have proper grounding outside the house and think that disconnecting the equipment will prevent damage you are in for a big surprise when the first strike occurs.

Jerry, K4SAV
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KD8MJR
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Posts: 2580




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« Reply #11 on: April 30, 2013, 06:36:18 PM »

K4SAV is correct.
Also I would add that most Hams will linger a little bit even when they hear thunder in the distance.  Forget about the equipment, just imagine what would happen if you had a sudden strike while touching the radio.  I have seen lightning striking miles away and then suddenly a bolt hits down a few hundred feet from my house.
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KC9NVP
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« Reply #12 on: May 01, 2013, 09:44:52 AM »

I'm sure many of us will fall into this camp, no damage to the radio equipment, but had a close strike some month ago, and it took out the controller four in ground electric fence.  the in ground fence loop consit of around 800 ft of wire in the ground.  the wave front thru the ground develop high voltage back into the controller taking it out.  $200 dollars later new controller) and two external .125 amp fuse, no damage to the controller, but have replace fuses after several storms.  So yes, close strikes can and will do damage, so grounding should be one of your line of defence to keep the energy out of your equipment.

David
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W6EM
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Posts: 833




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« Reply #13 on: May 04, 2013, 05:42:13 AM »

Quote
Why such an elaborate grounding system?  My approach is the opposite.  I do not ground my equipment.  As far as I can reason there is no need too.  Outlets are grounded and the lightening and RF grounding takes place outside.  That's all there is too grounding.  I've never had any issues or encountered any problems.  If someone thinks I'm missing some type of grounding concept then by all means say your peace.  I'm always willing to listen.

And here's a very important reason.  Before that, just because someone has operated for some number of decades and never had a lightning problem means absolutely nothing.  I operated for 40 years and pretty much disregarded grounding issues and then one day, a tree in my backyard was struck.  Damage to the station gear 150 feet away was $3,500.  Three years later, the same tree was struck again and this time the damage was about $200, and I attribute the difference to the attention I paid to grounding after the first strike. 

Lightning doesn't have to strike your antennas or your house to do extensive damage to your equipment.  I think of a lightning bolt being similar to a freight train - there's little you can hope to do to change its direction and to be sure if a bolt hits your station directly, it's all over.  But, tremendous damage can be done by a nearby strike that doesn't hit your antennas at all.  When a bolt hits a tree, your tower, etc., extremely high currents flow through the earth in all directions and will induce voltages in any metal in the path of the currents.  That might be a water pipe, underground telephone line, underground cable TV coax, etc.  In the case of my second strike, a significant voltage was induced in the underground telephone line and the wires were blown off the telco termination on the outside of the house.  The damage to the station that time was limited to things that were connected to the phone line.  Everything was connected to a ground buss on the rear of the operating table, but there was no protection installed on the phone line inside the station.  The telco box was properly grounded to the electrical panel ground.  The purpose of the ground buss is to equalize the voltages that appear on the cabinets of the gear.  If it weren't there, various pieces of gear might be at different potentials depending upon how they're connected to conductors coming in from outside.  That potential difference (many thousands of volts) will be resolved by an arc and a high current flow (enough to blow the traces off of a circuit board).  It's interesting to think that the ground on electrical outlets and a single point ground outside will result in no potential inside at the equipment, but that's not the case.

If you're single point ground is 50 feet away from the station equipment, you may still see a significant voltage at the  equipment depending on how your cables are routed.  For example, in this part of the world, almost all houses have a dirt crawlspace and cables are often left lying on the ground there.  A good opportunity to have a voltage induced in the cables with a high current and steep wavefront traveling through the soil. 

Unless your single point ground is right there at the electrical panel (and I'll bet most aren't), then you WILL have a potential difference between the ground on your antenna or control cables and the electrical outlet ground.  Of course, you might be one of the lucky ones who never has a strike nearby (and I hope you are).  I was successful in ignoring the problem for 40 years, but I wouldn't risk doing that again. 
One of the best explanations of the effects of indirect stirkes I've read.  Excellent. 
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K1CJS
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« Reply #14 on: May 04, 2013, 09:06:00 AM »

Why such an elaborate grounding system?  My approach is the opposite.  I do not ground my equipment.  As far as I can reason there is no need too.  Outlets are grounded and the lightening and RF grounding takes place outside.  That's all there is too grounding.  I've never had any issues or encountered any problems.  If someone thinks I'm missing some type of grounding concept then by all means say your peace.  I'm always willing to listen.

First, there is more than one type of ground for radio work.  The ground on the house electrical system is a safety ground ONLY.  It isn't a RF ground nor a lightning protection ground.  Keep in mind that the NEC does demand that no matter what type ground it is, all grounds 'serving' a building must be bonded together.  

True, if your antenna system is properly constructed or of certain types, you may not need a RF ground.  But if you do, that ground must be bonded to the house electrical ground.

If the antenna system is surrounded by taller structures or natural terrain, you may not need a lightning ground either--but it's still a good idea to have one.  Not only does that type ground provide limited protection from a direct lightning strike, it also provides protection from transient surges--which the antenna may pick up from a nearby lightning strike.  A lightning strike produces high amounts of RF energy--which is what an antenna system is designed to pick up!  Consider too that static electricity is generated by the simple fact of wind blowing by an antenna structure, and the ground connection will prevent that from entering your shack through the incoming antenna leads as well.

The more ground rods, the more the transient charges picked up by the antenna will be shunted away from your equipment, and the more protection you will have for that equipment.  Since that RF energy does have voltage behind it, that ground system must also be bonded to the house electrical ground for the safety of anyone in the house.  If it isn't, you may become that 'bond' simply by touching your equipment while you're in contact with anything else that is connected to the house electrical system.  At that point, you may well cease worrying--about anything!

In short, the more ground rods, the more energy that can be dissipated BEFORE it enters your shack/home, and the safer you will be from unexpected weather changes.  It also follows that if you have NO grounding of your antenna system, any static electrical charges will drain right through your equipment.  If you hear static sometimes while operating with such an antenna system, that is why.

Whatever you do, good luck and 73.
« Last Edit: May 04, 2013, 09:11:04 AM by K1CJS » Logged
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