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Author Topic: Shack grounding question  (Read 1215 times)
AC0JX
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Posts: 13




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« on: January 14, 2013, 06:50:59 PM »

At the risk of starting another long discussion on grounding, here goes.  I am installing my ham station on the third floor of a building.  I know ground floor or basement would be better, but it cannot be done.   This is new construction and I am able to run LMR 400 on a 100 foot run down the elevator shaft to the outside, to a DX Engineering copper grounding plate grounded with 2-8 foot ground rods 10 feet apart, each tied into the grounding plate with copper strap.  Plate is also connected to the ground for the electrical service with copper strap.  I have both coax (one HF, one for VHF) and the antenna control wiring (for Steppir vertical) runs through Polyphaser arrestors mounted on the copper plate.  Ok, here’s the question: should I run a #4AWG cable from the shack to the grounding plate to try to make sure all equipment is as close to the same ground potential as possible? The run would be about 30 feet vertical and then another 45 feet to the grounding plate.  The DC resistance is less than 0.1 Ohm, (http://okonite.com/engineering/dc-resistance.html) of course the AC resistance from a lightning strike would be higher.  The vertical antenna will be about 100 feet from the vertical aspect of this wire, I most commonly run 100 watts SSB.  I can see possible RF and other issues with the set-up and question what the best course would be considering all the trade-offs.  I could always install it and disconnect it if there are problems, but would prefer not to go through the expense and work if it's a no-go from the start from a design perspective. Thanks much for the thoughts and considerations.

AC0JX
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N4CR
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Posts: 1655




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« Reply #1 on: January 14, 2013, 08:14:01 PM »

Ok, here’s the question: should I run a #4AWG cable from the shack to the grounding plate to try to make sure all equipment is as close to the same ground potential as possible?

Are you looking for a lightning ground? A safety ground? An RF ground?

Wire gauge can play into the lightning ground, but it won't make a bit of difference for the other two assuming #12 or so.

A 30' ground is going to act like an antenna on every band, but what it does to you will change for different bands. If your ground plate is a quarter wavelength (or an odd multiple) from your rig, it's not an RF ground at all, regardless of the wire diameter.

Let's take 30 feet as an example:

30' is a quarter wave at 8.2 mHz and 3/4 wave at 24.6 mHz.

It could be very problematic on 12 meters and would probably not act like much of an RF ground on 40 or 30 meters.

I guess the next question is: Why do you think you need a ground?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K4SAV
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Posts: 1831




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« Reply #2 on: January 14, 2013, 09:12:31 PM »

Plate is also connected to the ground for the electrical service with copper strap.

Good.  That's what you need.  The idea is keep all the lightning current outside.  You don't want to route lightning current thru the building or your station, and from your description it seems like you have that.  If your grounding is configured to do that, then you don't need any big ground wires going to your rig.  Coax shields are sufficient for that ground, or you can add another small wire if that would make you feel better, but forget the big wire.

I would also recommend that you disconnect the SteppIR controller in bad weather.  It only takes a very few volts to blow the driver chips in that box.  Even a short to ground will do it when power is applied to the controller.

Jerry, K4SAV
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AJ4CU
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Posts: 75




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« Reply #3 on: January 15, 2013, 04:39:36 AM »

Before you start running non-elevator related items in a hoistway you may want to look at ASME elevator code which specifically prohibits anything not related to the elevator from being run in the hoistway.
Also the NEC is clear on this subject as well.

For clarity I know this from 8 years in the elevator servicing business, please be careful, this can cause you all sorts of code and safety related problems.

I am not trying to cause an issue here, I just want you to be informed...
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AC2EU
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« Reply #4 on: January 15, 2013, 07:06:25 AM »

The NEC ( National Electrical Code) has whole chapters on antenna grounding systems and feed lines.
However some states, like Florida, have even stricter requirements .
In your case, all of the grounding has to be accomplished BEFORE the feed line enters the building and have a ground point just before it enters the building as well,which is the basic philosophy of the NEC, anyway.
 Perhaps consult a local electrician who has installed antenna systems in your area. He should be knowledgeable about the specific codes used in your state and locality .

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W6EM
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« Reply #5 on: January 15, 2013, 05:55:17 PM »

One of the most effective schemes I've seen is in use at the multi-transmitter candelabra tower sight in Riverview, FL.  At about 1500 feet above average terrain, the broadcast antennas on it get nailed almost every day during the summer.  To keep lightning out of the transmitters a few loops of coax are just inside the building, and placed on big insulated standoffs above grounded plates.

Moral of the example:  add common mode impedance to feedlines to make them have more impedance to lightning current than tower legs, tower grounds, etc, so it will find a better path to ground some other way than back through radio equipment.

Lee
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AC0JX
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Posts: 13




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« Reply #6 on: January 22, 2013, 06:50:03 AM »

Thanks much for the thoughtful responses and good information and advice.  AJ4CU, thank you for your comments on the elevator code.  I should have been more clear on the placement of the cable, which is through conduit in the utility pass-through on the outside of the elevator shaft.  The intention of the ground was to make sure all the electrical gounds were equal, and that can be achieved through the coax to the copper wall panel in the room where the radios will sit.  I am fortunate in NC to have made some personal contacts with the people who care for the VOA headquarters here and have them and the electricians engaged in the project.  Not cheap, yet done in the right way.

Thanks again, I appreciate the thoughtful help.  I always learn something from the community on eHam.

David
AC0JX
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