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Author Topic: Is this right?  (Read 954 times)
K2YQ
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Posts: 16




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« on: December 15, 2003, 06:40:13 PM »

Building my first shack as part of a new office in my basement.

Question: I am planning to create a ground bus as illustrated in the ARRL Handbook. This will allow me to easily connect a DC ground for all the radio gear.

Rather than attach the bus to a new ground rod, I would like to tie the bus into the existing AC electrical ground rod.

Is this a safe practice?

My understanding is that all grounds must be tied together in any case. Therefore it seems alright to use the existing AC rod in lieu of adding a second ground rod, which would just have to be connected to the AC ground rod.

Is this logic sound or are DC grounds treated separately from AC grounds?
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N7NBB
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« Reply #1 on: December 16, 2003, 10:35:36 AM »

The "SHACK" ground (safety / electrical type) should be as short of a run of (ground) wire possible.  If you what you are proposing (hooking into the house ground)... **which I STRONGLY DO NOT SUGGEST YOU DO**  the "shack ground" wire has to go a few feet to reach your commercial power mains ground (at the wall socket).  Then it travels through your house inside the walls many tens of feet before it encounters your electrical service panel.  Who knows how many bends kinks, loops and other nests the wire takes on this journey.  Then, finally, the wire from your service entrance may or may not take a direct path back outside where it "should" be grounded via a rod into the earth.  It scares me to see how many aren't. Even if it IS, do you know for SURE what type of ground rod... and how long was used?  (AT Least 8 FEET - - 10 would be better)  I'm betting whoever installed the original electrical ground may have used a three or four foot ground... while it is better than nothing.. (in most cases) it is FAR from NEC code, nor realistic safety.

The point is this:

Your house wiring (ground wires throughout your home) will ((may under certain conditions)) now in effect become an antenna. That's why they say SHORTEST and MOST DIRECT ROUTE for Ground Wires.

In the event of a direct or very near lightning strike, you want the least number of turns, curves, bends, etc in the ground wire.  (else wise the lightning energy will skip your ground and jump to any other (more direct) route.

Now this is only the ELECTRICAL SAFETY GROUND... you also have to be concerned with an R.F. (radio frequency) GROUND.

There is tons of good practical info about grounding available on the web.  Do a search on GOOGLE.com for AMATEUR RADIO GROUNDING or similar.  Then after you have read up on the subject,   Do as much (or as little) as your conscience will allow you to do. But please don't cut corners just to save a few bucks.   That is wrong on so many levels.

73
CAM
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WB2WIK
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« Reply #2 on: December 16, 2003, 07:27:07 PM »

A "DC ground?"  I don't even know what that is...

The recommendation about minimizing bends and curves and keeping the wire short for lightning protection was good advice in general, but lightning protection grounding should *never* be attached to your station equipment.  

Lightning protection grounding should be located 100% entirely outside the shack, outside the house, and preferably not even near the house.  Lightning protection grounding should be installed on all antennas and supporting structures, and should already be in place protecting AC power mains, cable TV and telephone wiring, as installed by those utility companies -- and in all cases, those grounds are outside the home and never come inside the home.

The old saw, "ground is ground, the whole world 'round" is cute, but in reality ground has lots of resistance and is very imperfect.  But the earth is the only thing we all have in common except for the sky, which isn't a conductor; so, earth it is.  Plus, earth is a heck of a heatsink or energy sink.  You can pour thousands of Watts or Joules into it, almost any old place, and it's capable of absorbing or dissipating the energy.

I'd keep lightning protection grounds outside and not connected to my equipment in any way except coincidentally (continuity via cable shields and such), so that a strike or large EMP pulse can be transmitted via the lowest impedance path to earth, while a miniscule fraction of that energy might be conducted to my station via much higher impedance cable shields and such.

You don't need a "DC ground," at all.  You do need a mains safety ground (return path), which in all modern housing is the third wire in all your AC receptacles.  In very old homes, this may be missing or not working.  It's easy to test, and if you're unsure of yourself, hire a qualified electrician.

An RF ground is one that should be connected via the shortest possible path between your station equipment and earth, using the fattest, broadest, non-stranded and non-braided conductor possible.  Wide flashing is far better than any sort of wire, and if "wire" needs to be used, make it large, solid, and short.  Six feet of #6 solid copper isn't bad.  Twenty feet of #8 aluminum is pretty lousy.

WB2WIK/6
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WA4MJF
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Posts: 1003




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« Reply #3 on: December 17, 2003, 12:14:39 PM »

This lead me to a question.

Even with the lightning arresters grounded
seperatly from the equipment there is about
1 ohm resistance from the patch panel to
the grounds.   This is I believe because the
shield is connected to the ground of the
gas arc cartridge.

When it fires does it disconnect from
the shield or go right to the patch panel?

Happy Holidaze,

Ronnie

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WB2WIK
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« Reply #4 on: December 17, 2003, 01:08:59 PM »

I don't understand that question.

You should have any cable shields that add up to one Ohm of resistance, that's an awful lot.

I don't know what kind of lightning arrestors you have, but usually they don't "disconnect" anything in any way; they fire and create a short circuit between the line they're protecting and ground.  Some are designed to be reusable, some are one-time devices that once fired need to be replaced.

WB2WIK/6
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WA4MJF
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« Reply #5 on: December 17, 2003, 01:21:21 PM »

I guess you know what a patch panel is so I won't
go in to that.

Taking a VOM from the rak the pannel is in to the ground braid that the Alpha Delta Arrestors
are connected to I get about 1 ohm.  When going from
the shield of the coax to the stud (metal one,
not manly one) I get about 1 ohm.  I'm not quibbling
about the 1 ohm, as I should have thought I would have infinity.

The connection is through the shield of the coax.

So I wonder should I ground the panel rack to the
same ground as the arrestors are grounded too.

I always thought that equipment should not
be on the same ground.

Does this help 'splain?

Happy Holidaze,

Ronnie

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KD7PLU
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« Reply #6 on: December 30, 2003, 03:50:57 PM »

I went to a repeater site where the interior walls were covered in copper screen material. The Old-Timer Extra-Class guy I was with said that it was silly because the rips and poor overlap of the screens negated any effective use the idea may of had. Does anyone have experience with this type of concept, or something like it to make your shack 'RF Clean' to some degree- Or does it even matter/worth the effort?

Loren, KD7PLU
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WA6BFH
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« Reply #7 on: March 10, 2005, 09:36:16 AM »

Your ‘old time Ham friend’ was quite correct! Any break, poor bonding, or hole in the wall for coax etc, notwithstanding all sorts of energy will enter and leave the building riding on the outside of the coax shield -- negates such ‘screen shielding’. Even gaps around the door, cracks, negate this!

More on point with the topic, I would worry also about “Ground Loops”. I wrote an article several years ago about grounding Ham stations. Many of the responses I got amazed me how poorly “AC” circuits are mis-understood -- and I’m not talking about the 117 or 220 volt house mains “AC” -- I’m talking about RF!

Think about this simple example, why do you think tank coils in TransMatches etc have such fat wire, strap, or tubing used in those coils?
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