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Author Topic: Wood vs Metal Desk & bonding  (Read 3456 times)
KJ4ADN
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Posts: 39




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« on: July 29, 2012, 08:01:15 AM »

I've used a wooden desk for everything, it's a worn out antique...

There's a free metal desk, looks really nice for a station, so I thought I'd ask, several of you guys have what looks like metal desks.

Question #1:  Did you bond your desk to the ground, radio, coax, etc?
My ground rod(s) are less than 5' from the desk, thru the wall, outside.
all the coaxes are bonded together to a 1/4" plate (outside the wall), which is bonded to the ground rod.
The entire system is running on 12VDC, battery bank, charged by homebrew 80amp linear supply.

I have no intentions of cadwelding a #2 cable to the desk... metal or wood

Question #2:  What SHOULD I be doing with the end of the disconnected coax?
I've been disconnecting it during lighting attacks (ground strikes in the front yard... )
I don't want to smoke $$ expensive radios, so I disconnect them as a precaution - good idea, bad idea with my grounding?
A friend of ours had his disconnected, coax laying on the desk, took a near strike on his G5RV, looks like a cigar where the coax rested / vaporized, right in front of the radio!   A girl in the club puts her coax end in a large glass jar....?!

KJ4ADN - Bill
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WB4SPT
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Posts: 132




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« Reply #1 on: July 29, 2012, 08:58:00 AM »

If the shield of the coax is always grounded at the entrance plate, I'd say that you are MOSTLY covered.  A PL259 is good for what, maybe 5-10kv of voltage before it arcs??  Yeah, its best to get the radio out of the lightning equation by disconnection of all wires to it.  There are equally valid arguments for bonding or not bonding a metal desk.  I'd probably float it if possible, unless there were outlet strips screwed into it, or other lethal voltages attached.  Then, I'd use #12awg to bond it.  You don't want the desk to be a carrier to lethal voltages, on the other hand, its a great opportunity to create a ground path for your body when you contact voltage from something else.  Wood makes more sense, if you ask me. 
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K1CJS
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Posts: 5812




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« Reply #2 on: July 29, 2012, 09:10:05 AM »

If I were going to use a metal desk, I would take great care to make sure that there was no connection to that metal desk from any radio, power supply or other piece of equipment that was sitting on it or was near it.  I wouldn't want to get a zap from just touching that desk!

As to actually grounding that desk, I wouldn't.  It really isn't necessary. 
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LA9XSA
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Posts: 376




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« Reply #3 on: July 29, 2012, 03:05:34 PM »

On question 1, I don't have a metal desk but if you choose to bond it you don't need to use CADWELD (thermite welding) on ground connections indoors. CADWELD is mainly recommended for buried connections, or at least outdoor connections, where corrosion is more of an issue.

How about an ESD table mat? Could that be an idea if you're annoyed by static jolts and headphone pops or do a lot of electronic work?

On question 2: You should connect both the coax shield and the center conductor to ground, as near to the ground plate as possible. The glass jar is not thick enough. "Floating" feed lines is a bad idea in lightning. So either connect the coax to ground, or disconnect both ends, and perhaps even take the coax out of the shack.
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KJ4I
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Posts: 110




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« Reply #4 on: August 05, 2012, 06:44:12 PM »

In regards to question # 2. I have been working on this issue lately myself but I WOULDN'T put the feedline in a glass jar for sure. In my opinion that jar could become a grenade during a hit and glass would go everywhere. I wouldn't want to be anywhere near it when it exploded. I think some type of system to ground out the loose ends would be best but I do not have that in place yet. I do have everything grounded at the base of the tower well before it enters the house but I still am not happy with loose coax inside. In the meantime I ended up with some heavy plastic electrical enclosures with SO-239's fed through them. Right now I just disconnect my jumpers from there and secure the lid on the box until I can do something better. I guess that plastic box could explode also. Might be a false sense of security but for time being it makes me feel a little better having it secured in an enclosure rather than laying out on the desk.
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K1CJS
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« Reply #5 on: August 06, 2012, 08:01:30 AM »

If your antenna takes a direct hit from lightning, there isn't anything in the world that's going to help if there is a disconnected coax laying around.  That lightning just went through miles of air to get to where it was going, and a glass jar or a plastic box isn't going to stop it from seeking out a ground point.

The one thing that doing that will help is it will prevent anyone from accidently touching the live end and getting a pretty nasty shock from a simple static charge that may be present on the disconnected coax.  And that may be there during almost any kind of weather--simply from air blowing past the antenna!
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KJ4I
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« Reply #6 on: August 06, 2012, 09:54:48 AM »

Quote
That lightning just went through miles of air to get to where it was going, and a glass jar or a plastic box isn't going to stop it from seeking out a ground point.

It absolutely won't stop it from seeking out a ground point but the theory is that at least something is better than nothing at all. I think the box would be a much safer alternative than a glass jar. A good ground system before the feedline enters the house is a must in any situation. There are much better solutions than this one but in some situations those ideal solutions are much more difficult to work with than in others. Lightning is a powerful and strange thing. I think the best thing to do would be to ground out the terminated ends if possible. No ground system no matter how elaborate can guarantee 100% protection but every little bit helps.
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