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Author Topic: Best way to route copper strap to outside?  (Read 1582 times)
KC9IFF
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« on: May 16, 2009, 09:58:52 PM »

Hello,

I'm planning to put a small ham shack in my basement.  The room is almost entirely underground and the walls are concrete up to the floor joists for the main level.  At this point my only antenna will be a roof-mounted VHF/UHF vertical antenna, though I'd like to account for the addition of two more verticals at some point in the future.

For lightning protection I'm planning on three 8-foot ground rods appropriately inner-connected.  In order to get the coax into the basement, I'll have to drill through the rim joist and bring it into the ceiling of the shack.  For reference, this is approximately two feet above ground level.

I plan on putting my copper sheet single-point-ground on the wall right near where the coax will be entering the building (probably only holding 1 arrester at this point).  But my question is, how should I route the copper strap from the SPG outside to the grounding system?  I was thinking of an approximately 4-inch hole through the rim joist to the outside and into an enclosure.  Should I run the grounding strap through the same hole/enclosure as the coax?  What should I do with the copper strap once it's outside?  Is it safe to run it through a vertical segment of PVC conduit from where it exits the house down to the ground (for aesthetic reasons)?  I'm also concerned that the external enclosure might not be big enough to allow for a gradual radius in the copper strap, but that's probably another topic...

Thank you.
Luke
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K9KJM
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Posts: 2415




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« Reply #1 on: May 17, 2009, 01:48:03 AM »

Your plan sounds good to me.

Just install a plain 4 inch clothes dryer vent assembly, Put your single point ground panel up high, Very close to the entrance hole, (So your copper strap really does not have to go "uphill" before it starts heading out toward ground)
You are correct, Try to avoid any sharp bends.

For some good info:
http://members.cox.net/pc-usa/station/ground0.htm

For tips on how to do it on a budget, Pick up a copy of May 2009 Popular Communications Magazine.

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AA4PB
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« Reply #2 on: May 17, 2009, 06:29:24 AM »

Why not use a #6 or larger round copper conductor for the lightning ground? It'll go through a much smaller hole. Most ground systems, probably including your electrical system ground, use round copper wire.

Another option is to put the single point ground panel on the outside of the building. You can waterproof the connectors an/or enclose the panel in a waterproof utility box.

Don't forget that the NEC requires that this ground be electrically bonded to your home's electrical system ground with at least #6 wire.
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KB1NXE
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« Reply #3 on: May 18, 2009, 09:54:37 AM »

Round Copper Cable - while great for a 60Hz safety ground, exhibits a larger impedance to higher frequency currents.  Lightning is actually a rather high frequency current, as well as the RF generated inside your shack.  Due to the skin effect, wide copper strap is a much more effective conductor at these high frequencies.  This is why it is recommended for lightning protection.

I recently added the 4" dryer vent (as recommended in another post) to my new QTH with a twist.  I laid my copper strap in the bottom of the vent _after_ adding a layer of electrical tape to keep the copper off the aluminum.  To keep out creepy crawlies and weather, I mounted an external electrical box over the vent.  I also fitted the inside of the 4" pipe with 10 - 3/4" diameter pieces of PVC conduit.  I placed a piece of curled up newspaper inside the 3/4" conduit (think back to the days as a kid you made up a spy scope by making a tube out of newspaper).  I then filled all the holes with spray in insulating foam (Great Stuff was the brand).  

Now I have a weather/bug sealed and insulated, expandable entrance panel that looks like the vastly more expensive commercial units.  Feed the coax through a piece of the 3/4" conduit and seal both ends with Monkey - uhh - Stuff (Duct Seal).  

Because you lined the 3/4" conduit with newspaper, the foam plug made by filling it will push right out.  When it's there, it's an air tight, insulated plug.
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K9KJM
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« Reply #4 on: May 18, 2009, 11:29:21 PM »

VERY good ideas from KB1NXE!     Installing the 3/4" plastic conduit INSIDE the 4 inch dryer vent for future expansion is a really slick way to easily add more coax/wires later without disrupting the overall seal!

And thanks also for pointing out that the copper should be isolated from the aluminum. Most of us know that and do it in practice, But fail to mention it to others.

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KB1NXE
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« Reply #5 on: May 19, 2009, 07:17:54 AM »

Ken,

   Thanks for the comments.  I plan to write up the whole idea and submit to QST for a possible article.  I documented the whole thing with pictures.  One thing I did forget to mention is because I used an outside electrical box, I ditched the plastic hood.  Could have done the same thing with a 4" piece of PVC, but on the inside I have this handy flange to screw against the wall with the dryer vent.  Looks neater, but then again, it's covered up by the coax patch panel I built.  Anyway, the new shack is almost complete.  Anyone have Tower Seeds left over:~)

Jim
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AA4PB
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« Reply #6 on: May 19, 2009, 02:09:29 PM »

Round Copper Cable - while great for a 60Hz safety ground, exhibits a larger impedance to higher frequency currents
-------------------------------------------------------
But the fact is that most all building downleads use round cable. I expect it is a trade off regarding ease of installation. If you do use strap then I expect you still need to be concerned with its DC resistance because all of the lightning energy is not high frequency. The strap needs to be large enough that it isn't going to burn through, which could in turn start a fire.
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K9KJM
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« Reply #7 on: May 19, 2009, 10:50:28 PM »

Flat copper strap of about .022" or so thickness is the material of choice used at commercial tower sites for lightning protection.  In many years of working with this stuff, I have NEVER seen any flat copper strap "fuse" open from lightning.......

And that includes the "old" installations from many years ago when 2" wide strap was the norm. Nowadays commercial sites use 4" and 6" wide strap.
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KB1NXE
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« Reply #8 on: May 20, 2009, 08:28:25 AM »

AA4PB - Bob,

   I'm confused by what you are challenging.  The facts are:

A round copper conductor (for the sake of argument 0 gauge) will present a larger impedance value to a high frequency current than a 4" wide copper strap.  Skin Effect.  Physics.  Fact

Buildings MAY use a large diameter conductor for lightning Protection.

There is a DC product to Lightning.  It will not see either conductor as a difference in impedance.  But this is NOT what I was talking about.

Your point?
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AA4PB
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« Reply #9 on: May 20, 2009, 03:48:15 PM »

I'm not challenging the use of heavy copper strap, only whether is is really an absolute necessity given his difficulty with getting it through the joists. That's based on my observance of many building installations using round cable and a search of the internet for lightning suppression equipment showing mostly round cable supplied as standard.

My solution to the routing problem would likely be to put the single point ground plate on the outside.
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KB1NXE
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« Reply #10 on: May 21, 2009, 08:23:27 PM »

Bob,

   OK, I just didn't understand where you were going with your comments.  I truly was confused by your comment.

   I guess one way he could route it out using a copper strap is over a windowsill.  Kinda snaking it around and form fitting it to all the corners.  There's not a huge worry about lightning from inside the shack (unless he's real unlucky) needing to follow the path of the copper.  We all know that a lightning ground needs to be as straight as possible with gentle turns where needed.

   With the strap you still would be able to close the window.

   Still, for a permanent installation, a through the wall hole is the most convenient way to go.
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K4JC
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« Reply #11 on: July 24, 2009, 11:54:47 AM »

KB1NXE, that's almost exactly what I've been wanting to do! I already have a length of 4" PVC and a metal electrical box to mount on the outside wall, but I hadn't considered using the 3/4" conduits inside the 4" pipe. I may just have to steal your idea! :-)

My main issue has been finding someone who can drill a 4" hole through the wall (it's concrete block.) I don't have the nerve (or the tools) to do it myself and no one I've spoken with has the right tools. Anyone have ideas? Thanks!
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AB4ZT
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« Reply #12 on: July 24, 2009, 06:25:16 PM »

I just installed a new homemade cable entrance panel to route coax, switch, and rotator lines through my concrete wall.  It consists of 1/4 inch aluminum plate with connectors on each side, with the connecting cables running through a 6 inch square hole in the wall.  I made the hole using a hammer drill (one hole in each corner), an angle grinder with a concrete cutting blade, and a wide chisel.  You don't want to try an "drill" one big 4" hole at once, but rather drill several holes around the circumference and knock out the middle.  If you do not have a hammer drill and angle grinder, you should rent them - a very tough job otherwise.  And also - I agree with AA4PB - the grounding plate should go on the outside.
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WB5JEO
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« Reply #13 on: July 24, 2009, 09:09:08 PM »

If you read the technical articles associated with the "for real" suppressor makers, you mostly see the emphasis on surface area of the conductor and length. The emphasis is definitely on length, and it takes a lot of wire area to make up for length. Observing routine buildings and residences is no reliable guide. The residential norm of a driven ground rod with a longish copper wire is not adequate for what we're talking about. It's a safety ground only. That is quickly discovered when the crunch comes and you're filing an insurance claim for most of the electrical devices in the house.

Yes. Strap is a pain to work with, especially from equipment to nearby SPG, and I don't consider it a necessity at the equipment end. I use nickeled copper strap on some, because I found a big pile of it at the recycler, but it's just not practical for some of the equipment. I'd have to run rather long with strap (and deal with the problem it presents when you want to pull out equipment to get to the back). The shorter run with heavy solid copper wire is the trade-off. And anything is better than "grounding braid."

I think you have to look at this kind of like you should look at the priority of the antenna in the station system. If you can radiate, the best rig is crippled. If you can't dissipate the surge because the ground is inadequate at ground level, it doesn't much matter what you've used to tie the equipment to it. There are probably as many truly harmful errors made in ground system layout as there are attempts with poor materials. Things like grounds run down towers without being bonded to the tower, creating a fusable path that fails when it takes the load without the tower involved and leaves you worse off - or driving three 8-foot ground rods three feet apart, thus using three rods that just act like one.

Big, continuous conductor surface, as short as you can manage, in tough situations, opting for the big-surface conductor that give you a significantly shorter path.

The old lightning rod folks knew about such things and maximized the surface area of their rod supports and conductors by using a twisted star shaped or fluted cross section.
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N3OX
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« Reply #14 on: July 26, 2009, 11:01:36 AM »

"A round copper conductor (for the sake of argument 0 gauge) will present a larger impedance value to a high frequency current than a 4" wide copper strap. Skin Effect. Physics. Fact "

True enough, but it's also a PITA to route outside, and a short bonding conductor from SPG to ground system is... well, SHORT.

It is also physics and fact that a 12" wide copper strap is better than a 4" wide copper strap.  But that would be even less convenient.

It's reasonable thing to suggest the use of a short length of heavy round cable instead of copper strap for a short ground lead.  "Less impedance" is not always important if the impedance of both things is very, very low compared to some other impedance (like the impedance from the ground system to earth).  You don't want it to fuse open and you don't want it to be high inductance, but if it's short enough it doesn't matter in comparison to other impedances in the system.

Now, that said, using a dryer vent with a wide copper strap through it is also a reasonable suggestion that allows a lot of future expansion of lines in and out of the shack.  But it's not the only acceptable thing to do, IMO.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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