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Author Topic: RF Ground Strap  (Read 6926 times)
AD5X
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Posts: 1430




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« Reply #15 on: November 03, 2009, 04:18:16 AM »

I use the aluminum tape for a lot of my projects, especially for large ground areas inside outdoor elecrical plastic boxes.  To interconnect sections of tape, I overlap the tape.  For the upper piece of tape, I fold over the end of the tape so no adhesive is between the upper and lower tapes.  Then I use a couple of stainless steel screws/washers/lockwashers/nuts that pass through the upper and lower tapes and box to provide a good interface.

Phil - AD5X
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N2EIK
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Posts: 32




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« Reply #16 on: November 03, 2009, 05:27:24 AM »

Ok, CONDUCTIVITY...Here is my findings. I took two strips of foil tape, foun inches long. I peeled the backing off and stuck the first piece down on a piece of card stock. I pasted the second one down on top of it with a 1/2 inch overlap. rubbed it firmly with my bare finger to smooth it down. Then I tested the resistance from the farthest edge of strip "a" to the farthest edge of strip "b" with a Sencore DVM56A. Zero Ohms.

Anyone else care to confirm this?
And please lets leave lightning out of this thread because the topic is RF GROUNDING. Lets assume one had proper grounds for lightning.
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N2EIK
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« Reply #17 on: November 03, 2009, 05:34:22 AM »

To clarify this: the bottom of one (the adhesive side) to the top of the other, no foldover. just stick one down on top of the other.



copy:

RE: RF Ground Strap  Reply  
by N2EIK on November 3, 2009  Mail this to a friend!  
Ok, CONDUCTIVITY...Here is my findings. I took two strips of foil tape, foun inches long. I peeled the backing off and stuck the first piece down on a piece of card stock. I pasted the second one down on top of it with a 1/2 inch overlap. rubbed it firmly with my bare finger to smooth it down. Then I tested the resistance from the farthest edge of strip "a" to the farthest edge of strip "b" with a Sencore DVM56A. Zero Ohms.
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WX7G
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« Reply #18 on: November 03, 2009, 07:54:29 AM »

The world has 25 years proven reserves of copper. Be conservative with that copper ground strap.
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W8JI
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« Reply #19 on: November 06, 2009, 08:36:05 PM »

Why do you need an RF ground like that?

http://www.w8ji.com/rfi_rf_grounding.htm

Are you bringing a single wire feeder to the desk, and running a single wire feeder between equipment on the desk?

Or do you have an antenna mounted right on or near your desk?

I would not hesitate to use aluminum tape under a carpet to establish a groundplane for an upper floor radio room with a very close antenna or an end-fed like a longwire brought into my operating position. I doubt I would run aluminum tape all over my desk because it would be a PITA to get connections to it.
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W6BR
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Posts: 59




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« Reply #20 on: November 12, 2009, 09:42:20 AM »

For a good and reasonable Ground Strap go to your local Hardware Store and pick up a rool of copper plated Plumbers Tape. Works Great!!

Remember the KISS - keep it simple stupid!! :-)

Ron, W6BR
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K9KJM
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Posts: 2416




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« Reply #21 on: November 13, 2009, 12:39:03 AM »

Copper "plated" plumbers strap is just that. VERY thin copper plating that will not last more than a month or so outdoors, Not much longer indoors before it starts to rust. The steel perforated strap is also much too narrow, And full of holes that reduce it's surface area, Which is already far too narrow.  Not at all suitable for grounding.

For a decent ground strap, Get some copper roof flashing from the same home supply store. Usually comes in 10 foot long rolls 6 inches wide. Just take a tin snips and cut it into 2 inch wide strips for 30 lineal feet of nice .022" thick ground conductor that is the same as the pro's use.
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NA9Q
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« Reply #22 on: November 22, 2009, 07:38:12 PM »

The issue with grounding conductors whether for RF current grounding or lightning protection is not the cross sectional area of the conductor but rather the surface area.  That is why copper strap is the recommended material.  It has greater surface area for the weight of the copper.  A wire is a cylinder and minimizes the surface area.  With RF currents or a lightning impulse the current flows only in the first few mils of material below the surface. Hence the term 'skin effect'.  A current due to a lightning impulse behaves the same as an RF current since the wave front of the current impulse contains much high frequency energy.  The skin effect comes into play in either case and only the outer skin is carrying the current. The surface area is essential as what you are trying to achieve is low inductance, NOT just a low  resistance.  Both qualities of the path are important.  You can have a low resistance ground system but if it does not have low inductance as well, you may as well have left it out.  A lighting induced current impulse will not follow the high inductance path to earth.  The current impulse will find the lowest inductance path and if it isn't extremely low a pulse of high voltage will appear across the ground path and it is this voltage impulse that does the equipment damage, not the current flow.

I've spent 33 years in the manufacturing and engineering side of the broadcast industry and I can tell you it is impossible to over design a ground system.  Copper strap used is normally .030 to .060 thick and 2 inches wide at a minimum.  The thicknes is mainly for mechanical issues and to allow the strap to withstand the temerature of braising.  Junctions between grounding conductors must be braised or Cadwelded.  Braising requires much higher temps that soldering and Cadweld is best but requires special equipment and is done at extreme temps.  Strap of less than .020 cannot withstand these temps.  The thinner strap will be destroyed by the braising or welding process.  Soft solder (Tin/Lead) is never used as it will not withstand corrosion.  Soft solder will corrode over time when exposed to moisture and the result will be a bad connection.

These are commercially viable solutions but sometimes the cost of materials may be beyound the average ham.  But each station should have a ground system as close to these goals as possible.  Use the widest and heaviest copper strap you can find and afford from the station to the ground  rod.  Clamp it mechaically to the rod and then braise it to the rod.  If you have several pieces of strap to make up the requied length to reach the station, overlap the strap at each joint and bolt the joint together with brass hardware and then braise the joint. Once in the station it usually becomes difficult to connect each peice of equipment to the ground strap with strap.  Here use a #10 wire as short as possible and bolt and braise the connectio to the strap.  At the equipment end use a firm bolted joint with a crimp on lug.  Most transcievers have a ground post, use it.

That takes care of the inside.  Out side make sure your tower, antenna supports, lightning arrestor feed through panel etc. are also connected to the ground rod with strap.  Again it will not be possible to do a braised joint in each case but at least a firm mechanical joint should be used.  But these joints will require frequent maintenace due to weathering.

Mike NA9Q
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K9KJM
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Posts: 2416




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« Reply #23 on: November 22, 2009, 10:16:47 PM »

Very good explanation by NA9Q.

I have some copper ground strap here from a 1950's vintage AM Broadcast station that measures .038" thick.  Over the years I have watched it get thinner and thinner. It seemed to remain around .025" for quite a few years, Now the last time I checked some it was .022"  Just like lots of other products. Pay more for less product.

For Cadwelding, NA9Q is quite right, Even at .022" it is possible to burn through such thin strap.
A skilled welder can braze the thin stuff usually with no problems.

And I agree 100% that the "soft" lead/tin type, Or even the "silver solder" sold for plumbing use should never be used for grounding.

There are various trade names for the correct braze rod, "Silfoss"  "Silvalloy" and others. They are a silver/copper/nickel alloy rod with a very high melting point, But can be welded with a small hand held torch with MAPP gas.   Back in the olden days we called that "Silver Soldering" (Those were the days before the little hand held torches became popular, And it took an ox/acy torch set up, Or at least the "Prestolite" type torch to flow the rod.  I no longer call the correct rod "Silver Solder" because of the confusion with the soft type solders. It is actually a braze rod.  

The correct rod can usually be purchased from any well equipped welding supply store, And it is used by air conditioner repairmen. A skilled welder can get LOTS of joints out of just one rod, Making it a very low cost way to get a very high quality bond.

Cadwelding became popular because no special skill is needed to do it- But it is fairly expensive.
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