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Author Topic: RF Ground Strap  (Read 7030 times)
N2EIK
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Posts: 32




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« on: October 29, 2009, 04:34:37 AM »

I have always been told to use WIDE copper braid or WIDE strip of copper for an RF ground (like the Georgia copper adverts on eham), all for the reasons we all know concerning RF ground vs DC ground.
Heres the question...

Has anyone used aluminum foil tape?
Its easily obtainable at any home depot/lowes/hardware store. Its VERY wide.
Mounting is easy because of the foil backing.

I am a Television repairman by trade (30 years) and
all of the new LCD's and some plasmas are using a similar tape on internal shielding.

I have read that the conductivity is lower than copper, but NO CORROSION issues, and it's relatively cheap and easy to use.

Run one strip against the back wall or underside of your counter/self/desk and run a separate strip to each piece of gear using the adhesive to fix the strip to bare metal (leaving the paper backing still on the foil on areas that you dont want sticking to anything). Heck, for that matter run a long strip out of the shack to a ground rod.


OPINIONS?
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AA4PB
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« Reply #1 on: October 29, 2009, 05:40:21 AM »

Aluminum foil is usually very thin and not very strong. If it does take a large current surge and heats up, the paper backing is probably not a good thing. It might be useful for conduction normal low-level RF currents but I'd hesitate to use it in an area where it could be subjected to large lightning surge currents.
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N2EIK
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« Reply #2 on: October 29, 2009, 11:35:58 AM »

Well, like I said..RF ground. Not DC ground and I would think (imho) that if your taking a lightning whack strong enough to burn the foil backing, it would be the smoke coming out of your rig that you should be more concerned about. LOL.
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AA4PB
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« Reply #3 on: October 29, 2009, 12:46:14 PM »

Unfortunately, lightning doesn't know the difference between an RF ground and a DC ground :-) If you've got conductors connecting equipment together and have a nearby lightning strike they are going to have current flowing in them. The idea is to make the impedance low enough that you don't get much voltage difference from end to end so that you prevent currents from "smoking" your equipment.

You asked for my opinion, and it is that ordinary paper backed aluminum foil is not a good idea for a station equipment grounding system. Its okay for internal RF shielding where it does not become part of a station ground system. I've used copper foil for VHF/UHF antenna counterpoises, etc. I've seen it used to line the inside of plastic consumer equipment enclosures for shielding.
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K9KJM
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« Reply #4 on: October 30, 2009, 12:42:27 AM »

I agree.

Aluminum roof flashing (Which is much thicker and heavier) CAN be used for a lightning protection ground, But the problem is making a good electrical connection with either the foil or the thicker flashing material.

It is much easier to make connections with copper. Nice copper roof flashing (Of about .022" or so thick) can be cadwelded, Silver brazed, Or indoors in a pinch even soft lead/tin soldered, Where the Aluminum cannot.
Note that they also make an adhesive backed copper foil, Which could be soft soldered to for something like an indoor RF shield.

(Braid of any type should be avoided if possible, And only used where needed, Like to bond a moveable door to it's door frame, etc.)
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K9KJM
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« Reply #5 on: October 30, 2009, 12:47:12 AM »

For a reasonable priced source of the flat copper strap, Check with local upscale roofers, Or your local home supply store. Ask for copper roof flashing. If they do not have it in stock, They can order it.

Common sizes are 6 inch wide by 10 foot long roll, Sells for less than 30 bucks. (That could be cut down to 2 inch wide strips with a simple hand tin snips, Making 30 lineal feet of 2 inch wide strap for only a buck a running foot......)

Also available in wider sizes and longer rolls.

The stuff you want is around .022" or so thick.
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WX7G
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« Reply #6 on: October 30, 2009, 06:11:46 AM »

For RF grounding, aluminum foil will work electrically as well as copper strap. The inductive reactance of each exceeds the RF resistance and so there is little difference in the impedance of a 2 mil tape, for example, and a 10 mil strap.
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K9KJM
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« Reply #7 on: October 30, 2009, 11:37:10 PM »

While the thin aluminum foil will work as good as thick copper (Or thick aluminum flashing) For a small RF shield, The thin foil will NOT function as a good lightning protection ground, AND how do you get your low resistance CONNECTION between pieces of foil and the ground itself? The "sticky" side of the foil is an adhesive that is a NON CONDUCTOR. If you simply "stick" one piece of the foil on top of another, Odds are slim that you will have any type of electrical connection at all!

I have that thin aluminum foil in stock here, And it is nice for some applications, But there are serious problems getting good contact between pieces. It's best use seems to be for small projects to use as an RF shield.
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OLDGEEZER
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« Reply #8 on: October 30, 2009, 11:41:19 PM »

"Run one strip against the back wall or underside of your counter/self/desk and run a separate strip to each piece of gear using the adhesive to fix the strip to bare metal  
"

Will not the adhesive inhibit electrical contact?

Dave
VE9YA
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N2EIK
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Posts: 32




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« Reply #9 on: October 31, 2009, 04:25:38 AM »

This was a thought. Everyone has posed some great questions and responses. When I realized the manufactures are using this stuff I wondered about a ham radio application. The conductivity of the adhesive is (in my opinion) the questionable part.(wouldnt it be questionable for the television R&D engineers?) I may take this a step further and start testing the stuff for conductivity. Although there is a mighty fine post re:cutting roofing flashing, the aluminum tape would have taken it a step closer to "plug and play".
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WX7G
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« Reply #10 on: November 02, 2009, 11:21:11 AM »

The original question was about RF conductivity and not about the ability to withstand lightning current. Someone might want to start a lightning thread and we can explore whether copper strap is up to the task.

To improve the electrical bond between two sections of adhesive backed aluminum tape poke the overlap many times with a X-acto knife.
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AA4PB
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« Reply #11 on: November 02, 2009, 11:43:16 AM »

If, as he stated, you run aluminum foil behind your desk, connecting all the equipment together and then out to a ground rod how do you prevent lightning currents from flowing in that foil? Yes, it can provide a low impedance path for RF ground but how do you convince lightning surge currents not to melt it?
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WX7G
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« Reply #12 on: November 02, 2009, 06:35:21 PM »

How do you convince lightning currents to not melt copper? We might step back and determine what is really needed for the typical 300 kA, 1.2/20 us lightning strike.
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WX7G
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« Reply #13 on: November 02, 2009, 07:27:51 PM »

For lightning let's start with the National Electrical Code. It says a TV antenna shall be grounded using a #10 AWG (or larger) aluminum or coppper wire. #10 wire has a cross sectional area of 0.008 sq inches.

Compare this to Nashua 617020 322-3 aluminum foil tape. It is 3" inches wide with a thickness of 0.005". It has a cross sectional area of 0.015 sq inches. It is equivalent to #7 AWG wire.
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K9KJM
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« Reply #14 on: November 02, 2009, 11:13:31 PM »

Commercial tower sites use the .022" thick flat copper strap for lightning grounds from the "Single Point" ground panel out to the ground system.  I have never seen any of that copper strap fused open by lightning strikes.  (These tall towers take direct lightning strikes most every storm.)

Note that through "voltage division" The main part of a direct lightning strike is put to ground through the tower legs out to the ground system. Only a small portion of the strike energy is put to ground through the Single Point ground panel.

http://members.cox.net/pc-usa/station/ground0.htm+
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