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eHam Forums => Station Building => Topic started by: N2EIK on October 29, 2009, 04:34:37 AM



Title: RF Ground Strap
Post by: N2EIK 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?


Title: RF Ground Strap
Post by: AA4PB 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.


Title: RF Ground Strap
Post by: N2EIK 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.


Title: RF Ground Strap
Post by: AA4PB 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.


Title: RF Ground Strap
Post by: K9KJM 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.)


Title: RF Ground Strap
Post by: K9KJM 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.


Title: RF Ground Strap
Post by: KH6AQ 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.


Title: RF Ground Strap
Post by: K9KJM 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.


Title: RF Ground Strap
Post by: OLDGEEZER 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


Title: RF Ground Strap
Post by: N2EIK 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".


Title: RF Ground Strap
Post by: KH6AQ 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.


Title: RF Ground Strap
Post by: AA4PB 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?


Title: RF Ground Strap
Post by: KH6AQ 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.


Title: RF Ground Strap
Post by: KH6AQ 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.


Title: RF Ground Strap
Post by: K9KJM 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+


Title: RF Ground Strap
Post by: AD5X 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


Title: RF Ground Strap
Post by: N2EIK 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.


Title: RF Ground Strap
Post by: N2EIK 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.


Title: RF Ground Strap
Post by: KH6AQ on November 03, 2009, 07:54:29 AM
The world has 25 years proven reserves of copper. Be conservative with that copper ground strap.


Title: RF Ground Strap
Post by: W8JI 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.


Title: RF Ground Strap
Post by: W6BR 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


Title: RF Ground Strap
Post by: K9KJM 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.


Title: RF Ground Strap
Post by: NA9Q 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


Title: RF Ground Strap
Post by: K9KJM 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.