Copper tape shielding and capacitance

Hi All,

Just mooching around the shed and decided to experiment with something that has been bugging me.

I overlaid two pieces of copper tape:

Attachment 6133

As is done in most guitar construction efforts.

Now, a capacitor is essentially two pieces of metal overlapping each other separated by a dielectric material.

So two pieces of tape separated by the glue backing, does this equal a capacitor? In other word is the overlapping of tape inserting capacitance into our guitar circuitry.

A little surprisingly, the answer is Yes.

I simply measured the capacitance between the two pieces of copper.

In the randomly selected but probably indicative sizes used in my experiment it resulted in almost 0.4 nF (Nano-Farads).

A small amount, for sure, but in an AC circuit the reactance is also of concern. Reactance is the resistance presented by a capacitive circuit - as frequency decreases reactance increases, so if our signal is travelling across the shield, and as the ground component of the AC it can, will this affect our bottom end response?

I don't really know enough about the response side of things to say, and it would be difficult to objectively test.

So I'll throw it over to some of the more experienced wire fiddlers to shoot down my theorem - as the scientific method would ask me to do...

Oh, btw, the way the simple way to overcome this reactance issue would be to make a solder bridge across the joins, thus the path is no longer capacitive.

Quote:

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Originally Posted by Rabbitz

Oh, btw, the way the simple way to overcome this reactance issue would be to make a solder bridge across the joins, thus the path is no longer capacitive.

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And this is why I always solder the joins and only use good quality shielding tape... Not snail tape!

So after a little more experimenting albeit at radio frequencies I came up with some surprising numbers.

At 1.6MHz through 0.4 nF showed a reactance of around 450Ω. (this is the lowest frequency my test gear will measure)

I then started digging through my radio theory books to find the equation, then I had a brainwave, maybe the interwebs has the equation? Better still it has a calculator:
0.4 nF at 300Hz = 1.3 MΩ
0.4 nF at 3kHz = 1.32 kΩ

The amount of reactance/resistance surprised me.

Bottom line, follow DingoBass' advice = solder the joins!!!!!!!!!!

dude, you have too much time on your hands...

very interesting info though

As in solder where the tape overlaps Rabbitz ?

Spot on Jrod :)

You only need a dab of solder on each join, but if you are anally retentive like I am about these things do the whole join..

Quote:

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Originally Posted by stan

dude, you have too much time on your hands...

very interesting info though

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Well, today I've made rillettes, mowed the lawn, finished sanding the ST-1, potted some plants, put a clear coat on the ST-1, fixed a leak in the roof, worked out why the water tank doesn't seem to be filling - is it any wonder I was pottering in the shed (out of sight) waiting for the clear coat to dry?

:)

Hey Jarrod, yes a blob will do, essentially we want a low resistance path between the pieces of copper, kind of shorting out the capacitance/reactance.

Now I hope we don't have to worry about "skin effect" surface flow between the layers in the unbalanced circuits... ;)

Rabbits, I about to reply show my ignorance.
The tape is connected (or should be ) directly to earth., and the frequencies we would be draining out the earth would be either 60 Hz or 120 from some flouro's .?
So if we have a direct path for the pickups, and a filtered path for EMF, would it effect the signal from the pickups?

Hi Bruce,

The ground is part of the circuit, it is the "other half" of the AC cycle if you like.

A lot of the EMI (noise, hum, etc) is lessened due to the handling of the ground outside of the guitar. There is still part of the signal also travelling in the earth.

I gather the cavity shielding forms a type of Faraday Shield, which aims to prevent the electrical noise from getting into the signal and becoming part of the amplified signal.

I am just trying to really understand this myself (a bit of a new field for me and is an idle curiosity), but for me the "drain analogy" seems a little flawed when you really start to dig in. However, it is a useful simplification.

The issue here, if it is in fact an issue, is not the ability of the EMI to be removed it is the fact that the dielectric effect of the overlaps could make it difficult for the earthing to happen due to the high resistance (reactance) presented at the join. 0.4 nF @ 60Hz will be seen as 6.6 MΩ. There are a million other factors, such as alternate paths in the wiring, which will probably mitigate the effects, but I think we really want to get rid of the reactance if we want the Faraday Shield to work as it should.

The capacitance/reactance thing is really a bit left field and probably is of little real world consequence, but as it will take about 10 minutes to solder some joins. After all the fiddly mucking around with the tape, I think it is worth the effort.

Yep, it's a Faraday shield :)

Is all of this just a good argument for using shielding paint instead?

Time to get out a couple of theory books, if I still have them. :)

Because I seem to remember that a Faraday Cage works by ducting the unmatched charge around the surface of the cage and this can then be directed to earth (as opposed to signal ground). Maybe I shouldn't have joked about "skin effect" and surface currents earlier.

:)

One of the really good things about this forum, is that on other technical forums I have used in the past a troll would have abused me for being a Faraday illiterate or something similar by now.

Who wouda thunk buildin' geetars wud give the ol' brain box such a workout?

Quote:

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Originally Posted by wazkelly

Is all of this just a good argument for using shielding paint instead?

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Now where is the fun in that? :)

Hang on, is there capacitance between the paint particles? Hmmm...

Quote:

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Originally Posted by dingobass

Spot on Jrod :)

You only need a dab of solder on each join, but if you are anally retentive like I am about these things do the whole join..

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Basically welding the bugger together essentially... I always test the shield with my multimeter and I solder it up anyway, just to be on the safe side...

Quote:

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Originally Posted by wazkelly

Is all of this just a good argument for using shielding paint instead?

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That is a moot question indeed...
Some folk swear by paint, others swear at it.

In my experience it works ok but copper tape seems more effective

Quote:

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Originally Posted by dingobass

copper tape seems more effective

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Plus it is much easier to apply, won't spill on the body or anywhere for that matter and also once you are done with it it is ready to install the other electronics!

I would have no doubt that copper tape would be far superior to paint how ever most my guitars have neither and I simply wire it together and do not get any hum or interference. My Fender US strat does not have the cavities taped and neither does my LesPaul. My question is , is it really necessary?

It is possibly an old school thing, lets face it, just about every electrical do dad is shielded these days so the chance of picking up EMF is low.
Also, most pups are shielded so this eliminates another possible source.
However, it is probably a good idea to shield regardless.

I would be surprised, (but given the crap F&G make these days I shouldnt be) if the Strat doesn't have shielding under the pick guard and the LP wasn't shielded with paint..

Strat has yes under the pick guard but not the cavities. The LP is a custom Black Beauty and quite frankly is not crap. It is a nitro finished model and is nitro in the cavities too. So I'm guessing the pre shielded electronics is the answer. Also my strat is a 60th aniversary (you have seen it) and is also is a superior guitar (not crap) lol...

TBH with you Jrod, if you gave me the choice of one of your Nebbies or the Strat or LP, I would take the Nebbie any day...

Well I appreciate that bro, but I do love my strat. In truth the LP is not really my cup of tea. But I would trade the wife before the strat. The strats worth more then her. Lol....straight to the naughty room

I sure hope the Minister for War and Finance doesn't read this thread, Jrod....
You could end up sleeping in the man cave amoungst broken bits of Strat :p

I'd take a Neb over a Frickter or Grubson anyday

Quote:

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Originally Posted by dingobass

I sure hope the Minister for War and Finance doesn't read this thread, Jrod....
You could end up sleeping in the man cave amoungst broken bits of Strat :p

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Funny you should say that. There has been many hints about a fold away bed in there :)

Any hoos. This has been a great education about sheilding Rabbitz.

I too harbour doubts about the absolute need for shielding, however given the amount of cheap Chinese-made PA gear, the amount of things with little radio transmitters in them today - bluetooth, wi-fi, radio mics, IoT etc, the electromagnetic spectrum is awash with noise.

So for a few bucks and an hours work, I guess it is worth doing. Geez, after all we spend days and days sanding the damn things. At worst it won't make things worse.

I did a switch swap on a mate's LP Supreme a couple of months ago and there was no shielding in the cavities.

On the subject of Faraday Cages there is a very impressive one at Garden Island in Sydney, it's the navy war room. The roof, walls & floor of the room and the long winding corridor into it are all welded steel and the only penetration for communications is optic fibre.

I don't know if it's still there or been moved it was around 1991 when we did the comms upgrades there, even if it's still there I guess what I installed is long gone because technology has come a long way since then ( but it's still all just zero's and one's )

Quote:

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Originally Posted by dave.king1

I did a switch swap on a mate's LP Supreme a couple of months ago and there was no shielding in the cavities.

On the subject of Faraday Cages there is a very impressive one at Garden Island in Sydney, it's the navy war room. The roof, walls & floor of the room and the long winding corridor into it are all welded steel and the only penetration for communications is optic fibre.

I don't know if it's still there or been moved it was around 1991 when we did the comms upgrades there, even if it's still there I guess what I installed is long gone because technology has come a long way since then ( but it's still all just zero's and one's )

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I wouldn't be surprised if it was still there. There is a lot of stuff there, well between Garden Island and Kuttabul.

Besides we are still using 30 year old frigates...

You should see the set up at the CSIRO, they have gold foil on the Windows.
I have read about the need of not needing shielding, but isn't that why jumbuck ears were invented?
And dummy coils?
But not to start a war, I've played in some places were hums not a problem and others that sing like a banshee .

You are right, Alm..
Some venues that have fluro lighting can cause some dramas, saw this (or should I say heard?) happen a lot in Canberra when I was chief Roadie, drinks meister, Bass and Guitar tech for a mates band.
A lot of the older pubs would cause some major dramas, where as the newer ones not so much.
The worst one was an outdoor gig in Civic, that place had so much EMF is was near impossible for the sound guys to get the setups right...

wow DB you must have been Australia's busiest roadie with extra duties getting drinks and guitar/bass tech !