Coil shielding problems

[Stefano]

Hi all,
I've made a 10" mono coil which allows me to sample at 15-20 us and detect at a reasonable distance.
I' m planning to use it on beaches and possibly in sea-water, so I think i have to shield it.
Following the forum suggestion I tried several kind of alluminium foil shilds, from the thin alluminium foil used in kitchen for cooking to the thick one and also a type of 3M alluminium tape .
All them make the decay time much slower (20-30 us longer) and so the performance gets worst.
It seems to me that alluminium is not good at all for coil shielding no matter what the thickness is.
Unfortunately in the town wher I live I'm not able to find 3M copper tape like the one Carl suggests for his HH as well as conductive nickel or graphite paint.
I'm wondering if some of you experienced the same problems with allumium and if there is any other way to shield coils.
Thanks
Stefano

[jackdetect]

Re: Coil shielding problems

Are you make a space between shield and coil wire, if you not, you was get increasing of capacitance.
If you open battery 9V (be carefully), you will find conductive graphite dust. You can write on cardboard in thick skin, with graphite dust. After, you can lacquer cardboard to protect. Also you must put litz wire for grounding.

Jackdetect.

[Reg]

Re: Coil shielding problems

Hi Stefano,

There are several different strange things one can try to shield their coils that should work. The key is the material has to be conductive.

Remember, the object of the shield is two fold, with the first objective to eliminate the capacitance effect and the second to absorb external noise for noise reduction.

If a very thin shield material is used, then there should be little or no increase in the delay since any signal from the shielding would have disappeared before the desired sampling time. If the material is barely conductive enough, the capacitance effect can be eliminated but it might not work that well for the noise elimination. That is why there are preferred materials.

Again, the key to any material is that the material be conductive. A simple check with an ohmmeter will confirm this to be true.

One very simple material I have found in the US is a simple form of metallized ribbon. This is the kind used for bows and ribbons, party favors, etc. Generally, it is about 1/2" wide and is generally a silver color. Now, there are different kinds of ribbons that may look like they would work but many are not conductive so a person has to just check them with a meter until they find one that at least one side of the ribbon is conductive. Here, I have found some in our Walmart storess. This seems to work and is quite cheap.

This material seemed to be conductive enough to take care of the capacitance problem. The difficulty lied in connecting a wire to the ribbon. What I did was to simply take a piece of about 30 awg stranded wire, strip a couple of inches of insulation off and then tape the stranded wire to the ribbon, making sure the tape is tight.

Another simple material that seems to work is something we find on some of our insulation used in our houses. We have different sized foam boards with a very thin metallized mylar film attached to one side. This conductive mylar film can be stripped from the foam board and used as a form of shielding. One can wrap the windings, or simply lay a piece of the material on the bottom and top of the coil form between the windings and the form to shield the two sides.

Another material I have used is a conductive rubber tape used for high voltage splices. This can simply be wound around the windings.

In all cases, make sure you have some material as a spacer between the windings and the shielding. Spirawrap works well but if it isn't available, then one can split a thin plastic tubing and use it for a spacer.

If the material is to be glued inside an enclosed form, then there still needs to be a spacer between the windings and the shielding material. Something like a 1/8" to 1/4" foam or other rigid material could be used for the spacer.

Other strange things that could be tried might be some thicker litz wire, providing the individual strands are small enough. Just flatten and wind the litz wire around the coil windings leaving a small space between wraps. One would have to experiment with this technique.

There are some conductive cloths available so you might check with shops carrying sewing materials. Also, there are various types of metallized mylar materials available that most likely would work.

Finally, some people have used a stainless steel (ss) mesh or very small ss wire. I have not tried this so I don't know how well it works.

I hope this gives you some ideas.

Reg

[jackdetect]

Re: can not cancel noise with shield!

Hi Reg,
I agree with most thinks, you give a lot of ideas.
But shielding can not canceling noise. I always consider shield to cancel capacitance changes between ground and coil (change resonance). Maybe shield can cancel some electrostatic noise, but most important-electromagnetic, it is absurd.
Jackdetect.

[FJ]

Re: can not cancel noise with shield!

How can you make such a statement??? This type of shielding has been used in many disciplines since the beginning of electronics to shield external magnetic interference. Why would you say this is "absurd"??? This is fact.
FJ

[Reg]

Re: can reduce noise with shield!

Hi Jackdetect,

PI's have a broadband preamp and as such, are highly suseptible to external noise, both electrostatic and electromagnetic in nature.

The easiest way to know for sure if shielding reduces noise on a PI is to simply measure the difference. Hold the coil well above the ground so the ground capacitance isn't a problem and rotate or change the orientation of the coil. By observing the output of the preamp with a scope, and of course, listening to the audio, you will be able to compare the noise levels which normally will change depending upon the position of the coil in relationship to the loudest noise source. Do this using a coil with no shielding and then with one with shielding to compare the effectiveness of the shielding. As another simple test, one can just disconnect the shielding ground connection and watch the noise level rise considerably.

Reg

[jackdetect]

Re: Open loop is not fully faraday shield!

We are spoken about open shielding loop, not about closed faraday shield. It is a big different.
Any electromagnetic line of force who passed within coil will be induced, apart of they source ( Pi field or Em noise), and open shielding loop certainly can not blocked it.
Why then in serious geophysical locator designers using a differential receiving coils?
New Lorenz model, also.
Reg, I can not explain you experiment. Maybe electrostatic noise can be important factor near by ground. I was read somewhere you text about some differential coils to prevent EM noise (8 coil, I think).
Jackdetect.

[Eric Foster]

Re: Open loop is not fully faraday shield!

Hi Jackdetect and All,

It is perhaps incorrect to describe shielding as cancelling noise. It is better to refer to shielding as attenuating noise. We also need to consider what frequency noise we hope to deal with by this method. As Reg mentioned, the front end of a PI has to be broad band. Not necessarily down to d.c., but it could be from a few hundred Hz, up to 100kHz or more, depending on how fast a decay curve we want to look at. This means that any coil shielding must be transparent within that frequency band. The aluminium shielding referred to in the first post, obviously was attenuating frequencies at 50kHz as it was increasing the useable delay to 20uS or more. In this case it was impeding the higher frequency components of the TX pulse from getting out. If the shielding starts attenuating at 100kHz, then for higher frequencies, there will be ever greater levels of attenuation, depending on the skin depth characteristic of the particular material used, and how thick it is. Aluminium or copper tape could be used, but it would have to be very thin, more like the aluminium film deposited on Mylar, for decorative material.

Even when the shielding is giving high levels of attenuation, it depends how near the source of the interference is, as to whether there is any effect. i.e if the shielding reduces the interference amplitude by a factor of 100, but the source is brought nearer, so that there is 100 fold increase in signal, you are no better off. R.f. interference has two effects on a PI detector. One is a beat note effect, such as experienced in here in England with the high power 200kHz transmitter at Rugby. A PI detector running at a pulse frequency which is a close sub multiple of 200kHz will experience beat interference. e.g. 200Hz, 2kHz, 20kHz. A low frequency warble of the audio will be the result. That is why a TX frequency control is very useful, as a small adjustment can increase the frequency of the beat note to the point where the integrator smooths out any residual effect. Obviously any reduction in the interference amplitude as seen by the receiver input, helps enormously. You can certainly see the reduction in the 200kHz signal when a coil is shielded, as compared with an unshielded one.

The other type of r.f. interference results from much higher frequencies, even to 100’s of MHz. This is usually from very close range devices such as mobile phones, VHF transceivers etc. This results from the small amount of signal that gets through the shield and which is modulated with audio. The input circuitry of the detector, with its protection diodes, acts as a demodulator and presents the receiver with audio frequencies which are within its bandwidth. Even just switching a transceiver on without modulation can cause a dc offset at the receiver with a resulting bleep in the detector audio before the differential integrator cancels it. Again, any reduction rf pickup by shielding the coil is valuable here. Additional rf filtering is often used on industrial PI detectors in the form of ferrite sleeves and LC filters.

The effects of coil shielding is best observed on a spectrum analyser, and believe me, it does make a difference, particularly if you want to comply with EU rf emissions and immunity standards.

Low frequency interference within the passband of the detector is not reduced by coil shielding. It is then that you have to resort to figure of 8 or differential receiver coils in some situations.

Eric.

[Stefano]

Thanks!!

Many thanks to all of you for your replies.
The purpose of the shielding I'm going to use is mainly for ground capacitive effect.
I experienced last year with my first Pi that raising and lowering the head, even on dry sand at the beach, made the clicking requency changing a lot, and that coil was shielded with alluminium foil for kitchen!
So I'll keep on experimenting different new and possibly cheap materials like the ones Reg suggested.
Thank you all again.

Stefano

[FJ]

Re: Open loop is not fully faraday shield!

Thanks for the info Eric. I'm curious, if coil shielding is useful for ensuring compliance with the EU RF emissions standard, since you don't use shielding on the large 1meter coil type detectors, do they comply with the EU standards? It seems most of them are manufactured over in Europe.
Thanks,
FJ

[FJ]

Re: Open loop is not fully faraday shield!

Hi,
Does the Lorenz use small coils or only large 1meter and large size coils? They don't shield those large coils. A stacked set of them opposed acts like a gradiometer and cancels noise. Eric has often suggested wiring a coil in a figure 8 pattern to give the same kind of noise canceling properties. Maybe the lack of shielding on the large coils is the reason for 2 seperate coils to reduce noise.
Thanks,
FJ

[jackdetect]

Re: Valuable information, thanks!

Thanks Eric,
I agree with everything, and learn a lot about thinks whose can only guess.
Jackdetect.

[jackdetect]

Re: Differential coils!

Yes, you right. But such conception will still have a problem with capacitance changes to ground. I am very curious about EM61 and EM63 solutions. They patents are very poor with data.
I think 8 coil is very unsuited for search, because of second signal.
Jackdetect.

[Eric Foster]

A Question

Hi Stefano,

Was the foil grounded via the coax braid? It certainly shouldn't have reacted with dry sand; unless the sand contained iron minerals perhaps?

Eric.

[Eric Foster]

Re: Differential coils!

Hi Jackdetect,

It is unlikely that capacitive changes have an effect on large loop geophysical PI's. My early PI's had sample delays no shorter than 50uS and used unshielded coils. There was no noticeable capacitance effect on a wet beach, which is where it is worst. It wasn't until I made a unit that sampled at 20uS, that it became apparent that a shield was necessary. Most of the earlier PI's were overdamped and coil/cable capacitance was relatively high in comparison with the coil ground capacitance changes.

I have found that figure of 8 and differential gradiometer configurations are excellent at cancelling power line noise and are probably good up to a few hundred hertz. However, they are not much use at higher frequencies. I found that 200kHz noise was not reduced much by a differential arrangement.

Eric.