Shielding VS signal, how much loss

[wyndham]

From the small amout that I have absorbed on PI's and coils, I am wondering about how much Tx signal is lost due to shielding on the coil. I understand that ther are gaps in the shielding to allow the signal out but if some one might help out on understanding this, please help.
Is it a small fraction of signal loss or half or more. Are the gaps related to the signal strength or frequency? Thanks Wyndham

[Max]

From the small amout that I have absorbed on PI's and coils, I am wondering about how much Tx signal is lost due to shielding on the coil. I understand that ther are gaps in the shielding to allow the signal out but if some one might help out on understanding this, please help.
Is it a small fraction of signal loss or half or more. Are the gaps related to the signal strength or frequency? Thanks Wyndham

Hi Wyndham,
I'm not an authorithy in this field but maybe could give some enlightment.

GAP is needed to avoid a short conductive turn all arount winding(s). If you short the shield a too conductive path is enstablished and you have a big eddy current just over your coil and that's no good for good PI detection of small eddy currents due to targets: you saturate your rx stage with a big overshoot on decay curve.
So gap is needed in any case you have a conductive shield no matter signal strenght or frequency.
First gap meaning is avoid static charges (so static electric fields) interfere with detection due to capacitive coupling e.g. with soil, grass etc
Second meaning is radio- or bf-components noise reduction, but this work just on some contitions and gap extension determines how much effective could be this kind of shielding at various frequencies of external signals (other details involved too e.g. shielding material, dielectric constants, self-resonance etc).

For first point the more the signal emitted the more the eddy currents in the shield (at parity of shield). Also the more sharper the switch-off the higher the eddy-currents. Thin materials (e.g. mylar) are used cause eddy-currents depends also on conductivity of the shield, a thicker one develop higher currents at same conditions.

Your observation about frequency is right too. There are some hi-frequency PI (3000+ Hz range) that need special care on shielding material and thickness (e.g. I'm thinking at deep-starII that work around 3Khz) and so lead (really thin) tape is used cause have a better response (bad concuctivity) to higher frequencies.

In normal PIs e.g. for depth (DP, XR-71, PulseStarII, Lorentz etc) no shield is required (large coils keeped high on soil) or simple shields only e.g. on smaller e.g. 8'' or 10'' coils made of foil, conductive varnishes (nyckel , graphite) etc are needed and no special things (like copper or lead tapes are required).
This is due to low frequencies used but one can choose using lead or copper too, or graphite.

Graphite shields are really effective at higher frequencies cause they offer uniform shields without too eddy-current conductive paths inside the shell.
They are also used with success in vlf detectors for similar reasons also above 20Khz.

Best regards,
Max

[Max]

From the small amout that I have absorbed on PI's and coils, I am wondering about how much Tx signal is lost due to shielding on the coil. I understand that ther are gaps in the shielding to allow the signal out but if some one might help out on understanding this, please help.
Is it a small fraction of signal loss or half or more. Are the gaps related to the signal strength or frequency? Thanks Wyndham

Hi Wyndham,
I'm not an authorithy in this field but maybe could give some enlightment.

GAP is needed to avoid a short conductive turn all around winding(s). If you short the shield a too conductive path is enstablished and you have a big eddy current just over your coil and that's no good for good PI detection of small eddy currents due to targets: you saturate your rx stage with a big overshoot on decay curve.
So gap is needed in any case you have a conductive shield no matter signal strenght or frequency (but see later about graphite).
First gap meaning is avoid static charges (so static electric fields) interfere with detection due to capacitive coupling e.g. with soil, grass etc
Second meaning is radio- or bf-components noise reduction, but this work just on some contitions and gap extension determines how much effective could be this kind of shielding at various frequencies of external signals (other details involved too e.g. shielding material, dielectric constants, self-resonance etc).

For first point the more the signal emitted the more the eddy currents in the shield (at parity of shield). Also the more sharper the switch-off the higher the eddy-currents. Thin materials (e.g. mylar) are used cause eddy-currents depends also on conductivity of the shield, a thicker one develop higher currents at same conditions.

Your observation about frequency is right too. There are some hi-frequency PI (3000+ Hz range) that need special care on shielding material and thickness (e.g. I'm thinking at deep-starII that work around 3Khz) and so lead (really thin) tape is used cause have a better response (bad concuctivity) to higher frequencies.

In normal PIs e.g. for depth (DP, XR-71, PulseStarII, Lorentz etc) no shield is required (large coils keeped high on soil) or simple shields only e.g. on smaller e.g. 8'' or 10'' coils made of foil, conductive varnishes (nyckel , graphite) etc are needed and no special things (like copper or lead tapes are required).
This is due to low frequencies used but one can choose using lead or copper too, or graphite.

Graphite shields are really effective at higher frequencies cause they offer uniform shields without too eddy-current conductive paths inside the shell.
They are also used with success in vlf detectors for similar reasons also above 20Khz.

Best regards,
Max

[Max]

From the small amout that I have absorbed on PI's and coils, I am wondering about how much Tx signal is lost due to shielding on the coil. I understand that ther are gaps in the shielding to allow the signal out but if some one might help out on understanding this, please help.
Is it a small fraction of signal loss or half or more. Are the gaps related to the signal strength or frequency? Thanks Wyndham

Hi,
BTW from my experience on PI a good shield made e.g. of aluminium/mylar film cut less than 10% of total power at low frequencies (under 1Khz).

Best regards,
Max

[ApBerg]

Nice test to see what a short circuit does:

Take a cheap (silver) ring , make with a iron saw a gap in it. (It has now a opening in it). If you push the ring togeter it makes a short circuit, when you release it, it opens again.
Test the sens for the open ring and test it when pushed together.
Surprise !!
Nice test for your friends coils, .... but tell it after you had your fun ??!!!

Regards.

Ap

[Reg]

Hi Wyndham,

Your simple question doesn't have a simple answer because there are too many variables.

Now, one person who has done quite a bit of work in determining how much shielding affects a detector is Bill Hays of Hays Electronics. He mentioned to me some of the testing he did years ago on his TR detectors.

If it will affect the TR, it will effect a PI which is much more broad band.

What he mentioned to me was he found the graphite would reduce the sensitivity as well as the extra ground signals if the shielding paint was put on thicker.

Now, one thing he did find that worked well was the special conductive mylar layer that was applied to one side of foam insulating boards. He used to pick up sheets of the insulation up at one of the larger hardware stores I believe. Anyway, he would peel the metalized mylar coating off the foam board and use it. It worked well and cause very little reduction in sensitivity, if any. The coating was so thin that it couldn't be detected.

Later he tried some of the reflective mylar sheeting use in hydroponics and found it would reduce the sensitivity. This same reflective mylar was also coated on both sides and could be detected quite easily. He also found he could cut thin strips out of the reflective mylar sheet and reduce the losses, but that was too time consuming.

I never did try to see what depth loss occurred but I found if I simply made slice marks in the mylar sheet such that the sheet looked more like a thin center strip with a bunch of fine thin strips much like tinsel hanging off of it, the stuff still worked well for shielding and wasn't detected. I ended up gluing a piece of this mylar to a round piece of thick paper the size of the coil housing and then cutting thin slices in the mylar. I would check to see if it I could detect it and once it was weak enough, I tried it for a shield. It worked fine.

My guess is if the shielding is detectable, it is reducing the depth capabilities. Also, I would guess that the better it is detected, the more signal absorption occurs, which results in depth loss. However, this is a simple guess on my part.

Reg

[ivconic]

I noticed certain attenuation when shield exist on PI coil. Less sensitivity, slower reaction...kind of inertion?

Thats why i run my DP's on low freq. and use unshielded coil with it...


regards

[Esteban]

This Wilson detector has coil housing with a certain kind of paint and with some marks. I think one of the best solution is to shield only the coil of the sensitive part, this is the receiver coil, and leave free of shield the transmitter coil.

Also a low impedance coil based on aluminium loop can be a solution. But here you need a system for to eliminate vibration of the loops...

[Max]

This Wilson detector has coil housing with a certain kind of paint and with some marks. I think one of the best solution is to shield only the coil of the sensitive part, this is the receiver coil, and leave free of shield the transmitter coil.

Also a low impedance coil based on aluminium loop can be a solution. But here you need a system for to eliminate vibration of the loops...

Hi Esteban,
I think depends much on type of detector. I mean there are different VLF types too. If you think at phase signals no problem having tx unshielded cause these are relative signals involved and just all is related to shift between one and each other (rx , tx). I totally agree here.

If you think at tx absolute amplitude or frequency things could be absolutely bad without shielding the tx coil too.
I've seen many older VLF types with unshielded monocoil using the PLL approach to detect small frequency shifts but built one some years ago(electronique pratique's one) and then realized they completely missed the point about stability cause I have to keep the coil ever at same height, and was almost impossible to do a real search. Too much falsing that way...then I put a foil shield on coil , gain stability, but lose much detection depth !

Now I think that if tx frequency or absolute amplitude at tx osc is critical also for rx/detection section a shield is mandatory.

Have to test e.g. on bandido but I think I'll get much more false without tx shield there.

Best regards,
Max

[Max]

I noticed certain attenuation when shield exist on PI coil. Less sensitivity, slower reaction...kind of inertion?

Thats why i run my DP's on low freq. and use unshielded coil with it...


regards

Hi ivconic,
I've noticed too same things. All due to capacitance increase.

What's the height you normally keep the DP coil over the ground ?

Best regards,
Max