Graphite shielded coils

[Tony60]

Good morning everyone,
as raccommended in a previous thread by Dbanner I removed the kitchen foil from the coils and painted with Graphit33 from Kontack Chemie. I wanted to ask if I should still wrap a thread with very spaced coils as a ground return respecting the gap.
Thank you in advance and have a good weekend.
Tony60

[dbanner]

Good morning everyone,
as raccommended in a previous thread by Dbanner I removed the kitchen foil from the coils and painted with Graphit33 from Kontack Chemie. I wanted to ask if I should still wrap a thread with very spaced coils as a ground return respecting the gap.
Thank you in advance and have a good weekend.
Tony60

If you haven't already, then watch this video:https://www.google.com/url?sa=t&rct=...0oOrH1bT55dWUP

Wrapping the coil with wire is supposed to limit the resistance of the graphite shield, but this all depends on the type of graphite spray you have used and how thick the layer is. In the video, the graphite layer measured 1.5K per cm(increased by 1.5K for every cm), and then he wrapped a thin wire around it afterwards, in order to limit the resistance of the shield.

You can measure your resistance of the graphite shielding, if it is anywhere near 1.5k per cm or higher then a wire wrapped around(leaving gap of course) could be done. There should be no connection between the coil and the shield, you must check this. If I am not mistaken, in the video the windings are spaced 8mm apart.

[waltr]

I have watched that video many times and it is well done BUT:
He does not fully describe what he uses for the graphite shield (does state 1.5k Ohm/cm).

Does seem he only uses epoxy as an insulating layer between coil and shield.

The very thin 'drain' wire decreases the total series resistance, else 1.5k/cm will be mega ohm from ground connection at cable to end of shield. I think very thin wire could be important. He also winds the ;drain' wire in one direction then back in the other direction created a 'crossed' pattern.

Another reason I can think of to use a drain wire is in case the shield layer cracks. This way a discontinuity in the shield will be bridged by the drain wire.

[dbanner]

I have watched that video many times and it is well done BUT:
He does not fully describe what he uses for the graphite shield (does state 1.5k Ohm/cm).

Does seem he only uses epoxy as an insulating layer between coil and shield.

The very thin 'drain' wire decreases the total series resistance, else 1.5k/cm will be mega ohm from ground connection at cable to end of shield. I think very thin wire could be important. He also winds the ;drain' wire in one direction then back in the other direction created a 'crossed' pattern.

Another reason I can think of to use a drain wire is in case the shield layer cracks. This way a discontinuity in the shield will be bridged by the drain wire.

Very good points you raised.
Most of the stuff I've read on shielding for the VLF/IB coils are a bit of black art "trial and error" methods.
There were plenty misgivings on exactly what is the best resistance for the graphite layer.
But there are some very good "proven" solutions on the forum.

There is one strange issue that you are sure to encounter when you are dealing with tesoro circuit like TGS and the likewise TGSL.
That has to do with grounding the Rx coil or not. I recall Dbowers and others having a lot to say on this issue. In the stock factory coil, the Rx coil is grounded.

[dbanner]

However, in the Tesoro Aztec circuit, the Rx coil is left floating. The issue is discussed at length here:https://www.geotech1.com/forums/show...ght=ungrounded

[kt315]

this is just not showing the ground, not an example. William Lahr's every schematic has 1...2...3 mistakes as rule.
you must trust just you keep in your hands - pcb and coil. not schematics floating around.

[dbanner]

this is just not showing the ground, not an example. William Lahr's every schematic has 1...2...3 mistakes as rule.
you must trust just you keep in your hands - pcb and coil. not schematics floating around.

I agree, it could be an omission on Lahr's part BUT:

If you are familiar with the thread "grounded or ungrounded Rx coil" , then you will know Dbowers had best success with UNGROUNDED Rx coil on WET GRASS.

Same for me as well.

[kt315]

ok. George Payne, Copyright (c) 2002 will be telling now with you.

Generally the Receive is about half the diameter of the Transmit. So if you choose an 8 inch Transmit use a 4 inch Receive. Wind the Feedback Coil on top of the Receive coil. The wires are insulated so it ok to have the Receive and Feedback touching. The Transmit and Feedback coils can have the same wire gauge. Here is a very very important point! The end of the Receive wire nearest the Feedback Coil must be connected to ground. In other word it must be connect to the loop shield and to the ground in the circuit. If you don?t do this the completed coil will not operate correctly. The R null component of the coil will be excessive and may overdrive the detector. This has to do with the high capacitive coupling between the Receive and Feedback windings. Connecting the coils as I have outline above will solve the problem.
--- end of cut---

sorry, but you do not see A POINT inside a coil grounded.

[waltr]

ok. George Payne, Copyright (c) 2002 will be telling now with you.

Generally the Receive is about half the diameter of the Transmit. So if you choose an 8 inch Transmit use a 4 inch Receive. Wind the Feedback Coil on top of the Receive coil. The wires are insulated so it ok to have the Receive and Feedback touching. The Transmit and Feedback coils can have the same wire gauge. Here is a very very important point! The end of the Receive wire nearest the Feedback Coil must be connected to ground. In other word it must be connect to the loop shield and to the ground in the circuit. If you don?t do this the completed coil will not operate correctly. The R null component of the coil will be excessive and may overdrive the detector. This has to do with the high capacitive coupling between the Receive and Feedback windings. Connecting the coils as I have outline above will solve the problem.
--- end of cut---

sorry, but you do not see A POINT inside a coil grounded.

That is a CONCENTRIC coil.

Discussed here is a DD Coil. Also the OP did not state WHICH detector so this could affect the grounding scheme.
On a TGSL (the one I am familiar with) The Pre-amp is a fully differential circuit. The non-inverting input IS grounded through the 220k resistor. I do not see any reason the ground the RX coil if shielding is done properly. This is how I built coils for my TGSL and it works very well without falsing.

However, I have built and use concentric coils on my PI detector and the RX Coil is NOT Grounded inside the coil housing. The RX and TX coils are grounded on the PCB and the coils have NO shield. This coil works excellent but it is not used on a VLF detector.

Now since the subject of this thread is the coil shield. The reason for the shield (really a bad term) is to keep the detector's internal 'ground' (circuit common is a better term here) at the Same potential as the earth or grass under the coil since any small difference will cause a false detection. Remember that less than a mVolt difference can be a valid target.

The coil shield is NOT a shield to eliminate EMF or ESD but is capacitive coupling to the earth. Remember that the circuit is 'floating' (no direct connection to earth) and it is trying to only pick up magnetic field changes (due to eddy currents) but it also will pick up electrostatic field changes. The shield is to eliminate the electrostatic potential difference so only magnetic field changes produce a change at the pre-amp. This is also why a graphic 'shield, with 1.5kOhm/cm works. It is conductive enough to equalize the potential Voltage difference between the Earth and the detectors 'common'.

[kt315]

floating RX coil is not working. you must transfer a potencial in relation to a power line of the opamp. JUST TO RELATION TO GND, +E OR -E OF THE OPAMP.
floating coil has not relation to a power line nowhere.
russians checked that 'floating' many years ago with no result.
on pics right ground connection of TESORO RX coil you do not want to notice.





That is a CONCENTRIC coil.
--
thats just geometry. omega coil, dd and concentric all are working in ONE WAY. NO DIFFERENCE

[dbanner]

Payne specifically stated the reason for grounding the Rx wire that is nearest to the feedback coil. In a concentric arrangement there is HIGH CAPACITATIVE COUPLING BETWEEN THE RX AND FEEDBACK COILS.

In a WIDESCAN coil, no such condition exists. The R null component is NOT affected by the presence(capacitative effects) of a feedback coil. There IS NO FEEDBACK COIL.

It seems that all concentric designs which use the buck coil feedback method, the Rx coils are ALWAYS grounded at one end, within the coil housing.

A Widescan coil on a tesoro with its front end preamp topology might be another issue. THAT IS WHY TESORO WIDESCAN COILS ARE CENTRE-TAPPED RX COILS, WITH CENTRE TAP TO GROUND!!!!

If you make ordinary DD coil for tgsl (non centre-tap), and you connect one end of Rx to ground(the end that connects to the non inverting input, as shown in the schematic), you will get terrible falsing on wet grass, as others have noted. I don't know why, but the concentric tesoro coils have no such problem. Maybe it is the shield or the cable???

[kt315]

THAT IS WHY TESORO WIDESCAN COILS ARE CENTRE-TAPPED RX COILS, WITH CENTRE TAP TO GROUND!!!!

---
oru bl ))) and where your theory 'floating RX coil' is here if you SEE YOURSELF THE CENTRE TAP GROUNDED???

[Tony60]

Hello everyone,
sorry the delay of the answer but I was two days in the mountains. You have been very clear, I do not yet know how much is the resistance of graphite that I used but I still decided to
wrap the ground wire and put another layer of graphite on top.
Have a good week and thank you again.
Tony60.

[dbanner]

THAT IS WHY TESORO WIDESCAN COILS ARE CENTRE-TAPPED RX COILS, WITH CENTRE TAP TO GROUND!!!!

---
oru bl ))) and where your theory 'floating RX coil' is here if you SEE YOURSELF THE CENTRE TAP GROUNDED???

My dear KT, you fail to grasp the nuances of english language.

Let me explain more clearly.

First, I am saying that a NON-CENTRE TAPPED RX coil in the DD style, does not work well if grounded according to the scheme. It falses on wet grass, Why, I don't know.

Second, ALL TESORO DD COILS, aka WIDESCAN, are having a CENTRE-TAPPED RX COIL which is grounded by the centre tap connected to the shield.

DO YOU THINK THIS IS JUST A BIG COINCIDENCE???

I DO NOT.

ALL TESORO WIDESCAN COILS MADE TO BE USED WITH THE TGS/ROYAL SABRE/SILVER SABRE and such models ARE HAVING A GROUNDED CENTRE TAPPED RX COIL.

Conclusion: if making a DD coil for your TGSL, make a RX with a centre tap!!!!

With ordinary DD RX coil, you get falsing on wet grass unless you leave the coil floating!!!!

[kt315]

falses on wet grass because the water for HV static discharge is good conductor. for low voltage we know the water is dielectric.

https://www.youtube.com/watch?v=gKhL4WqkiOU

ie poor shielding gives falses on wet grass. it must be clear for understanding.