grounded vs ungrounded RX coil

[dfbowers]

i've been experimenting with a couple of HH versions and have been using the 'audio' grounding techniques that I was taught as a lad.
this doco ( fig 10.7 on page 3 ) has a drawing showing current flow and earth paths. you'll also need to find some info on shielding technuques to add to this knowledge.

http://www.analog.com/static/imported-files/analog_dialogue/5467026043687049331665676350Grounding.pdf

my idea is that the coil is just like an extra component of the metal detector but it has a high level signal ( tx ) and a low level signal ( rx ) located at the end of a cable. and i treat the shield on the cable in the same way as i'd treat the electrostatic shield on a coil. my earth common point is as close to the detector electronics as is practical.

Thanks..

That seems to be my conclusion by arriving there the hard way.

[greybeard]

i've been experimenting with a couple of HH versions and have been using the 'audio' grounding techniques that I was taught as a lad.
this doco ( fig 10.7 on page 3 ) has a drawing showing current flow and earth paths. you'll also need to find some info on shielding technuques to add to this knowledge.

http://www.analog.com/static/imported-files/analog_dialogue/5467026043687049331665676350Grounding.pdf

my idea is that the coil is just like an extra component of the metal detector but it has a high level signal ( tx ) and a low level signal ( rx ) located at the end of a cable. and i treat the shield on the cable in the same way as i'd treat the electrostatic shield on a coil. my earth common point is as close to the detector electronics as is practical.

[Monk]

My first "inclination" is to have separate return paths for Tx and Rx coils, i.e 2 ground returns. But as with all things having to do with EMI and grounding, it's 20% inspiration and 80% perspiration......

[simonbaker]

Simon,

I did test both coil in the wet surf. Both coils reacted when touching salt water, but the ungrounded one would still be my pick as the reaction was minimized somewhat. I was able to nearly reduce all reaction to hunting over wet, salty sand by bumping up the disc a little... around where thin foil is rejected.

Now, where I had the most trouble was the area between where the sand was completely wet and completely dry.. but the TGSL was still very usable.

Don

Ok, encouraging, sounds better than my Compadre.

-SB

[Qiaozhi]

One of the earlier post suggested that maybe the difference between the "3 wire + cable shield" and the "4 wire + cable shield" had some effect on grounding. I used the 4th wire for the connection to the cold end of the oscillator and Tesoro does not. Electrically, I think it's the same thing.. just a redundant wire (The black lead inside the cable).

Although I cannot see why it would make a difference, it's something to try.

[dfbowers]

Monk,

So you are suggesting that disconnecting the extra Tx wire buys us nothing.. correct?

Don

One of the earlier post suggested that maybe the difference between the "3 wire + cable shield" and the "4 wire + cable shield" had some effect on grounding. I used the 4th wire for the connection to the cold end of the oscillator and Tesoro does not. Electrically, I think it's the same thing.. just a redundant wire (The black lead inside the cable).

[Monk]

So you are suggesting that disconnecting the extra Tx wire buys us nothing.. correct?

I don't understand you question. What do you mean by "extra Tx wire"?

[dfbowers]

That's interesting demo, certainly shows your point.

I didn't realize your TX coil was not grounded at the coil shields either - interesting.

The funny thing is that your "ungrounded" coils are the wiring I would have expected naively -- the TGSL standard grounding always puzzled me but apparently is the Tesoro way. Only concern would be common mode spikes, but others feel that is not an issue with LF353.

I'm still interested if you were able to test both coil types over wet sand. Wet sand (like when surf recedes) is a mess for my Compadre, but an area I'd like to be able to hunt.

Your making a real case for "ungrounded" coils. And you appear to be in your element at the beach... I'll pretend I'm not sweating at a hot desk....

Thanks for the tests and videos.

Cheers,

-SB

Simon,

I did test both coil in the wet surf. Both coils reacted when touching salt water, but the ungrounded one would still be my pick as the reaction was minimized somewhat. I was able to nearly reduce all reaction to hunting over wet, salty sand by bumping up the disc a little... around where thin foil is rejected.

Now, where I had the most trouble was the area between where the sand was completely wet and completely dry.. but the TGSL was still very usable.

Don

[dfbowers]

FYI

Monk,

So you are suggesting that disconnecting the extra Tx wire buys us nothing.. correct?

Don

[simonbaker]

Simon,

Here are a few details on my grounding experiment. Attached are the two grounding schemes that I have tried. Coil details are as follows:


* 5 complete coils.

* 4 of them have been wired per the "TGSL coil making" pdf.

* One of them that I made has not been tested in wet grass (the one I
sent you) but is wired per the TGSL coil making pdf.

* One has been wired without any active parts of the coil attached to the
shields in any way (see attached). This one is quiet in wet grass..

* That leaves me with 3 coils that behave badly in wet grass.

* All 4 of my 27cm coils airtest at over 30cm for 1e. (one is a smaller DD
test coil).

* For all practical purposes, there is no real difference is detection depth between grounding schemes.. just ungrounded is quieter.. I have about 10 hours of
actual use on the ungrounded one at the beach.. I found 115 coins total in a week.



* All are shielded the same, have the same type of shell, epoxy and cable.

See attached links to videos for wet grass demonstration..

Sensitivity was at minimum in both videos.
GB on ferrite.
Disc set to MIN.

Don


Links to videos.

http://www.youtube.com/watch?v=XhRboS639no (Grounded)

http://www.youtube.com/watch?v=7UBOca3btMw (Ungrounded)

That's interesting demo, certainly shows your point.

I didn't realize your TX coil was not grounded at the coil shields either - interesting.

The funny thing is that your "ungrounded" coils are the wiring I would have expected naively -- the TGSL standard grounding always puzzled me but apparently is the Tesoro way. Only concern would be common mode spikes, but others feel that is not an issue with LF353.

I'm still interested if you were able to test both coil types over wet sand. Wet sand (like when surf recedes) is a mess for my Compadre, but an area I'd like to be able to hunt.

Your making a real case for "ungrounded" coils. And you appear to be in your element at the beach... I'll pretend I'm not sweating at a hot desk....

Thanks for the tests and videos.

Cheers,

-SB

[Monk]

FYI

[dfbowers]

That's correct.

I'll give it a try..

[Qiaozhi]

Qiaozhi,

Do you mean just disconnecting the "hitch hiker" wire (the redundant Tx finish wire)? I suppose I could do that by just snipping the connection from one of my existing coils.. So the cable shield would just carry the Tx finish connection. Is that what you mean?

Don

That's correct.

[dfbowers]

Hi Don,

OK - now I can see a difference between your TGSL "original" wiring and the wiring of a commercial Tesoro coil. In the commercial version there are only 3 wires used in the cable. The fourth wire remains disconnected. This would be equivalent to breaking the connection for the black wire (TX Finish) at both ends of the cable. Perhaps you could try this in your next coil and let use know the results.

Qiaozhi,

Do you mean just disconnecting the "hitch hiker" wire (the redundant Tx finish wire)? I suppose I could do that by just snipping the connection from one of my existing coils.. So the cable shield would just carry the Tx finish connection. Is that what you mean?

Don

[Qiaozhi]

Hi Don,

OK - now I can see a difference between your TGSL "original" wiring and the wiring of a commercial Tesoro coil. In the commercial version there are only 3 wires used in the cable. The fourth wire remains disconnected. This would be equivalent to breaking the connection for the black wire (TX Finish) at both ends of the cable. Perhaps you could try this in your next coil and let use know the results.