PI coils- Shielded or Unshielded?

[bklein]

Does a shield degrade the TX signal power?

[green]

Does a shield degrade the TX signal power?

I didn't see a drop in signal with the tests I tried.

[surfdetector]

From my notes on sample delays:

8-10us: gold nuggets - coil must be shielded (eliminates electrostatic interference at low delays)
>15us: beach
30us to 50us: coin shooting
Coils for longer delays do not need a shield

I am reading this as with delays of >15us you do not necessarily need to shield your coil. Is that correct?
Cheers

[Qiaozhi]

I am reading this as with delays of >15us you do not necessarily need to shield your coil. Is that correct?
Cheers

In my personal experience, I've not always found it necessary to add an electrostatic shield, although it does help to eliminate external emi. At delays above 50us, a shield doesn't seem to add any benefit.
For a VLF, an electrostatic shield is mandatory.
I'm sure others may have some different views.

[waltr]

I have built three types of coils for my HH2 PI detector. A 10" Mono, a 10" DD and an 8" concentric. None of them have shielded coils and work well at 10-15 usec delay to first sample.
It is the pre-amp that limits this first sample delay not the coils.

[Polymer]

I have built three types of coils for my HH2 PI detector. A 10" Mono, a 10" DD and an 8" concentric. None of them have shielded coils and work well at 10-15 usec delay to first sample.
It is the pre-amp that limits this first sample delay not the coils.

Did you do the testing on real ground?

As some members have mentioned, testing in real conditions shows what?s up. I fully agree to this now.

I built a PI coil without a shield which worked fine on the test bench. Outside it was a squealy-piggy as soon as I lowered it within 1-2 feet of the ground.

I put an impromtu mesh shield around the outside of the coil for testing. This solved the squealy-piggy problem and showed that I definitely need a shield.

Hoping to get around implementing a shield is understandable, as it is quite a bit of extra work and may cause other (new) problems along the line.
It also opens up a new "rabbit hole" of testing and experimenting - which we do somehow like in weird way - but sometimes you just wanna finish off and go detecting.

@surfdetector

I would agree with Qiaozhi?s info as a loose rule of thumb, BUT, it comes down to how it works in your ground, sample delays used and what you are looking for.

In my experiments shielding did not influence search depth negatively, it actually got better due to way more stability of the threshold.

[surfdetector]

@surfdetector

I would agree with Qiaozhi?s info as a loose rule of thumb, BUT, it comes down to how it works in your ground, sample delays used and what you are looking for.

In my experiments shielding did not influence search depth negatively, it actually got better due to way more stability of the threshold.

I would definitely like to test the coil before and after shielding but as I do all of my detecting at the beach and 90% of that in the water it isn't possible right now due to the Covid19 closure of the beaches.
Cheers

[waltr]

Yes, All three coils have been in use for nearly three years at Old farm sites, parks and the beach (not in the water but on dry & damp sand).
Remember that my HH2 uses the Three Sample GEB method that does cancel mineralzation. The one beach in RI I detected had areas of black sand that made my pin-pointer go nuts. The HH2 with the DD coil had no issues.

[Polymer]

@waltr

Thank you. How easy it is to get apples and oranges mixed up. You have quite a different PI setup to mine. A very refined version if I may say so.
I forgot to specify which PI my shielding trials and tribulations were made with.

@surfdetector
The PI I am using is the MPP version E.
Hope your beaches open soon again!
The sand here is mineralised, but not black sand.
There are also a helluva lot of old black iron flakes.

[waltr]

@waltr

Thank you. How easy it is to get apples and oranges mixed up. You have quite a different PI setup to mine. A very refined version if I may say so.
I forgot to specify which PI my shielding trials and tribulations were made with.

@surfdetector
The PI I am using is the MPP version E.
Hope your beaches open soon again!
The sand here is mineralised, but not black sand.
There are also a helluva lot of old black iron flakes.

Ahhh.. I would not have thought that it would make a difference but maybe it does.

GEB control is useful to eliminate those mineralised areas.
Before I added this feature the PI would respond to old red bricks found at a n 1800's farm.

[dbanner]

Am I correct in saying that undersea conditions, shielding is useless? (Pi machine)

[Polymer]

Ahhh.. I would not have thought that it would make a difference but maybe it does.

GEB control is useful to eliminate those mineralised areas.
Before I added this feature the PI would respond to old red bricks found at a n 1800's farm.

Thanks for mentioning the red bricks - might need a PI with GEB for my situation.
It?s not red bricks, but fired red clay blobs and "rocks" from the last few millenia.
The MPP E reacts to these, not very strong, but enough to wonder if its a deeper ring.

[Polymer]

Am I correct in saying that undersea conditions, shielding is useless? (Pi machine)

Hmmm, a really interesting and logical assumption from my perspective.

Since seawater is very conductive, I would guess that the capacitive charge potential to the coil is "null".
So I would wager that your statement is most likely true. Have not tested this directly though.

BUT - yeah, always these buts ... gotta leave some wiggle room ...

One of the things my "PI building adventures" has tought me
is that there are more quirkies or devil in the details than I could have ever imagined.

Hope one of the more seasoned PI passionista give some input on this.

[dbanner]

I was under the impression that very short sample delay requires very good fast coil. For a fast coil it is inferred low self capacitative in coil and cable, and shielding. Yes, shielding is a must.

But what is the useable sample delay in undersea conditions? Or even at water's edge in wet sand.

If you intend to use the machine for inland detecting, then shielding is most wise move. Then you can do deep dive or scavenge around the inland parks.