Just how good are commercial coils?

[WM6]

Lower Ohms also = Faster Eddy current decay

.

Faster eddy current decay than what?

Than in higher ohm shielding?

In higher ohm shielding eddy currents are not build or not significant to take in account.

So what is sense of your claim?

[detectormods]

There may be confusion where i compare 30 ohms with 700 ohms.
The difference in particular to eddy current decay is in the standard coil shield of 80-100 ohm to the vey low 30 ohms. The 700 ohm shield may have next to zero residual current.
All this was a question to be answered, not a statement of fact as I not tested the 700 ohm shield. Testing with a HE probe the lower ohm material has faster decay than the 80 ohm shielding but the lower ohm material consumes more energy.

[turtlebowl]

I have experience only with PI mono coil, which has it's own particular needs, that may not be relevant to those two coil detectors .. but...

Looser turns have less capacitance and faster discharge, and also flatter discharge..eg they allow higher frequencies (if you look at the discharge curve in the frequency spectrum/domain).
Because of this they can 'hit' smaller targets better, I have found.


.

Tec,, I value your observations on loose/verse tight winding for PI coils. Thanks for joining in here.

Do any of you here have experience / observations that apply to IB coils in regards to loose/tight windings? Many of the old school manufactures used self binding loop wire which translated to tight windings. So was this for easier manufacturing (easy potting), or better coils?

[turtlebowl]

Sorry satdaveuk, I didn't notice till now you had commented on the winding tensions. Thanks bud.
Anyone else have any knowledge on this tension question?
John Earle , a true Coil guru, seems to favor Steve's thoughts as well on wire thickness relationships. His Compass coils, at least, are just as Steve describes.
Phil M

[golfnut]

Phil, regarding IB coils - they are a low frequency device operating at the tx frequency - sub 20kHz - so can tolerate tightly grouped turns and potting.


I tie up the strands into close nit bundles, then flood for electromechanical stability using polyurathane resin (2 part). This gives a stiff shape - which is beneficial during final potting/nulling.



Pi coils, on the other hand, have to operate over a broader range of frequencies. The Tx spike contains frequencies over a broad frequency range. These coils are 'relatively' wide bandwidth devices and cant tolerate high interwinding capacitance or potting very well.


Not obvious - but a strand of straight wire, no turns, has inductance per unit length.
Thin wire has - more L per unit length, more R, and when wound - more interwinding capacitance, plus less eddy currents in the wire cross section.

Revese is true for thicker wire.

Steve

[Melbeta]

DetectorMods, I have a question...

There may be confusion where i compare 30 ohms with 700 ohms.
The difference in particular to eddy current decay is in the standard coil shield of 80-100 ohm to the vey low 30 ohms. The 700 ohm shield may have next to zero residual current.
All this was a question to be answered, not a statement of fact as I not tested the 700 ohm shield. Testing with a HE probe the lower ohm material has faster decay than the 80 ohm shielding but the lower ohm material consumes more energy.

Did you tune the Tx/Rx coils to a precise zero balance as the Nexus machine designer indicates he does?
Melbeta

[gsignal888]

I have learned my lessons in regards to publishing information on the internet, the commercials will just take it and run. Years ago i was discussing new idea's for metal detecting on some forums only to see the information collected and some new patents are applied for. Publishing information on the internet is like giving a thief access to your bank account. Like my mod on P.I detectors for a variable gain front end, now others see and try to copy into their design. Sometimes it is best to watch and say nothing.

Publishing your info is good for the community in terms of sharing and others do the same. It also depends how much detail you disclose and the more the better. It is now time stamped and public.

Companies that go out and patent it just wasted their $$$ in doing so. Why? Anything that is public can not be patented. That company that got the patent did not disclose your public post on the internet showing them how to do it. That's why they got the patent because the Patent office saw theirs was the original.

All you have to do is contact the U.S. Patent office and disclose your pre-dated internet post, provided it has detail. The Patent office can make the decision to invalidate (make null) their granted patent. You can go to the U.S. Patent website and find all this.

[Tec]

Coils

I have shielded my coil with Graphite powder on paper. The the graphite is dipped in isopropyl and painted on until it is the right resistance. It is all held in place afterwards with spray paint and then epoxy.

The shield is connected with bare wire taped to the bare graphite and then epoxied.


The shield is like magic. It stops noise from wet sand and wet grass.

The reistance of the shield is I used is 1k ohm.

That is 1k ohm per square.

(1 square inch, or 30 x 30cm square paper, or 1 square killometre all have the same resistance =1kohm, this is how resistances of surfaces are measured)

I have found tiny silver jewelery (nothing super valuable) in wet low tide sand with this setup.

The graphite powder is from the hardware store (used as dry lubricant).
It is messy stuff.

[Carl-NC]

All you have to do is contact the U.S. Patent office and disclose your pre-dated internet post, provided it has detail. The Patent office can make the decision to invalidate (make null) their granted patent. You can go to the U.S. Patent website and find all this.

Unfortunately it's not that simple. You would need to file for re-examination, which cost several thousand dollars ($8800 for inter-partes, which is the correct way to do it).

Also, until recently, a patent could possibly be defensible based on first-to-invent rules, which are extremely subjective and based on notebook dates which are easily faked. Rules recently changed to first-to-patent, making first public disclosures tough to ignore.

- Carl

[Aziz]

Re patent trolls:

"Throw the dogs some bones to chew" and make totally different and secret project.

"Throw more bones to the dogs" to confuse them while you're working on the secret project.


Never give any hints to the dogs. They will bite you while doing this.


Aziz