Just how good are commercial coils?

[detectormods]

A few years ago I asked a fellow from a well known metal detecting company why their coils performed very poorly. The answer that came back was an interesting one and gave me an insight into the commercial detector market. The answer was " we are in the business of selling detectors, not coils".

With that statement I persued many experiments with coil design. So how good are the coils that are being used? I have to say that the majority of coils are absolutely woeful in terms of performance. I will say that they are mostly well constructed, have ample Faraday shielding and look the part.

Performance.....cough hmmm, Why is it that for a given size many commercial coils are way below many homebrew coils? Today I built a new modern 12" and a 14" coil to test head to head with a popular brand of 12" and 14" coil. Using a popular brand of detector, the difference between the home made and commercial coils was......Staggering!

The coils are all Mono types and being tested under repeatable conditions, I am not going to make any mention of brands as it will start WWIII but it does show that there are people running around with equipment of questionable performance.

As an example the 14" commercial was straining to hear a reference target at 2 cm while the exact same size, winding count and inductance homebrew could obtain a clear signal at 9 cm. This performance tracked for various sized test targets as a percentage of output signal.

So, in all of this there may be a few lemons on the end of some shafts.

[turtlebowl]

An interesting post ModMan.
Can you share what What you feel makes the difference between MP coils and your handmade?
What did you spend the extra time getting "just right"?

[detectormods]

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.

[mario]

A few years ago I asked a fellow from a well known metal detecting company why their coils performed very poorly. The answer that came back was an interesting one and gave me an insight into the commercial detector market. The answer was " we are in the business of selling detectors, not coils".

With that statement I persued many experiments with coil design. So how good are the coils that are being used? I have to say that the majority of coils are absolutely woeful in terms of performance. I will say that they are mostly well constructed, have ample Faraday shielding and look the part.

Performance.....cough hmmm, Why is it that for a given size many commercial coils are way below many homebrew coils? Today I built a new modern 12" and a 14" coil to test head to head with a popular brand of 12" and 14" coil. Using a popular brand of detector, the difference between the home made and commercial coils was......Staggering!

The coils are all Mono types and being tested under repeatable conditions, I am not going to make any mention of brands as it will start WWIII but it does show that there are people running around with equipment of questionable performance.

As an example the 14" commercial was straining to hear a reference target at 2 cm while the exact same size, winding count and inductance homebrew could obtain a clear signal at 9 cm. This performance tracked for various sized test targets as a percentage of output signal.

So, in all of this there may be a few lemons on the end of some shafts.

I have often wondered about this...

Do you think the companies don't have the necessary information to make a superior coil? or is it more of a production issue? It does take a considerable amount if time to finish a good coil.

Why is there such a discrepancy between home made coils and the ones made by the manufactures?

[detectormods]

Hi Mario,

I would classify some sections of coil design as a black art, there are plenty of books and information on inductors and transformers for RF and power conversion but I have never seen the Engineers Handbook of Metal Detector Coil Design. When you try to find information it is very hard to locate. There are some good articles on coil design but all of these except for a couple are written by hobbiest experimenters. Also there is not a lot of information on performance metal detectors, most information is closely guarded and there is not much in the public domain. One thing with that I have found with detector coils is that in most cases a good transmit coil makes a poor receive coil and vice versa, any person that is conversant with the art would not consider this to be consistant with nuts and bolts electomagnetic theory.

[IBGold]

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.

Yes there are thieves every where Woody.

Ian.

[turtlebowl]

So is this thread "wrapped up?"

[satdaveuk]

SEF coils are proberly up amongst the best to date thats all they make, but of course very exspensive.
Ive made coils for the tgs which are as good but not better, can make them more sensitive in air tests but they fail big time in real ground conditions mainly with small silver thats why air tests mean very little to me.

interesting subject hope you get more feed back on this one so we can all learn that little bit more but can we stick with ground satistics please we can see plenty air tests on utube

Thanks again
Regards

[moodz]

... ah I love this discussion ... lets ignore the elephant in the corner of the room .... the main reason that coils have to be made so well is to compensate for the poorly designed front end electronics in almost every detector design ( mainly .. due to a lack of engineering knowledge of balanced circuits and sampling theory)

moodz

"good engineers make what they want .... they dont buy it"

[maikl]

SEF coils are proberly up amongst the best to date thats all they make, but of course very exspensive.
Ive made coils for the tgs which are as good but not better, can make them more sensitive in air tests but they fail big time in real ground conditions mainly with small silver thats why air tests mean very little to me.

interesting subject hope you get more feed back on this one so we can all learn that little bit more but can we stick with ground satistics please we can see plenty air tests on utube

Thanks again
Regards

This applies to the "fresh" buried silver coin. Silver money was for many years in the soil creates a patina that still creates a chemical reaction that still gives a better detection of those in the air...What is valid for all detectors.

[turtlebowl]

Air tests are boring and that is a given. Appearantly the author doesn't want to talk anymore about it.
This is a facinating topi and lets not let this die. Lots of guys here including myself feel the coil is the key.
the variables are really divearse...... grounding means, shielding means, capacitance, precise turns, null adjust technique, front ends, behinds, lol etc.
there are really many 'secrets"....... But all are strangely specific to the application. We are all just trying to help each other learn bits and pieces. Are there really secrets that are specifically across the board?. maybe.
personally I don't see the sampler as a limiting pro/con device vs balanced front ends and noise. But thats why we are all here talking. Obvoiusly others see the Choppers as important and I would like to hear about that so I can learn more.
I just wanted to hear what the author thought the MP's lacked verses handmades.
Manufacturing tolerances can't be avoided in mp devices so is that really it? Or is there something else? i.e carefully adjusting by hand the "tuning loop" etc., precisely appliing the graphite shield layer, PC compensations, I dunno. Even windings, tight windings,,,,,,Teach me,, geo guru.

[satdaveuk]

Air tests are boring and that is a given. Appearantly the author doesn't want to talk anymore about it.
This is a facinating topi and lets not let this die. Lots of guys here including myself feel the coil is the key.
the variables are really divearse...... grounding means, shielding means, capacitance, precise turns, null adjust technique, front ends, behinds, lol etc.
there are really many 'secrets"....... But all are strangely specific to the application. We are all just trying to help each other learn bits and pieces. Are there really secrets that are specifically across the board?. maybe.
personally I don't see the sampler as a limiting pro/con device vs balanced front ends and noise. But thats why we are all here talking. Obvoiusly others see the Choppers as important and I would like to hear about that so I can learn more.
I just wanted to hear what the author thought the MP's lacked verses handmades.
Manufacturing tolerances can't be avoided in mp devices so is that really it? Or is there something else? i.e carefully adjusting by hand the "tuning loop" etc., precisely appliing the graphite shield layer, PC compensations, I dunno. Even windings, tight windings,,,,,,Teach me,, geo guru.

Good points and one of them regarding tension of the windings done lots of experiments involving lots of time and money spent and my opinion unlike what I thought say six months ago is that making the coils tightly wound is the way to go, but with my experience to date its not so, what I have found is you need to find a happy medium with constant continuity with the windings, I have coils here that are pretty well in the limits far as inductance and resistance but no matter what you do with them they are poor performers and all tightly wound coils with very close luming and hard as a rock.
Has anyone else here noticed this or is it just me?
Another thing ive started to do now is started to put the CX components external because me personally have never had two pairs the same so rather than tune a detector just to work with one front end you can swap and change with various coils.
The manufacture Tesro coil I have im going to build a bigger diameter shell for the plug to mount components in far as my home made ones a thin strip of vero board between the RX/TX Ds seems to do the job with no adverse effects although you do need a extra core on the down lead for the TX which is fine on the TGSL because we are using 5pin plugs and pin2 is spare if wiring the Tesro manufactures way which I found more sucessful anyway.
Couple of pointers like to here from anyone else gone down that road.

Regards

[golfnut]

In MP, I think for ease and speed and cost most Tx and Rx coils are made with the same diameter wire.

I believe Tx coils should be fewer turns of thicker wire. All about generating magnetism with high currents.

Rx should be more turns of finer wire to generate more signal voltage from the tiny magnetic returns from targets.

So the world has to change.


Steve

[Tec]

Tightly wound coil

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.


Also, the Damping resistor affects sensitivity to grass and ground.
If the damping resistor is too high (under damped), then there is the beginnings of ringing which is affected by capacitance (between coil and ground), so the coil becomes more sensitive to touching grass and ground.
Over damping (lower damping resistor), makes the coil less sensitive to touching the grass.

The faraday shield puts a 0V between the coil and the ground.
That is, and charge flowing from the ground (capacitance between coil and ground) will have to go through 0V, it will be at 0V and not be seen as a signal across the coil. It might be pushing the 0V up and down in voltage, but the detector system is referenced
to 0V, i.e it is on top of the 0V .




Note: there is no such thing as an expensive magnetic field or a cheap magnetic field, and the target can't tell the difference between a litz wire magnetic field or a copper wire magnetic field.

[detectormods]

A very important aspect is the sheilding, the new Minelab coils are 700 ohms per 5 Cm while Nugget Finder are at 30 Ohms per 5 Cm , both these coils are designed for ML GPX5000 so why the difference? Lower Resistance can have a High frequency sheilding effect, while the higher ohms can reduce the losses for the target signal.

Lower Ohms also = Faster Eddy current decay in the shield material but possibly at the expense of target signal reduction. Lower Ohms also can make a better more effective Faraday sheild.

The proximity of the windings to the sheild also comes into play, especially on the ground facing side.