Minelab Go-Find

[Qiaozhi]

By the way , if you have this Go-Find device in your hands , you can make some experiments to find out the coil construction . Just use a little probe coil connected to the scope and explore the field lines concentration across the main coil . If the lines are stronger on the outer edge of the coil - it means that the TX winding is located there , with less diameter RX . But if the field is uniform outside and inside of the coil - it will definitely show the mono-coil construction ...

Yes - I'm planning to do that in the next couple of days. No time at the moment.

[Davor]

It is quite difficult to obtain coupling anywhere near 1 for coreless transformers, and balancing of such may be provided by a cored or even coreless transformer. X-ray could give insight to this. Perfect coupling would transform Rx high impedance to Tx, rendering it useless.

[golfnut]

My guess is the Rx is bifilar. So 1/2 of the turns go CW and the other go A/CW - so the Tx is cancelled without a target nearby.

[DOOLEY]

my guess is it's a square wave forced TX on a mono coil,

and the square wave is used as a reference.

mono coil VLF if you will.

[deemon]

My guess is the Rx is bifilar. So 1/2 of the turns go CW and the other go A/CW - so the Tx is cancelled without a target nearby.

Bifilar winding is a good idea ( it helps to solve some problems ) , but it's not enough to make a mono coil balance ...

[Tinkerer]

Bifilar winding is a good idea ( it helps to solve some problems ) , but it's not enough to make a mono coil balance ...

To cancel the TX signal, we need to precisely match the receive coils for current, voltage and phase, then subtract.
At the same time we need to be careful not to eliminate the target signal.

[deemon]

To cancel the TX signal, we need to precisely match the receive coils for current, voltage and phase, then subtract.
At the same time we need to be careful not to eliminate the target signal.

Of course you are right , when you use a separated RX coil ... but I say that we can obtain a balance of a mono coil as well . I explained this in my topic - what we need there is a bifilar wound coil and something else , to get the compensation signal - http://www.geotech1.com/forums/showt...3-mono-coil-IB
But if Minelab uses their digital VFLEX technology , they theoretically can obtain this compensation signal by digital algorithm , and perform all this stuff in digital domain , that's my point ...

[crane]

Minelab says it's an un-nulled Rx coil and Tx coil wound together as an effective mono,
and then nulled electronically, ie, no mechanical nulling at all.

[Tim]

All well and good, but the end result that matters is its' price for performance, and robustness.. If it can't see a wheat penny/ Aussie 1c piece through the trash/background, then there is little point in analysing its' complexity??
I do like the compactness though.
Any nulling, whether mechanically or electronically is dumbing down, in my opinion and definition. After all, if it eliminates the target signal at high trash level, any electronic advantage is compromised altogether.
For an electronically adjusted system I would suggest the hottest coil the electronic advancement could tolerate. That may mean RX/TX being vastly different, or infinitely close, depending on the method of signal analysis employed.
But hey, what do I know? Apparently my BB is a BFO. But it would not surprise me if the advantages of my/other/all analogues could and have been made accessible to the lazy and brain-dead via reliance on digitalisation.
Perhaps the spiralling of the bifliar and/or eddy current effect of the combined coil(s) can be advantageous to the electronic analysis of the signal, through fast compensation on motion.
I apologise in advance for the offence my lateral yet brutal thought process will obviously cause for SOME..........

[kt315]

Minelab says it's an un-nulled Rx coil and Tx coil wound together as an effective mono,
and then nulled electronically, ie, no mechanical nulling at all.

do not believe in ML's statements. they said that vflex is fully digital tech while a RX signal from coil goes
to be having the analog sinusoidal form. LIERS. they always put opamp in RX coil, so any test
shows superiority vs rivals in the depth, 1-2", but they NEVER tell you about this trick.

[Tim]

It's called "corporate dis-information" KT. And anyone here that says it doesn't happen is FULL OF SCHIZEN!

One day soon this sort of corporate dishonesty will be punishable by lynching,capital punishment,or worse. I hope. The global ponzi scheme is a recipe for the next great depression and then the corporate and other parasites living on the high hog will be rightly targeted.
When the fit hits the shan I shall carry on regardless.

[Tim]

Today I got hold of a Minelab Go-Find 40.

First impressions - it's very plasticy, and doesn't look like it will have a long lifespan. I've already heard that the handle can be broken quite easily, and I'm not surprised. Also, the stem wobbles around a lot.

On the plus side - it folds down small enough to fit in a rucksack, so it's also easy to tuck away in the car when you go on holiday. it's really simple to use, and with respectable depth. It can get a Victorian penny at 13" in an air test on maximum sensitivity. I did a quick test in my test garden, and it was able to find 2 of the 5 coins buried there. To be fair, no detector ever tested there has been able to find all 5. In general, only 2 coins are ever detected. However, the 3rd coin is sometimes detectable, but only because I know it's there. In practise, it's not a signal you would decide to dig. With one of my homemade detectors I can just about detect the 4th coin with a 4" coil. The problem with the test garden is the huge amount of ferrous trash. Some really cheap detectors can only find the 1st coin, and one detector failed to find any at all. That was a Viking 5.

One thing that interests me about this Go-Find detector is the coil. It looks like a mono, but it reacts like a concentric. According the technical specifications, the detector runs at 7.7kHz, and Gary Schafer (Vice President of Minelab Americas) stated that the Go-Find uses their VFLEX technology, which is single-frequency VLF (same as the X-TERRA). He also stated that the coils operate from a pinpointing perspective similar to how a concentric coil would, but they are actually mono loop coils. So my question is: Does this mean the coil is being used as mono coil all the time? This is supposed to be a VLF machine and includes ferrous/non-ferrous discrimination ... so a mono coil doesn't make sense.

Anyone care to speculate?

Perhaps you should try Colin Mitchells curcuit 8 if you have so much trash. I'm digging the brass and copper out of burnt tins and batteries every day,

[Tinkerer]

Of course you are right , when you use a separated RX coil ... but I say that we can obtain a balance of a mono coil as well . I explained this in my topic - what we need there is a bifilar wound coil and something else , to get the compensation signal - http://www.geotech1.com/forums/showt...3-mono-coil-IB
But if Minelab uses their digital VFLEX technology , they theoretically can obtain this compensation signal by digital algorithm , and perform all this stuff in digital domain , that's my point ...

In your thread http://www.geotech1.com/forums/showt...3-mono-coil-IB there were a lot of good ideas. I will post a balance compensation circuit there, not to hijack this thread.

[Carl-NC]

Any nulling, whether mechanically or electronically is dumbing down, in my opinion and definition. After all, if it eliminates the target signal at high trash level, any electronic advantage is compromised altogether.

"Nulling" refers to induction-balance nulling, not target nulling. It substantially increases dynamic range.

[Qiaozhi]

I've just probed around the coil with a small ferrite-cored sensor, and it looks like the TX loop is situated at the outer edge of the coil shell. This leaves plenty of room to position an RX loop inside the TX, spaced apart by (let's say) 10mm. The TX frequency was measured as 7.86kHz, and displays as a very pure sine-wave.

There are two possibilities here:

1) The GO-Find coil is a coaxial type, but I think this is unlikely due to the close proximity of the two RX loops, which would be necessary to fit the two loops into the coil shell.

2) The GO-Find coil is a coplanar (concentric) type. This would make more sense, even though the RX loop would still be close to the TX. Although Minelab are referring to this coil as a "mono", it clearly is not a mono in the usual sense, since this is a VLF machine. Since VFLEX technology is claimed to be used by the GO-Find, it should be noted that the VFLEX patent cites this paper on the GEM-3 from Geophex Ltd. -> http://www.geophex.com/Pubs/GEM-3_mo...et_al_1997.pdf
This paper proposes the use of a "monostatic" coil, more commonly known nowadays as a concentric. Therefore I suspect the use of "mono" by Minelab in this context is purely a marketing tool, when they actually mean "monostatic" (i.e. concentric). Due to the closeness of the RX loop to the TX loop, it does have the appearance of a [rather thick] mono coil.

If option 2 (above) is correct, loop balancing would be rather tricky, so there's most likely an element of electronic nulling in this design. One advantage of this approach is that it would require much less overlapping of sweeps than would be necessary with a conventional concentric,