CONCENTRIC COIL MADNESS

[SWL]

Thanks Carl! This is what I wanted to hear ...

[gurdal]

Hello master carl,
I could not pass underground 25 cm depth with 28 cm central coil for idxpro.
How should I go to go deeper?
I stick to the values given for the coil
Best regards

[dbanner]

Also, DD cancels using a magnetic null just like a concentric; the opposing field is the field on the outside of the TX coil.

Thanks for that, it was never intuitive, at least not to me.
Two circular coils or DD for that matter can be balanced by placing the RX coil in a region where the TX fields cancel. But this is geometrically always the same. Whereas in the concentric coplanar, you could pretty much place the RX coil anywhere within the confines of the TX coil so long as an appropriate bucking coil is accompanying. It doesn't have to be in the dead center. So this got me thinking about a distinction between a single tx coil and the feedback coil being regarded as two separate coils even though they are series connected.
But I take your point, in both cases, they are balanced by way of magnetic null.

[Carl-NC]

You could say that the DD is a geometric null, whereas the concentric is a mechanically-contrived null.
You can also use a bucking coil with a DD arrangement if, for example, you want a broader overlap.

[dbanner]

Carl, you do have a way with words. A " mechanically contrived" null, is just what I meant to convey. I wasted a whole paragraph when all I needed were just two simple words

Now perhaps you could expand a bit on the thermal drift you mentioned above. Are you saying that the more the thermal drift, the greater the angle between voltage and current. Thinner wire in the context we speak of is more susceptible?
There is so little info on this in relation to search coils and ambient temperature, I'd like know a bit more, in layman terms of course.

[Carl-NC]

Now perhaps you could expand a bit on the thermal drift you mentioned above. Are you saying that the more the thermal drift, the greater the angle between voltage and current. Thinner wire in the context we speak of is more susceptible?
There is so little info on this in relation to search coils and ambient temperature, I'd like know a bit more, in layman terms of course.

Good question. In a normal VLF the TX voltage triggers the demods, usually through a variable phase shift circuit that allows us to adjust the phase to achieve GB. If the TX coil has no R then the relationship between the TX voltage and the TX current (and hence the magnetic field) is a perfect and constant 90 deg. But with R it is



Suppose your 10kHz design has a 10" 1mH 18awg coil; that's about 40T and comes to 105 feet of wire. Copper resistance is 0.6686 ohms @ 27C. Let's say you're hunting on a summer day, it's 27C (~80F) in the shade and 32C (~89F) in the sun. The TC of copper is ~0.4%/C so a 5 deg temp shift results in 0.6834 ohms. Plugging those numbers into the equation above you get phi=89.39 in the shade and phi=89.38 in the sun. That's a phase shift of 0.01 deg which is not going to be noticeable.

Now take the same coil wound with 27awg. The resistance is 5.3900 ohms and with a 5 deg temp shift becomes 5.4977 ohms. The new phase shifts are phi=85.1 (shade) and phi=85.0 (sun). That's a 0.1 deg phase shift and you will likely hear that over mineralized ground.

[dbanner]

Good question. In a normal VLF the TX voltage triggers the demods, usually through a variable phase shift circuit that allows us to adjust the phase to achieve GB. If the TX coil has no R then the relationship between the TX voltage and the TX current (and hence the magnetic field) is a perfect and constant 90 deg. But with R it is



Suppose your 10kHz design has a 10" 1mH 18awg coil; that's about 40T and comes to 105 feet of wire. Copper resistance is 0.6686 ohms @ 27C. Let's say you're hunting on a summer day, it's 27C (~80F) in the shade and 32C (~89F) in the sun. The TC of copper is ~0.4%/C so a 5 deg temp shift results in 0.6834 ohms. Plugging those numbers into the equation above you get phi=89.39 in the shade and phi=89.38 in the sun. That's a phase shift of 0.01 deg which is not going to be noticeable.

Now take the same coil wound with 27awg. The resistance is 5.3900 ohms and with a 5 deg temp shift becomes 5.4977 ohms. The new phase shifts are phi=85.1 (shade) and phi=85.0 (sun). That's a 0.1 deg phase shift and you will likely hear that over mineralized ground.

I said in layman terms, nevertheless, point taken. I suspected some TC ( coefficient) to be semi dominant. But you layed it out like no other.
Thanks again Carl, your the best.
It is indeed amazing that we have you. Not intuitive at all!.
The temp variation is going to be more than 5 degrees celcius. But I suppose poor people in the temperate zone can't afford to buy a metal detector. I added that last comment because I could.

[Kev]

I'm sorry I missed this thread while it was still hot, but I have a few points to add anyway.
6666 the original thread you linked to is very interesting because I have found some things contrary to what was asserted there. Qiaozhi's comment "This may be the difference between theory and practice" is so true, indeed when one considers the value of models (and also Prof Neil Ferguson comes to mind) the results are only as good as the completeness of the algorithms.

I recently experimented with actual coil configurations and found very little difference in the performance of "figure 11" and "figure 12" arrangement, just one was easier to adapt to Minelab requirements.

All the simulations had equal Tx/Rx inductances where in the real world that is not normally the case. My concentrics are 300uH Tx and 500uH Rx. This makes an enormous difference. For instance the standard 14x13 GPZ coil can detect a 5 cent around 45 centimeters, and my 17"concentric around 53 centimeters, in the house and on a gain of one. The concentric Rx is 8 inches in diameter, but the results are as good as a 4500 with an 18" mono attached.

On that thread it is stated that the Rx only sees what is inside of it. This is true with a concentric and is what makes it so good at pinpointing, but depth cannot be inferred from this, its response cone resembles a dunce hat and not a beanie. The DOD sees way out to the side of the Rx coils and I have located targets under rocks from the side. So there is much more going on with the coil interactions than these simulations account for, really it's all groping in the dark.
Cheers
Kev.