AMX coils

[Tinkerer]

A variety of coils are possible with the AMX, but all coils need to have separate TX and RX coils.

A concentric design gives the greatest depth for a given coil diameter.

A DD design gives the best ground coverage per sweep but needs to be of bigger diameter than the concentric for similar depth.

We will consider oOo coils and other exotic and specialty coils at a later time.

Initially I propose to start with an 8" DD coil, as I am about to build #5 experimental 8" DD coil for a CCPI system. I plan to post pictures and explanations along the build, but lets hear some comments and opinions about AMX coils in general.

Choices:

• High TX current and low turns, like 4A and 25 turns
• High TX turns and low current, like 250mA and 100 turns

Both varieties give about the same Ampere turns field strength.
Each has advantages and disadvantages.

Many more choices and compromises need to be made.

Comments, please?
Opinions, please?

[eclipse]

Option A /less turns/ is way better just because of increased weight in the other option. I would take this even if it performs worse. At the same time probably losses would be lower ?

[Tinkerer]

Option A /less turns/ is way better just because of increased weight in the other option. I would take this even if it performs worse. At the same time probably losses would be lower ?

Thank you eclipse, you make a very good observation.

Less turns = less resistance = less weight of wire = less TX losses
However, 4 times more current = thicker wire or increased losses and increased power consumption (W). I have not made the calculation yet, but for the AMX purpose, the power consumption is an important factor.

As you mention, the weight of the wire in the coil is also an important factor.
Here are some options to weigh:

Magnet wire, cheap, easy to source, more (skin effect) resistance, higher capacitance than fine thread tinned, insulated wire.
Fine thread tinned, insulated wire is more bulky, more expensive.
Genuine Litz wire, the most expensive, least resistance (skin effect, specially important with the high PPS of 25k and resonant frequency of 200 to 300kHz) least capacitance.
A few calculations to be made there, to find the best compromise.

And then, there still are the TX losses in the ground and the targets. (the ground has always some conductivity and is a very large target) These losses also need to be compensated. One compensation circuit will do it all.

[Tinkerer]

The resonant frequency of the AMX coil
A resonant frequency of 166kHz corresponds to a half sine wave of 3us or a change from +1A to -1A in 3us. With a TX coil of 25 turns = 50A turns in 3us or a rate of change (di/dt) of 16.6A turn/s.
For a target with a TC of 1us this is good.
For a target of with a TC of 10us a longer TX pulse like 10us would be better, but would make it more difficult to resolve the 1us TC target.

So, with a 25 turn, 300uH coil, we can have a TOTAL distributed capacitance of 3.3nF to achieve the resonant frequency of about 160kHz.
The distributed capacitance includes:
Interwinding capacitance between the wires of the coil
Winding to shield capacitance
Cable capacitance
Mosfet COSS capacitance
Other stray capacitances
We can add a dedicated high voltage capacitor to adjust the total capacitance.
If we want to use a higher resonant frequency we will induce slightly more eddy currents in the target.

The amount of energy or Joules stored in this distributed capacitance increases with the voltage of the Flyback.
The 1A current in the coil, connectors, cable, Mosfet and board copper traces generates a magnetic field. The transient generated by the di/dt induces stray currents in the surrounding of the path the current passes. THIS IS NOISE.

The power in the TX coil pulse is limited by the Flyback voltage. If we keep the flyback voltage limited to around 1000V, there is a choice of relatively cheap Mosfets available.

What about the wire and cable for 1000V?
Most 300V rated wires are tested to 2000V. We have not encountered any problems with that.
The layout on the PCB has to take the Flyback voltage into account.

[Carl-NC]

For a target of with a TC of 10us a longer TX pulse like 10us would be better, but would make it more difficult to resolve the 1us TC target.

I didn't understand this statement. By "TX pulse" I assume you mean the slew rate. But a faster slew rate should always make a stronger response, no matter the target TC.

[Tinkerer]

I didn't understand this statement. By "TX pulse" I assume you mean the slew rate. But a faster slew rate should always make a stronger response, no matter the target TC.

I agree that a faster slew rate will make a stronger response, with the same power.
However, how does it work, if there is more power?
As an example: (not real numbers) TX is 25 turns at 2A = 50A turns = 1000V limit Flyback = 16A turns /us
Or TX is 75 turns 2A = 150A turns = 1000V limit Flyback = 30A turns/us, reducing the Flyback by adding capacitance thus reducing the resonant frequency.
Would the higher A turns/us give a stronger response with a target that has a TC of 10us?

[Carl-NC]

Would the higher A turns/us give a stronger response with a target that has a TC of 10us?

Yes, but I don't think it will diminish the response of a 1us target. Maintaining the same peak flyback voltage implies that the transition slew rate is constant whether it's a fast transition at a lower peak current or a slow transition at a higher peak current. A constant transition slew rate induces a fixed EMF on the target. For a fast TC target the eddys build up quickly so whether it's a fast transition or a slow transition doesn't matter, the decay response should be the same. But for a slower target, yes, the slow transition allows more time for the eddies to peak and give you a stronger decay response. This is why traditional PI detectors with fast slew rates suck on high conductors.

[Tinkerer]

Winding the AMX coils.
About 8" DD, TX with 70 turns AWG 22 1x19 tinned wire gives me a dry run of 1539uH and 2.1 Ohm.
I use a turn counter so that I don't loose count.
On the coil winder you see the the coil winding form. I make a dry run first to check if everything is correct. Shape, amount of turns, inductance, bundle size etc. Then I unwind and rewind with epoxy resin filled with glass micro balloons.

For the RX I use Kynar wire wrapping wire AWG 30 and 25 turns.
The RX coil and TX coil are going to overlap about one inch to achieve a coarse induction balance.
This coil should give a sweep cover of near 8". The expected depth is not that great but sensitivity to small nuggets should be excellent.

A DD coil with a diameter of 12" or 14" will give more depth and sweep cover. TBD

[Tinkerer]

And here is the 8" TX coil.
2.06 Ohm
1513uH
70 turns AWG22, 1x19 wire

[Tinkerer]

Here is the DD coil I used for the tests of the AMX frontend.
This is the bare coil. I used a temporary bottom and top graphite shield for the tests. Now I will add the final shield and seal the coil.
Coil measurements:
33cm x 38cm
TX = 2040uH, 2.74 Ohm, SFR = 170kHz, wound with 60 turns of AWG 22, tinned 1x19, insulated 600V wire
RX = 342uH, 8.7 Ohm, SFR = 881kHz, wound with 22 turns Kynar AWG 30 wire wrapping wire
After adding the shield, the RX SFR dropped to 484kHz

[Tinkerer]

Now, looking at the specifications of this coil, quite a few questions could be asked:

Why 60 turns on the TX and 22 turns on the RX?
Why not 60 turns on the RX and 22 turns on the TX?
Or, what would the result be of the coil if it was 33cm x 33cm
Or 300uh TX and 300uH RX?
Or thicker TX wire or thinner TX wire?

So many possibilities with different results.

[lxs17757414132]

Can AMX use a single coil

[Carl-NC]

No, it requires an IB coil.

[lxs17757414132]

I am ready to try building AMX. Can someone tell me the coil data? Do you need a preamplifier inside the coil

[lxs17757414132]

I still need a parts list