Garrett Axiom Coil Pin-Out and Measurements

[Az_Ed]

I am volunteering with an archaeology search effort in Arizona and have been using the Garrett Axiom to search for artifacts that are about 480 years old. The Axiom is out performing other detectors used, especially on depth with larger targets. It appears increased detector depth is needed on larger targets like horseshoes, swords, chain mail, armor, etc. I'm currently using the Axiom 16" mono coil but need to search at depths beyond this coil. I'd like to design/build a 24-36" diameter coil for the Axiom. Fortunately the 4-pin connector used on the Axiom is a common microphone type connector and I have several in the junk box.
Does anyone have knowledge of the pin-out, coil design and typical parameters for Axiom mono coils? Here is what I have discerned so far:
11" mono coil
Pin 1-2: 13 Ohms resistance, 171.9 uH at 200 kHz, 215.7 uH at 15 kHz, unmeasurable Q. (Used a Peak LCR45 inductance meter)
Pin 3-4: 0.6 Ohm resistance, 196.9 uH at 200 kHz, 246.5 uH at 15 kHz, very low Q of 2 or so. (Used an HP 4342A Q-meter)
16" mono coil
Pin 1-2: 0.6 Ohms resistance, 156.4 uH at 200 kHz, 196 uH at 15 kHz. Did not measure Q.
Pin 3-4: 0.6 Ohm resistance, 189.2 uH at 200 kHz, 238.6 uH at 15 kHz. Did not measure Q.
I'm a novice to PI detector coil design so my guesses may be wrong. It appears that Garrett is using two coils, one for transmit and another for receive. I'm guessing that Pins 1-2 are the receive coil and Pins 3-4 the transmit coil.
At the coil connector I noticed that there is no shield on the 4-wire cable. Pin 1 of the connector is also connector case/ring or presumably ground. Perhaps the shield is grounded at the coil and un-terminated at the connector of the detector housing? Maybe Garrett is just using twisted pair in the coil cable to reduce RFI?
Any help or suggestions of material I can study will be appreciated. Ed, Saddlebrooke, Az

[Az_Ed]

Additional data on the coil 4-pin connector:
Pin-1: common with the connector ring or shield, presumably ground.
Pin-2: -0.283 VDC, about 0.3 VAC
Pin-3: +29.7 VDC, 26.2 VAC w/r connector ring/shield
Pin-4: +33.0 VDC, 30.6 VAC w/r connector ring/shield
Pulse period is about 640 us or 1.5 kHz PRF at over 50 V p-p w/r connector ring/shield. Not certain how to measure pulse amplitude. It may be a balanced signal that can only be measured via an isolation transformer.

[Carl-NC]

Most likely you're right about the coils. I know the Infinium uses separate TX & RX coil windings even though it looks like a mono coil. Most likely your pin guesses are correct, too. I don't have an Axiom to verify.

[pito]

Show pictures what you are doing, make x ray of the coil, can you buy a big coil make testing and returned after few days ? Make a pickup coil for Tx EMF strength, 100turns, any wire, coildiameter 10cm for standart.Show pictures what you are doing, make x ray of the coil, can you buy a big coil make testing and returned after few days ? Make a pickup coil for Tx EMF strength, 100turns, any wire, coildiameter 10cm for standart.

Look post #416 https://www.geotech1.com/forums/foru...e28#post421167



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Garrett ATX Pulse Induction Metal Detector with 11″x13


Canadian Treasure Seekers
https://canadiantreasureseekers.com › product › garrett-...

It features advanced pulse induction technology, a durable design that meets military specifications, and a new DD coil design for extreme and uniform ...
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actually x ray is not necessary, they say it is DD, so make copy of the coil you have and when you have the same results then make a big one.

using the Axiom 16" mono coil =? , x ray to confirm it is mono not a DD

[Az_Ed]

Thank you Carl and Pito for the comments. I will try measuring the pulse amplitude using a coupling coil. I do not have access to any Xray equipment. The voltage spike is so high I'm at risk of blowing my oscilloscope front end unless I rig up a voltage divider. Using my pin-pointer it seems the mono coils are configured as DD, but I can't really tell due to the broad response. I think I can inject a little signal onto the coils, one at a time, and possible sense the wiring layout.
One of the coils having 13 Ohms or resistance implies a lot of small diameter wire. That would equate to about 80 feet of AWG 32 or 200 feet of AWG 28. It is interesting both coils show exactly the same resistance on one coil. I'll try to test the 13" DD coil today and see if the pin-out and measurements are similar.

[Carl-NC]

You can buy a 100:1 scope probe off eBay for $20 or less. If the assumption is that the mono coil is actually co-wound separate TX & RX coils, the only concern I would have is that the preamp is designed to handle the large coupled flyback. To verify, I would put the RX signal on the scope and see if it has a large flyback. If it does, start building a new coil. If it does not, then the assumption is wrong and you need to figure out more about the coil.

[pito]

You also trace beginning of connection on PCB, maybe you can locate diodes or damping resistor.
and possible sense the wiring layout. = You can trace Tx coil shape without X ray, just use small pickup coil ( inductor with ferrite core )

[Az_Ed]

Thanks for the suggestion on the 100:1; I'm going to order a couple. My HP 54502A scope (scrounged surplus and repaired) can only handle 5 Vrms max. I don't really like digital scopes but it sure has a lot of nice features.
I checked the fly-back. It is about +/-8V peak. This could be handled by limiting diodes or something.
Data from the 13" DD coil:
Pin-1: Black wire, about 18 mils OD, maybe AWG 24, common with the connector ring or shield, ground.
Pin-2: Red wire, about 18 mils OD, maybe AWG 24, -0.29 VDC, about 0.30 Vrms.
Pin-3: White wire, about 28 mils OD, maybe AWG 20, +30.0 VDC, 25.3 Vrms w/r connector ring/shield.
Pin-4: Green wire, about 28 mils OD, maybe AWG 20, +33.5 VDC, 31.8 Vrms w/r connector ring/shield.
The cable does have a copper braid shield. The shield is connected to the Pin-1 (black wire) inside the coil. There is no braid connection at the detector housing. I guess the goal was an electrostatic shield while avoiding a ground loop.

[pito]

I read Axiom specification, it is very similar to regular IB metal detectors, wires diameters( post # 8 ) also similar to regular IB. Read again post #4.
My conclusion;
1. this is Induction Balance detector driven by pulses, which have wide spectrum, therefore it is easy to make multi frequency machine .....
2. the coil is a regular DD

about 0.30 Vrms. = in DD nulling acceptable range, show picture
pin3, pin4 probably have anti-phase voltages, so the voltage will be around 65V = regular DD voltage

I checked the fly-back. It is about +/-8V peak. = not sure it is fly-back, show picture.

without pictures 50% it is a guesswork.

[Az_Ed]

Thanks Pito for the comments. I have a 100:1 on order at only about $23. It is amazing how prices have fallen. O-scope probes from Tektronix used to be prized possessions and hard to find. Now we buy Chinese junk and hope it works. I have some Tektronix scope probes that are 50 years old and still excellent.
I'll try to get some photos included.
Can someone recommend how to measure the impulse applied to the transmit coil with an oscilloscope? It has DC voltage on it and no ground, so I guess it is some sort of balanced line driver that is not referenced to ground. I can't place a scope probe on it without seeing only one-half of the picture. Perhaps I could capacitively couple the ground lead of the scope to one side of the coil and the probe to the other. Detectors are interesting technology.

[Az_Ed]

I used a 900uH small flat wound RF choke as a pick-up loop for the O-scope. I placed 91 kHz on the coil loop leads at about 0 dBm, 135 Ohms Z. The mono coils only show signal only around the perimeters, for both coils on the connector. The 13" DD coil is indeed a DD configuration. It appears that the two coils on the connector, Pins 1/2 and 3/4 are wound on the same perimeter or DD layout. I must have been detecting shielding with the pin-pointer.