IGSL

[HECTOR]

An unshielded coil is practically useless in the field work. I use shielding to both coils, TX and RX. I prefer the aluminum adhesive tape, because it handles very well and does not usually cause any problems in the field.

Can you please show an example?
I have two types and they are both too thick and wide for coil shielding. The stuff I have works good for inside plastic housings. Speaking of which, do you think it would be any benefit to shield the inside of my Cibola? I was thinking that extending a shield on the top end wouldn't help unless it was true earth grounded. No?

[bernte_one]

hello davor

i must see when i return to home
i will try to measure pinouts of my musketeer x coil (the concentric white one)

they have 5 pin microphone plug (i think always better than expensive switchcraft plug from teosro coil)

regards

[Davor]

BTW, can anyone point me to a connector pinout of an original Musketeer coil? It is unlikely that I'll ever buy one, but it doesn't hurt to make it "standard" way and being able to swap my coils with someone else (in case I can even adopt that pinout for my differential Rx).

[Davor]

It is funny but making a true differential input requires just adjusting a few resistors values - nothing else. It goes like this... The inverting side resistors R1 and R2 set the gain, and the noninverting side resistors just divide the input voltage so that R1/R2=R3/R4. It is a common practice to use R1=R3 and R2=R4 because in ancient times of lousy opamps it was the only way to keep the output offset at bay. Even today with bipolar opamps you get minimum offset this way.

In case of AC coupling to the next stage (✓) you can freely play with values R3 and R4, and the worthy goal is to achieve a symmetric impedance to both inputs against the ground.

By virtue of feedback the inverting input impedance of symmetrically fed opamp is ~R1/2, and at noninverting side it is R3+R4. Matching these gives you a true differential preamp. With a little offset, of course.

I am using 6k8 330k 68ohm 3k3 values to get amplification of 48.5 and impedance mismatch of ~2% (which is within the component tolerances anyway). My total input impedance is ~6k8, while the original IGSL is over 220k. I expect some noise improvement this way, but also somewhat more demanding GEB tuning range - for which I know the cure.

[ivconic]

Well, I can understand why, but my approach is a tiny bit different. Shields, as applied to MD coils and LF, are a remedy for static fields, BUT not required with center ground tap. A prerequisite for such setup is a true differential input, or a symmetric/asymmetric transformer. My choice is a true differential opamp input.

Many of coils i know do have also center tap and still those are good shielded.
But ok for that, it is interesting approach and i am very interested in what you will achieve with it. All the new ideas are always very interesting and attractive.
But i thought first you gonna make genuine IGSL setup to have reference and later you will make your idea. But now i see that you actually don't have intentions to make original IGSL at all.
That's situation when we all will not be able to compare our IGSL's with your modification.
Ok, it's your choice.

[Davor]

Well, I can understand why, but my approach is a tiny bit different. Shields, as applied to MD coils and LF loop antennas, are a remedy for static fields, BUT not required with center ground tap. A prerequisite for such setup is a true differential input, or a symmetric/asymmetric transformer. My choice is a true differential opamp input.

[eduardo1979]

An unshielded coil is practically useless in the field work. I use shielding to both coils, TX and RX. I prefer the aluminum adhesive tape, because it handles very well and does not usually cause any problems in the field.

[satdaveuk]

Nope
We got a bit of a weather here, strong storm grade Bora and wild fires. My portable "lab" is outdoors so instead of IGSL i made some flapjacks.

I realised that cable gland is upside down, and that shaft fitting may require some extra and non-metallic support so I have something to play with.

Anyway, this is how it looks so far. I'll pot it with hot glue at the end. I don't intend to apply any foil. My Coil is center tapped bifilar, and my preamp is true differential.

Looks nice
Why dont you just put the cable jonny the correct way round and drill a hole through the side and give yourself side entry, then just seal with hot glue.
Dont know what country you live in but shielding is a must over here unless you have a magical formular.
Tryed once just shielding RX which was fine in open field until it faced the ground which rendered it useless.
All the best
Dave

[ivconic]

As a matter of truth; there were several genuine metal detector in the past with unshielded coils. I can't exactly recall those models right now. But fact is that those vanished in time.
Everybody now days are making ONLY shielded coils. Must be a very good reason for that.
Once even i tried to make one unshielded coil. Was omega coil for my Micronta 4003 variant, called MD3007.
That coil indeed worked like a charm on the bench, in the air. Perfect.
But later was useless on real soil.

[Davor]

Plastic part is taken from vacuum cleaner i suppose?

No, but close, it is the flat mop with foldaway head - you just dismantle everything and there you have a shaft fitting and a head that is moveable front-back and sideways. Got it cheap at fair.


I won't apply any shielding for now. I'm quite confident that this setup will perform without any. While nulling the coil I did not notice any proximity effect due to hands etc. so I guess it will be OK.
When I (finally) finish it I'll know for sure.

Plastic body is plexiglass sheet that I cut into an ellipse (drawn by classic method with two focal points) + an ear for a cable gland, using an arc saw. I got 2 bodies out of 50x25cm plexiglass sheet. Very tight.

[ivconic]

Nope
We got a bit of a weather here, strong storm grade Bora and wild fires. My portable "lab" is outdoors so instead of IGSL i made some flapjacks.

I realised that cable gland is upside down, and that shaft fitting may require some extra and non-metallic support so I have something to play with.

Anyway, this is how it looks so far. I'll pot it with hot glue at the end. I don't intend to apply any foil. My Coil is center tapped bifilar, and my preamp is true differential.

Yes we here also had pretty stormy night! Thunders and lightnings all night long!
Looking neat and clean work! Nice!
Plastic part is taken from vacuum cleaner i suppose?
You don't intend to apply any foil? Ok, but you certainly must apply any other form of shield!?
Otherwise such unshielded coil will not perform calm on real soil.
Maybe you are lucky and can obtain special filler with iron oxide particles inside?
Any method you use to form Faraday cage is better than no one, trust me.

[Davor]

Ok, coil is almost there, what's about electronics? Alive? Kicking?

Nope
We got a bit of a weather here, strong storm grade Bora and wild fires. My portable "lab" is outdoors so instead of IGSL i made some flapjacks.

I realised that cable gland is upside down, and that shaft fitting may require some extra and non-metallic support so I have something to play with.

Anyway, this is how it looks so far. I'll pot it with hot glue at the end. I don't intend to apply any foil. My Coil is center tapped bifilar, and my preamp is true differential.

[golfnut]

c;s;NSiuebfvperybqpcEIUGFY

Sometimes I got a dual sig on mine, It was almost always Iron.



I am making an IDx - whats the feedback on those currently. ?


Not that many posts praising these designs.. I saw a YT vid of a guy with an real IDX and everthing he was getting was up to about 4".



If somebody had a clone of Goldmaxx power Id build one of those right away. Tidy kit

My wishlist for next one is.
Balanced front end so Differential out to Dets
Balance dets to go with differential preamp.

Stop the design at filtered and amplified Det outputs. Then Go SW / A2D at tht interface.

I would zone return phase shifts into bands - say four and depending on phase value - play a tone for each phase. This would reduce the need for a display.

Id like a small meter dislaying detected ground signal level, so I could trim it low as I roam - So I could keep the gain on disc ch UP.

[ivconic]

I finally balanced my coil. A small step for mankind, but a milestone for my IGSL. I tuned it using a non-calibrated Visual Analiser hooked directly to a microphone input, so I have no idea how well it stands. OK, initially I placed an attenuator, but I had to hook it directly soon after. Tx coil was hooked to its oscillator at IGSL board.

At one point I got complete cancellation of Tx, but the moment I glued it - it was gone. I guess capacitances take care you have some residual signal to play with.

Both coils are unshielded, and Rx coil is bifilar wound with center tap hooked to ground. I did not notice any static related coupling, so I guess it is going to be OK this way.

Anyway, it took me several hours until I finally figured that the complete cancellation is not going to happen again, and I simply fixed things as they were after confirming a minimum with a wire curl. I was able to see amplitude changing when I waved some tools at hand, so I guess it will do. I left a small portion pliable so that I can optimise - or not - when everything else works.

I'll make another coil in a few days - just for the fun of it. If this bifilar thing works as I wish, I'll do another coil the same way. It is quite simple to form a coil this way, and gradually fix it as you go.

Things are looking up

Depends of your conditions; whether you will later apply Al foil shield on coils or anything else - but be sure applied shield will also affect null you previously achieved.
That's why is better to apply shield just after coils were made. But this also can be conditioned by type of coil you made. What is the type of your coil, DD or CC?
In case of CC than you made no mistake, you can apply shield later and by adding small loop to feedback, you can do additional nulling and compensate small jitters that appeared from shield presence (in case of Al tape).
But if your coil is DD than you may have a bit more of delicate work later.
That's why i always suggest shield to be applied before final nulling (at DD's).
Center tapped RX is ok, makes sense. Later at any time you can omit center tap connection and try different setup. Or use center tap. As you prefer.
Ok, coil is almost there, what's about electronics? Alive? Kicking?

[satdaveuk]

I finally balanced my coil. A small step for mankind, but a milestone for my IGSL. I tuned it using a non-calibrated Visual Analiser hooked directly to a microphone input, so I have no idea how well it stands. OK, initially I placed an attenuator, but I had to hook it directly soon after. Tx coil was hooked to its oscillator at IGSL board.

At one point I got complete cancellation of Tx, but the moment I glued it - it was gone. I guess capacitances take care you have some residual signal to play with.

Both coils are unshielded, and Rx coil is bifilar wound with center tap hooked to ground. I did not notice any static related coupling, so I guess it is going to be OK this way.

Anyway, it took me several hours until I finally figured that the complete cancellation is not going to happen again, and I simply fixed things as they were after confirming a minimum with a wire curl. I was able to see amplitude changing when I waved some tools at hand, so I guess it will do. I left a small portion pliable so that I can optimise - or not - when everything else works.

I'll make another coil in a few days - just for the fun of it. If this bifilar thing works as I wish, I'll do another coil the same way. It is quite simple to form a coil this way, and gradually fix it as you go.

Things are looking up

Hi Davor
Have you any pictures of your coil process, be interesting to see

Regards