CONCENTRIC COIL MADNESS

[6666]

In the past never had a lot of interest to make a vlf concentric coil , but with the voodoo it has sparked my interest.

Been reading lots of things, in one thread Aziz says a ratio of about .8 between the larger TX coil and the smaller RX coil seems about right (with bucking coil)

But what happens if the TX coil and RX coil are the same size, does it become a useless coil because of the high number of bucking coil turns presumably required to null the TX signal ?

[dfbowers]

That?s an interesting ratio. 1/2 diameter RX always makes it easy to calculate the bucking coil.. just divide by 4 and you can ballpark pretty easy. I always wondered what a larger RX would do.. Those later Tesoro coils are way smaller and worked great. Go figure..

[6666]

Here is the reference thread, what is your interpretation ?
https://www.geotech1.com/forums/show....google.com.au

[Skippy]

Round bundles of wire near each other don't couple as well as you would think. Even if they're touching, they are technically two radius apart. I think you could successfully make a cc coil with the RX at over 90% of the TX diameter, though choosing 80% would be my upper limit, unless mechanical constraints forced my decisions.
A key point is the RX and bucking coils MUST couple together well.

I'm currently experimenting with a 'Sunray Probe' type IB coil, and all the unknown coupling coefficients make designing the thing quite troublesome. Likewise. this will be a problem if you decide to make an unconventional cc coil.

One other thing I had pondered: is mechanical stability best with the common 50% RX coil ? If a 80-90% RX was used, would the resulting coil be more bump-sensitive?

[Qiaozhi]

That?s an interesting ratio. 1/2 diameter RX always makes it easy to calculate the bucking coil.. just divide by 4 and you can ballpark pretty easy. I always wondered what a larger RX would do.. Those later Tesoro coils are way smaller and worked great. Go figure..

The small RX loop in the Tesoro coils results in much better target separation.
If the idea of making the RX loop larger is to increase ground coverage, then why not put the RX loop on the outside and the TX loop on the inside?

[green]

Charted a test I did awhile back. Mono and concentric with a PI. Been thinking Rx diameter controlled slope. Looks closer to Tx(mono)slope. Not what I was expecting.

Added chart I posted awhile back.

[Semih BULGUR]

Thank you there l found realy good tech details about coils here. But l also need and appriciate further info for more sensitive target saperation with RX-TX loops or more coils...

[MXT SNIPER]

I find it interesting concentric coils are being made for both the gpz and gpx by X coils, and being tested now with very good results. Tiny gold doesn't seem to drop out on these coils.

[Tinkerer]

I find it interesting concentric coils are being made for both the gpz and gpx by X coils, and being tested now with very good results. Tiny gold doesn't seem to drop out on these coils.

Are these induction balanced coils like DD coils or the original GPZ triple coils?

Or just separate TX coils and RX coils?

[dbanner]

In the past never had a lot of interest to make a vlf concentric coil , but with the voodoo it has sparked my interest.

Been reading lots of things, in one thread Aziz says a ratio of about .8 between the larger TX coil and the smaller RX coil seems about right (with bucking coil)

But what happens if the TX coil and RX coil are the same size, does it become a useless coil because of the high number of bucking coil turns presumably required to null the TX signal ?

Isn't the purpose of the bucking coil to cancel the tx signal in the vicinity of where the RX coil is located?
Therefore if both tx coil and RX coil were co-located, then the buck coil would cancel the tx signal in the vicinity of the tx coil as well. Rendering it non-radiating. ( If you want to cancel the TX signal in the vicinity of the tx coil well duh). That, of course, is in the traditional vlf IB setup. A switching tx circuit might be a different matter. But then I would have to question the concept of inductively balanced( nulled as a result purely of geometry) eg. DD, as opposed to the use of a cancellation field( buck) ( magnetic null). The latter is not inductively balanced but rather, it is balanced(nulled) by an opposing field.( electroMagnetic null?)

[dbanner]

That?s an interesting ratio. 1/2 diameter RX always makes it easy to calculate the bucking coil.. just divide by 4 and you can ballpark pretty easy. I always wondered what a larger RX would do.. Those later Tesoro coils are way smaller and worked great. Go figure..

The first time I saw this business of " half the size, quarter the turns" was in a write up by a Mr. Dave Emery. And of course there is little surprise when people find that nulling the concentric coil is a big pain in the ***. That's because in the article, Mr. Emery uses same gauge wire for both tx and RX. #31.
So everything become quite simple.( The article is really supposed to help the novice builder) which I give praise to Dave for taking the time to put together).
But it is not the way a commercial coil for a metal detector is constructed.
TX coil wire is usually thicker, ( power considerations to reduce battery drain current), and the RX wire is a little thinner( inductance requirements and coil Q considerations given phase response) also transformer action)
Obviously a larger RX coil increases sensitivity but there are constraints on how large. The optimum coil should be no more than 8 inches( TX coil) with the RX around 4 inches. But the buck coil should be tx coil turns/3.3 and of the same gauge wire as the TX coil. This holds true only for coils 7-8 inches in diameter.
This is what George Payne discusses in his statements. The ratio of 3.3
Now I suppose 3.3 is close to 4, so in the end it's just a ballpark figure. Hence one is always fiddling around with a small loop of wire, looking for that " null". What a waste of time.

[dbanner]

The small RX loop in the Tesoro coils results in much better target separation.
If the idea of making the RX loop larger is to increase ground coverage, then why not put the RX loop on the outside and the TX loop on the inside?

For a given target inductance and Q , so for bigger RX, you'd reduce the number of turns using presumably the same gauge wire, but then what happens to it's resistance? I suppose one could recalculate the wire gauge although using the same guage wire for a coil twice the size should not alter it's resistance by large amount.
There would be greater sensitivity with a larger RX coil, but also sensitivity to ground.?
There are pictures of such a coil on forum, I think made by Garrett.

[Carl-NC]

Therefore if both tx coil and RX coil were co-located, then the buck coil would cancel the tx signal in the vicinity of the tx coil as well. Rendering it non-radiating. ( If you want to cancel the TX signal in the vicinity of the tx coil well duh). That, of course, is in the traditional vlf IB setup. A switching tx circuit might be a different matter. But then I would have to question the concept of inductively balanced( nulled as a result purely of geometry) eg. DD, as opposed to the use of a cancellation field( buck) ( magnetic null). The latter is not inductively balanced but rather, it is balanced(nulled) by an opposing field.( electroMagnetic null?)

True, you can't balance an RX co-located with the TX with a traditional bucking coil. You could make them the same diameter in a coaxial stack, or you could move the cancellation to a separate (and maybe remote, even in the control box) transformer like A.G. Bell did with his first attempt. Also, DD cancels using a magnetic null just like a concentric; the opposing field is the field on the outside of the TX coil.

TX coil wire is usually thicker,

A big reason for using a heavier gauge on the TX is to minimize phase error between the TX current and TX voltage. The voltage is what we use to trigger the RX demods and (with a little phase shifting) gives us ground balance. The absolute TX resistance is easy to adjust out in the GB phase shift but the thermal drift is not.

There would be greater sensitivity with a larger RX coil, but also sensitivity to ground.?

Mathematically a larger RX coil always wins, assuming a constant inductance and a homogeneous vertical target field. But target fields don't behave that way, they diverge and very quickly the larger RX coil loses its advantage. This is why small coils have better sensitivity to smaller shallower targets and large coils to larger deeper targets.

[SWL]

And so ... one question. Please specify. Is it necessary to make a gap between the compensating winding and the RX or wind it around the turns of the receiving coil?

[Carl-NC]

And so ... one question. Please specify. Is it necessary to make a gap between the compensating winding and the RX or wind it around the turns of the receiving coil?

You can wind it directly on the RX coil but you need to make sure the outer part of the RX coil is RX-ground and the inner part of the bucking coil is TX-ground to minimize capacitive signal coupling. Personally, I prefer a small gap.