Dual coil for mpp

[Denots]

Hi
i have a dual coil housing id like to use for my mpp, please see the attached photo


looking around it seems most people use a DD coil but I already have this housing so would like to use it if possible.
There are a couple of things that I?m wondering-

1. Is there a reason nobody seems to use a coil like this? Possibly not compatible with pi detector?

2. Is the smaller coil tx and bigger coil rx?

3. Could someone give me a rough base line to start with for the inductance and resistance of each coil please, from what I understand each coil has a different inductance reading?

many thanks

[godigit1]

Hi Denots,
Looks exactly like a dual field coil.
Ive wound one dual field it was a experimental two 3dss coils.
Actually easy to build and tune.
I dont have a way to measure the values of the individual dual field coils I have but I just split the desired Inductance between the two coils.
You can do the coil bundle the way you desire as long as it fits your shell and leaves room for shield spacing.
Predamp the inner coil before assymbly( resistor in coil). then final damp when the second coil in attached.
I heard different opinions of the dual field coil but I think its a very good beach coil. I like it on my Mpp.

Just double checked the 12 inch dual field. Im gettin 3.1 ohms so im not sure the two coils are damped seperatly or co wound on the actual dual field.

[waltr]

Hi
i have a dual coil housing id like to use for my mpp, please see the attached photo
[ATTACH]47462[/ATTACH]

looking around it seems most people use a DD coil but I already have this housing so would like to use it if possible.
There are a couple of things that I?m wondering-

1. Is there a reason nobody seems to use a coil like this? Possibly not compatible with pi detector?

2. Is the smaller coil tx and bigger coil rx?

3. Could someone give me a rough base line to start with for the inductance and resistance of each coil please, from what I understand each coil has a different inductance reading?

many thanks

My favorite coil to use with my PI detector is an 8 inch concentric 'dual' coil. It actually detects deeper than my 10 inch DD coil, pin points easier and is responses to 'good targets' around junk better.
Down side is the smaller area of detection, it is only the size of the RX coil which is 4 inches on my coil. This means each sweep must over-lap at 4 inches so it does not cover a lot of ground. My 10 inch DD does not go as deep on the same target but covers more ground per sweep so I use the DD when coin shooting parks and beaches and use the concentric for old farm sites.

I have details on building in my thread here, post #23 which includes a link to a good article on building concentric coils.
Note: Linked article is for VLF type detector as inductance value (number of turns) is different but method of building and nulling is the same.
https://www.geotech1.com/forums/show...ake-on-the-HH2

The outer coil is the TX and inner the RX.
I did both coils to have nearly the same inductance (TX=470uH, RX =407uH). Higher RX inductance increases the rx signal (more sensitive) but also increases noise pick-up so it is a trade-off.
Since your coil housing has an outer and inner coil space you will need to be a little creative to adapt the build methods in my thread to this housing.
So be sure to play with the coils and nulling before gluing the coils into the housing.
Also remember that the coil inductance for PI detectors is not overly critical so the main thing to get right is the Bucking coil and null.

Here is another good article on concentric coil building:
https://www.geotech1.com/cgi-bin/pag...anar/index.dat

Note: I do not shield my coils and have not had any interference or ground effect issues.
This could be due to the TX PPS is tuned to a multiply of 60Hz and I use the 3-sample GEB method (both are discussed in my thread).

If you need or want to shield then you will probably need to use the conductive spray method.

[green]

My attempt at a concentric coil https://www.geotech1.com/forums/show...556#post205556

[green]

Hi Denots,
Looks exactly like a dual field coil.
Ive wound one dual field it was a experimental two 3dss coils.
Actually easy to build and tune.
I dont have a way to measure the values of the individual dual field coils I have but I just split the desired Inductance between the two coils.
You can do the coil bundle the way you desire as long as it fits your shell and leaves room for shield spacing.
Predamp the inner coil before assymbly( resistor in coil). then final damp when the second coil in attached.
I heard different opinions of the dual field coil but I think its a very good beach coil. I like it on my Mpp.

Just double checked the 12 inch dual field. Im gettin 3.1 ohms so im not sure the two coils are damped seperatly or co wound on the actual dual field.

I've been measuring coils with separate Tx and Rx by shorting the Rx coil while measuring the Tx coil, shorting the Tx coil while measuring the Rx coil. Measure the same way as I do a mono coil. Think it is a good way, maybe someone could reply if it isn't.

[Carl-NC]

Denots, respondents are discussing 2 entirely different solutions. Let's start with your questions, assuming your intention was an IB coil.

1. You can easily build a concentric IB coil for a PI detector. Most people use DD instead because concentric has a stronger ground response in nasty ground, and nasty ground is often where PIs are used.

2. Large coil = TX, small = RX and bucking.

3. You would typically wind the TX just like a mono coil, i.e. 300uH and 1-3 ohms or so. The RX has a little more leeway, but 1mH might be a practical max. Then you also have a bucking coil wound on top of it. There is lots of info on these forums for how to do this.

The other solution is a non-IB "dual field coil," where you wind large and small mono coils and wire them in series. The trick is that the inner coil needs its own damping resistor, and the overall connection also needs a damping resistor. This may require trial & error.

If you want "easy" I would suggest the dual field coil.

[dbanner]

The Whites dual field patent has all the math information required to make calculations for the damping resistors for a dual field coil. It also shows how the coils are connected in series and the relative dimensions of the two coils, namely, the inner coil and the outer coil.
The outer coil measures about 12 inches, but the inner coil is just a little short of half the diameter of the outer coil. If I were to build one of these search coils, I would use a ratio of 2:1.
Now if somebody could provide the inductances of the outer and inner coil, along with type and size of wire, well.......

[Denots]

Thank you everyone you are all amazing, loads of information here for me to read through, I’ll have a good read through before I go any further with this.

For my windings to start with I used two core bell wire and made two monocoils, one 300mh the other 500mh, at the time of testing I didn’t notice any difference between those coils. Lately I’ve been using 34 awg enamel copper wire and made a coil that is 300mh and it works really well and is fast at picking up bits of metal.
Just earlier I decided to make another coil using the same wire to fit in the existing housing but this one would be 500mh, now I’ve hooked it up it’s really slow on both my working detectors, so am I right in thinking the more windings you have the slower the coil will be?

[Denots]

For reference this is the link to whites dual field search coil patent
https://patents.google.com/patent/US7994789

[Carl-NC]

Now if somebody could provide the inductances of the outer and inner coil, along with type and size of wire, well.......

I don't know the particulars of the White's coil, but I would start out splitting the inductance. If you do 150uH/150uH then the total will be higher than 300uH due to mutual coupling, so maybe start with 100u/100u.

You can always skew the coils to optimize for bigger/deeper or smaller/shallower.

[Denots]

Made a start
Big coil 136uh, there are some extra windings that I have left loose from the rest of the coil and should be able to drop it to 120 if needed.
Small coil 120uh
Together in series 308uh


thats it for tonight I?m going to bed, I?ll carry on with this tomorrow if there?s time

[waltr]

Good start Denots.

I didn't think of the Dual field so thanks Godigit1 & Carl.

[dbanner]

well, for a 150uH inner coil, with an estimated capacitance of 150pF, the calculated value of the damping resistor is 488 ohms, so I guess 470 ohms would get the ball rolling as far as damping the inner coil is concerned. I suppose the other damping resistor(the one which is across the overall connection) would be set using the conventional method oscilloscope and damping tool setup.
Just how would one determine if the inner coil was ringing is beyond me at the moment.
Carl, you got to know the answer to this.

[dbanner]

Setup and proper damping of a dual field coil can be tricky.
I propose the following method:
First, calculate the critical damping resistor(the resistor which goes across entire connection referred to as resistor #1 from the diagram in the patent. This value is calculated using the total inductance and stray capacitance of both coils.(use the formula in the patent)
with this resistor in place, check waveform on oscilloscope, there you will observe ringing due to effects of undamped inner coil.
Now place a damping resistor across the inner coil and adjust its value until the ringing observed on oscilloscope goes away. This will be the value of the inner coil damping resistor. The coil is now set.

[Denots]

For my windings to start with I used two core bell wire and made two monocoils, one 300mh the other 500mh, at the time of testing I didn’t notice any difference between those coils. Lately I’ve been using 34 awg enamel copper wire and made a coil that is 300mh and it works really well and is fast at picking up bits of metal.
Just earlier I decided to make another coil using the same wire to fit in the existing housing but this one would be 500mh, now I’ve hooked it up it’s really slow on both my working detectors, so am I right in thinking the more windings you have the slower the coil will be?

What a noob, I just remembered there’s a damping resistor inside the metal detector box, I then went and attached a damping resistor to the new coil I was testing, 2 x damping resistors is what caused the monumental slow pickup speed lol.

Electronics for dummies book turned up today so will be reading through that tonight, I need to clear some things up in my head before I progress, back to the basics