Figure 8 coil

[6666]

Hi all, just found a video on youtube in which a fellow says he is using a figure 8 coil on his whites detector.

Has anybody experimented with figure 8 coils ?
thanks

[Rov]

What Whites detector?TDI ?
figure 8 coils work on many PI detectors.
SEF coils for Pi Minelabs and Coiltek AI coils I suspect are a style of figure 8.
I have built figure 8 coils myself to suit many PI detectors.
Inductance,resistance and turns values with wire types vary from standard mono PI winds.but is achievable.
the results on PI for EMI rejection etc with figure 8 looms/winds makes a significant benefit. this was the primary reason I built several figure 8 coils for my Minelabs PI use and also for Salt Lake detection use,where the Minelabs PI was unstable and barely usable.
regards rov

[Jose]

Hello Rov
I would ask if a coil "8", produces two tones to a single target, such as for a dime?
Gracia

[bklein]

So is getting the 8's two loops to be the same size very critical?
What happens if the halves are folded over on each other to make a single loop?

[Rov]

No,the size of the two looms in the figure 8 shape is not critical to average the response. Basic the figure 8 is just a full large Mono loom ,flipped and inverted to show the shape.Just that the response from targets over the field produced from the two looms is different.
If you want you can alter the top half of the loom to be bigger or much smaller to the bottom half of loom for different target response.
Or even flip the smaller portion of the loom into itself(the larger portion of the figure to create a smaller within a larger loop.
as per appearance of a twin field coil with two looms. This does work,providing your PI detector accepts the overall inductance/resistance and coil capacitance values of the larger original large mono loop to Work.that you have created it from.
As to the two halves equal folded onto themselves,creates a different situation again. In some Vlf DD coils the one half of a DD is bi wound or is two loops or looms on each other ,essentially almost as a figure 8 on itself,but with a different gauge of wire for this half.This is for just One half of the DD loom. You may find a configuration similar to this in a new Gen. Whites Goldmaster DD coil for full funtionality? But hese are not a figure 8 as such.
rov

[6666]

Hi yes it was a whites TDI, thanks for the info on experiments with figure 8 coil

[Vortxrex]

Just for fun I took my surf pi coil which is 9.5" and twisted it into a figure 8 & tried it out on a small nugget. It did not work very well

Next thing I did was wind two 10 turn coils & connect them in series (DD or OO style) this worked a lot better & performed about as well as a 20 turn mono coil

Looking fwd to more experimenting !!!


Dave Frank

[Hannu]

I'm trying to make figure-8 for IB VLF (not PI). TX side (pic. A) is done and next step is make RX side. What is good angle (pic. E) for for crossing point? 150 degrees?
How about models C) and D). Are those even worth of trying?

[Davor]

C and D will be perfect with a 4-quadrant Rx. The angle E is of no consequence either way.

[Reg]

Rov is correct. The figure 8 coil in a mono form is used to minimize the external EMI noise. Yes, the two halves need to be very close to the same size for the best noise reduction. Since the two halves are out of phase, the external noise is cancelled or at least, partially cancelled.

Now, if one builds a standard TX coil and a figure 8 receive winding, you get both noise reduction and ground balance action. Just how well it ground balances will be determined by the two receive winding portions and how evenly they are held above the ground. Also, on this design, you get a signal reversal as the target passes from one receive half winding to the other.

Since there is a cancellation going on in both designs, the figure 8 will not get the same depth as a standard coil of the same size. This is why, such coils are not used on a regular basis. Instead, they are designed to be used in very noisy environments or very bad ground conditions.

Reg

[Humanise]

Since there is a cancellation going on in both designs, the figure 8 will not get the same depth as a standard coil of the same size. This is why, such coils are not used on a regular basis. Instead, they are designed to be used in very noisy environments or very bad ground conditions.

Reg

With Induction Balanced (IB) VLF design there is always cancellation going on so I don't see why the figure 8 can't potentially get a similar depth to other IB VLF designs. The current figure 8 coils don't maximize use of the magnetic field. I believe that by changing the design of the figure 8 we can get a better depth.

Below is a picture of a figure 8 modified to be more like a Dual double D receiver that I am hoping to build. Note that both the D s of this coil make up the receive coil.



There are some usability issues with the coil. You are only going to see an anomaly when it is over to one side of the coil or the other. An object directly under the middle of the coil will be canceled out.

I also expect that there may be problems trying to get a fully balanced version of this to be used on a current commercial detector that was not specifically designed for this coil. As the object goes from one side to the other, the Rx signal gets a 180 degree phase shift. The detector needs to work with both of these phases. It should also be able to tell the user which side the object is on.

I gather that a lot of commercial detectors are not really made for a fully balanced IB coil. Rather, they prefer at least a small signal to be present all the time.

These problems are worth persisting with as this coil has got some great advantages. These include:

- It automatically balances out any effects of the ground so long as the ground is reasonably consistent.
- It automatically balances out external EMI interference as long as the source of the EMI is a reasonable distance from the coil. In most situations in the field you will get this distance from the sources.
- It automatically balances out noise internally generated in the creation of the Tx signal.
- The symmetry of the coil makes it easy to manufacture. The Tx coil can use the Rx coil as a former or the Rx D subcoils can use the Tx coil as a former. It doesn't matter which one goes around the outside but they should be close together.
- It is easy to change the coil's sensitivity, regardless of size. To increase the sensitivity you just increase the number of windings on the Rx coil, that is equally on both the D subsections of the Rx coil.

I'm hoping to design my own IB VLF detector (PC + sound card etc) but this is the coil I am currently planning to use for testing it.

I would be interested in any comments you have on this version of coil.

Ken

[Aziz]

...
I'm hoping to design my own IB VLF detector (PC + sound card etc) but this is the coil I am currently planning to use for testing it.
..
Ken

Hi Ken,

I'll disclose the simplest VLF oscillator detector controller circuit just for this purpose soon.
This little beast achieves incredible good performance for it's simplicity. The DSP processing software is trivial too and one can use one of the available software interfaces from the open source community.

Cheers,
Aziz

[Humanise]

Hi Aziz,

How do you think that the above series double D receiver compares with your magic top hat coils?

My expectation is that your top hat coils should cancel out noise and eternal EMF but would not cancel out ground noise as this one should.

I'm hoping to use the above without using any shielding.

I'll talk to you later about the PC/sound card project

Ken

[Sergey_P]

I would be interested in any comments you have on this version of coil.

[Humanise]

Sergey_P,
I love your ability to create diagrams. It is something I have done little of and struggle to do. Thank you for those.

I make the assumption that the figure 8 has to be used so that it is on its side and moved left to right so that no anomaly can go through the center undetected as shown in your first figure.

I'm interpreting your graphs as being the amplitude of the signal generated in each half of the receive coil. With your third set of graphs for the situation of the overlapping halves, you have each of the amplitudes the same as for when they are not overlapping. I believe that this is wrong.

In the middle, where you have the standard figure 8 on its side, you would have the Tx around the outside. Each circle of the Rx takes up less than half of the Tx area.

When we move to the overlapping 8 Rx coil on the right and place a Tx coil around the outside, each circle of the Rx coil would be something like two thirds or three quarters of the area of the Tx coil and a lot more of the Rx coil would be up against the Tx coil.

I believe that the increase in Rx signal would be a lot more than the simple linear increase with increasing size of each Rx circle but I must admit that I find the maths and equations for this confusing. The equations for induced voltage from change in magnetic field have a 'curl' operator which is a circular operator. The magnetic field needs to be circular around the Rx wires.

For a simple circular Rx and Tx coil, the maximum induction with an air core is when the Rx coil is wound tightly next to Tx coil. As the Rx coil decreases in size, you not only have a smaller field within it but you have a field between the outside of the Rx and the inside of the Tx which is effectively in opposition to field within the center of the Rx coil. Consequently, as you decrease a Rx coil size, leaving the Tx the same, the change in signal would be a lot more than the linear decrease with change in size. I'm not sure whether the final changes, the voltage, the current or both as far as the received signal.

I would love to hear from others with better knowledge of the physics involved.

Ken