DD Winding Instructions needed

[Naildigger]

Hello,

since there is no DD-Coil available for my Detector (X3), it looks as I had to build one myself.
Has anyone experiences on that.
I measured 2 different Inductances at my concentric coils, abt. 1,4mH and abt. 9,9mH.

I thought a DD-Coil is made from 2 Coils with same Inductance. How can this problem be solved ? Do I need to put a fixed inductance in series of one coil ?

Has anyone experiences on that problem ?

Please help. Some general Infos on DD Windings would be helpful too...

Thanx a lot, and sorry for my bad english...

Manuel

[Carl-NC]

Re: DD Winding Instructions needed

For my X5, I get about 1mH (TX) and 6.5mH (RX). DD coils can have different inductances, and often do. Simply match the inductances you measured.

- Carl

[Naildigger]

Re: DD Winding Instructions needed

I have the 7" Coil and the 9" Coil (which is X5 Coil) And both measure the same inductance.
But that´s not the point.
If I understood you, I should wind two different coils, each, with the inductance needed, and put them together in DD-Style ?
And this will work? Do you have a DD-Coil for your X5 ? Do you have some additional Info about your coil ?

Thanx a lot,

Manuel; Naildigger

[Reg]

Re: DD Winding Instructions needed

Hi Manuel,

It has been a long time since I built a DD for a VLF machine and I have never built one for the X5, so I am not sure just what problems you might run into, but I will try to offer some advice.

First, a DD coil generally consists of two separate coil windings placed in a housing in a DD fashion. One winding is the tranxmit (xmit) and the other the receive (rec). Normally, the receive has much more inductance than the xmit.

Now, on a VLF, the wire size of the xmit is quite critical if efficiency is a concern. The lower the operating frequency, the larger the wire required. I have no idea what would be the right size wire for the X 5 but my guess is you would be safe using a 22 awg.

The receive winding can be much smaller wire and generally is. Something like in the 30 to 34 awg would probably work.

On most VLF's, there is a capacitor across the xmit winding to "tune" the coil. I am not sure that is the case on the X5 but I suspect it is.

When placing the two coils so they overlap, genrally care is taken to get the positions absolutely correct to null the signal. By this, I mean, you would check the receive winding and adjust the overlap for the minimum signal in the receive winding. Care also has to be taken to assure you have the correct polarity also. If it is incorrect, then reversing the wiring on one of the windings will correct the problem.

Normally, the windings are epoxied in place so they can't move, once they are aligned. This is important, otherwise, things will move and the receive signal my be increase and overload the preamp. Even the slightest Movement can cause problems. Some manufacturers will heat cure the epoxy to "set" the temperature the epoxy will get soft. If this isn't done, then some epoxies may soften and distort if the coil is left in a car on a hot day, which could cause a coil to not work correctly.

Building a DD coil for a VLF can be fun, or frustrating, or both. It is much easier to build one for use on a PI than on a VLF. On a PI, one doesn't have to be as critical about alignment, but still has to be careful to make sure the windings will not move once the coil is completed. One other thing I have found is it is easier to make a DD using a round housing, rather than an elliptical. For some reason, the ones built in a round housing seem to be more stable over the long run.

I hope this helps.

Reg

[Reg]

Re: DD Winding Instructions needed

Hi All,

When trying to add a DD coil, it is a good idea to match the inductance of a standard factory coil, so on a VLF, one would make the xmit coil of the DD match the inductance of the xmit of the factory coil. The same holds true for the receive winding.

Now, on a PI, generally, there is no separate receive winding, so one has to decide what they want to do. The exception is the Minelabs, and they are a different beast. If a person is going to try to build a coil for a ML PI, then extreme care has to be taken to match everything.

If a person builds a PI, then usually no original info is available, so one simply has to select the values they desire for the application.

Now, since many try to build a nugget hunting PI, then one might try to duplicate the values of an existing coil such as those built for the ML PI's. Some vary a little, but generally, they are 300 uh for the xmit and about 450 uh for the receive.

In the past, I have simply made the xmit and receive windings the same inductance simply to try to minimze the problems I might run into. Well, I am not sure just what is the best solution, so I recommend people try different receive windings to see just what works best.

I have recently built a couple of PI coils that basically match the ML, 300 uh for the xmit and 450 uh for the receive. Prior to making them, my coils ranged from about 500 uh for both the xmit and receive down to about 300 uh for both. The transistion down in inductance was done to allow the coil to work at shorter delays, with the lower inductance allowing delays shorter than 10 usec to be used.

I am still trying to compare coils made with equal inductance for the xmit and receive against coils having different inductance for the receive and xmit. Right now, it is too early for me to say which is best. Part of the reason is because I do not see any real dramatic difference. I need to make a couple more DD coils of different size with windings that are not equal and compare all my coils. Since I cannot control the environment (noise) where I normally test them, I cannot do a careful comparison at home. I have to find a spot away from all noise, or with minimal noise.

I can say that it has been easy to build a DD coil with a rec winding having more inductance than the xmit. I was concerned about that prior to making one. Now I am wondering about the interaction of the two coils and whether it might be better to have the two different inductances to minimize possible direct interaction problems.

Reg

[Naildigger]

Re: DD Winding Instructions needed

Hello Reg,

First of all, thank you for your answear. You seem to know what you are talking (writing) about.
So please allow me a few more questions.

When the winding is ready, I read it has to be shielded. So what´s that shielding thing about ? What´s the advantage in shielding a coil ? I mean, the coil is some cind of antenna. Why would you want to shield your antenna ? Sorry for that question, I´m Radio HAM, an I just cannot understand...

The Diameter of the wire should be no problem. When I measured the inductance, i also measured the resistance.... And after calculating the amount of windings of the new coil, one could calculate the needed diameter to fit the measured resistance... so the Q must be equal.

I heard, that troy uses two receive coils in series, of which one is wound against the direction of the TX Coil. I wonder if that fact might have any influence on the measured inductance.

At last, could you give any advice, how that "nulling" thing is done? Should I use the Detector as signal source and mesure the output signal at the RX-Coil or what ? And by the way, does anyone know, where to get those plugs for the Troy-Shadow? I think they are the same used in Fisher detectors. Are those any US-Norm Plugs? Because here in Germany I don´t know what to look for....

Manuel; Naildigger

[Reg]

Re: DD Winding Instructions needed

Hi Manuel,

I am certainly no expert on building a VLF DD coil, but I have built a couple and they worked. I have built quite a few DD's for PI's, but they are much easier to work with. Now, I will try to answer the questions I can. As for the connectors, I do not know what Troy uses, but you are probably right about using the same as the Fisher. The last thing I heard was Fisher was building the X5 for Troy. I am not sure that is the case, but I think it is.

All VLF concentric coils have some form of an extra winding which is used to "null" the receive signal. It can be done two ways. One is to add a few turns of xmit winding on the receive coil and the other is to wrap a few turns of receive on the xmit windings. Both work. Now, the inductance of those turns is usually quite small and shouldn't alter the measurement too much. I personally wouldn't worry about it on my initial design.

Now, to answer your question about shielding, yes, all coils should be shielded. The reason is because the shielding minimizes the ground capacitance which would cause the detector to be very erratic if the shielding isn't there. Basically, the shielding eliminates problems caused by the E field of the coil.

On a VLF coil, the coil is typically shielded using a carbon loaded paint. Quite a while back, Dave Johnson (I think he designed the X5) posted the name of a good paint to use. You might want to do a search for it.

I have used a conductive paint normally used for picture tube coating and it worked ok on VLF coils. I do use a different technique on PI coils.

One thing you might want to do if you use a plastic housing and decide to use paint is to put a primer on the inside of the housing and then spray the shielding paint on the primer to minimize any cracking or separation. This cracking is common when paint is applied to plastic. Normally, paints do not stick well or coat plastic evenly.

Attaching a wire to the shielding can be done several ways. I have seen conductive glue used to attach the wire. The wire has been taped to the sprayed surface with a good quality tape. As an example, lets say you use a 30 awg stranded wire to connect the shielding to the common lead of the coil. You might strip 1 inch of insulation from the wire and then fan and tape the strands to the painted surface. In the case of a large coil, one might want to attach a couple of wires on opposite sides to assure a good connection across the entire coil.

Another technique might be to use a small piece of adhesive backed copper foil and solder the wire to the foil. The foil is then placed on the inside housing surface and the paint applied over the foil, wire and the surface.

It is a good idea to shield both top and bottom of the coil housing and attach wires to both. If you only shield the bottom, then grass or weeds touching the top surface can cause erratic noise in the form of static. The results will be a whole lot of false signals. Some people attach a wire to one side and use strips of conductive foam in different locations to electrically connect the top and bottom halves of the sielded surface.

As for applying the paint, one just has to try it. Too much paint or too little will cause problems. So, it is critical as to how much paint is used. On a very sensitive detector such as the X5, too much paint becomes a "target" so any movement of the plastic housing with respsect to the windings will or could cause a response. Too little paint and there will be inadequate shielding, so even things like touching the grass will cause false signals.

Now, I have seen other techniques used for shielding. One other technique used still used paint, but painted a special paper that was taped on the top and bottom of the windings. Actually, the windings were embedded in a foam/epoxy form and the paper applied to the top and bottom of the form.

There should be a space between the windings and the shielding. Something as simple as strips of rubber tubing placed around the windings would be sufficient. cut the tubing in half inch lengths, split one side, and slip it on the windings.

One could take a piece of foam, route out the area for the windings, leaving some foam under the windings as a spacer.

So, as one can see, there are several ways one could do this. However, telling someone how to shield a coil is sort of like trying to tell someone how to drive a car. The only way to know for sure is to simply do it and observe the results.

As for nulling the coil, I would look at the output of the first amp stage (the preamp) and adjust the windings to minimize any signal. What I have seen done is to glue the windings in place for all but a very small area once you have the coil nulled and then carefully adjust the small part for the final null.

If you are concerned about connecting a scope to anything inside the detector, you might build a simple preamp and xmit circuit operating at the same freq and use it to make the alignment. Again, I would look at the output of the preamp and not at the output of the windings. The scope might introduce some problems that could cause the null to be incorrect.

Again, I hope this helps. There are other ways to shield a coil and hopefully, others will jump in with suggestions.

Reg

[Carl-NC]

Re: DD Winding Instructions needed

Reg has addressed most of your questions. Here's some more feedback...

The TX coil is usually used as part of a Colpitts oscillator, so the inductance needs to be reasonably close. But even 10-20% off is no big deal. True, TX wire is larger, 22-26 AWG (that's American Wire Gauge, SWG is slightly different) will work, and 30-34 AWG on the RX. Matching the Q is generally not critical. Just get it decent.

Reg covered several shielding methods. For coil alignment, another option is to cement the coils in place as close to balanced as possible, but leave a single loop of wire free on either coil, and use it to fine tune, then cement it in place. Since all induction-balance coils are really just transformers, you could rig up an audio signal into the TX, and have the RX drive a buffered speaker. Then you could adjust the balance until the audio nulls.

The Shadow concentric loops have a large TX coil and a small RX coil, and then has an RX bucking coil just outside the TX coil. Most other concentrics use a TX bucking coil close to the RX coil. Either way will work, and when you measure the inductances, you are getting the bucking coil as well. Although DD doesn't use the bucking coil, don't worry about trying to subtract it from the coil values.

Coil connectors on Troy, Fisher (who does make the latest Shadows), Tesoro (who made the X2), and Whites are all made by SwitchCraft.

- Carl

[Beachcomber]

What is the advantage....

of using part of the receive winding as the bucking coil rather than using part of the transmit coil as the bucking coil? Or is it just simply another way of doing it. Since the X-5 is one of the deepest detecting VLF's I am wondering if that isn't part of what gives it the advantage over other machines. Just some early morning thoughts. Thanks!!!!

HH

Beachcomber

[Carl-NC]

Re: What is the advantage....

It's mainly another way of doing it, although the Shadow way may be slightly better. I once did a side-by-side comparison of the S9 and Tesoro's 8x9 (both with the same Bandido II uMax)... the "detection cone" of the S9 was a little less tapered than the 8x9, so you would get better coverage a depth. It wasn't a huge difference, but was noticeable.

- Carl

[Beachcomber]

Thanks!!!!! n/t

n/t

[LUDMIL BAROSSOV]

COIL/ DD/

Hello,

YOU CAN BUILD /DD// -TX TWO PATRS/THE SAME/
BETWEEN THEM RX .THE FESE IN DEGREE WIL BE
MORE SENSIVITILY.




BAROSSOV.

BEST REGARDS.


[email protected]

BULGARIA.



























hallo.
you can build double D TX-TWO THE SAME INDUCTANCE
the rx between them.the fase of degree wiil be more
sensivitily.

best regards

barossov

[email protected]

BULGARIA.














since there is no DD-Coil available for my Detector (X3), it looks as I had to build one myself.
Has anyone experiences on that.
I measured 2 different Inductances at my concentric coils, abt. 1,4mH and abt. 9,9mH.

I thought a DD-Coil is made from 2 Coils with same Inductance. How can this problem be solved ? Do I need to put a fixed inductance in series of one coil ?

Has anyone experiences on that problem ?

Please help. Some general Infos on DD Windings would be helpful too...

Thanx a lot, and sorry for my bad english...

Manuel[/QUOTE]