Where can I find information on

[Beachcomber]

making and tuning a Concentric Coil?

Thanks

HH

Beachcomber

[C.Pruysers]

Re: Where can I find information on

Hey Beachcomber. That info is the hardest to get . Coils are more important than the detectors themselves . I built all kind of detectors but cannot find any good info on the coils . If we were lucky somedays that info would be handed down to us . There MUST be people that have this info . I'll bet they do not want to share it with us though .They are a little selfish , but hey ! some day we will get it . Kees

[Beachcomber]

Re: Where can I find information on

Thanks...hopefully someone will be willing to share that info!!!

HH

Beachcomber

[Jeff Kinzli]

Re: Where can I find information on

Beachcomber, I have a lot of info in my brain about building coils (concentric, DD and others) but I haven't seen much in print specifically about metal detector coils. There is a good amount of theory on air-core inductors, EM theory, but you'll have to make that theory fit with your intentions for a MD coil. This forum is about the best place too, with some very clueful and experienced people frequenting it).

While I can help and am very willing to help those interested, sitting down and writing about how to make a coil from scratch is a little more than I have time for at the moment, but as I said I'm more than willing to help by posting on the forum here or by emailing privately.

For what it's worth, I'm trying to start a little side-business of making innovative metal detector accessories (coils and related toys) but have found that it's a whole lot more work than just coming up with a good idea or design for a product. Actually manufacturing, getting the right parts in stock, and a myriad of other things really slow things down. That's another story though

Post away with questions or email me privately if you'd like...

[Beachcomber]

Re: Where can I find information on

Hi Jeff,

My biggest area where I lack info on coil building is in the theory behind and the method used in setting up a Cocentric Coil. (Tuning) I have a good idea on the actual physical constraints involved. I am still looking for someone who sells premade cocentric coil housings. I am looking for 12" spider style housing. I have found a place that makes all different size "DD" housings and sells tham at a very reasonable price. But for some reason they don't make concentric. I am looking to make two 12" concentrics. One for the Explorer which will be my 2nd coil. My first will be for the X-5. It's been almost a year since the X-5 hit the market and still no cocentric coil larger than 9" is available for it. Basically when I run out of patience waiting for a manufacturer to make something they said they would have I decide it's time to try making it for myself. I decided if I want a larger coil for the X-5 when the lakes open this Fall the only way will be to start now and see if I can make one myself. If I am successful I will then tackle making one for the Explorer.

So if you can just skim over the tuning and balancing of a Concentric Coil I would really appreciate it. No need to go into great detail just an overview of the procedure would be great.

Thanks

Beachcomber

[Jeff Kinzli]

Re: Where can I find information on

Hi Beachcomber,

I'll give it a whirl here and we can start a dialogue for the parts that I leave out if necessary.

I'm assuming you know and understand the basics of how a concentric coil is built, about the TX coil, the RX coil and what is commonly called a "bucking coil". I'm also assuming you know about the balancing procedure, but if not, ask and I can fill in some details.

Firstly, when you say tuning, I think of "tuned" coils, ala Whites 6.5kHz coils, which are basically part of the TX circuit (it's a Colpitts oscillator, and the TX coil, capacitor and resistance complete the oscillator circuit). However, neither the X5 nor the Explorer use this type of tuned coil, so this is basically fluff for this conversation.

So, you have a couple detectors which don't expect tuned TX coils. You make a TX coil that is basically within spec of the stock coil (I think the X5 is ~1mH but I'm not sure, I don't have one to measure). You then want to build a receive coil that's probably on the order of 15-20mH but again I don't have an X5 to measure, I'm just guessing here, regardless, start with the inductances of the stock coil for some rough guidelines. You'd then wrap part of the TX coil in antiphase around the RX coil, and leave a bit of extra wire. This extra wire is going to allow you to get the null that you're looking for. Watching your o-scope (connected to RX) move that last piece of wire around until you get the smallest null you can find and tack it with tape or glue. That's it!

In a nutshell that's it, but there's a lot going on. As I said, I'm assuming you know the basic setup of the different coils, and also you're looking for the smallest RX nulled signal possible, but in reality, you're not going to get a flatline, but you can get pretty darned close. I'm guessing that the X5 is a lot like the MXT, in that it is extremely sensitive to coil balance. This is where some of the coil "artwork" that I've talked about comes into play

Does that help any, or did I completely miss/dodge the question?

[Beachcomber]

Re: Where can I find information on

Thanks Jeff,

That is the kind of info I was looking for. I wasn't sure if the X-5 was or wasn't a tuned coil like the older detectors. I assumed the Explorer wasn't as it would be hard to have tuned to cover a frequency range of 1.5-100khz. So basically and this is very simplified is that for the X-5 and Explorer the coil just basically needs to be nulled rather than tuned and nulled. Since the X-5 and the MXT were designed by the same person I would expect that the X-5 is going to be every bit as sensitive to coil balance as the MXT.

Thanks for the info and it's nice to know that there is a place to go for this type of information!!!! Once I have found the coil forms I am sure I will have some more questions.

The next step in my earch will be to find a place where I can get an 11 or 12" open style coil forms.

Do you know anyone who makes or sells them?

Thanks

HH

Beachcomber

[Jeff Kinzli]

Re: Where can I find information on

Beachcomber,

I am guessing that the X5 will be somewhat easier to make a coil for than for the Explorer, since the X5 is a single frequency detector. When you bring multiple frequencies into the picture nulling the coil and getting things working is significantly more difficult.

I'm personally working on DFX coils now and they are, well, interesting to say the least. DFX coils are about the trickiest ones out there today, and while the MXT is super sensitive and very sensitive to coil balance, it's not as hard to make it work as the DFX coils are.

As for coil forms, there's a couple places you can get them - Hays Electronics has a few, and also there's a chap on this forum that is making coilforms too I believe (Rick?). You might also be able to con Charles(NY) to make you one

Have fun, post what you learn, we all learn from each other!

[Beachcomber]

Re: Where can I find information on

Hi Jeff,

THe coil forms that Hays sells in the larger sizes are all "DD".

Hopefully if "Rick" sees this and if he is making 11" or 12" Concentric Forms maybe I will be able to get a few from him. I saw the huge "DD" coil form that Charles(NY) made. That thing is awesome as well as very well made!!!

Thanks Again Jeff!!!

HH

Beachcomber

[Carl]

Hays coil shells

Here is a pic of the larger coil shells that Bill Hays has. The largest suitable for concentric is 8". The large oval DD is 10x13.5, the round DD is 10", I believe the smaller oval is about 9x11.

Rick is sending me samples of what he's making, but I believe they are DD and mono.

Perhaps Bill, Rick, or Charles is reading this, and will step up to make some additional sizes.

- Carl

[Beachcomber]

Re: Hays coil shells

Hi Carl,

I had been to Hays website yesterday. He has some nice size coils for DD and PI's but as you mentioned nothing larger than 8" that can be used to make a Concentric.

Hopefully one of the other people you mentioned will have or can make a open style 11 or 12" Concentric Form.

Thanks Again

Beachcomber

[Chris]

Re: Where can I find information on

Hi Beachcomber

A few post's down in the forum is a link with images & description how to make and setup a concentric coil. Check it out....(see link). The side is in Russian but someone was kind and made a translation into english. Info on that is also in the forum.


Chris



Coil construction

[Carl]

X5 coil values, plus questions

I measured two 7" X5 coils, they were:

TX = 1.079mH, 1.073mH
RX = 6.568mH, 6.561mH

Couldn't find the 9" coil.

The RX inductance is very straightforward -- for most concentrics there is only one RX coil. However, the TX coil inductance includes the main coil, plus the bucking coil, as they are usually connected in series inside the coil shell. But the bucking coil is 180 degrees out-of-phase, so the main TX coil would have to be larger than 1mH, with the bucking coil value bringing the sum total down to 1mH.

So, how do you figure out the values for the 2 TX coils? The value of the bucking coil depends on the value of the main coil (remember, the same current flows through both), plus it depends on how close you place it to the RX coil, and probably whether you place it on the inside, outside, or even stacked. I would think that you first need to wind a main TX to something larger -- say, 1.5mH -- then experiment with bucking coils to find out what gets you close. Probably a lot of tweaking. Jeff, is this right?

Regarding tuned coils, I view coils as being either narrowband or wideband. NB coils have resonant caps internal, and can only be used with a NB detector design. WB coils are just inductors, and can be used with WB detectors, such as the DFX & Explorer, or NB detectors, such as the MXT. The MXT should have the resonant caps inside the detector. So, besides internally resonating the NB coils, what is different about building & balancing the two different types?

- Carl

[Jeff Kinzli]

Re: X5 coil values, plus questions

Hi Carl, one of the things that makes the DFX coils difficult is nulling at both frequencies, as this requires you to be careful of some things that aren't very important in the single frequency coils. One of those things is the capacitance between the TX and RX coils.

[Carl]

Re: X5 coil values, plus questions

Gotcha... so, if you shield the RX coil (which should be done anyway), then wouldn't that eliminate the capacitance problem?