Coil question

[Frank-S]

Winter project, Trying to make concentric coil for a VLF detector useing same restance from
orginal coil,it worked for a DD coil.haveing a problem with coicentric coil. QUESTION
Is there a formula where to locate center coil, as to location to outside coil.and size of each
coil. I made 18" out side coil and a 6" inside coil.it dont work with small coil in the center,it
does work when I move 6" coil about 3"to inside of 18" larger coil.Poor dept.
Is there a formuler, as to size and location of each coil.
Thanks.

 

[Reg]

Frank,

There is no formula that I know of as to the size of the receive coil. However, the general standard is to make the receive coil diameter 50% of the transmit but there has been various other sizes with most between 35% and 70% of the TX for the RX windng.

There is a discussion going on right now on the Geotech forum as to what might be the best size ratio of the tx and rx coil. The discussion is more about the ideal SNR ratio, but all aspects more than likely follow similar results. As for where the center coil is placed, the normal placement is centered inside the outer transmit winding. Based upon what is coming out of that discussion, the 50% ratio is a good safe size to use. So as an example, if you build a 12" coil, then the receive winding should be 6" diameter. In your case, the 18" coil with the 6" diameter receive probably should have worked while centered. Since it didn't I suspect the inductance is way off on one or both of the windings.

Now, with that said, I don't recommend you try to match the resistance of your orignal coil. Furthermore, I am not sure how you do this for a concentric tx/rx coil? Matching resistance is only important for ML PI's and doesn't mean that much for most other designs. Even then, are you matching the wire size? Resistance isn't relative to anything without the size of the wire used.

What detector are you trying to build this coil to use on? Most PI's are very forgiving when it comes to coils but there are some basics that should be thought about. First, work with inductance and not resistance. Second, is the detector a low power PI and if so, the resistance isn't much of a concern.

As Eric Foster mentioned very recently on the Geotech forum, an inductance meter is a must. Considering the fact you can buy one for about $40 off ebay, it makes sense to have one.

What inductance do you have on your 3" coil? If you don't know, then how many turns are in the small coil? Did you use any program to determine the inductance?

As to why your coil didn't work, well it sounds like you don't have the right inductance combination as well as a good tx to rx ratio.

You need to read the various coil building articles on the Geotech forum and a lot of the discussions. There is a lot of good information for a beginner on that site.

One last note, when trying something new, I recommend you stay with safe values such as the same size as your factory coil. In other words, if your factory coil is 11" diameter, then try to build a concentric coil the same size first and get it to work right. From there you can try different sizes. By following this procedure you will most likely have better luck.

One last note, concentric means everything shares a center focal point or axis. Thus all windings are centered within any larger one.

Reg

 

[Carl-NC]

I made 18" out side coil and a 6" inside coil.

Did you include a bucking coil?

 

[Frank-S]

No Carl, I did not use a bucking coil. tell the truth,I dont know what a bucking coil is.I used 2 coils
with same restance as the coil that came with Tecknetic G2 detector,it has a 11" DD Coil.I did make
a DD coil that worked.Thanks for your in-put

 

[Frank-S]

Thanks Reg.
If you rember years ago when you helped me modify CS-6,I am not school trained in electronics.
I am way past 70, cant swing a detector like I used to.Tinkering with this keeps my mind working.
The detector that I am tinkering with is a teknetic G2, with a 11" DD coil. I removed the electronices from
a 5" DD coil.Useing same size wire and restance from both coils,not sure what coil is tx transmetting coil or
rx coil.For now I am useing the coil with most restance 57.9 ohms as center coil and outer coil 2.7 ohms
larger wire as outer coil.I got to a place in the coil where the sound nulls out, but it wont detect a target.
Next I will swith coils around.PI coils are easer to make.
Years ago Dave Emery was working on a Co-planer coil did he get it working.

 

[Reg]

Hi Frank,

Boy, I owe you an apology. In your first post you mentioned VLF and to be honest, I am so used to this forum being a PI forum I flew right by the VLF part. So, my original post isn't totally true.

Sure, I remember you and the earlier detector and you are right, working on projects does keep the mind working. So, keep up the good work.

As for Dave Emery and the concentric coil, he was working on one for a PI but much of what he wrote will work fine for a VLF also. In fact, Dave wrote an article on how to build a Concentric coil complete with a bucking coil. Keep in mind this is for a PI and some changes will have to be made for a VLF. Here is a link to that article. The link will get you to the page, but then you have to drop down to the MISC section and select the article on the Constuction of Coplanar search coils.

http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=projects.dat

One thing when bulding a coil for a VLF is to make sure you use the proper size wire for the transmit, otherwise coil current can go up unnecessarily if the wire size is too small.

Now, Carl asked about if you built a bucking coil, which is what woke me up that you were referring to a VLF, so I will let him explain that to you. He is far more familiar with VLF coil design than I am.

Reg

 

[Frank-S]

Thanks Reg.
I switched coils around in VLF concentric coil that I am working on,it also did not work.I have a 20"
round template.I made a 17" DD coil same restance, same wire,The price of wire, so I use same wire over.
The DD coil works,dept not to good.it detects a flying nickel at 13''air test not good for a 17" coil.I can make
it go deeper with less restance on larger shorter wire.
I will ask Carl-NC about bucking coil.

 

[Frank-S]

Carl,if you get time a fue questions,it might help some-one else also.
Bucking coil,is it a coupling coil or does it connect to transmitting coil.
how does one determine number of turnes of wire in a bucking coil
The coil with less turns of wire does it go in center of a concentric coil.
Wire,on a working DD coil will the same number of turnes work in a concentrjc coil.
useing same Tx and Rx coil.I have made a fue coils that worked,still dont know much.
Thanks Frank

 

[Reg]

Frank,

I see that you didn't read the article I linked to in a previous post. In that article it talks about the bucking coil and how to figure out how many turns are required. So, I am linking the article again and hoping you take time to read it.

http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=/projects/coplanar/index.dat

Hopefully, the link I just posted will take you directly to the article. Once you read it, you will probably have more questions but will know more about the bucking coil.

Reg

 

[Frank-S]

Thanks, I did read the one about coplanar coil by Dave E. it dident sink in,guess I am a little slow.I will
read it again.I did get the concentric coil to work,by lowering restance in center smaller coil.not pleased
with it but it works,got all wenter to make it better.

 

[Reg]

Frank,

Please read the article again because it tells you about the bucking coil and how to figure out the number of turns.

Now, the bucking coil is used and designed to cancel out the transmit signal that is seen by the receive coil. By adding transmit coil wraps right around the receive coil but wired such that these windings cause a signal that is opposite the signal from the transmit coil, the bucking coil cancels out the transmit coil signal. Thus, only a target or the ground signal will cause a response or stated another way, create a signal in the receive coil.

So, the bucking coil is really part of the transmit coil and is wired in series with the main transmit winding.

Reg

 

[Frank-S]

Reg, from what you said a bucking coil is not the same as a buster coil.
I went back to the web site you posted.I use public libary so I made a copy of Coplanar coil.
My PI detector has a 2 pin connector from coil to controal box,the coplanar coil might need a 4 pin
connector.my PI has one wire going down to coil winding and back to connector.
the coplanar coil has 2 sets of wires, one set for TX one set for RX coils.,to make it work my piranha PI
would need to be modifyed to use a DD coil.2 sets of wires
Dave, uses same size wire and numbers turns in coil as I do.only differance I can test coil on my templet to see
how well it works befor it glued-up.
My problem is with VLF coil, my formular works for a DD coil ,VLF detector and half as good on a VLF
concentric coil.Same detector.

 

[Reg]

Frank,

You are mixing apples and prunes.

You were working on a VLF and having difficulty with the coil you made. A question was asked if you used a bucking coil. My response was to that question. Now, you are trying to discuss using a Concentric coil on a Pirahanna PI. There are significant differences between the two types of detectors and how they work.

BTW, I have no idea what a buster coil is or what it does. Where did you read about a buster coil?

Most basic PI's do not have any form of ground balancing and are not designed to use a Concentric or a DD type coil. Instead, they are meant to use a mono type coil. Some people have modified their basic PI's so they can use other types of coils but in most cases, the PI doesn't come that way.

So, you are right, if you want to use a concentric coil or a DD coil on the PI you mentioned, then you will have to modify your detector.

VLF's are built differently and require a concentric or a DD coil to work and won't work with a mono coil. So, you can't use a mono coil on a VLF and you can't use a DD or a concentric coil on many of the PI's made today, at least not without some modifyfing of the detector. Since I have never seen a schematic of your PI, I can't tell you how difficult it might be to modify.

Now, building a DD coil, in some ways is easier to build than a concentric coil. This is true for a PI type detector but not necessarily true for a VLF. Balancing the coil is difficult to do and keep in adjustment on a DD. However, if the design of the VLF is such that it allows for some error, then the DD might not be as bad.

Reg

 

[Frank-S]

Thanks,Reg.
I find it easy to make coil for a PI detector.
VLF detector not so easy,I will keep tinkering with it.
VLF, DD coil not to not to hard,
VLF concentric coils are somethings-else,its not easy to get number turnes of wire for
TX and RX coils. Thanks