Concentric Coil Design

[coolhotboy]

Hi all,

I've been trying to build a concentric coil arrangement. But I'm not exactly sure as of how the connections should go.

Attached below is the arrangement which I've used, but i haven't been able to null the receiver coil.

Is this connection right? If so, then I can be sure that I'm going wrong in the number of turns, etc.

all the capacitors are chosen to make the coil resonate at around 17KHz.

Thanks a lot in advance...

[Qiaozhi]

Hi all,

I've been trying to build a concentric coil arrangement. But I'm not exactly sure as of how the connections should go.

Attached below is the arrangement which I've used, but i haven't been able to null the receiver coil.

Is this connection right? If so, then I can be sure that I'm going wrong in the number of turns, etc.

all the capacitors are chosen to make the coil resonate at around 17KHz.

Thanks a lot in advance...

Have you read this article? -> http://geotech.thunting.com/pages/me...planar_300.pdf
I suspect your nulling coil has the wrong number of turns.

[Carl-NC]

Polarities look fine, must be the turns.

[Reg]

Hi,

The drawing looks correct, but the only way to assure the windings are actually like the drawing is to check using a couple of quick tests. First, disconnect the null coil and measure the receive signal. Then connect the Null coil back in place and see if the receive signal increases or decreases. Also, note the polarity of the receive signal.

If the null coil is wired correctly, the receive signal will reduce significantly. If the null coil has too many windings, the receive signal will reverse polarity. So, if you have not changed polarities and have not nulled completely, then more null coil windings are required.

Reg

[coolhotboy]

thanx a lot

thanks very much for the help....

I went through the dave Emery artcle on coplanar coils. i used 120 turns for the transmitter and 30 turns for the bucking coil, which is 1/4th of the transmitter coil. the radius of bucking coil is half the radius of the transmitter coil. So I did follow the guidelines. The only change I made was I used a 200 turn receiver coil which I already had made for another test. According to the Dave Emery article, I am supposed to use twice the no. of turns for receiver as of the receiver, which in my case would be 240 turns.

Do you think this might cause the problem? Although I thought that 200 turns( less than double) would actually decrease the received signal ampliture.

I'm yet to try what Reg said... will do that and get back to the issue soon.....

thanks again for the quick help....

Cheers

[Reg]

Hi,

Using fewer turns for the receive isn't the problem. I suspect you will have to add more turns to the bucking coil. Also, if you are building this coil to use on a VLF, you might want to read something written by George Payne some time ago.

Hopefully, this link will work.

http://jb-ms.com/Baron/gb.htm

It is a good article on coils for VLF's as well as ground balance designs. Some tips on coil winding can be found near the end of the article.

Reg

[coolhotboy]

Do I still need more turns??

I tried out what Reg said... When I brought the bucking coil, the amplitude did decrease... But it still doesn't seem to null out... I've already added lots of bucking coil windings and still its not nulling out.

I think I'm not resonating the transmitter as well as bucking coil. I measured the Inductance of the combination of transmitter and bucking coil connected in series and connected an appropriate capacitor to resonate the whole setup at 17Khz.

But even when the bucking coil is nowhere near the transmitter and the receiver, I'm just coupling 1 or 2 V from the transmitter. I am feeding a 10V peak to peak sine wave into the transmitter.....

Do I need to still keep adding more and more windings to the bucking coil to get the null signal???

[Reg]

I suspect you did not read the article by George Payne that I linked to. He is the expert on the subject. On an 8" coil, George recommended something totally different in terms of the number of turns needed.

George recommended the bucking coil have about 1/3 the number of turns of the transmit for an 8" coil. So, you might try additional turns. Also, if this coil is to be used on a VLF, you might want to read the article I recommended. It does discuss the best way to connect the coils to minimize problems.

Reg

[coolhotboy]

decrease in magnitude!!!

Thanks heaps for helping out...

This time, I didnt worry about the number of turns in the bucking coil. I just kept adding turns to the bucking coil continously till the receiver showed a null. It worked to some extent. I was able to bring down the receiver amplitude to less than 50mV considering the 20V i feed to the transmitter and bucking coils. Is this null good enough ? or has it got to be even less than that.,...

Another behaviour I noticed was, when the object was brought closer to the coil, the 50mV decreased to about 30mV. I thought when the target is present, the amplitude on the receiver coil would increase. I think the target is getting influenced more by the bucking coil, hence reducing the amplitude at the receiver. Does this make sense? Is it OK to proceed or am I on the wrong track?

[Unregistered]

Something Wrong......

Either I am looking at your drawing wrong or everyone seems to have missed that fact that you have the bucking coil and the Rx coil reversed. The Bucking coils is supposed to be between the TX and RX windings. You have the Rx winding between the Tx and Bucking coils. I doubt that you will ever get it to null that way. The Rx winding should be the centermost winding. Then the Bucking coil. Then the Tx winding. At least that is how I have made mine in the past and have never had any trouble nulling them.

[Carl-NC]

Either I am looking at your drawing wrong or everyone seems to have missed that fact that you have the bucking coil and the Rx coil reversed. The Bucking coils is supposed to be between the TX and RX windings. You have the Rx winding between the Tx and Bucking coils. I doubt that you will ever get it to null that way. The Rx winding should be the centermost winding. Then the Bucking coil. Then the Tx winding. At least that is how I have made mine in the past and have never had any trouble nulling them.

The bucking coil can be inside, outside, above, or below the RX coil... it really doesn't matter. You can even use an RX-connected bucking coil placed out at the TX coil.

- Carl

[Unregistered]

Hi Carl,

My understanding was that the Bucking coil was supposed to be between the TX and RX on a normal Concentric coil and if you wanted to make a widescan Concentric coil like Troy uses you put the bucking coil around the outside of the Tx winding. Whenever I have wound a Concentric I would ti around the of the Rx winding. Never had a problem getting a null so far. I have never tried winding the bucking coil inside the Rx winding yet.

JMHO

[Carl-NC]

There are many ways to buck a coil, see my coil article for some diagrams. Depending on how you do it, winding ratios will vary. I would guess that placing the bucking coil just to the outside of the RX coil is the most popular way.

- Carl