Since things have been a bit slow over here.....(kind of long)

[Anonymous]

and my brain has been working overtime lately I thought I would start a discussion about PI's and Concentric coils. Ever since reading Dave Emery's Post on making a Concentric coil I have had all kinds of thoughts and theories running around in my brain <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> This isn't necessarily a good thing <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> So I thought I would post some of these thoughts and see what everyone here thinks about these ideas.
First since this Forum deals mostly in PI design and use for those that may not know that much about VLF detectors which usually use Concentric coils I will start from that viewpoint. I keep this part as short as possible. A VLF detector has it's transmitter section on continuously unlike a PI which switches the transmitter off during receive. This being the case there had to be a way to keep the transmit signal out of the recieve section as it would easily overload the receiver and preclude the possiblilty of any targets that were detected from being heard. This feat is accomplished in the coil design and tuning. A concentric coil has 3 windings. A Tranmit winding, a receive winding, and a feedback winding. The feedback winding is wound in series with the transmit winding except that it is would in the reverse direction of the transmit winding and physically placed between the receive and transmit windings. The feedback coil produces an electromagnetic field opposite to the transmit field essentially blocking the transmit signal from reaching the receive coil and it's associated circuitry. This is a simplification but is close enough for use for the rest of the discussion.
You may be thinking well how does this have anything at all to do with a PI's which for the msot part use a single winding coil. Well here is where my brain went into overtime <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> For the purpose of this discussion please assume that the ground conditions are of an average level of mineralization or even an average mineral freshwater lake. For the momeent forget about salt. You'll see why I am saying that shortly. Back to the PI and the Concentric coil. I am no expert on PI's!!! But it seems from everything I have read here and in most other places concerning PI's one of the biggest problems in getting shorter pulse delays is the time it takes to shut down the transmit pulse. The harder and faster you shut down the pulse the shorter you can make your pulse delay. But in doing this you create a higher back emf to deal with. Now let's say you change from a monoloop design coil to Concentric style coil. Since a properly designed and tuned Concentric coil is not going to allow the receive section of the detector to see any or very little of the tranmit signal you shouldn't have as much of a problem achieving even shorter pulse delays. Also the fact that there is a feedback winding in the coil should also help since it should create a feedback signal to the transmit coil when the tranmit section is turned off which I would think would reduce the back EMF to a more easily managed level without wasting as much energy as heat through high current high voltage damping resistors. Another benefit would be that since the receiver doesn't see the transmit signal I would think that you be able to see much smaller pieces of gold than can now be seen with a good PI. Currently the GQ SS seems to be able to see gold down to about 5-6 grains. This is large by VLF standards. A good VLF can see gold down to 0.5 grains under the ground conditions which I mentioned above. Two current detctors which are capable of this are the Tesoro Lobo ST and the Troy X-5 and both these machines are capable of doing this in the Discriminate mode. However neither of these two VLF's are very good on a Saltwater Beach. But I am getting away from this discussion.
I am very interested in hearing other people's thoughts on the use of a Concentric Coil on a PI and the reasons that it would or would not work or where there is a flaw in my thinking on the subject. As I said I really do not know a lot about the intricacies that go into the design of a PI and which of those may make the use of a concentric coil inadviseable. Any and all comments are welcome!!!!!
Thanks and Happy Holidays to all!!!!!!!!
HH
Beachcomber

 

[Anonymous]

[No message]

 

[Anonymous]

Hi Beachcomber
I don

 

[Anonymous]

Hi Beachcomber,
Your discussion covered two aspects, the ability to find small gold and concentric coils. I am going to concentrate mostly on the gold portion at this time and very little on the concentric.
The reason for lack of discussion on the concentric coil is my first real attempt was less than successful. At least, I know more about what I need to do now, but will leave it at that.
As for finding small gold, there was a discussion on Bill Southern's Nuggetshooters forum regarding certain nuggets that are invisible to PI's, even the new ML 3000.
As a result of the discussion, I contacted the person who initiated the discussion, John Blennert, and asked if I could borrow a couple of those nuggets. He was nice enough to send me a couple, one weighing in at about 10.5 grains and the second, 11.7 grains as measured on a powder scale. On my cheap electronic scale, the weights were both 0.7 grams. These nuggets were huge when compared to a couple I found last spring with my PI detector.
Now, we are not talking flyspecks that can't be seen, but nuggets maybe up to a couple of grams or so that can't be "seen" by a typical PI. To get a better idea, I have attached a link to the forum thread. John posted 3 pics of different nuggets that are "invisible" to the PI. In his middle pic, the nugget directly above the lettering on the dime and one below, almost opposite the first are the two nuggets I received. The second is one of the darker nuggets that looks like it might be an inch or from the dime but is down and a little to the right. I tried to upload a copy of this pic with the nuggets circled but the file was too large. I will try to correct that later.
First tests of the nuggets were quite surprising. The only way I could get even a very faint signal was if I rubbed the nuggets across the face of the coil, and then one had to really be listening for it. There would be no way to find them with the setup I had and it was operating at less than 10 usec delay.
Being more familiar with the DD design of coils, I elected to try to modify my detctor and a coil to see just what I could accomplish using my GQ clone as the PI.
Well, rather than drag this on forever, I will just say that when done, I was able to detect the same nuggets with a decent signal at least 2" from the coil in my house. Outside, was a little better. The coil worked great and now with the nuggets lying on the ground, I could raise the coil a distance that approached 3" and still get a response I would recognize in the field.
Reg

 

[Anonymous]

Hi Reg,
Thanks for the response! Thanks for the info on the small nuggets. I kind of got off the track a little in my post. I wanted it to be mainly about the concentric coil when used on a PI and what might be possible. It seems that PI's have always been optimized for high mineral or saltwater locations since a pulse delay much below 10us allows the salt to be detected it doesn't seem anyone has gone much beyond that point.But then I am sure I ahae not read about everything that has been tried when it comes to PI's! However in Freshwater applications or in areas of low mineralization it seems that there is still room to make improvements. I thought that maybe a Concentric coil might be a way to go to achieve higher performance in those situations just as you found you could increase the response to the nuggets by going to a DD coil of your own design.
Thanks
HH
Beachcomber

 

[Anonymous]

Hi Mr. Bill,
I was aware that the DD's that had been used were of the two coil design as I had followed closely the progression of modifications that Reg and Scott-Me previously of AZ were doing over the Summer.
I didn't know that Eric had any PI's that would go down to that short of a pulse delay. As you said in high mineral conditions that low of a pulse delay would allow ground pickup but in low mineral condtions I would think that delays shorter than 10us would work and be beneficial. But I could be wrong...it wouldn't be the first time <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> I am still learning about PI's!!!
Thanks
HH
Beachcomber

 

[Anonymous]

Hi Beachcomber,
One of the reasons concentric coils are more popular than DD's is a concentric coil is easier to "null" and will stay in null better. That is probably one of the big reasons for the transition from the DD to the concentric. However, a DD will do a better job of reducing the ground signals and the overlap zone can approach the diamater of the coil housing, so ground coverage can be better.
Concentric's appear to be slightly more sensitive when compared to a DD on a VLF, but there are a few things most people do not realize about them.
For example, a typical concentric design has the receive coil about 1/2 the diameter of the xmit. Now, when detecting real small objects, the detection zone is the zone under the receive coil and not the whole coil itself. The signal betwen the xmit and the receive windings will produce a response that is opposite of what occurs directly under the receive, when the small object is very close to the coil.
Larger objects do not display this characteristic as well, and as the object gets farther away, the real detection zone seems to expand some, so it is larger than the receive coil but still is generally less than the xmit windings.
One a PI, one of the advantages of either a concentric or a DD is the isolation of the receive coil from the xmit. This appears to significantly reduce the noise level, which becomes very important when trying to "hear" marginal signals. So, it is quite possible that a concentric coil will produce better results than a mono on a PI, at least in certain locations or under certain conditions.
I am hoping to get back to working on a concentric in a couple of weeks that will detect small gold. My first attempt worked ok but I couldn't get the delay down to where I wanted it. At least now, I have a better idea of what I need to do.
Reg

 

[Anonymous]

Hi Beachcomber
Eric uses them in industrial design. Environment is a lot more controllable in industry.
Recently when Reg., and Scott were doing the coil thing, Eric did do sampling at faster pulse delays on tinny, small pieces of gold. From what I recalled the gains he saw were slight, and certainly not cost effective to manufacture as such. I noticed almost no change at all going from 10/11uS. to 7/8 uS. with a 3 grain nugget doing test in my shop.
It

 

[Anonymous]

Hi Bill,
I have been pushing my PI down to the 7 to 8 usec range and it is extremely tough to get a coil to work right. However, I am seeing good results.
Like you said, building a coil to operate consistently at anything less than 10 usec takes quite a bit of effort. In fact, I have found it necessary to fine tune the coil to the particular detector on which it will be used.
If the detectors are set up the same, a coil made for one particular detector will work on another but it is quite possible it will not work at the same minimal delay when the delay can go less than 10 usec. The difference could be as much as a usec or two when the delay is as short as I have been experimenting with.
Reg

 

[Anonymous]

Thanks Reg,
One other characteriistic of a Concentric over a DD which doesn't come into play witha PI but does with VLF is that Concentric coils have better discrimination characteristics which may also have contributed to them becoming more popular on many of the VLF's being marketed.
HH
Beachcomber

 

[Anonymous]

I agree with you in that the machine has to work as close to universally as possible from a sales point of view. But for the purposeds of this discussion sales and working everywhere are not the main ponit of the discussion. It is more about theory and possibilites as to what may or may not happen when a Concentric coil is used on a PI.
However making different coils for different uses and types of ground has long been an accepted way of doing things in the VLF world. Maybe it can carry over into the world of PI's as well. Still we are getting away from the point of the whole discussion which was about whether a Concentric coil would work or not work on a PI and if there are advantages that might be gained under certain conditions that might make it worthwhile. That is why I prefaced my remarks in saying that I was assuming average mineralization for the use of the Concentric coil. One very good point that Reg made was that when he went from a Monoloop to a DD there was a large reduction in background noise making it easier to hear marginal signals. I would think this would also be the case with a Concentric Coil maybe even more so since there is an even higher level of isolation between the transmit and receive coils due to the presence of the feedback winding. It's fun getting into this kind of discussion <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="">
HH
Beachcomber

 

[Anonymous]

Hi Beachcomber,
Concentric coils were used on the very first PI

 

[Anonymous]

Hi Eric,
Thanks for the response and the history lesson on PI coils. I would think that anything that helps to keep the transmit signal and it's remnants out of the recieve section will help to lower the noise level and help in hearing the weaker/deeper signals.
Thanks for the response!!!
Happy Holidays!
Beachcomber

 

[Anonymous]

<EM>"I agree with you in that the machine has to work as close to universally as possible from a sales point of view. But for the purposeds of this discussion sales and working everywhere are not the main ponit of the discussion. It is more about theory and possibilites as to what may or may not happen when a Concentric coil is used on a PI". </EM>
<STRONG>I thought we were both on the same page ? We were discussing the possibilities of using a concentric coil on PI