Just some thoughts

[Ray(NM)]

Last night before I went to sleep I was thinking about DD coils and Concentric coils. Correct me if I am wrong. instead of a bucking coil for the concentric could you feed a signal from the tx circuit to the rec circuit that is 180 out of phase and adjust the amplitude to get the null you are seeking in the bucking coil effect. Another thought is if you could adjust the phase of this signal, what would be the effect.Or is the timing more critical than I am thinking. The second thing I thought about is everyone makes the DD in the shape of a D. Would the effect be the same if both coils were circular and overlaped or does the area between the overlaps create the best area for detecting? and the 2 sides of the d coils have to be parallel? I have not tried this just been wondering about it.
Just thinking outside of the coil or is that the box.

Ray(NM)

[Reg]

Re: Just some thoughts

Hi Ray,

Actually, some early detectors did inject an inverted transmit signal to "null" or minimize the receive signal, so that has already been done.

The reason for using a bucking coil is all coils work fine, regardless of the machine they are placed on. When a feedback from the transmit is used, then normally the feedback is adjusted for a particular coil. That is fine until coils are changed. Unless, the coils are exactly the same, the machine would have to be adjusted on every coil change.

This is much more critical on a VLF type machine than a PI. PI's are quite forgiving in this regard. In fact, I do not make any attempt to "null" the DD coil signal on my PI and I have DD coils working down in the 6 usec range. It definitely is harder to make a mono coil work at that short of a delay, so I just shoot for a mono coil that works at less than 10 usec.

Now, as for your wondering about DD coil winding designs, different manufacturers use somewhat different shapes. In some cases, the overlap side is just part of an oval so it is wider in the middle than at either end. On other coils, the overlap zone is relatively a constant width.

Both styles work, so I don't know which is really the best, if, in fact, one is better than another.

Now, Both coils could be round on a DD coil. One manufacturer or guy in England is doing that now.

Charles of NY also tried something similar and built a DD coil for his Explorer (I think) that had windings that were more round than oval. Maybe he will let us know his feelings on using more of a round coil than an oval. Actually, the windings were a little more on the square side, but the principle was the same.

I think the idea was one could obtain a greater depth if the DD windings were round, rather than oval. This would make the housing wider than it is long. My guess is a greater depth could be obtained. It would just look a little strange.

Reg

[Carl-NC]

Re: Just some thoughts

DD coils can be any shape you want... "D", round, square, rectangular (a la Cleansweep), hexagon. Very early IB designs (WWI era) had round DD coils. I think "D" shape became popular because it makes an overall round coil, and creates a long detection zone. I think Reg might be right, completely round might actually have a very slight depth advantage.

Yes, instead of a bucking coil you can use a bucking signal, perhaps inject an AC current. You could even calibrate this signal, by holding the coil in the air and performing an electronic auto-balance. Not only for amplitude, but phase as well.

This auto-balance method allows you to use any coil without worrying about making the coil an exact match. In fact, the coils could have quite a bit of variation, depending on the dynamic range of the bucking signal. This is an advantage over the traditional bucking coil design, which should be manufactured to a tight tolerance, but often is not, resulting in some performance variation even between coils of the same size. Also, traditional coils can shift over time, reducing performance. Dynamic bucking is exactly calibrated every time you turn the detector on.

- Carl

[Ray(NM)]

Re: Just some thoughts

Thanks Reg and Carl.
That clears up some of my questions. I figured out when I come up with something someone else has already thought about it or done it. But for some of us we are still on the bottom end of the learning curve. The sharing of information is what makes this all so interesting.
Ray

[Charles (Upstate NY)]

W-DD Coils

The subject of wide DD coils just came up on the PI forum, here's a link to my post on them. Wide aka square, oval, e.g. not your typical round DD.



Click Here

[Reg]

Re: W-DD Coils

Hi Charles,

Actually, I mentioned your wide DD coil setup on this forum, hoping you would respond with how it had worked out. Now, you mentioned that you even modified it since I last saw any pic of it. I would like to see a pic of the new and improved design. BTW, did it work out better by detecting deeper than the original 18" wide coil?

Reg