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Date Posted: 02:02:48 08/16/02 Fri
Author: Reg
Subject: Re: I think you're on the right track......
In reply to: Ralph 's message, "I think you're on the right track......" on 12:07:23 08/15/02 Thu

Hi Ralph and all,

I am really not asking Dave J. to reveal any real secrets, just want to know the basics. Is the trick mainly in the filter design, or is the operating frequency the main reason, or is it both. If necessary, I will take time to partially reverse engineer the thing and look at the various waveforms to see what makes it tick.

Now, as for the nail through the ring, I would recommend you try the same test comparing detectors and use a solid gold nugget rather than a ring type object. For some reason, circular objects create different responses. Part of the ability to "see" the ring may be the result of that.

I started using one test nugget that was a reasonable size for US type hunting, a 1/4 oz nugget. I have tested quite a few detectors using this as my standard test and like I said, the GB 2 has been the only one that will detect the nugget with a strong response when the nugget is close to a nail.

I will do more testing with the GB 2 in the near future to see what I can find out. Like I mentioned before, I have looked at the waveforms of various iron targets and seen the amplitude responses of those targets, so I know how easily a low conductive signal like that from my gold nugget could easily be masked. However, my experimenting has been with lower frequency detectors, and what is have seen on them is probably quite different than what I might see on a higher freq detector.

Since my posting, I did try a similar test using a piece of magnetite and found that the magnetite will cause the nugget to be ignored. However, if there is just a little separation between the nugget and the rock, maybe a half inch to 1 inch, the GB 2 will respond to the nugget from certain directions.

Example, if the coil is swept with both the nugget and the rock under the coil at the same time, the nugget is ignored. If, however, it is nugget first then rock, even though they are very close, the nugget is heard. If it is rock, then nugget, one may get a signal depending upon sweep speed and distance between the nugget and the rock.

Ok, what does that mean? Well, before answering that question I tried something else, I tried passing the nugget very slowly past the coil. In doing so, I didn't get the typical long response, but more of a multiple pulsing.

Without looking inside, I suspect one of two things, the filter speeds are quite fast and/or possible it is a 3 filter detector. Both could make a difference.

Now, couple those ideas with the fact that at a higher operating frequency, the signal from gold will be stronger and I would suspect the signal from iron would be weaker. This could make the ultimate difference.

Anyway, that is my best guess at this time that it is a combination of all the items mentioned. Whatever it is, it works.

Now, as for operating frequency, it is my understanding that increasing the frequency makes it much more difficult to separate the higher conductive targets, so there is a frequency limit for detectors that are going to be multi-purpose. It is quite possible that the 20Khz is that limit. Without jumping into the math, which I don't remember how to do any more anyway, the basics do indicate smaller low conductive targets should respond with much stronger signals on higher freq detectors, which is what we see on the GB 2.

I guess now the question is, what would be the ideal frequency of a gold only VLF or LF detector. Several years ago I had a conversation with George Payne about this topic. At that time, he indicated, if I remember correctly, that probably an operating frequency around 100Khz would be best, especially if one were looking to readily detect small gold. However, there were concerns about ground attenuation at this frequency. Now, I am curious if any of the manufacturers have experimented with this frequency. It is possible that Fisher did and then settled on the 70khz instead.

Reg

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