Can I make a 6 foot wide coil?

[Anonymous]

Hi JVP,
Like I said in my post below, I haven't used the Bigfoot and don't know much about it. I did look it up on the net the other day to see how long it was.
A friend of mine has one and he really likes it. You would be amazed to see what he has found with it. Last year he found a really unique watch fob made of gold nuggets that he estimates the value to be around $50,000.
My friend claims the Bigfoot does have reasonable depth. Maybe it doesn't go as deep as the standard coil, but goes deeper than many people might expect.
I like your idea of scanning a larger area at a time when hunting a field for meteorites. Great idea. Now, we need to find the best solution.
The general coil idea I mentioned in my other post might also have some basic discrimination capabilities also, ie ferrous and not ferrous. I would think so. Hopefully, we can get more input as to what coil design might be the best. Who knows, there probably is a better design than I proposed. It was the thought that came to mind for working a wide open area using the general dimensions you mentioned.
Obviously, the one that Thomas Bruer proposed works well from the pictures he posted, but in a large open field miles from power lines, I am not sure that design is needed. I would think it would, however, work very well in conjested areas.
As for the "harvested ground", the first thing I thought of was a friend of mine who, in his words, "harvested an area" of meteorites. He is pretty evasive but mentioned he has done real well. My guess is he has found well over a thousand, maybe closer to two thousand of them.
I need to check the time constants of some of the larger stony meteorites I have. It is my guess, that there would be no probem with the iron and stony iron types if a very long coil is used. However, right now, I would guess that the there would be difficulty with many of the smaller stony types. However, I would think that baseball size and larger wouldn't be a problem, but golf ball size and smaller might not generate much of a signal. Again, I need to do some experimenting.
Reg

 

[Anonymous]

Hi Carl,
If you take a separate coil of wire, load it with a resistor and then place near near your pi coil, you will get a response similar to what is xmitted with the same polarity. Now, when you flip the coil over, the polarity of the received pulses are also flipped.
If half of the received signal is sensing a positive response and the other half, a negative response, the net result should be zero, or close to it, depending upon the actual alignment. As such, I would think any ground signal would also be cancelled since it would be the only "object" within the field of the whole receive coil. If the coil is relatively level above the ground along the lines of the elongation, the net ground signal, I would think, would cancel, providing of course that the ground signal is a relative constant matrix.
I would think that any sensing would be done by an object entering one half or the other half of the receive coil, but not both.
Is my thinking correct or do I need to take more vitamins to increase the circulation to my lonely remaining brain cell.
Reg

 

[Anonymous]

Hi JVP and all others,
We made some tests today with different coil shapes and sizes as well as different objects. BTW, the problem with the insensitive area of a compensated figure 8 coil is almost negligible for smaller objects close to the coil; only large objects at a greater distance will cause a weaker signal between the two coil sections.
<BLOCKQUOTE><EM>JVP: At this point I am trying to determine if a coil can be built to cover a fairly large area but still be sensitive to small targets. T R Fisher offers a coil for their Pulse 8X that is 8" x 48". It is advertised to be sensitive to small targets. </EM></BLOCKQUOTE>
What are small targets? The coil I suggested in my first posting (1m x 3m or 40

 

[Anonymous]

Hi Thomas,
I wish to thank you for the thorough and enlightening response. It sure has given me more to think about. It is also good to know that my remaining brain cell hasn't given up completely. The poor thing will probably really get a good workout now trying to digest all that was posted and different ways to possibly apply it.
With some meteorites going for several thousand $ per gram, it could be quite rewarding to find different effective methods of detecting them. Obviously, not all meteorites are that valuable, but even a $2 to $5 a gram, a large iron meteorite would be worth a fair amount of change, not to mention its scientific value.
I just did some very quick testing on a fair size(when compard to most found there) stony Chondrite meteorite found in Gold Basin. It is about 190 grams in weight. The physical size is about half the size of the palm of one's hand.
A quick test indicates this type of meteorite requires a fairly short time delay time for adequate detection. I would guess it would have to be 20 to 25 usec or less to detect it at all and obtain any depth of detection. A delay of 15 usec or so would be much better and gives a nice strong response several inches from the coil. Just thought I would toss this info into the pot.
Thanks again for all the information you provided in your thorough response.
Reg

 

[Anonymous]

We have been building these for use on Salt lakes and under power lines, for the last year, - they are also excellent on sensitivity to the smallest of targets. For example a 4 foot by 2 foot version has picked up a .5 gram nugget whilst being towed on a salt lake. With respect to ground noise, - the irritating surface hot rock problem is not eased, - in fact the sensitive area where the Rx swaps from one phase to the other (the reason for the good sensitivity) is also a problem for strong ground changes. It would be possible for a great ground covering coil to have good sensitivity, - but I beleive it will need to be of this type, - or some other multi coil set up. John

 

[Anonymous]

Hi John,
Thanks for the additional info. Is the mono tx, figure 8 receive coil you make the one they call the salt coil or something like that? I have heard of them but have never seen one or knew how they were made.
A couple of my friends swear by your coils and have quite a collection of nuggets to back up their claims of how great they work.
I was hoping a friend of mine had his latest nice find posted on his website but he doesn't. It is a decent trophy as U.S. nuggets go. Until he does I can't say much except it was several ounces and found using one of your coils.
Reg

 

[Anonymous]

Yes Reg, - that is what we call the 'Salty'. Its a bizzare thing, - but it is the only coil that will work dead quiet on the heavily saturated salt lakes in the WA goldfields, (and of course in bad electrical environments). We have guys using them in Europe also in very noisy areas (normally the SDs achilles heel) and are probably the most underrated coil we build - with the most potential.
John

 

[Anonymous]

Hi John,
Interesting info on the different uses of the "Salty". Thanks for the explanation. How does it compare to the DD in the really "hot" ground? I would think it would have some advantages simply because of the additional ground noise reduction capability.
I actually got my idea quite a while back about building this type of coil from looking at the pulse waveforms using an external coil with a loading resistor. I had no idea how much work had already been done with this concept, let alone the results you mentioned.
A couple of months ago, I passed the idea of building a coil of this basic design by a friend of mine who is and has been active in designing metal detectors for many years. He told me that he tried the general concept maybe 25 years ago but abandoned it because of the different response one gets from the different coil halves. He was working with VLF's at the time and felt it would be confusing to an operator.
It is strange how this particular design now has found such a good application. I wonder how many other coil designs have been experimented with over the last 50 years or so that didn't work for the initial application but would work exceptionally well under different conditions.
Too bad forums like this one weren't around at that time. Who knows where we might be today, technology wise, if information was exchanged at that time like it is today.
The more I experiment, the more I appreciate and respect those of you who have taken the initiative to go forward with your ideas and introduce new products.
Thanks again for the information about your salty coil and its merits. Hopefully, this information will reach more people and possibly provide a solution to their particular problem.
Reg