"Long Tom" Boat Towed Underwater PI M/D??

[Anonymous]

I remember a post about a large coil called "Long Tom". I have recently made a boat towed mag and of course it only detects ferrous objects. I am interested in making one or more boat towed metal detectors to pick up non-ferrous and ferrous. I would like to make something similar to the JWFishers Pulse 12 with the 24"x36" search coil.
I have a known wreck site and would like to search and detect gold and silver coins and relics at depth. Maybe even make a sled.
If anyone has any schematics or ideas on how to get started, lets see what we can come up with. I would like to know how to make the coil as well as any schematics on the Pulse Induction control box.
Thanks,
Robert

 

[Anonymous]

OK, no response yet.
How about this. We start out with the GoldPic PI kit. How would I go about constructing a large coil say 24" x 36" to detect objects while being pulled behind a boat up to 100 feet away?
How would I connect the coil back to the main control box? Would I get some signal reduction and how would I overcome this?
Any help anyone?
Thanks,
Robert

 

[Anonymous]

How would I go about constructing a large coil say 24" x 36" to detect objects while being pulled behind a boat up to 100 feet away?
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Go buy some PVC pipe and elbows and make a coil with the same parameters that the original goldpic specs out. This might be 7 to 9 turns of wire to get about the same inductance.
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How would I connect the coil back to the main control box?
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Use 100feet of low capacitance coax connected just like the standard coil would be connected.
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Would I get some signal reduction and how would I overcome this?
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Yup, the signal is extremely low level on a long coax run. The extra capacitance of the coax is gong to affect your signal as well. The good news is that you should still be ok though. IF you think that you want to overcome this then you can put some of the electronics down there in the water with the coil. Is it worth it? Probably not. Why? Because you need a multiconductor cable, including coax, to run between your top side unit and coil, you need a waterproof case with connectors or glands, you need to condition the signal to tolerate this long run, etc.
I suspect you didn't get a lot of answers because this has all been covered before. I'm not the expert here, Eric is, and if you do a search you will find his answers on this forum. Heck, I might have been the one asking these questions the first go round. <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="">
Good luck,
FJ

 

[Anonymous]

Hi Robert,
Sorry I could not answer sooner, as I am training another engineer at the moment, which is full time and intensive.
What FJ says is correct and I will add another few points.
Use a heavier coax like RG 213 to cut down on resistance losses. Also best to run the coax in a plastic hosepipe for protection. The extra capacitance of long cables means that the sample delay will have to be set back considerably. Usually no shorter than 100uS. With a large coil, you would get too much sea water signal anyway, to use shorter delays.
Check that the Goldpic, or whatever you use, has dual sampling to counteract low frequency signals. Without this, the movement of the coil in the earth's magnetic field will generate a lot of noise.
Ideally, you want to pick off at dc signal at the end of the receiver chain, and feed it to a chart recorder. This way it is easy to see in retrospect, what are likely signals against the background variations you will get from changes in water depth and sand cover on the bottom.
The coil/sled needs to be heavy enough so that it does not start to "fly" when you are towing. A heavy chain mounted on the tow rope, sufficiently ahead of the coil so as not to be detected, is an effective solution. Tape the coax at intervals to the tow rope, leaving sufficient slack so that if the coil/sled hits an obstruction and the rope pulls tight, the cable is not stressed.
If I think of any more points, I will post them later.
Eric.

 

[Anonymous]

Hi Robert,
I built the Gold Pic and it didn't have a second sample. I don't think it has changed.
If you are going to use a kit type design, you might want to consider Carl Moreland's Hammerhead PI. This circuit does have dual sampling to compensate for the earth field effects.
Reg

 

[Anonymous]

I like Carl's work. Good to know about the sampling. I am going to give it a try and see what I come up with. I will look a little harder for more answers.
Thanks, Guys.
Robert