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I read where it cancels noise and can reject ground. Would it also cancel earth field?
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do not keen foot style and oval coils in PI. those coils has sharply low sens be cause the form.
1/3 sizes of diameters is too critical. may be 2/3 still good. I do not remember who wrote about that. so believe me now on words.
Anatloy -
Not exactly bigfoot, but I'm trying big. Had it working this morning, now dead.Comment
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Yes, a Bigfoot-style PI coil should also cancel Earth field.Comment
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how use DD coil for pi machine?
can explain?Comment
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Mohandes,
I can't explain, I'm just trying to make a coil for my Garrett ATX, which comes stock with a DD coil.Comment
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I'm unclear on what constitutes a BIG FOOT COIL. Is it applicable to a mono coil as well as a DD coil? Is it just any coil with two parallel sides and either rounded ends or square ends in a 3:1 or 4:1 length to width ratio? Does it look like the coil in post 216 of the CHANCE PI COIL thread of the COILS forum?
Thanks,
DanComment
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Thanks Chet. So from what I read a Bigfoot is a figure 8 coil.Comment
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One difference between the Bigfoot figure 8 and your Racetrack coil is the field patterns. The Bigfoot coil has two sensitive detection points with a null in between. This makes it harder to pinpoint than with the Racetrack coil which has a pattern similar to an oval or elliptical shaped coil. Long narrow Bigfoot coils are popular with coin detectors for covering ground faster especially in competition hunts where the coins are planted shallow.
Regards,
ChetComment
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Yes the Racetrack 3:1 coil appears to have a very consistent blade of detection about 8"long and it is not difficult to pinpoint using one of the ends of the coil. I'll be posting more construction details and photos soon.
Regards
DanComment
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Want to make a big foot coil using two round coils and a race track coil. If it works I'll try a 3 to 1(length to width). Charted the coil responses to compare the big foot against when finished. Big foot (two Rx, opposite phase, One Tx around both Rx coils, IB)Comment
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Hi Green
Thank you for another great test. Since the receiver coils will be inductively balanced you might be able to increase the number of turns to increase received signal strength. And earlier sampling may be possible.
Have another great day,
ChetComment
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Hi Chet, Finished the big foot with two 110mm round coils for Rx.and a racetrack coil around them for Tx. I changed the coil driver to current control(constant rate, 12500 amps/sec), 80 usec on time, 1amp peak current. Gives a flat response during on time for ferrite and ground. Planned on taking a sample at the beginning and end of on time to discriminate ferrous, non ferrous. Got some oscillation at the start of on time.Don't know why yet, maybe doesn't mater. Looks like ferrous signal change is opposite polarity at end of on time and first sample of off time. Lost some signal strength over mono coil, but the meter readings were more stable. With amplifier gain of 300 the 100u volt digit changed 1 count where it changed a few counts, varied with time with the mono coils. The Rx coils are 210 uh each giving 420 uh in series. Almost not able to sample at 6 to 10 usec. Think I need to use a difference amplifier and grounded center tap for the Rx. The US nickel didn't show during on time because it decays before the oscillation ends. A reply in another thread stated the big foot coil works good for pin pointing. Looking at the chart I would agree if you can see both polarities. The null in the center is quite small. Looks good on the bench, hope it's as good on the ground. The coil is IB, but I still get a spike at turn on and turn off. Scope traces are amplifier out. Forgot to turn scope frequency display off, doesn't mean anything.Originally posted by Chet View PostComment
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Hi Green
Very good results. Looks like it works better than the Racetrack coil alone. The ferrite and other target responses during on time is especially interesting. It would be interesting to see the effect of lead and aluminum targets during the on time.
Could you spread the two round coils apart until they achieve individual inductive balance above each end of the Racetrack coil. This would be a rough simulation of a DODD coil as found in a mikebg post below. The fourth coil L3 is not needed. The individual inductive balances might reduce the spikes and ringing.
Thank you for the fine work,
Chet
http://www.geotech1.com/forums/showt...0200#post70200Comment
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