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I have been off the board for about a year, when I left there was great interest in basket coils an testing over all performance. I built several, an found the results to be very good. I was using a very a large multi- wire Litz wire. Is there still interest in this type of coil or did a new idea surface. Best to all Jim
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I haven't heard of newer ideas.Originally posted by James View Post
Just that most of us do not require sub 10us sampling for small gold.
There are however a few members that are working on detecting short TC targets like small gold nuggets.
Unknown what coils they are using but if remember correctly are using better front end designs. -
I'm interested !Comment
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https://www.youtube.com/watch?v=a41bERTFBUI
This looks unreal:-)). What we've got from the rivers is really small so i feel pity how this guy is even ignoring it... If its not hoax... note the comments are disabled. he do not need any delay below 10;-)))He do not need metal detector either.
Sorry for the offtopic.Comment
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Gold Gold Gold.....
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LOL, NOT ! Those in the KNOW would know it would be IMPOSSIBLE for gold of that size to be just sitting atop alluvial gravel like that. I can see where the uninitiated would fall for this one. Specific Gravity Rules.Comment
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So these videos are crap?
Why is someone making such videos?Comment
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Panning on plate cannot lie - it is pure gold.Comment
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Basket coil construction is designed to reduce the coil turn-to-turn wire capacitance that would be higher if the wires laid parallel to each other. Once advantage of reducing coil capacitance is the potential for that coil to sample faster due to less capacitance in the TX/RX circuit. When delays get below about 10 uS the type of wire needs to be considered so that the wire itself does not retain any eddy currents and then look like a target. That is why Litz wire makes a good coil wire when attempting to achieve low sampling time. The value of the damping resistor is a good indicator of how much coil and TX/RX circuit capacitance needs to be damped.Originally posted by James View Post
The ability to fully stimulate a target with a TX pulse requires that the coil discharge TC be five times faster than the target TC. Here is a thought experiment. A charged 2 uS target, fully stimulated, will fall to near noise in about 5 TCs or 10 uS for that target.That is why you would need to sample at less than 10 uS for a 2 uS TC target.
Now, to fully stimulate a 2 uS target requires a coil discharge TC of 2/5 or 0.4 uS. The coil discharge TC is determined by the coil inductance divided by the damping resistor value. A 300 uh coil would need a 750 ohm damping resistor to meet that requirement. Now you can see why making fast coils requires using many techniques to reduce capacitance and allow a higher value damping resistor to be used to fully stimulate small, low TC targets.
In mono coil PI metal detectors the op amp input resistor, typically 1K ohm, is ahead of two clamping diodes and while these diodes are conducting with a voltage above about 0.6V, the input resistor is effectively in parallel with the damping resistor thus effectively reducing the damping resistor value until the voltage falls below 0.6V. That is why DD coils that separate the RX and TX circuits are easier to operate at lower delays.
Optimizing coils should be done for a class of and small range of target TC ranges to have the best results. This includes the PPS rate, coil current, flyback clamping, damping resistor value, delay time, number of samples being integrated, coil size, coil sweep speed and coil's response to the soil and detecting environment.
The basket coil is just one way to attempt to reduce capacitance but there are many more things that need to be considered.
Joseph J. RogowskiComment
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