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There is other information about coils that I am still looking for:
Ergonomics – this involves size, shape and weight of the coils.
·How can we optimize the weight of the coils?
·What shape and size of coil gives the most depth?
·What size and shape of coil gives the most efficient ground coverage?
What is the best “all round” coil, the coil that gives the best compromise between all of the above?
Tinkerer
Tinkerer,
·How can we optimize the weight of the coils?
Use the thinnest and lightest wire at a modelate to high TX PPS frequency.
Fill coil housing with foam to keep the coil rigid but light, with no movement inside the housing that might cause false signals.
·What shape and size of coil gives the most depth?
This depends on what you are looking for. Eric Foster made a chart that shows the relationship of target size to coil radius and suggests ways to optimize a coil size to detect a particular size target. Coins are pretty common targets so the rule of thumb is to use a coil that is in the 10" to 12" range for these common targets. PI machines are pretty sensitive to coil size when trying to operate a large coil at low delays. Eric Foster mentioned many times the difficulty in getting a coil larger than about 11" to operate at 10us. The capacitance of the coil shield seems to be the controlling factor. If I were to attempt making a coil in the 14" range operate at low delays, I would use it with a shaft mounted control box to minimize the coax capacitance. I would use all the techniques that I have shared to make a fast coil like using AWG 30 Teflon wire, Teflon spiral wrap or polyethylene spiral wrap, a low capacitance shield like Scotch 24, and a short coax 30" to 34" long.
·What size and shape of coil gives the most efficient ground coverage?
I do mostly beach hunting so this tip is related to my beach experience. Those who hunt for gold nuggets may want to share their experience.
To cove a lot of ground try a coil that is 18" long by about 6" wide. Hays Electronics make one that is about this size. The rule of thumb seems to be: make the coil width about 1/3 the length. When you wind a coil that is a circle and compress it into an oblong shape, the inductance is reduced between about 5% and 10% . One of my 10.5" coil is 270uH in a round circle. When I squeeze it into a 13" by 5" oval it is 250uH and when I squeeze a little more to be 4" wide it is 240uH. This means that if you plan on making a oval coil, make it about 10% higher inductance so it will be your desired inductance in it's final oblong shape.
What is the best “all round” coil, the coil that gives the best compromise between all of the above?
Eric Foster mentiond that an 11" coil is the best compromise for general purpose use.
Here is a coil making idea that I have kicking around in my head. Maybe some forum members can improve upon it. This should make a rigid, waterproof and relatively light coil for beach hunting.
Bend a .5" ID PVC plastic pipe into a 12" ID circle with a heat gun and by filling the pipe with sand, capping the ends to keep the pipe from kinking.
Insert into the bent .5" ID pipe a flexible pipe 3/8" OD X .25" ID with Scotch 24 around this like a long sock. Solder a ground lead to the Scotch 24.
Insert and glue a T connection. Use a guitar string as a long needle and feed about 20 turns of Teflon insulated wire through the inner shielded .25" ID pipe, one turn at a time at the T connection opening. This will probably take two people to keep the wire from kinking or getting tangled. When finished, spray expandable foam into the inner .25" pipe to secure the wires.
I hope this stimulates some tinkering.
bbsailor
Ergonomics – this involves size, shape and weight of the coils.
·How can we optimize the weight of the coils?
·What shape and size of coil gives the most depth?
·What size and shape of coil gives the most efficient ground coverage?
What is the best “all round” coil, the coil that gives the best compromise between all of the above?
Tinkerer
Tinkerer,
·How can we optimize the weight of the coils?
Use the thinnest and lightest wire at a modelate to high TX PPS frequency.
Fill coil housing with foam to keep the coil rigid but light, with no movement inside the housing that might cause false signals.
·What shape and size of coil gives the most depth?
This depends on what you are looking for. Eric Foster made a chart that shows the relationship of target size to coil radius and suggests ways to optimize a coil size to detect a particular size target. Coins are pretty common targets so the rule of thumb is to use a coil that is in the 10" to 12" range for these common targets. PI machines are pretty sensitive to coil size when trying to operate a large coil at low delays. Eric Foster mentioned many times the difficulty in getting a coil larger than about 11" to operate at 10us. The capacitance of the coil shield seems to be the controlling factor. If I were to attempt making a coil in the 14" range operate at low delays, I would use it with a shaft mounted control box to minimize the coax capacitance. I would use all the techniques that I have shared to make a fast coil like using AWG 30 Teflon wire, Teflon spiral wrap or polyethylene spiral wrap, a low capacitance shield like Scotch 24, and a short coax 30" to 34" long.
·What size and shape of coil gives the most efficient ground coverage?
I do mostly beach hunting so this tip is related to my beach experience. Those who hunt for gold nuggets may want to share their experience.
To cove a lot of ground try a coil that is 18" long by about 6" wide. Hays Electronics make one that is about this size. The rule of thumb seems to be: make the coil width about 1/3 the length. When you wind a coil that is a circle and compress it into an oblong shape, the inductance is reduced between about 5% and 10% . One of my 10.5" coil is 270uH in a round circle. When I squeeze it into a 13" by 5" oval it is 250uH and when I squeeze a little more to be 4" wide it is 240uH. This means that if you plan on making a oval coil, make it about 10% higher inductance so it will be your desired inductance in it's final oblong shape.
What is the best “all round” coil, the coil that gives the best compromise between all of the above?
Eric Foster mentiond that an 11" coil is the best compromise for general purpose use.
Here is a coil making idea that I have kicking around in my head. Maybe some forum members can improve upon it. This should make a rigid, waterproof and relatively light coil for beach hunting.
Bend a .5" ID PVC plastic pipe into a 12" ID circle with a heat gun and by filling the pipe with sand, capping the ends to keep the pipe from kinking.
Insert into the bent .5" ID pipe a flexible pipe 3/8" OD X .25" ID with Scotch 24 around this like a long sock. Solder a ground lead to the Scotch 24.
Insert and glue a T connection. Use a guitar string as a long needle and feed about 20 turns of Teflon insulated wire through the inner shielded .25" ID pipe, one turn at a time at the T connection opening. This will probably take two people to keep the wire from kinking or getting tangled. When finished, spray expandable foam into the inner .25" pipe to secure the wires.
I hope this stimulates some tinkering.
bbsailor
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