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Offline tommybTopic starter
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« on: July 09, 2011, 03:45:37 pm »
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Has anyone mapped the changes in sensitivity of a detecting a small object using a coil wired in opposing polarities (two different conditions)? I would guess there should not be any detectable difference, but based on how the field would be collapsing, there may be a different peak field distribution and hence more sensitivity to smaller objects?

Thomas

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Offline GoldDigger1950
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« Reply #1 on: July 09, 2011, 06:02:18 pm »
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Tommy, I am not really sure what you are asking here. A single, mono coil wrapped in the opposite direction to another coil will not make one bit of difference. What will make a difference is the orientation of any Faraday shielding on the coils. The left hand versus right hand field will still work the same as long as all other factors are taken into consideration.

A single coil with half of its windings split off and folded over will result in what is commonly called a Mono Double D configuration or if not folded over, a Figure Eight configuration. Characteristics of these babies will vary and experimentation is the best tool for learning. In other words, build a coil and try it.

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« Reply #2 on: July 09, 2011, 08:03:15 pm »
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In practicle use , you are right with PI  and mono coil  with deep target.... many times you have more detection  in one orientation (ej: north- south)  and in another  oreintation more less ...i have many times see this in  a test field ....
It is for that point  that  it is best to always  pass the coil  making a cross  when you are searching  with this method  you  will not loose any target !!!!!

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Offline tommybTopic starter
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« Reply #3 on: July 10, 2011, 10:14:30 am »
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Hello - thanks all for the replies.
Based on the N/S vs E/W comment, does this imply that if the coil polarity were changed, the N/S sensitivity would flip with the E/W sensitivity? Or is there no difference at all if the polarity is swapped. This is not what I expected, but since I am just a newbie building my first MD, it something to try and explore a little more.
My assumption is that there is a possibility that the detection profile would change (shorter and squater vs deeper and more conical) but not rotate a lobe as described above. Maybe this lobe effect is created due to the field disruption of inner lead wires crossing over the coil from the center? What is the best (easiest) way to map the field in free air and get repeatable results?

Today I will build my first coil, wish me luck!

GoldDigger - which direction should the Faraday shield be wrapped? For example if the ground side lead of the coil is on the outer diameter and the coil is wrapped in a clockwise direction looking down on the coil, should the wrap go clockwise or counterclockwise and why? Does a mono coil really need a shield?

One more question, if the flyback voltage that is generated is really > a few hundred volts DC, how likely is arcing across the lacquer insulator of the magnet wire. Does anybody add extra insulation (tape) between the lead wires for example (assume highest voltage delta is between lead wires).

Thanks for the help in advance,
Tommy

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Offline GoldDigger1950
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« Reply #4 on: July 10, 2011, 02:00:53 pm »
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Quote:Posted by tommyb
GoldDigger - which direction should the Faraday shield be wrapped? For example if the ground side lead of the coil is on the outer diameter and the coil is wrapped in a clockwise direction looking down on the coil, should the wrap go clockwise or counterclockwise and why? Does a mono coil really need a shield?

One more question, if the flyback voltage that is generated is really > a few hundred volts DC, how likely is arcing across the lacquer insulator of the magnet wire. Does anybody add extra insulation (tape) between the lead wires for example (assume highest voltage delta is between lead wires).

There are two types of Faraday shield in use. One type wraps the entire coil with a single layer of very thin foil leaving about 5cm (2 inches) of open space near where the wires come out. The other is like lining a Frisbee and putting the coil below that.

The voltage can indeed be as high as 200v in a very short burst but will not harm the insulation. The current is miniscule.

Let me point something out to you here. Most of the detectors we discuss here have been built by one or more people. Some even use existing coils from older machines. While your questions are informative to you and other beginners, just remember that this is not the first time the models you are looking at have been built. They're not going to suddenly melt down on you. Build according to the plan and you will be fine.

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« Last Edit: July 10, 2011, 02:03:40 pm by GoldDigger1950 »
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« Reply #5 on: October 04, 2011, 05:12:03 am »
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very good job Detecting

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