ELLIPTICAL COIL QUESTION

[baum7154]

I was thinking of how to get a geometry that gives a good linear blade or fan of energy into the ground without having a large 'footprint for susceptability to ground effect.

Something like this where you essentially have numerous paralell isolated wires ( like litz ) without the burden of 'full loops' picking up all in sundry. S

[ATTACH]30581[/ATTACH]

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Is your attachment a dual coil like a double D? I think the wound coil on a straight line form will only set up a field similar to a bar magnet. The elimination of almost all width goes beyond the 3:1 described in earlier postings in this thread and I think will give a much diminished blade field and depth. The best way to know is to build it and see.

Regards,

Dan

[green]

I recorded some data. Made a single layer coil 10.3 inch diameter. 21 turns awg 28 enamel coated wire. Made it so I could slide the coil off the round to put it on the other coil forms. Same coil for all tests. Plotted change in amplitude out of integrator vs target distance from coil. Coil not shielded. Circuit not stable as it needs to be to plot the lower amplitudes. Will do again and add the rectangle if I solve the drift problem. Probably not stable enough to define .25 x .25 in aluminum can. Wondering if I should have done the test different.

Recorded the rectangle (coil comparison_3). Corners fairly square. Recorded nickel and 9x9mm square aluminum coke can mentioned in another thread as a good target for small gold. Used a 12 volt supply, so had to divide by two to match original data (coil comparison_2). The square corners looked different than the half round ends used in _2.

[CAS]

I see you are using aluminium as a test. Would it be better to use a small piece of lead to simulate gold? I have always found that small lead shot and gold seem to give same readings on my other detectors.

[green]

I see you are using aluminium as a test. Would it be better to use a small piece of lead to simulate gold? I have always found that small lead shot and gold seem to give same readings on my other detectors.

[A 0.47 gram nugget (7.25 grains) can be emulated with a 9mm x 9mm aluminium square from the side wall of a Cola can. Your 10mm x 10mm square of aluminium foil is much thinner, so I doubt you'll be able to detect it.] from Repliis to Minipulse Plus #343 by Qiaozhi

Don't know what is a good test target. The above statement referenced a gold weight to a target size, so I went with it.

[baum7154]

Recorded the rectangle (coil comparison_3). Corners fairly square. Recorded nickel and 9x9mm square aluminum coke can mentioned in another thread as a good target for small gold. Used a 12 volt supply, so had to divide by two to match original data (coil comparison_2). The square corners looked different than the half round ends used in _2.

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Sorry I did not get back to you on this sooner been out of pocket for a few weeks. From the attachments it looks to me like the 4.5 round end coil (280 uh) is a bit more sensitive than the 10" round coil (310uh).

If I am interpreting your charts correctly the rectangle coil is worse than the 10" round coil and the 4.5 x 13.5 round end coil. Do you agree or am I completly misreading your charts?

Good work!

Dan

Just noticed that in test 2 (round end coil)you used a 1/4 x 1/4 (.0625 sq in) Al target, and in test 3 (rectangle ends)the target is a 9 X 9 mm (.1255 sq in) Al target. Was this difference accounted for in the results?

After rereading your charts it appears that your method is to use the 10" round coil normalized to a strength of 10 units (10uv?) set by target distance for a given target. Then other coils are tested with same target at that distance and the signal strength is recorded, correct? If so then the reference between charts 2 & 3 is the 10" coil, correct? Then from that the highest signal strength for a given target defines the best coil, correct? Whew! Please let me know.

Dan

[green]

[From the attachments it looks to me like the 4.5 round end coil (280 uh) is a bit more sensitive than the 10" round coil (310uh).]
At short distance it looks to be true.

[If I am interpreting your charts correctly the rectangle coil is worse than the 10" round coil and the 4.5 x 13.5 round end coil. Do you agree or am I completly misreading your charts?]
I agree. Something I didn't do was plot signal strength away from center. The rectangle coil looked good as I moved the target towards the end. Probably needs to be done for the rectangle,round and 4.5 x 13.5 round end.

[Just noticed that in test 2 (round end coil)you used a 1/4 x 1/4 (.0625 sq in) Al target, and in test 3 (rectangle ends)the target is a 9 X 9 mm (.1255 sq in) Al target. Was this difference accounted for in the results?]
The comparison in test 2 should be valid, same for test 3. The rectangle coil and the 4.5 x 13.5 round ends are both better than the round coil at 1 inch with the aluminum can target. It looked like I used 12 volt supply for test 3 and 6 volt supply for test 2, so I divided the results for test 3 by 2 to get values to compare with test 2.

[After rereading your charts it appears that your method is to use the 10" round coil normalized to a strength of 10 units (10uv?) set by target distance for a given target. Then other coils are tested with same target at that distance and the signal strength is recorded, correct? If so then the reference between charts 2 & 3 is the 10" coil, correct? Then from that the highest signal strength for a given target defines the best coil, correct? Whew! Please let me know.]
Plot is voltmeter reading vs distance for each target. Lowest digit = 100 uvolts, 9500 = .9500 volts. I need to put more information on the charts explaining how they were done.

Interested in your rectangle coil with rounds ends.

[baum7154]

Thanks Green,

So have you come to a conclusion on which coil config is the best for depth and small target detection?

Dan

[green]

Thanks Green,

So have you come to a conclusion on which coil config is the best for depth and small target detection?

Dan

Depending what the signal strength is off center I would try the rectangle with round ends. Don"t know if how they act on the ground vs air might change the selection. Been wanting to do discrimination. Have to use a DD or build a Chance or figure out how it does it with a mono coil.

[baum7154]

Thanks.

After studying your plots again it looks like the round ended 3:1 coil does have a bit of an edge over the others for what I want to do. I actually built another round end 3:1 coil that is 3.5" X 11" that I did not post and it performs quite well on the small stuff. It detects the gold flakes at about 2" in air but then I started tweaking the filters in the preamp and went too low @ 425kHZ. Ran a MATLAB sim yesterday and didn't like the signal delay added by the 425kHz filter stages. Also the A/D offset level needed to be reset with the changes so now I have to backtrack a bit to get to best operating conditions with the CHANCE PI. A/D offset level is critical to getting discrimination to work properly. Also coil damping always needs to be optimized. It is challenging when there are so many variables in play.

Regards,

Dan

[baum7154]

Below is a repost of testing in the CHANCE PI Coil thread. The reason for the repost is it is directly related to the 3:1 Oval Coil discussions previously on this thread.

Originally Posted by baum7154
I have built a 4" X 12.5" oval coil with 8.5" parallel sides in accordance with Eric's 3:1 profile recommendation. This coil is a fast spider coil using the instructions I posted in post #126 using only simple tools. The construction pictures are posted below. I will post test results for this self shielding coil in the next few days.Attachment 31039Attachment 31035Attachment 31036Attachment 31038



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Well it's been much more than a few days since I posted the above info. About 4 months have passed since the above post and I finally got the time to complete testing on this new coil today. Nothing worth while is ever easy and this is no exception. I had to remove the 430 kHz filters from the CHANCE PI and installed the 530 filters but still did not like the results on small gold so then installed the original 680 kHz filters. Also the adjustable damping network had to be reworked to properly damp the coils. The new damping network adjusts from 1075 to 1245 ohms.

The object of these tests was to compare my 335 uh round coil to the new 4" X 12.5" 328 uh parallel side 3:1 oval coil on small gold targets. These 'small gold targets' are the most difficult I have and are as follows:

1. Gold flakes in glass vial - largest piece is .95 grain, 2 pieces - .19 grain, 1 piece - .12 grain, 1 piece - .11 grain. Total weight of all dust and flakes 2.92 grains. With the flakes loose in a glass vial it is hard to present the target consistently to the coil as the pieces can shift or be on edge.

2. 4.5 grain, 14 kt jewelry gold head for mounting a 4mm stone. No earring post or other metal.

3. 1/4" X 1/4" .004" thick piece of aluminum soda can.

All tests were conducted at a Guard Interval of 10 (8us) on the CHANCE PI detector. Battery voltage throughout the tests was 12.6 volts. Only repeatable test results are accepted in these tests. In many cases greater 1st Detection Distances were observed but not consistently repeatable and so were not included in the results. Test coil was placed horizontal 20" above table and test was conducted in air. Table was slate with no other metal in the area of the coil.

1st Detection Distance Tests:

Target ---------------------------8" Round Coil Distance ---------------------Oval Coil Distance

Gold Flakes ---------------------1.75" -----------------------------------------1"

4.5 gr Head ---------------------4.75" -----------------------------------------3.5"

1/4" X 1/4" AL ------------------4.5" ------------------------------------------2.75"



Solid Detection Tone Distance:

Target -------------------------------8" Round Coil Distance --------------------Oval Coil Distance

Gold Flakes -------------------------0.5" -----------------------------------------.375"

4.5 gr Head -------------------------2.75" ----------------------------------------1.75"

1/4" X 1/4" AL ----------------------2.75" ----------------------------------------1.75"

Conclusion: It is clear that the 8" round coil has a depth advantage but its field is a virtual pinpoint at the greatest distances recorded, making it difficult to quickly cover an area in the field. While the Oval coil has a bit less depth it has an 8" blade like field that will cover an area much more quickly. It will be interesting to see how these coils perform over soil once the snow clears here.

Regards,

Dan

[mohandes]

hi all
i want make dd coil 27cm and 45cm for Delta pulse 2 . i want add discrimination too it.
so i want use mono and dd coil . i want use mono coil in 110cm so it is no hard and i want put the frequency on 100hz.
but i dont know about the ohm and inductance ??? and more important thing about the frequency mode. what frequency need to use , 500hz is good ?
i really need data for you and need more help.
if i want use dd coil need change the schematic or no?
regards

[baum7154]

The following is a repost of testing some additional targets from the CHANCE PI BUILD thread because the coil used is a 4"X12.5" oval coil discussed in this thread.
---------------------------------------------These 'small gold targets' are the most difficult I have and are as follows:

1. Gold flakes in glass vial - largest piece is .95 grain, 2 pieces - .19 grain, 1 piece - .12 grain, 1 piece - .11 grain. Total weight of all dust and flakes 2.92 grains. With the flakes loose in a glass vial it is hard to present the target consistently to the coil as the pieces can shift or be on edge.

2. 4.5 grain, 14 kt jewelry gold head for mounting a 4mm stone. No earring post or other metal.

3. 1/4" X 1/4" .004" thick piece of aluminum soda can.

All tests were conducted at a Guard Interval of 10 (8us) on the CHANCE PI detector. Battery voltage throughout the tests was 12.6 volts. Only repeatable test results are accepted in these tests. In many cases greater 1st Detection Distances were observed but not consistently repeatable and so were not included in the results. Test coil was placed horizontal 20" above table and test was conducted in air. Table was slate with no other metal in the area of the coil.

1st Detection Distance Tests:

Target ---------------------------8" Round Coil Distance ---------------------Oval Coil Distance

Gold Flakes ---------------------1.75" -----------------------------------------1"

4.5 gr Head ---------------------4.75" -----------------------------------------3.5"

1/4" X 1/4" AL ------------------4.5" ------------------------------------------2.75"



Solid Detection Tone Distance:

Target -------------------------------8" Round Coil Distance --------------------Oval Coil Distance

Gold Flakes -------------------------0.5" -----------------------------------------.375"

4.5 gr Head -------------------------2.75" ----------------------------------------1.75"

1/4" X 1/4" AL ----------------------2.75" ----------------------------------------1.75"

Conclusion: It is clear that the 8" round coil has a depth advantage but its field is a virtual pinpoint at the greatest distances recorded, making it difficult to quickly cover an area in the field. While the Oval coil has a bit less depth it has an 8" blade like field that will cover an area much more quickly. It will be interesting to see how these coils perform over soil once the snow clears here.

Regards,

Dan[/QUOTE]
---------------------------------------------


These are some additional tests with the 328uh oval coil with 1075 ohm damping on some Aluminum simulators of small gold and some dental gold targets from about 1 gram to a bit over 1/2 gram.

Target--------------------------------1st detection dist------------------Solid tone distance

1/4" X 1/4" X .004" AL------------------2.75"-------------------------------1.75"

3/8" X 3/8" X .004" AL------------------5.5"--------------------------------4.0"

1cm X 1cm X .004" AL------------------5.75"-------------------------------4.12"

1/2" X 1/2" X .004" AL------------------7.5"--------------------------------5.375"

1" X 1" X .004" AL-----------------------11.75"-----------------------------9.75"

.7 grain 1/8" dia AL slug-------------3.25"------------------------------1.75"

1.4 grain 1/8" dia AL slug------------4.25"------------------------------2.375"

2.1 grain 1/8" dia AL slug------------5.5"-------------------------------3.0"

Dental Gold of various geometries and unknown gold percentage.
8.6 grain------------------------------4.5"-------------------------------2.25"

10.6 grain-----------------------------4.75"-----------------------------3.25"

11.8 grain-----------------------------4.25"-----------------------------2.5"

15.4 grain-----------------------------4.75"-----------------------------3.0"

Regards,

Dan

[Hayden]

I have a question. It may make no sense but I have one Everyone keeps talking about insulated stranded wire to reduce capacitance. If you squeeze the circle into a 3/1 rectangle, aren't you adding capacitance back to the coil? For instance, a square antenna cut to freq will have a feedpoint of 102 ohms. Make that same antenna into a 2/1 rectangle and you reduce the feedpoint to 51 ohms because of the added capacitance even though you use the same amount of wire. That is how we run full wave loops without needing an antenna tuner. It would seem that a 3/1 would add even more capacitance back reducing the feedpoint by several ohms. Say to 30 ohms. The same happens to a dipole as you dip the ends toward the ground making an inverted V. You take it from 75 ohms down to about 36. What retuning would you have to do inside the detector to acomodate this change in feed point? Just asking

[baum7154]

By changing the profile to 3:1 you are actually changing both inductance and capacitance. However in the case of the 3:1 coils I have built they are wound on an oval form to a target inductance of about 320 to 330 uh. Every measure is taken to wind them with the least capacitance possible thus making them decay fast enough to be used at about 8us sample delay. I have not had any problem with a 3:1 coil that would not operate at this delay. The manifestation of a coil with too much capacitance or too low of a Self Resonant Frequency is that the coil will not allow operation at very short i.e. <10us delays. The slower they get, the longer the delay required to allow detector operation. So to answer your question, a slow coil will require the PI detector sample delay to be adjusted longer than a fast coil does. Short sample delays are necessary to detect small gold in the sub 1/2 gram range. Hope this helps.

Regards,

Dan

[Hayden]

Thanks Dan. I was thinking more about damage to the transistor or IC from to high SWR. And also the ability to transfer power to the coil as in a 1 to 1 match. Doing the same to a radio would blow the finals in just a few seconds. Antennas have to achieve maximum radiation out into space. You want all power put into it to leave. I had no idea that the critical thing here is decay time. Is that the reason for so few turns on the coil, to stop the flyback reaction? A television flyback that will not ring 10 times is useless. Aparently that is not wanted or needed here.