PI coil

[ApBerg]

Hello Friends,
Just for fun I made a coil from a old coax cable ( ca. 15 meter), striped it from the shielding and outside PVC, so only used the inside wire with its tick isolation. Look at the photo’s, not the most beautiful coil…, I made it with some hot glue and a old 24 cm coil form .
For testing I connected it on a Surfmaster-PI… the results for small gold are very good ! The small very thin gold medal can be detected ! ….. Other tested coils ignored it !
So if you have some old coax…

[WM6]

Very good idea ApBerg. Congratulation.

[Esteban]

Hello Friends,
Just for fun I made a coil from a old coax cable ( ca. 15 meter), striped it from the shielding and outside PVC, so only used the inside wire with its tick isolation. Look at the photo’s, not the most beautiful coil…, I made it with some hot glue and a old 24 cm coil form .
For testing I connected it on a Surfmaster-PI… the results for small gold are very good ! The small very thin gold medal can be detected ! ….. Other tested coils ignored it !
So if you have some old coax…

Hi Appie. Very good info! I think can be used monofilar coax here. Maybe you obtain good results because the spiral form.

My best regards

Esteban

[bklein]

Umm are you saying the PI wouldn't detect the gold medal with the standard coil?

[ApBerg]

Hello Bklein,

Well……it was my intension to show how a simple good working coil can be made. The photo’s give some more info how to make the coil, no secrets al very easy. And yes…. the other coils they did not detected that very thin gold medal…. Keep in mind that this gold medal is very.. very thin ! Also some VLF detectors do not detect it !
(photo shows the detector with 3 coils)
Best regards.
Ap

[bklein]

Hello Bklein,

Well……it was my intension to show how a simple good working coil can be made. The photo’s give some more info how to make the coil, no secrets al very easy. And yes…. the other coils they did not detected that very thin gold medal…. Keep in mind that this gold medal is very.. very thin ! Also some VLF detectors do not detect it !
(photo shows the detector with 3 coils)
Best regards.
Ap

Very cool then. I know my Surfmaster would not detect a small gold nugget but I would expect it to detect a gold film of what looks like 1" diameter - hmm. Do you expect then your coil will detect small nuggets?
Mine sure finds small iron crap well....
Did you readjust your 1st sample time for the coil? If so, how did you set it?
What do you recommend for the coax - or does it matter much? Stranded isn't too common in my garage....
How did the coil do for just coin detecting? Would you then reset the 1st sample out further (or use the control).
Did you wind the coil for the coilform or intentionally wind it for some specific inductance (how best optimize the coil design for a variety of coax choices).. Sorry for so many questions - I don't have any hands-on time with coil making yet. It is my next move when I get some play time.
Was to start making a coil winding platform then try and decide upon wire.
Your example here looks simple enough but I don't think the coax I have is stranded. Need to do some shopping.

Barry

[ApBerg]

Hello Barry,

Well, I have to test the coil in 'real live' over 2 weeks give it a tray on the beach.
Think your Surfmaster can detect small nuggets if you use a different coil. And set the delay critical.
In the past I have read BBsailers pdf, very useful. This is way I tested the coax cable, I think it is the isolation around the wire that is important !

Perhaps somebody who knows can explain this !

Best regards

Ap

[Tinkerer]

Hello Barry,

Well, I have to test the coil in 'real live' over 2 weeks give it a tray on the beach.
Think your Surfmaster can detect small nuggets if you use a different coil. And set the delay critical.
In the past I have read BBsailers pdf, very useful. This is way I tested the coax cable, I think it is the isolation around the wire that is important !

Perhaps somebody who knows can explain this !

Best regards

Ap

ApBerg,

When you have the coil windings very close together, like when using magnet wire, you get more inductance, but also quite a bit of inter wire capacitance. the capacitance increases the Flyback decay time.
Thicker wire also increases capacitance.
As you increase the space between the wire turns, the capacitance decreases, making the coil faster.
some time back, I posted pictures of coil building with thick magnet wire. To reduce the capacitance I used a Polypropylene string wrapped around the magnet wire.
Air is the best insulator.
Polypropylene is a very good insulator.
The polypropylene string as a spacer between the coil windings uses air and PP and produced a very fast coil.
I don't remember the title of my post, it was something like "coil with polypropylene spacer".


Here, I managed to find the thread. http://www.geotech1.com/forums/showp...98&postcount=1
Note the sampling time of 2.5uS. very good for gold nuggets.

Tinkerer

[bbsailor]

Hello Barry,

Well, I have to test the coil in 'real live' over 2 weeks give it a tray on the beach.
Think your Surfmaster can detect small nuggets if you use a different coil. And set the delay critical.
In the past I have read BBsailers pdf, very useful. This is way I tested the coax cable, I think it is the isolation around the wire that is important !

Perhaps somebody who knows can explain this !

Best regards

Ap

Ap,

What is the self resonance of the coil in the housing with the shield connected? This should be measured at the connector end of the wire only.

This one measurement tells you much about the coil wire-to-wire capacitance, shield-to-coil capacitance and coax capacitance.

Your use of the thick insulation and the spiral wind of the coil with 23 turns should produce a very low capacitance coil. The coax cable capacitance is based on the dielectric constant of the insulation and the thickness of the insulation. Use the shortest coax that is convenient. Typically, the coax cable only needs to be about 32" to 34 long to reach the coil. Normal coax cables are about 27pf per foot and a low capacitance cable would be about 16pf per foot.

Coils with the highest self resonant frequency have lower capacitance and require less damping in the form of a higher value damping resistor which makes a more sensitive PI detector for smaller targets when operating at lower delays.

If the coil operated at the lowest delay, do some research on your schematic to find a way to reduce the delay a little more by about about 2us to 3us to make your PI machine more sensitive to smaller objects. Making a faster coil only provides the potential to detect smaller objects if your PI circuit can use that potential. typically only a resistor change can reduce the delay. See Carl's Hammerhead Article and look at the modification section and find the same delay resistor in your PI machine.

Make sure that you take advantage of your fast coil design by optimizing the value of the damping resistor. Increase the damping resistor value in 100 ohm increments and look at the output of the first amplifier stage with an oscilloscope to see that the pulse damps a fast a possible with no ringing. My guess is that you could try a damping resistor about 200 ohms higher in value. This is one part value that must be fine tuned on the coil, and coax used to make the coil to ensure that it is optimum for that coil.

If you change coils, you can make the damping reistor on the circuit board a little higher than optimum discussed above by about 100 to 200 ohms for a underdamping situation, then use a higher value (2000 to 5000 ohms) in the plug to fine tune each coil to be optimal. This way you can change coils and all will be optimally adjusted to your PI machine.

Nice Job!

bbsailor

[ApBerg]

Hello Friends.
Thanks for the replay, very useful !
Some questions:
Thinkerer : In the past I have seen some ‘hollow’ coax. Do you think this is better than the one I used?
‘BBSailor: For measuring the self resonance from the coil; I connect a function generator at the cable to the coil and searchs, looking on the scope, for the highest amplitude? If so I can tray this next week. Or is there a better way ?
Best regards.
Ap

[bbsailor]

Hello Friends.
Thanks for the replay, very useful !
Some questions:
Thinkerer : In the past I have seen some ‘hollow’ coax. Do you think this is better than the one I used?
‘BBSailor: For measuring the self resonance from the coil; I connect a function generator at the cable to the coil and searchs, looking on the scope, for the highest amplitude? If so I can tray this next week. Or is there a better way ?
Best regards.
Ap

AP,

Connect a signal/function generator to the coil through a 1 meg ohm resistor to isolate the high impedance of coil at resonance from the low impedance of the signal generator output. Connect your scope probe on the 10 to 1 setting to ground and the coil after the 1 meg resistor. Sweep the signal generator between 500KHz and 2 MHz, observe and note the frequency of the highest signal peak at resonance. Suspend the coil in mid air, away from other metal, to get the most accurate reading.

The scope probe at the 10 to 1 setting has a lower loading capacitance and will make your measurement more accurate. See the illustration in my fast coil article to put a low pf value (1 to 3 pf) gimmick capacitor, made with about 3 cm of twisted wire, between the probe and coil. This will put even less loading on the coils resonance point and make the reading even more accurate. My guess is that your coil resonance should be between about 700KHz and 800KHz.

I hope this answers your question?

bbsailor

[Tinkerer]

Hello Friends.
Thanks for the replay, very useful !
Some questions:
Thinkerer : In the past I have seen some ‘hollow’ coax. Do you think this is better than the one I used?
‘BBSailor: For measuring the self resonance from the coil; I connect a function generator at the cable to the coil and searchs, looking on the scope, for the highest amplitude? If so I can tray this next week. Or is there a better way ?
Best regards.
Ap

You can try the hollow kind of coax or the foam kind, but probably there will not be much difference.
The spiral wound coil has very good sensitivity at the very center, but it's detecting field gets very narrow some distance from the coil. Like a funnel shape.
This means that you have great sensitivity at a good depth, but only about the diameter of the center hole in the coil.
You can also wind a bundle coil with the same cable. The capacitance will be about the same, because of the shielding. The spiral coil has more shielding surface, that adds some capacitance.

Tinkerer

[ApBerg]

AP,

Connect a signal/function generator to the coil through a 1 meg ohm resistor to isolate the high impedance of coil at resonance from the low impedance of the signal generator output. Connect your scope probe on the 10 to 1 setting to ground and the coil after the 1 meg resistor. Sweep the signal generator between 500KHz and 2 MHz, observe and note the frequency of the highest signal peak at resonance. Suspend the coil in mid air, away from other metal, to get the most accurate reading.

The scope probe at the 10 to 1 setting has a lower loading capacitance and will make your measurement more accurate. See the illustration in my fast coil article to put a low pf value (1 to 3 pf) gimmick capacitor, made with about 3 cm of twisted wire, between the probe and coil. This will put even less loading on the coils resonance point and make the reading even more accurate. My guess is that your coil resonance should be between about 700KHz and 800KHz.

I hope this answers your question?

bbsailor

BBsailor;

Thanks for the 'manual' very usefull !

Best regards.

Ap

[ApBerg]

Hello Tinkerer,

The spiral coil detects over the whole coil, not only in the mid-opening
(small medal for ca 80 % )
A cola can is detected at almost 1 meter and gives even a good response when it’s at the side from the coil.

To see the effect from the coax cable I made also a square coil ca. 25 cm, ..... It has the cable bundled in the tube (what a work this was)
.....It did not detected the small gold medal .....
So it seems that the spiral coil is in favor for detecting small gold.

Best regards.

Ap

[Gosta]

3 coax coils

Hello Friends,
Just for fun I made a coil from a old coax cable ( ca. 15 meter), striped it from the shielding and outside PVC, so only used the inside wire with its tick isolation. Look at the photo’s, not the most beautiful coil…, I made it with some hot glue and a old 24 cm coil form .
For testing I connected it on a Surfmaster-PI… the results for small gold are very good ! The small very thin gold medal can be detected ! ….. Other tested coils ignored it !
So if you have some old coax…

Hi ApBerg I wonder how to connect 3 coils to an recieve transmit on pi detector.Please help Regards Gosta.