Shielding or No???

[Qiaozhi]

I tried shielding my 16" circ mono coil and the result was absolute silence. The shielding was just out of curiosity so it is not mandatory to have it as coil works well without, but it would be nice to find out what went wrong.

I used aluminium tape throughout the coil and attached the ends to coaxial outer lead.

Should it have gaps, should only the other end be connected to ground, or what...?

Thanks for any ideas,
Markus

If you did not leave a gap, then no wonder there was complete silence. Without a gap the shielding will act as a single shorted turn. Only one end needs to be connected to ground.

[porkluvr]

You should probably consider using something besides aluminum tape because it is very conductive, more than needed. Something like Scotch 25 braid, or look at the paint recipe here:
http://www.geotech1.com/forums/showthread.php?p=100499#4

Do a search on 'shield' or 'shielding' and get some other ideas. I remember seeing another paint recipe either in the Tools or else the General section, but this may be even better.

Before I knew any better I bought a roll of aluminum tape, but now I won't use it with what I know. Or, maybe I could cut narrow strips and make a loose wrap?

[Tinkerer]

Perpendicular coils

Hi Geo,
I can give 5 answers:
1. Small coins.There are ancient Greek coins with unusually small diameter, which are very valuable. If you increase the diameter of the sensors, the machine will lose its sensitivity to them. Do you looking for large objects in places where there may be such small coins?
2. Coil diameter. The diameter of the TX coil sets the ground timeconstant. If you increase the diameter, the machine will receive a stronger and slower decaying response from conductive ground. Do you use large sensors on the sea beach?
3. Timeconstant of shield signal. Conducting shield creates a signal like a metallic object located too close to the coil. The timeconstant of the shield needs to be done other than timeconstant of the desired object.
4. Gain of preamp. In addition, the created from shield parasitic signal requires to reduce the gain of the preamp. The depth of detection depends mainly on the gain of preamp.Let us reflect on how it should work an ideal metal detector having real preamp. When no target, at the input of the preamp there is no signal. Therefore, we can increase its gain until output voltage begins to saturate input of synchronous demodulators. The output voltage is only large chaotic noise, but it does not interfere with the work because synchronous demodulator acts as a lockin amplifier.
This can not be obtained in a real metal detector, but designers should strive to be close to the ideal case. In real metal detectors the gain of preamp is not limited by termal noise, but by parasitic signals in input. These are two parasitic signals: a direct transfer of energy from TX coil to RX coil and the ground signal. They reduce the modulation index of target signal.
5. Coil configuration. The worst sensor for metal detection is monocoil, because with it, the mentioned two signals are strongest. With monocoil, the target signal attenuates by 6th power of depth.
If you want to discover larger objects at greater depth, my advice is to make sensor type "perpendicular coils" (Depth Multiplier). U.S. traders and unkle Garrett incorrectly call this sensors TWO BOX, but this term means quite different sensor system where the mutual inductance between the TX and RX coils inevitably vary during operation. In Depth Multiplier two coils are fixed in two opposite end of carrier rod and their mutual inductance is set at a minimum. This allows us to increase the gain of the preamp.

Hi Mikebg,

a friend from the forum has asked me if the TINKERERS-V1
http://www.geotech1.com/forums/forumdisplay.php?f=53
could be adapted the use Depth Multiplier Coils.
I have never worked with this type of coil before, so I would much appreciate if you could give me some advice of how to implement that.

All the best

Tinkerer

[Agraz]

Hello Geo:

You are OK, but if you want ask to my friend Gary Chemelec, in Canada, him build many experiments in PI, find the page in web. and send one e mail. he is good person.
www3.telus.net/chemelec

[Geo]

Hello Geo:

You are OK, but if you want ask to my friend Gary Chemelec, in Canada, him build many experiments in PI, find the page in web. and send one e mail. he is good person.
www3.telus.net/chemelec

Hi Agraz.
In this forum, all we know Mr Gary, years ago..
But i wanted an answer from a member that knows about the current of the coil so to determine the wire diameter