monocoil advice needed

[joop]

Hi have just finished building a monocoil using thin teflon wire. Inductance is 333 uH with self-resonance at 1.1 Mhz which is pretty good... However, ohmic resistance is 5.6 Ohms. I suspect this is a little to high. I intend to use it with a SURF-PI. Can anyone advice me on this? Thanks.

[ApBerg]

Hello Joop,

Think you used thin wire..well the current the surf takes will be low... look here http://www.geotech1.com/forums/showt...highlight=coax if you have some old coax..(as a radioman I think you have
And make some tests with coils like that..

Friend Nakky made a coil as on the photo..also verry good..

[WM6]

However, ohmic resistance is 5.6 Ohms.

.

Superb. Your battery will last twice as more.

[joop]

Thanks Ap! Yes, I used thin wire, the only available teflon wire I had.

It is just an excercise, my first mono coil. I want to use it with screening, so your coax coil is no option for me. Maybe I need to do some tests first, to see if it detects small stuff. I guess the higher resistance will affect depth?

[ApBerg]

Look again at the coax coil... it has a very good screening...

If you have a lot of thin wire then use 2 or 3 wires parallel (1) . ..works also good (if not better) then make it the Nakky style...use some doubleside stiking tape to keep things in place and graffiet spray to shield..
If ready ...fill the coil with to component pur foam as used in surf boards.
For small stuff, with fast sampling needed ,the best way to make a PI coil... ( I think..)

Succes

Ap

(1) so 2 or 3 wire's from around 18-20 meters parallel, at the ends connected so it become's one wire..

[joop]

okay, graphite spray for screening! Have to buy a can of that... What kind of coil form are you using?

But... first would like to try metal mesh tape screening. Have found some teflon mini-coax here which I plane to use as coil wire.

[maikl]

Surf PI does not have ground balance, so there is no real reason to be "fast coil"... because you'll detect the ground ...
Best regards.

[joop]

According to the document "Making a Fast Pulse Induction Mono Coil" by J.J.Rogowski:

Shielding the coil is required for delays below 30 uS. The shield does two primary things: It eliminates the coil responding to the ground at low delays when the coil is lowered to the ground and it protects the coil from picking up electrical noise.

Since I want to use this coil to find gold on the beach (e.g. gold chains and rings) I will need a small delay...

[maikl]

The shield is necessary under 30us but...
You then make a "fast coil" to delay below 15us... there is a small gold (necklaces, earrings...) and you will see that this is not helping Faraday shield the coil... only helps "ground balance".
Try it and you'll see...
Best regards.

[bbsailor]

Joop,The longer delays in the approximately 30us range were used with larger coils and lower frequency pulses. After about 25uS to 30uS depending on the ground type, the response of the ground is not detected at this leter delay. These types of PI machines were typically looking for larger objects. When you seek objects that decay faster such as small gold chains where each link is loosely connected to each other, the delay of the PI machine needs to be in the 10 to 15us delay range to detect these small targets with low response times. Try using various size of pieces of alunium foil such as, .5 inch square, 1 in square, 2 inches square up to about 5 inches square. As you raise the delay setting, you will only be able to detect the larger pieces of foil at the same fixed distance. This test demonstrates that lower delays are needed to detect smaller targets with shorter total decay times.On the dry upper beach you can detect with the delay at the lowest settings, but as you approach the wet saturated sand, you begin to detect the wet sand at settings at, near, or below 10 uS. By using a shielded coil, the capacitance of the coil remains fixed even when the coil is held in mid air or lowered to the ground. Unshielded coils can even detect you hand when brought close to the coil but a shielded coil does not sense your hand at all. Some highly mineralized soil can be detected with a shielded coil as the soil looks like one giant target. If you run into black sand on the beach, take a magnet and run it through the black sand and see if any of the black sand sticks to the magnet. This can be a problem so back off on the delay a bit to see if you can still detect desired targets but the sensitivity to very small gold will be reduced by this condition. Remember, the total TX coil current is determined by the total resistance of the TX circuit which included the MOSFET on-resistance, coil resistance, and cable coax resistance. The coil TX time constant (TC) will be the coil inductance divided by the total TX circuit resistance. In 1 TC the coil will raise to about 64% of maximum, in 2 TCs 85% and in 3 TCs about 95% of maximum. Example: a 300uH coil that has a 6 ohm total TX circuit fed by 12V DC will have a max current of 2 amps (12/6) so the TX pulse width tell you how much the current will rise. The TC of this coil is 300/6 or 50uS so a TX pulse width of 150us will be equal to three TCs and cause a coil current of about 95% of 2 amps. The more current that flows in the coil causes a higher flyback voltage and requres a smalller damping resistor which will sample a little later. Minimizing the coil-to-shield capacitance allows you to critically damp the coil with a higher value damping resistor and sample a little faster. Coil current, pulse width, TX pulse frequency and sampling delay are interrelated and requires a balancing act to optimize the PI machine parameters for the target types you are seeking. Read Carl's excellent Hammerhead PI assembly document to see how all this works.I hope this helps?bbsailor

[joop]

Thanks guys for explaining. There are many paramaters in coil design, even for a simple monocoil... I will eventually get a feel for it. Just need to build a couple of coils myself I guess...