Chance PI coil...

[6666]

I wound the wire onto a spool made of scrap MDF to make it easy to move the wire in and out of the former.

[6666]

After one turn.

[6666]

Now here is the trick to making this coil, after 5 turns the wire will fill the same slot from where you started, move the wire one slot and continue winding another 5 turns and then move the wire one slot and continue , you should not have any parallel wires, and the wire should stack up in the slots.

[6666]

I finished the last few windings in white wire so you can see the pattern.

[6666]

When completed 33 turns gave me 320uH.

[baum7154]

Good demo!

[kt315]

Now here is the trick to making this coil, after 5 turns the wire will fill the same slot from where you started, move the wire one slot and continue winding another 5 turns and then move the wire one slot and continue , you should not have any parallel wires, and the wire should stack up in the slots.

its called 'transfer on following/another layer'

[6666]

Interesting useing an un calibrated MPP, this spider coil does not detect my hand if I fully grip the coil, and I can belt it pretty hard and no falseing, but it does detect a 1/2 red house brick, but that has to be fairly close, so a bit more testing tomorrow in my beach sand box.

[baum7154]

Interesting useing an un calibrated MPP, this spider coil does not detect my hand if I fully grip the coil, and I can belt it pretty hard and no falseing, but it does detect a 1/2 red house brick, but that has to be fairly close, so a bit more testing tomorrow in my beach sand box.

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Have you tested the Self Resonant Frequency and set tha Critical Damping yet? If so what values?

Regards,

Dan

[6666]

According to the very broad scale of my GDO the SRF with out any feedline is about 1.4 MHZ, which is higher than my bunched coils made from solid copper wire
I went to calibrate the delay pot on my MPP last night and discovered a fault with the delay circuitry, as I cannot alter the sample delay so got to get that working and then I can get things like the damping right.

[baum7154]

According to the very broad scale of my GDO the SRF with out any feedline is about 1.4 MHZ, which is higher than my bunched coils made from solid copper wire
I went to calibrate the delay pot on my MPP last night and discovered a fault with the delay circuitry, as I cannot alter the sample delay so got to get that working and then I can get things like the damping right.

—________________________

Your 1.4 MHz is right in the ball park for a 320uh spider coil of this type without the added capacitance of the feed line. With a 33" feed line and the capacitance of your detector your damping should fall between 1k to 1.15k ohms. Are you using the high speed diode in series between the Tx and the coil? If so that damping range should be correct.

Dan

[6666]

Thanks Dan, the fault with the MPP has slowed me down , but I have a surf and a barra here , no HIGH SPEED diode on any MD so far, but I do have some to try.

[baum7154]

Thanks Dan, the fault with the MPP has slowed me down , but I have a surf and a barra here , no HIGH SPEED diode on any MD so far, but I do have some to try.

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I highly recommend the series diode as its low capacitance in series with the high capacitance of the TX mosfet will significantly lower the capacitance contributed to the coil by the detector electronics and speed up the coil performance. This same technique is used in the Chance PI detector to improve performance.

Good Luck,

Dan

[baum7154]

Looking at the data sheet for the HER208 diode it shows about 48pf of capacitance for a .1volt reverse voltage on the diode. I wonder if 2 HER208 diodes in series would drop that about in half to 24pf and give even faster coil performance, but at what other performance penalty? Maybe the 0.6 volt additional forward voltage drop for the 2nd diode isn't a good tradeoff.

Just a thought

Dan

[Davor]

Actually, the diode will not get anywhere near 0V during Rx sampling period, and it is because drain capacitance keeps ample charge for quite some time. Trouble is in discharging that happens in exponential fashion, and thus it interferes with targets reception. It can be fixed by clamping a drain to a fixed potential via a resistor or otherwise.
Few MOSFETs have capacitances modelled correctly, so this mechanism seldom appears in simulation.