Magnet wire coil

[Chet]

I'm going to guess it's because the coil with fewer turns is bifilar. Starts out with two wires twisted together and then wound, then connected tin series. More capacitance than the other coil where each turn just sees the turn before and after. To me it looks like a spiral coil wound spiderweb. I expected more capacitance with the bifilar coil, but wondered how much less noise it would have. I couldn't see any advantage other than noise. Maybe there is?

Hi green
Thank you for the explanation. I think the higher self resonant frequency is more important.
Chet

[green]

Comparison of 3,4,6,and 8 inch ID spiderweb coils. Not all had the same inductance, so I tried to correct for it. Formula on chart. Still not stable enough for a good reading at 1 uv. Trend should be ok.

[Chet]

Hi Green,

Of the coils tested, looks like at depths of 5 or more inches the 8" coil is the coil of choice.

Thank you,
Chet

[green]

Made a DOD coil, scaled from a photo in the forum. O inside length is 8 inches. D is 330 uh each, O is 340 uh. Don't know what to expect. With the 2 D coils in series (Rx) same polarity, signal strength about the same as a 8 inch ID mono coil, high in the center. With one inverted about 1/2 the signal strength, null in the center. I had a hard time getting the tooth picks out of the foam this time, same glue different can. If you try to follow my instructions(Big foot style for PI) I would coat the tooth picks with something, maybe wax.

[Chet]

Hi Green,

Great looking coil. Did you null balance each receiver coil with the transmit coil?

Thank you,
Chet

[green]

Hi Green,

Great looking coil. Did you null balance each receiver coil with the transmit coil?

Thank you,
Chet

Don't know if correct. Connected Tx(O) and one Rx(D). Balanced for zero volts when coil on. Connected 2nd Rx(D) in series and positioned it for zero volts when coil on.

[Chet]

Hi Green,


That nulling procedure should work. With the receiver coils nulled they should not load or slow down the transmit coil. I think you could make up the reduced target voltage by increasing the number of turns in the receiver coils by a factor of 2 to 4. Also I don’t think the receiver coils need to be spider wound or fast coils.


Keep up the good work,
Chet

[green]

Hi Green,


That nulling procedure should work. With the receiver coils nulled they should not load or slow down the transmit coil. I think you could make up the reduced target voltage by increasing the number of turns in the receiver coils by a factor of 2 to 4. Also I don’t think the receiver coils need to be spider wound or fast coils.


Keep up the good work,
Chet

Hi chet, What you say makes sense. I can null the voltage with coil on but I always get a signal when the coil turns off. Tx 340uh, 2.23 Mhz resonance. Rx 329uh, 2.5 Mhz resonance each. Including scope traces with Tx and only one Rx. Little change with amplifier out zero and +- 1 volt. 1k damping resistor across Tx. 1k damping resistor across Rx, one side to common, the other 1k in series to + input with back to back 1N4148 diodes to ground. Turn off current discharge rate over 100 times charge rate, but it still seems like it should null. Anyone know what I'm doing wrong?

[Chet]

Hi Green,

I suggest that you try a lower damping resistor on the receiver coil. There may be some kind of receiver coil and op amp oscillation trying to start up?

Have a good day,
Chet

[green]

Hi Green,

I suggest that you try a lower damping resistor on the receiver coil. There may be some kind of receiver coil and op amp oscillation trying to start up?

Have a good day,
Chet

Hi Chet, Thanks for the reply. Tried 500 ohms, took about 1usec longer, then I tried something should have tried a long time ago. Shorted the coil wires, still had the +- full scale with faster decay to zero. Need to work on circuit and power supply.

[green]

Made a DOD coil, scaled from a photo in the forum. O inside length is 8 inches. D is 330 uh each, O is 340 uh. Don't know what to expect. With the 2 D coils in series (Rx) same polarity, signal strength about the same as a 8 inch ID mono coil, high in the center. With one inverted about 1/2 the signal strength, null in the center. I had a hard time getting the tooth picks out of the foam this time, same glue different can. If you try to follow my instructions(Big foot style for PI) I would coat the tooth picks with something, maybe wax.

Is the commercial coil wired adding or subtracting?

[Chet]

Hi Green


The receiver coils should be opposing. Shallow targets will experience a null at the center of the transmit coil. They will be detected near one of the windings. At deeper depths the targets will be detected in the center of the transmit coil. More information is attached.


Have a good day,
Chet


http://www.geotech1.com/forums/showt...154#post128154


http://www.geotech1.com/forums/showt...179#post128179

[6666]

The improvement gif needs Mikebg's explanation

"An improvement can be made by adding second TX winding in opposite direction as shown in the right side drawing."

[green]

Hi Green


The receiver coils should be opposing. Shallow targets will experience a null at the center of the transmit coil. They will be detected near one of the windings. At deeper depths the targets will be detected in the center of the transmit coil. More information is attached.


Have a good day,
Chet


http://www.geotech1.com/forums/showt...154#post128154


http://www.geotech1.com/forums/showt...179#post128179

Hi Chet, The inside length of the Tx coil is 8 inches. Monitoring integrator out, a nickel at 4 inches reads the highest at the crossover of Tx and Rx. At 8 inches the center of the Rx is a little higher than the crossover point. Null in the center of Tx at 4 and 8 inches. Maybe I'm doing something different.

[Chet]

Hi Green


I don’t have an explanation. Maybe it requires a differential receiver similar to the ones shown in some of the other threads.

When I get the time I will give it a go with a differential receiver. I will probably try twice as many turns in the receiver coils.


Have a good day,
Chet


PS: Check your PM