Building better coils

[Turjake]

Hi!

Let's try to demystify coil making by sharing our coil data (coil size, wire diameter, shielding material, inductance, resistance, frequencies, peak voltage, phase information). I'll start by posting data on 11" tesoro widescan DD and
adequately performing homemade 14" Double D. I'm hoping for lot of pictures and also post your failures, maybe we'll eventually see what works and what doesn't. General tips and tricks also welcome. Here's some stuff:

My trusty IGSL:

Test setup for 14" homemade coil:

Coil comparison:

Oscilloscope screens:

TX vs RX preamplifier output

Observations I made:
- Although three inches bigger, my coil performance is only marginally better than the tesoro coil. I have no idea why. At least I'll cover more ground.
- Tried different nulling voltages: measured from rx out, best performance was found at 200-300mV, 400-500mV was good with only about one or two cm loss of depth, at 600 mV significant loss of depth. Below 200mV the phase difference
starts to rapidly grow: at 180mV the phase difference was 70 degrees with bad sensitivity.
- Earlier made TX coil with 0,2 mm wire and over 30 ohms was bad.
- Too low TX resistance resulted in imperfect sine wave as seen on scope picture. Cured with 10 ohm resistor.
- Ground balancing can take off 5cm of maximum depth
- It's very hard to desolder components from silverdog boards
- Shielding didn't noticeably affect performance. Tested spray graphite and aluminized tape. I remember I had reduced sensitivity in past with aluminum foil tape.
- Air testing is inaccurate. Results vary wildly with groundbalance settings. I used only solid beeps on every swipe.
- Owners of concrete floor BEWARE! Had to raise coil 1.5 metres off the floor because of the steel reinforcements. Recommend watching amplitude on scope and moving your coil around.
- Making a very hot coil remains a mystery. I will throw some tuning capacitors and resistors at coil and will post if I find the critical parameter.

[WM6]

Air tests worth nothing in real world.

Rethink about better (computerized) digging tool.

[Skippy]

Quote:"Although 3" bigger, my coil performance is only marginally better than the Tesoro coil. I have no idea why"
If you're talking about real world targets, in simple terms, it picks up (say) 3 times more target signal, and also nearly 3 times more ground signal. So your detector still can't distinguish between the blip of a real target and the continuous wobbling up and down of the ground signal.
If you're referring to your air test table, you need to know that bigger coils work best for bigger targets, and vice versa. So your smallest targets, like the U.K 1 pence coin, is of a size that a 12,13,14 inch coil is best. A 15 inch coil would lose depth. If you were to try your tests with a small target, like a 0.22 airgun pellet, or a Medieval cut quarter farthing coin, you would find the big coil significantly worse than the stock coil.

[Koala]

"- Tried different nulling voltages: measured from rx out, best performance was found at 200-300mV, 400-500mV was good with only about one or two cm loss of depth, at 600 mV significant loss of depth. Below 200mV the phase difference"

Not built the Tesoro coil. But I am building a IDX coil at the moment. I found that the phase did not alter much +- 200mV from the center null. Best results were with the smallest null and then the phase fetched back correct by altering the RX cap. You can go beyond where the factory coil phase is and the depth will be more. You will end up with a unstable and unusable coil in the real word.

I built a 10" DD and performance is twice that of a factory CC 8" on a silver cut half.

[specialstar]

thk you for the infos of coils