Multiple questions about coils

[Nozimo]

Hello everyone, I spent many hours reading through hundreds of posts on this forum and other information online regarding PI MDs. I?m new to PI detectors, but I?m not new to MD and have experience building RF devices.
My main interest is hunting for coins and jewelry on the beach. I don?t have any expectations, just want to have some fun building my own detector and hunting. But I also love planning and getting into a project knowing exactly what I?m doing and why.

I ordered an MPP Rev E board + components. I won?t do any mods at this time to keep things simple. Hence why a coil post.
I wanted to compile some info and do a sanity check with you regarding the coil. My plans are the following:

• Shape/type: mono round, to keep things simple.
• Diameter: about 9 in, I?m not interested in very small stuff like nuggets and this seems to be a common size for beach hunting.
• Winding: Basket weave/3DSS, this should be easy to make (I can 3D print it) and should give me a fast enough coil (low C) for beach hunting.
  Here?s doubt/question #1: I?m not sure about the ?self-shielding? part. On one side, I can see how that would work, the winding sort of wraps around itself. However, I didn?t find good consensus on this forum. Some found that no shielding worked just fine. Others found that all sorts of problems came up without shielding, from RF interference to ground proximity effects. I know that shielding will inevitably increase capacitance, slowing down the coil. However, the added shield capacitance may be negligible for beach hunting?
  Question #2: If the self-shielding effect is real, should I connect the ?self-shield? side (the outer turns) to the low impedance side of the board as done in radios and other RF equipment, so ground?
• Inductance: 400 uH. I noticed that most builds on this forum use 250-450 uH coils. The MPP build instructions recommend 300-500 uH. My understanding is that larger inductance will yield more sensitivity, but at the expense of coil speed due to an inevitably higher interwinding capacitance and larger coil TC. However, inductance increases faster than capacitance with additional turns (power of 2 instead vs roughly linear).
  So, question #3: Is there a reason for not going over 500 uH if a very fast coil is not a priority? Does it boil down to large coil TX TC requiring very long TX pulses?
• Wire: 22 AWG equivalent Litz. This is another puzzling point. My understanding is that Litz wire could help with the higher frequency components of the pulse by minimizing skin effect and thus reducing AC resistance. However, at the us level (so 100s kHz) the additional AC resistance in any coil wire is very small, often <1 ohm. I also read in a few posts that Litz wire should reduce eddy currents. I?ve seen plots shared by green (thank you!) where 24 AWG magnet wire appears to behave like a target for up to 2 us, while 28 AWG magnet wire almost doesn?t.
  Question #4: Litz wire appears to be the best choice, but maybe the benefits are minimal for long sample delays as in beach hunting?
• Feedline: I was thinking to use a continuation of the coil wire in a twisted pair as baum7154 and others did to keep capacitance low.

One more general question, #5: are there benefits in building a coil that is faster than needed? I read that beach hunting typically imposes sample delays of 15-20 us (I also read >12 us) due salt water. In other words, is there an advantage in building a coil that can recover in 10 us vs let?s say 20 us for beach hunting?

[Qiaozhi]

Q1) Personally I would not bother with basketweave/3DSS for use on the beach. These coils are complex solutions designed to reduce interwinding capacitance. Unless you are intending to search for gold nuggets I wouldn't waste time on this.

Q2) Some people say the 3DSS is self-shielding, and others do not. The problem with all spiral and basketweave types is that any additional shielding adds a huge amount of extra capacitance that can completely negate any potential benefits.

Q3) You can certainly go over 500uH if you want, but this may adversely affect how early you can sample, bearing in mind the sample time limitation imposed by the salty wet sand at the beach.

Q4) Litz wire is expensive and arguably not worth it for beach hunting.

Q5) The MPP does not include ground balance, so it would be pointless to build a complicated low capacitance coil that can sample below 10us.

As with a lot of things, you will come up against the law of diminishing returns. My advice would be to build a simple 300uH 9" diameter mono coil using hookup wire. The thicker insulation will help to reduce interwinding capacitance, and the dual-stage preamp of the MPP will allow you to sample early enough for salt to become a problem.

[Nozimo]

Thank you for the advice Qiaozhi!