Very large DD coil

[kt315]

i can show you a megastrike if you need. its not problem for me. i think you do not present what is electromagnetic field
and what is electrostatic in nature.

a detector coil is protected ONLY from electrostatic field influence and NOT from EMI.

https://www.youtube.com/watch?v=M4LiPvE9X1Q

[Davor]

Suppose you are right, and your coil is shielded, and you get the microstrikes you predicted. Those will ground into the shield and... will still give you noise because it is RFI.
In reality it is the field you worry about, and in that case balance works just fine.
Reality check: see about the professional microphones, and stage equipment, and be amazed how balanced arrangement works well for those.

Shield is only helpful against static fields and EMI with an unbalanced coil, and not at all against RFI. So any discharge will produce noise anyway.

[kt315]

any coil moving in electrostatic field around. a coil accumulates electrostatic charge on itself.
if you do not protect a coil you get microstrikes between enveronment and winding.
every strike will give a noise and impossibility to work.

any another opinion IS THE FAULT.

[Davor]

No, you don't. If you build your own detector and your own coil, it may as well be balanced, and in that case you don't need a shield.

The other schematic is better noise-wise, but VLF is also limited with front end distortion, and that circuit sucks at distortion. Sorry.

DFX coil must be terminated with 1k resistor, which is not optimal for squeezing the maximum off your coil.

[kt315]

I've seen a few photos of several different coils and I'm "a little bit surprised" with their shielding, because I've often seen windings without screening EMI (???) - Is it common practice?

we do NOT SAY about EMI screening. you mislead something.

yes, pulse induction coil may be without screening, but your... a VLF type coil... needs always in antistatic field protection.

[Krzychu_]

For example, would such a differential amplifier with a biffilar winding coil be more appropriate?

[Krzychu_]

The presented preamplifier is the input stage of the DFX detector designed by Whites and, if possible, I intended to use it without any changes. I am more interested in why it is not a good choice and how to redesign it to work together? The shielding I saw was done carelessly as a graphite layer painted on the inside of the case, only for the relative electrostatic charge compensation, not as an EMI screen, and as you can see in most solutions it probably works (?).

[Davor]

Your Rx preamp in a previous post is not a very good choice.
Regarding shielding, If you build a centre-tapped balanced coil for Rx, you'll be just fine without shielding, but your Rx front end must be able to accommodate such symmetrical arrangement, say, by configuring a preamp as an instrumentation amplifier. Tx coil is always shielded in commercial builds because those must go through EMI certification, and shielding is the way to pass without breaking any sweat. In your personal build you don't have to shield a Tx coil at all

[Krzychu_]

I've seen a few photos of several different coils and I'm "a little bit surprised" with their shielding, because I've often seen windings without screening EMI (???) - Is it common practice?

[Krzychu_]

Is it therefore possible to wind the receiving coil with a wire with a diameter of, for example, 0.22mm and obtain an inductance of around 100mH at a resistance of approximately 315Ohm may be the right solution? The input circuits look as follows:

How to choose the right inductance and RX coil resistance? What criteria and assumptions are important?

[Davor]

...or not. Basically you gain nothing with Rx coil resistance below about 50 ohm. It is because the input noise of 1nV/sqrt(Hz) is just as noisy as a 50 ohm resistor.

[Krzychu_]

The weight of the coil is not a limitation. It can be heavy because it will be on wheels. I intend to duplicate the transmit coil (TX) to preserve the "factory" parameters L = ~ 540uH and R = ~ 2,7Ohm. I hope that the 4 ... 5 section of such coils will work in parallel. I will use only a higher frequency - 15kHz. My RX DeTech 13 "DD coil has, for example, about RL = 45Ohm and L = 13.65mH, and Eclipse 6X10" DD RL = 60Ohm and L = 13.9mH, which means that the Q-factor should be larger for a larger coil, that is, the diameter of the wire should also be larger. The calculations show that such RX rectangular coil with dimensions of 20X200cm would have about 60 turns of wire. With a diameter of 0.5mm, its resistance would be around 24ohms, and at 0.3mm around 63ohms, so theoretically the variant with a thicker wire seems to be better ... (?)

[Davor]

There are 2 conflicting constraints with Rx coils of "normal", and 3 in multifrequency machines.
An Rx coil has its inductance and resistance. When you increase the number of turns by 2-fold, you get twice the resistance, and 4 times inductance (N^2), but also twice the voltage. Effectively you have voltage transformation by the square root of Rx to Tx coils inductances' ratio. Since the Rx input circuitry has a certain input noise equivalent resistance, and only the best of the best rigs have it down to 50ohm or 1nV/sqrt(Hz), it is opportune to raise the voltage and resistance up to the value close to that noise related resistance. However, as you go up with inductance, your coil's self resonance goes down, and there's your constraint.
The other constraint is a coil's weight. With a certain wire thickness your voltage to resistance ratio is constant, and it increases with wire thickness. However, pushing it to extremes would lead to a heavy coil. Reducing the coil's resistance below, say, 30 ohms is largely pointless because there are no front end circuits capable of exploiting lower input resistances noise-wise. If there is an imaginary Rx front end with input noise equivalent resistance at, say, 10 ohm, the best coil would have resistance at about 10 ohms.
The third constraint relevant for multifrequency is related to the need to maintain amplitude and phase across the used frequency range, against the voltage applied to the Tx coil. This has a lot to do with the Tx coil's Q factor on the lower bound and Rx coil's self resonance on the upper bound. Both have to be quite good.

[Krzychu_]

What about RX coil?
What determines the inductance and resistance of the receiving coil in multi-frequency detectors? In DFX the same non-resonant coil operates on 2 frequencies (3kHz and 15 kHz). Is the induced voltage the most important (or for example the Q-factor of the RX coil)?

[Davor]

Every coil can be scaled, but mechanical vibrations will create increasingly more of false signals with size. Tx coils of free running oscillators are critical for their Q factor, but with a thicker braided wire it should work fine.