just outa morbid curiosity...

[grungymike]

1) If a transmit coil was wound with the individual loops in parallel instead
of series, would this result in a faster rise and decay of the transmit
pulse? What effect would this have on the EMF?

2) if a transmit coil was wound with two wires wound in opposite
directions, interlaced, with inputs and outputs siamesed, would this
have the same effect as a bucking coil on a concentric or Tx+/Tx-
coils on a coaxial? Or would this kill the field?

3) Is a bucking coil necessary for a concentric PI design?

4) Two coils are wound, both with the same length and dia of wire. 2nd
coil is smaller dia with more turns. What differences exist in the
inductance/capacitance/resistance characteristics between the two
coils?

5) A wirewound coil, with it's roughly circular cross section,has a certain
shape to the EMF. Tesoro's Sandshark uses a spiral wound printed
coil, which has gotta have a different shape for the generated field.
How do the two designs react to ground effect? Are there any other
advatages/disadvantages between the two designs?

6) Bbsailor calls for 19 turns of #30 for his coil design for the
hammerhead. If the same coil was wound with fewer turns of a larger
dia wire, wouldn't this result in a quicker rise and decay of the xmt
pulse, due to lower resistance? How would this affect the capacitance
and induction characteristics?

7) I'm new at this stuff, so I saved this one for last. Is there really such
a thing as an induction meter???

[PVGOLD]

Hi Grungymike
I'm still at the shallow end with building coils, but I can tell you that
inductance meters are available on ebay for about 35.00-55.00.
Hope this helps a little.

Herb

[Carl-NC]

1) If a transmit coil was wound with the individual loops in parallel instead
of series, would this result in a faster rise and decay of the transmit
pulse? What effect would this have on the EMF?

Theoretically I think you can do this, but no one does, and there must be a practical reason.

2) if a transmit coil was wound with two wires wound in opposite
directions, interlaced, with inputs and outputs siamesed, would this
have the same effect as a bucking coil on a concentric or Tx+/Tx-
coils on a coaxial? Or would this kill the field?

I'm not sure I understand what you're saying, but it sounds like you will get field cancellation.

3) Is a bucking coil necessary for a concentric PI design?

No.

4) Two coils are wound, both with the same length and dia of wire. 2nd
coil is smaller dia with more turns. What differences exist in the
inductance/capacitance/resistance characteristics between the two
coils?

Same inductance, same DC resistance, smaller coil has higher self-capacitance.

5) A wirewound coil, with it's roughly circular cross section,has a certain
shape to the EMF. Tesoro's Sandshark uses a spiral wound printed
coil, which has gotta have a different shape for the generated field.
How do the two designs react to ground effect? Are there any other
advatages/disadvantages between the two designs?

Spiral coil will have a more disperse field and slightly less sensitivity.

6) Bbsailor calls for 19 turns of #30 for his coil design for the
hammerhead. If the same coil was wound with fewer turns of a larger
dia wire, wouldn't this result in a quicker rise and decay of the xmt
pulse, due to lower resistance? How would this affect the capacitance
and induction characteristics?

Keep the turns the same for same inductance, larger gauge wire will improve turn-on rise depending on other circuit factors, but no effect on turn-off. Larger gauge probably increases capacitance.

7) I'm new at this stuff, so I saved this one for last. Is there really such
a thing as an induction meter???

Inductance meter? Yes. Field meter? I suppose, but I've never looked into them.

Good questions...

- Carl

[grungymike]

I imagine some of these questions are pretty ignorant, and a couple got answered when I read the article on concentric coils, thanks for your patience...

[simonbaker]

4) Two coils are wound, both with the same length and dia of wire. 2nd
coil is smaller dia with more turns. What differences exist in the
inductance/capacitance/resistance characteristics between the two
coils?

Same inductance, same DC resistance, smaller coil has higher self-capacitance.

- Carl

Same inductance? Surprising to me. I tried with Qiaozhi calculator and it was very different, but I may have made mistake.

Regards,

-SB

[WM6]

Same inductance? Surprising to me. I tried with Qiaozhi calculator and it was very different, but I may have made mistake.

Regards,

-SB

Same inductance claim can be correct, but not at the same frequency.

[Qiaozhi]

Same inductance, same DC resistance, smaller coil has higher self-capacitance.

Same inductance? Surprising to me. I tried with Qiaozhi calculator and it was very different, but I may have made mistake.

Personally I would expect the inductance to be higher, and the DC resistance to be the same. I would think that since there are more overlapping windings the capacitance would also be higher.

I tested my assumptions with the coil calculator. By starting with a coil of 5" (127mm) radius I calculated the length of wire for 100 turns, and found (for 0.25mm wire) an inductance of 7.8mH. Using the same length of wire I calculated the inductance for a 3" (76.2mm) radius coil, which turned out to be 11mH.

[Carl-NC]

Same inductance? Surprising to me. I tried with Qiaozhi calculator and it was very different, but I may have made mistake.

I admit I did this in my head and for a solenoidal coil, where

L = u*N^2*A/l

And I kept l (length) equal. For a typical detector style coil, I agree L will vary somewhat. Sorry, I was doing a fast reply and did not do a thorough job.

- Carl

[foxyrick]

Thanks for asking those questions Grungymike, and thanks for your answers Carl.

I hadn't realised that a spiral coil would be less sensative, although I suppose it makes sense now I think about it.

I'm planning to build a Goldscan 4, and am also playing with my own design ideas at the same time. The last thing I want is to lose sensitivity for the sake of saving a few pF. You stopped me wasting my time winding a difficult coil!

Good timing too - I was sat here half an hour ago wondering why I hadn't seen so many spiral coils if they were simply better.

I've just realised this is my first post here... hello everyone!

Rick.

[Tilladude]

I'm a little late to the party but you know the old saying so I won't bore you.

As to question #1 like Carl said you could do that but the result wouldn't be usable. The reason being a coil with virtually no inductance because inductors in parallel are like resistors in parallel. in other words the resulting "L" would be less than the "L" of a single loop which would be next to nothing.

2 thru 6 are beyond my meager knowledge of detector coils.

I have a nice little Tenma C, R, L meter, it uses 1khz to determine the "L" of a coil.

Dennis