DD Coil construction queries

[raygdunn]

Hi all,

I'm working on coils for the TGSL-EDU. My first attempt seemed to show the electronics are basically OK, but there is only about a four inch range! I discovered that by the time I'd tightened the coils with wrappings and hot gluing on the housing, I had ended up with two very close inductances. The Tx was a little high, but scope measurements indicated the frequency was about right. So I'm winding another receiver coil.

Queries:

1. Why include a screen on a Tx only coil? (I've not spotted anything to the contrary) Surely it's only the Rx coil signal that gets amplified. It feels like it could be counter productive to the transmission.

2. How critical is the value of the inductance of the receiver coil in the TGSL? I noticed a reference to tuning the receiver circuit frequency by adjusting the capacitor part of the tuned circuit.

3. Starting my second attempt, I tried some plumber's joint tape. Being very thin and slightly elastic this produces a nice tight neat wound finish. The catch is that the inductance went up by about 70%! This starting with a coil tied tightly with button thread every couple of inches. Apart for the obvious failure on my part, is it actually bad to wind the coil too tightly? This results in one reducing the receiver coil length, which I presume reduces sensitivity. Is a loose (but rigid) coil better? Is this why embedding the coil in epoxy or similar is usual?

Ray

[kt315]

4. if vlf coil is basically a trasformer with poor link why i must consider the coefficient of transformation.

[kt315]

[Skippy]

One advantage of binding the coil tight is it makes it almost rigid, so it's not going to go out of shape over time, or indeed whilst you're making the coil. And it will also be resistant to false-signalling if you bump it against a rock/tree etc.
Another advantage of a tightly-bound coil is consistency. Another coil built the same way should end up with very similar inductance. Wheras if there was this random element of 'how tight', you might make another coil the same that's a failure.

[homefire]

Shielding on the Tx coil helps with EMI issues. The EMI Carried by the Tx signal can be a issue. Both should be shielded.

[raygdunn]

DD Coil construction queries

Hi Guys

Thanks for the replies.

I tried some plumber's joint tape. Being very thin and slightly elastic this produces a nice tight neat wound finish. The catch is that the inductance went up by about 70%!

Plumber's joint tape is not a material I'd now recommend. The reduction in tension and inductance by the following day had me doubting the accuracy of my previous meter readings! I can see now that there is a general problem with wrappings that might sag later altering the inductance value.... I see now why "the book" recommends duck tape for the outer wrap.☺

Ray

[homefire]

This is the ticket. http://www.banggood.com/search/condu...pper-tape.html

[Sean_Goddard]

I prefer aluminium tape for faster coils.

[homefire]

What is the Advantage of Aluminum ? Magnetic Permeability ?

[Koala]

Hi all,

I'm working on coils for the TGSL-EDU. My first attempt seemed to show the electronics are basically OK, but there is only about a four inch range! I discovered that by the time I'd tightened the coils with wrappings and hot gluing on the housing, I had ended up with two very close inductances. The Tx was a little high, but scope measurements indicated the frequency was about right. So I'm winding another receiver coil.

Queries:

1. Why include a screen on a Tx only coil? (I've not spotted anything to the contrary) Surely it's only the Rx coil signal that gets amplified. It feels like it could be counter productive to the transmission.

2. How critical is the value of the inductance of the receiver coil in the TGSL? I noticed a reference to tuning the receiver circuit frequency by adjusting the capacitor part of the tuned circuit.

3. Starting my second attempt, I tried some plumber's joint tape. Being very thin and slightly elastic this produces a nice tight neat wound finish. The catch is that the inductance went up by about 70%! This starting with a coil tied tightly with button thread every couple of inches. Apart for the obvious failure on my part, is it actually bad to wind the coil too tightly? This results in one reducing the receiver coil length, which I presume reduces sensitivity. Is a loose (but rigid) coil better? Is this why embedding the coil in epoxy or similar is usual?

Ray

1 works better. Tried just the Rx results weren't good.
2 there is leeway. However too far out it will not ground balance and the discrimination will be off.
3 more efficient. Loosely wound you would need more wire for the same inductance and more glue or resin or what ever you use to stop the moving when you bump the coil.


The winding really need to be glued together as well as binding otherwise the false signals will drive you round the bend

[raygdunn]

Thanks for that feedback Koala.

Metal detector construction is an amazing collection of limits and compromises.

The DD is much more of a challenge than the simple coil of 12V thick insulated wire I used on my first PI detector. So for all this one gets less depth, but hopefully much less of the attached☺

Thinking along "The Detectorists" series lines, perhaps I should make a collection "Horseshoes through the ages". A steep field I'm currently working has very much less iron.... too steep for a regular tractor.