problem tuning concentric coil

[Koala]

Having problem with a concentric build. 10" Tesoro Bandido

When nulled to 9mv depth is very poor

At around 24mv it sort of works okay. But very twitchy on ground balance. Using a 10 turn POT 1/4 a turn either side of ferrite is enough to loose a couple of inches on most targets.

[dfbowers]

Hi Koala.
Is everything OK if you substitute a known good coil?
Don..

[Koala]

Hi Koala.
Is everything OK if you substitute a known good coil?
Don..

Yes 5.75" works fine.


10" I have just made is useless. Spent quite a few hours trying to resurrect it.

[Sean_Goddard]

Check your screening is NOT a complete circle around the Rx coil. You should have AT LEAST a 1/8" GAP in it somewhere, run the ground wire from one side of this gap.

Also, check the frequencies each coil RESONATES at byt swapping the Rx and TX coils. It they are the same frequency, then you will get exactly the problem you describe.

[Koala]

Check your screening is NOT a complete circle around the Rx coil. You should have AT LEAST a 1/8" GAP in it somewhere, run the ground wire from one side of this gap.

Also, check the frequencies each coil RESONATES at byt swapping the Rx and TX coils. It they are the same frequency, then you will get exactly the problem you describe.

I have used a large cardboard disc covered in a graphite/PVA mix. Normally works fine on other coils.

Measured the inductance and it ground balances fine so guess the windings aren't to far out.



Update.


No idea what's going on. Gave up with this coil. Left it where it was. Been using my other coil. Thought I would given it another try. Just plugged it back in and its working great. Nothings moved even the cable is tapped to the side of the upturned bucket.

Getting 5.5" on a 10mm square from alloy can with clean tone. 6" with broken tone but regular two way signal.
8" on a cut half penny.

Large pre decimal coin is a little disappointing tops out at 11.5"

Seems I have reached the limit as an 8" coil gave very similar results.

[Koala]

Nope still no idea what's going on.


Same target blue tacked onto a piece of wood.

Sometimes you switch it on and you only get 7" with a hard cut off at maximum depth.

Try a few hours latter 10 inches with a soft cut off


Nothing moved but nearly 50% increase.



Could it be interference from a neighbour that affects the larger coil ???

[Koala]

Been tying to resurrect this coil.

Thought maybe it would be okay outside away from interference.

found two problems fixed one


If I pushed the coil into wet grass it was fine but swinging through the grass was very flaky.


Added a copper slug strip around the outside of the coil and connected it to the cable shield. Works perfect through the grass.



Second problem. Not fixed yet.


Ground balance on ferrite everything is fine. But there is an area about 1/2 a turn away where the distance drops by nearly 50%. But if I turn the discrimination up I get most of the lost distance back.

[sinclairuser]

hi koala, a tip i discovered with copper slug strip is, leave it hung up outside to oxidize for a couple of months for some reason it is more stable.
just clean a small patch to solder to.

[Koala]

Cheers.


I suspect the problem is where it ground balances. My other coil balances two turns further away from the dead spot so never see problem.


If I add a small capacitor across the Tx and the ground balance point alters but the dead zone stays still. If I can move phase enough the dead zone would be beyond normal daily adjustment range.


I have tried moving both a small ferrous and none ferrous around the coil still can't get enough phase movement

[Koala]

Tried again on this coil. Giving up and starting again.


when its work its working very well. Too good. Out performed a T2 quite easy.


Tried adding a small capacitor to the Rx or Tx.

Tried moving a small piece of ferrous/none ferrous or ferrite around the coil

Just can't get it to run stable and very dependent on ground balance. A few digress of a 10 pot and it changes from great to useless.


Also seems to struggle with a silver ring? But a small silver coin is fine