Wet grass Chattering with VLF

[Henrikras]

Hello all.
Is there anyone who has knowledge or a good idea, what to do with a Stock coil, with some water/wet grass chatterings problem..? Its one of those Very sensitive deep detectors from the eastern countries 18 khz.
Not all owner got the problem or noticed it, but some had. They pretty much had to lower the sencitivity adjustment (properly GAIN of the return signal), to get it calm. Sheilding problems I would say, but how and why, and can it Be Helped?

Thanks.

Henrik

[Koala]

Take a look at the IDX long thread. Been talked about many times. As far as I know there is no fix. Most single frequency VLF detectors suffer to some degree with wet grass.

[Davor]

That would depend on the coil configuration. In case Rx coil leads are connected in balanced configuration (not likely) it then could be significantly improved by a balanced Rx input. A transformer or a balanced preamp.
If it is unbalanced, and the shield is connected to one of the coil's ends (more likely) it could be helped only by improving the coil's shielding, but that's most likely not feasible.

In any case, for an amateur it is very advisable to connect Rx coil as balanced regardless of the shielding, as such decision lets you decide later on what configuration you wish to have. You can as well connect a shield and one of the coil's ends in a connector, and also change your mind at any moment. Doing the same in the potted coil body is not at all practical.

[Qiaozhi]

Hello all.
Is there anyone who has knowledge or a good idea, what to do with a Stock coil, with some water/wet grass chatterings problem..? Its one of those Very sensitive deep detectors from the eastern countries 18 khz.
Not all owner got the problem or noticed it, but some had. They pretty much had to lower the sencitivity adjustment (properly GAIN of the return signal), to get it calm. Sheilding problems I would say, but how and why, and can it Be Helped?

Thanks.

Henrik

It's probably a combination of poor electrostatic shielding and too much gain.

[Henrikras]

Thanks Davor and Qiaozhi.
The searchcoil got a preamp inside the searchcoil.. I'm thinking that Qiaozhi said it, but when you have to lower sens. much, just because the grass got wet after som rain, it say to me that something should Be done. (Buy another Detector or coil, stupid to waist time searching with Very low sens. / depth, or come back at a sunny day).
What can I do to find out the configuration / balanced or not, I dont got the diagram.
Can you recommend something (else that give up), Ive be so happy.
Davor.. Not sure that I did understand everything you said.. Sorry If I lost something.

Henrik

[Qiaozhi]

Can you recommend something (else that give up), Ive be so happy.

What is the make and model of the detector?

[Silver Dollar]

If you can get a good one and bad one together switch coils and see if the problem
remains with one coil.

[golfnut]

Ive never owned a commercial machine. Are there machines on the market with single frequency that are not affected by wet grass?

I don't hear GMP, or Deus owners complain these are 18 and 17k models . Ive seen chatter from Tesoro kit on wet grass.


I guess, as these machines respond to phase changes, it could be....

Tx signal phase drift wrt to Rx tank

Rx " " " " " Tx

Rx signal phases from two sources - soil and wet grass ( ie can only cancel the soil one )


Anybody scoped X,Y on wet grass to see what happens?

[Henrikras]

I was Maybee a little naive, hoped that somebody had said.. Spray the coil with graphite and make good connection to the grounding in the coil cable... And finish with a solid spray clear lacquer or spray paint.. But I can see ist no so easy, and it properly relies in what kind of chatter the wet grass is causing. I had only seen or heard this kind of problems with high resonans VLF detectors with a sensitive fine tune GB adjustment and never on those turn "on" and go detectors.
Not long ago the Deeptech Vista Gold (25khz) had a production with a eliptical coil with wet grass problems. I and other owners did get a new coil Very Quick free of charge. The brand and model in this thread is the Nokta Velox one (17khz), and this isn't the same Big problem like the Vista gold.. Properly two different kind of causing. !

[Qiaozhi]

The problem of signals from wet grass is usually the result of either poor coil shielding, or (if it was working ok previously) that the shield connection has come loose. You can also get this problem in some poorly designed detectors with excessive gain to make them perform well in an air test.

[Davor]

Davor.. Not sure that I did understand everything you said.. Sorry If I lost something.

If your preamp is inside a coil, there is nothing you can do, unless you can get inside.
I still don't understand if this is a new problem, or it happens from the beginning. Some detectors are simply nervous.