choice for a 18 khz

[lamoiz]

You say you can calculate Tx to run at 18Khz "no problem"? But you can't calculate Rx mods? That's odd.

that's right ... but can you give me the solution? I do not think so .

[blobber]

IDX original coil calculation is approximately:
TX 0.6mH/1uF/6.497kHz
RX 35mH/33nF/4.683kHz

So I would try this:
TX 0.6mH/122nF/18.602kHz
RX 35mH/2.7nF/16.372kHz
I am sure, that this will work, but not sure about performance, phase..etc.

[lamoiz]

IDX original coil calculation is approximately:
TX 0.6mH/1uF/6.497kHz
RX 35mH/33nF/4.683kHz

So I would try this:
TX 0.6mH/122nF/18.602kHz
RX 35mH/2.7nF/16.372kHz
I am sure, that this will work, but not sure about performance, phase..etc.

thank you blobber yes for my idx these are the values that I use.
tx 0.583mh / 1uf / 5.59khz and rx 36.5mh / 33nf / 5.58khz ...
why is there such a big difference between 18.6 and 16.3 khz? There is something that I do not understand in the calculation and realization of a dd coil ... if you want to explain me it would be really nice .. thank you.

[lamoiz]

oupss rx..4.58khz no 5.58....

[lamoiz]

..6.59 tx ...

[blobber]

Yes, now your values are correct, you must have a difference in RX calculation, of about 2 kHz lower, than your TX frequency.

[blobber]

In your case you should try 133nF in TX and 2.7nF in RX tank. That will match better the 2kHz diference.

[lamoiz]

thank you blobber, I will try immediately on an idx test coil.
can you explain to me why it takes a difference of 2 khz between rx and tx? This question is a bit off topic, but I never understood the why not 1khz or 3 or 5 or 10 ..... is this related to the working frequency of the preamp? 2khz for idx? For example?

[blobber]

That is a good question, but unfortunately I can not explain the reason for the difference, maybe some experts here can do this?

I have only noticed, that many VLF detectors are working with this divergence, like IDX, Terminator, SMW, Krot, Fortuna, Quasar and I wonder, why TGSL and Bandido have it to the oposite way. There is TX lower than RX.

[waltr]

VLF chapter of "Inside the Metal Detector".

[lamoiz]

thank you blobber for your answer. I ask myself the same question .. why some detectors work with this shift and others not .... but the great specialists of this forum certainly have the answer, dbanner, waltr .... those who can read george overton ..... inside metal ..... it's so easy to send us back to this book ... without telling us the reason ..... I will continue my experimentation and also learn to read.

[blobber]

Yes lamoiz how pity, an answer should be easy possible, but maybe some people are involved te earn some profit in selling this basic book.
Anyway such answer confirm in my opinion, that you are on the right way.

[lamoiz]

then, test result ..
10cm on a mini piece of 24carat gold ... against 2 cm on a tesoro cibola 14khz .... it starts really well !!

[blobber]

That seems good!

[blobber]

This small piece of gold was not detectable with IDX, in original coil setup right?