Tx and Rx coil inductance ratio

[Carl-NC]

Good article, but a lot gets lost in translation.

In thinking of the coil as part of a transformer (actually, a double transformer), maximum transformer "gain" seems to be the obvious answer. Some of the old White's coils were as high as 8:1, and many other coils are 3:1 or higher. Yet a lot of Tesoro coils are approx 1:1, and Minelab BBS/FBS is less than 1:1, and no one is complaining they lack depth.

The reality of transformer ratio is that the "gain" you get applies equally to targets and ground. That is, a 3:1 coil gives you a 3x signal for targets over a 1:1 coil, but also a 3x signal for ground. Although external EMI isn't affected by the transmit side, a higher RX turns also couples in proportionally higher EMI levels. So as far as incoming SNR, transformer gain is a wash.

When I was at White's I did an experiment. The V3 coil has TX=540uH and RX=14.6mH (relying on memory here!) which is ~ 5:1 ratio. I replaced it with a Minelab Explorer coil which has TX=580uH and RX=470uH; the TX-L was close enough to the White's TX-L that no changes were needed on the TX side; on the RX side I bumped up the preamp gain by 5x. And because the V3 has all-electronic calibration, re-calibrating to the Minelab coil was simply the push of a button. The results: exactly the same performance, I could not (1st order) tell a difference.

High transformer gain does help in one respect: in the V3 example I had to increase the preamp gain to compensate for the lower transformer gain. This will increase thermal noise. However, the lower RX winding also has less resistance, which helps thermal noise. But, in general, higher transformer gain does slightly benefit thermal noise.

My own personal preference is toward a low transformer gain, between 1:1 and 2:1, especially for multifrequency designs. MF requires a wideband RX coil and that gets really difficult in a hurry for a high-turn coil because for self-capacitance. For single frequency, I would still stick with a low transformer gain because, for a production detector, repeatability is much easier. Also, low transformer gain coils have less copper and are lighter.

[Nandor]

interesting article!

[Altra]

Kev, here's some more bits and pieces. George Payne discussing some thoughts.

http://cleotusrus.tripod.com/Baron/gb.htm

[Kev]

Thanks Guys for posting this.
It is a bit hard to follow the translation, which is a shame since Kostin has done lots of experimentation and thinking.
I apreciate what it's like for others here whose mother tongue isn't English.

Anyway, is the gist of what he's saying something like - a front-end fully resonant to the fundamental highly sensitive but impractical in the field, but an rx coil resonant to the first upper or lower tx harmonic optimal?

Kev.

[Polymer]

Thanks KT

See attached pdf in english

Thanks KT and Altra for posting the translation. I really enjoyed reading it.

[Davor]

Easy: just see what is on offer at fleabay, and adopt it for your project.
30 years ago you couldn't afford a low noise front end to use low impedance coils, so the coils were not low impedance. Now you can.

[Nandor]

After reading the article there is one question that bugs me, how to decide the coils inductance?

[Altra]

Thanks KT

See attached pdf in english

[Nandor]

Try the pop-up window, mine has one. I click on ''translate'' and it appears in hungarian. I just have to scroll a bit slower so the text gets translated.
This article is very interesting!

[Nandor]

Very interesting article, and has many good points. It even speaks about coils I have at home. The phase relation part is the most intersting I think.
Now I understand why Tesoro is less more sensitive than Whites with the same filter gain.

thank you!

[6666]

Thank you very much, gugel translate works okay!!!

Ok glad you got it to translate, I was trying via google Docs but the process was only working 50%

[kt315]

our big and absolutely insoluble problem is people are operating by 'Ohms and L' and do not wish to operate by the transform ratio.
so we still do not have full data on all coils. people do not wish to learn what is transform ratio.
they just want to get '100 turns for TX and 250 turns for RX' (for example) level knowledge and nothing more.

[Nandor]

Thank you very much, gugel translate works okay!!!

[6666]

Thanks KT, I can see all the content as htm page, but I can only translate half of it but no graphics, I will have another look at it tomorrow

[kt315]

ok. try.