the right factor ..?

[lamoiz]

Good morning all,
I wonder.
I have to make a new coil, example 16.5khz tx.
I have several solutions: 423uh / 220nf or 282uh / 330nf or 930uh / 100nf ...... here is the question, what is the determining factor? how to choose the right combination?
thank you all.

[waltr]

For which detector circuit??

sounds like a VLF detector and TX coil is part of the Oscillator tank.
Therefore the LC combination MUST allow the circuit to oscillate.
This depends on what type of oscillator circuit you are using.
Post schematic and then we will have a clue.

[Carl-NC]

For which detector circuit??

sounds like a VLF detector and TX coil is part of the Oscillator tank.
Therefore the LC combination MUST allow the circuit to oscillate.
This depends on what type of oscillator circuit you are using.
Post schematic and then we will have a clue.

His 3 examples all resonate at 16.5kHz. For the most part it doesn't matter what circuit he's using, so I think his question is: since all of these have the same resonance, should he use high L or low L?

For a fixed TX power supply a lower L produces a stronger TX field but probably at the expense of higher power consumption. You also want to keep the SRF of the coil about 10x higher than the TX freq so that limits how high you can make L. Personally I lean toward ~500uH as a good compromise TX inductance for sub-20kHz .

[Skippy]

Whatever combination you choose, I would suggest using a capacitance slightly higher than the stock E6 values you've proposed. For example 240 nF , which would ideally be 220 nF + 20 nF ( or 22 nF ). You then have the option of fine-tuning the capacitance to get the precise resonant frequency, eg. you could use 10 nF in place of 20 nF if your coil turned out with slightly higher inductance than you expected. And obviously, you can still add capacitance, just as you would've if only 220 nF total was chosen.

[lamoiz]

thank you Carl. you understood my question perfectly. I think that automatic translators have real problems with the subtleties of the French language.
thank you too skippy.
I will take your advice.but the srf at 165khz? Why ?
thank

[Carl-NC]

I will take your advice.but the srf at 165khz? Why ?

The coil's parasitic C is not necessarily stable, it can vary with temperature as the coil expands/contracts. If this is a major part of the overall C, then frequency will drift. It may or may not be a problem, but it's usually best to avoid it.

[lamoiz]

thank you Carl, I just understood a lot of things there.

[Sergey_P]

what is the determining factor?

calculate the coil (q-factor, ampere*turns), the capacitance determines only the frequency

[Sergey_P]

calculate the coil

https://my-files.su/zq7340

[profesor]

For which detector circuit??

sounds like a VLF detector and TX coil is part of the Oscillator tank.
Therefore the LC combination MUST allow the circuit to oscillate.
This depends on what type of oscillator circuit you are using.
Post schematic and then we will have a clue.

Thank you Carl.

I have a question. How do we create the ratio between tx to rx. sample whites for 6.59khz in coils.
tx 540uH 0.7mm cu.
rx 21mH 0.2mm cu.
how is this rate adjusted?

[profesor]

G?? kaynağı i?in daha d?ş?k g?? kaynağı, ancak muhtemelen daha y?ksek g?? t?ketimi pahasına. Ayrıca bobinin SRF'sini TX frekansından yaklaşık 10 kat daha y?ksek tutmak isteyebilirsiniz, b?ylece L'yi ne kadar y?ksek yapabileceğinizi sınırlar. Şahsen ben 20kHz altı i?in iyi bir uzlaşma TX end?ktansı olarak ~ 500uH'ye eğiliyorum.



I have a question. How do we create the ratio between tx to rx. sample whites for 6.59khz in coils.
tx 540uH 0.7mm cu.
rx 21mH 0.2mm cu.
how is this rate adjusted?

[Carl-NC]

Thank you Carl.

I have a question. How do we create the ratio between tx to rx. sample whites for 6.59khz in coils.
tx 540uH 0.7mm cu.
rx 21mH 0.2mm cu.
how is this rate adjusted?

If you are asking what is the best Rx/Tx turns ratio, then there is no clear answer. A higher ratio (such as the 38:1 ratio in the White's coil) gives you "free" transformer gain which seemingly should increase sensitivity. But it increases sensitivity equally for targets, ground, and EMI so it seems to be a wash. And a high ratio is also harder to achieve a good null point. A high inductance RX coil also has high parasitics which can be difficult to deal with, and more weight.

However, a low ratio (such as 1:1 in Minelab BBS/FBS coils) requires more preamp gain to compensate for lost transformer gain, and this usually results in a higher thermal noise.

Most of the designs I'm working on a wideband so I tend toward a low ratio like 4:1 or 2:1.

[profesor]

thank you Carl, I just understood a lot of things there.

[dbanner]

There is also a question of why rx coil is detuned from tx frequency. Surely there will be less gain, but I'm sure there are very good reasons having to do with stability and ambient temperature variations all of which reside squarely in the domain of the engineers. Maybe one day Carl will put the technical reasons to explain this.

[waltr]

I haven't yet understand why the different approaches to coil TX/RX #turns and offset resonance.

For some background reading study to two common VLF project here in GeoTech. These are the TGSL and the IDX. Lots of threads on building these.

As for why TX/RX off resonance here is a paper that has been posted with math and experiments. Interesting read.Two inductively coupled RLC circuits.pdfVLF coils Kostin.pdf