What is the value of the capacitor?

[WM6]

. try making a more realistic 25cm - 30cm coil first.

A suggestion worth considering.

For 25cm coil diameter you need about 42m of (0,45mm) wire length, winding about 40 turns , with what you get about 5,5 Ohm DC coil resistance

Number of turns for 275mm coil diameter should be about 38 turns

Number of turns for 30cm coil diameter should be about 36 turns

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[ivconic]

I'm so grateful to everyone who has shared their insights. I'll definitely keep your thoughts in mind as I continue my search for a living horse (a newer technology). Thank you all for your help!

Additional note: Despite Japan's world-class reputation in electronic engineering, I find it intriguing that there appears to be a complete absence of domestic companies producing metal detectors specifically for treasure hunting.

That's because the Japanese are very smart people who don't want to waste time on stupid things!



As for your desire to make a good BFO... that's a pretty easy task. You have a "million" BFO projects both on the forum (older topics) and on the Internet.
You don't even need schematics and other people's ideas. The BFO operating principle is as simple as it could be.
You need a rock soild oscillator, the best is crystal based. Let's say with IC circuit CD4060. At the output you get a "lower" frequency. Several tens to hundreds of kHz.
On the other side, you need a "variable" oscillator with a coil (search coil) that will change its frequency in a narrower range under the influence of the presence of metal near that coil.
Then you need a "mixer" (CD4013 or similar flip-flop comes to mind) into which you introduce both signals and mix them. You try to make the mixing product fall within the audible range (100-3000Hz for example).
The output from the mixer goes directly to the audio amplifier. And that's it.