The Ideal, The Perfect, The One!

An ideal dream machine will be a metal detector with a 'Medusa' coil. Basically a coil that could specifically detect and probe automatically, seeking the object, via an extended test prod with a miniature camera on the tip (like they use in surgical operation) connected to an LCD display (modified from a car DVD player monitor) in order to verify the object underneath, before the real dig! My wishful thinking...

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		Quote:			Posted by kendavid
	He said "MegaHertz" but that was just a slip of the key. GD meant GigaHertz. I researched resonant frequencies and gold resonates at roughly 1.7 to 3 GigaHertz. So I'm still worried about my feet, Brian. Microwaves start at 1Ghz (technically) and extend to 300 Ghz (depending on the application). GD, I hope we can make this dream a reality!

Ok, just build a small UHF oscillator and a small UHF receiver, almost identical circuits, tune them both with a small varactor on each board, and if you have to, place a small capacitance in series with the varactors if thhe tuning range is too wide.

tune your inductances, at the low end of the band and rig the receive with a small tunable capacitance, at the high end, so it all tracks. Feed the receive to a UHF detector circuit and run the resulting voltage to a meter of some kind.

All you need for a Tx antenna is a piece of pipe, with a 1/4 wave antenna, in one end, at least a 1/2 wave from the closed end, and a flared, funnel end, at the open end. Same for the receive.

A 3.5 inch aluminum heater pipe should work, you can use a 1/2 wave antenna, from a coax panel connector, just solder the wire antenna into the coax center..... a 2 inch antenna, at 1.7gHz, for 1/4 wave, it is going to be MUCH longer, electrically at the higher frequency... like half wave at 3.4 gHz, that means an impedance change of some magnitude. There should be a way to get around this. Only thing I can think of is too big to go in the pipe

You could use a rhombic for the TX, it is very broadband, and use the pipe for receive only, or a rhombic  for both, but with less pinpointing. A rhombic is diamond shape, with an 800+ ohm termination at the far end , so the TX impedance is the same as the termination, if you use 820 ohm carbon, 1/2 watt, resistor, you can put out a 1/4 watt Tx, should be lots.

You can find the UHF boards in all kinds of places. Seems to me Ramsey Kits used to carry some. Or you can roll your own, with the right UHF transistors. Try an oscillator with a piece of small twin lead and the tune cap at the outer end, away from the transistor.

I used to have some 2N710s... very obsolete, very low power, but the frequency cutoff was over 900 mHz, as an amplifier, but as an oscillator, most transistors will operate beyond cutoff. Look for something with MRF as the prefix.

I just have never monkeyed with UHF much.... VHF is high enough. Impatt diodes. might work, they are an oscillating diode, That is all I know about them.

Oh, yes, sweep with a sawtooth wave or a triangle wave, the saw tooth will only sweep up, the triangle will sweep up and then down. And if you can not get a very small varactor, ise a 1N914, reverse biased. The higher the bias the smaller the capacitance. I just calculated that a 1 pf change would only give 263 mHz sweep, hardly enough....hmmm. have to think about this... a piston cap, 1 to 5 pf....

goldigger

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The middle of that band around 2.4GHZ is frquency of of AV senders could modify, modulate and amplify transmit.
Receive with usual simply VCO output for experiment.
As GD was suggesting what is penetration of ghz signals like, poor,might be good for probe.
Mobile phone might go with boosted Ghz

Regards Jeff

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		Quote:			Posted by kendavid
	Jeff, If this Dream Machine can help me find gold rings quickly and easily I will be glad to leave the mobile phone in the car!  Just 10 years ago I didn't even own a cell phone

SSSSSHHHHH! I am thinking! 

goldigger

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Yeh my mobile phones been drivin me for about that long  .
Dug out the old AV sender/receiver from the shed.
Trans has an internal wire antenna about 12 cm approx wavelenght at 2.5Ghz.
Not sure of method of modulation for these things,
might be able to use some kind of signal strenght meter to measure dip?



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