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I have been adding to my armoury of coils for the Vallon electronics. I now have one in a 12in shell, which I think would be my preference for beach hunting, and a 5.5in coil squashed into a small elliptical housing. These two latest coils worked first time with no problems, provided I kept the inductance, resistance and resonant freq. with certain limits. The Vallon coils I have range from 1.493mH - 1.512mH with resistances of 4 ohms and 160kHz f(res) with cable. It seems that you can go outside these limits to some degree as my 15in coil reads 1.556mH, 3.7ohms and f(res) 171kHz. That works very well.
The 12in coil was a bit closer with L = 1.493mH, R = 3.3 ohms and 160.3kHz. The 5.5in coil has L = 1.446mH, R = 2.6 ohms and 172kHz. The only worry was the low R, so I added a 1.0 ohm metal oxide resistor in series with the coil to bring it up to 3.6 ohms. Running the electronics on a bench psu set to 4.5V gave 372mA draw current for the 2.6 ohms, compared tp 332mA with the additional 1 ohm, so the resistor was cemented into the end of the coil housing. The latter level of current is similar to the standard Vallon coil at this supply voltage.
To get L close as possible to 1.5mH the 15in coil requires 39 turns; 12in coil 44 turns and 5.5in coil, 70 turns. Spiral wrap on the coil, then copper fabric tape as a shield. Nickel plated copper fabric tape is to be tried on the next coil but connecting the ground wire to it requires a different method as soldering is not possible. A thin drain wire is one possibility as the sample I have has conductive adhesive. The drain wire can be between the tape and the underlying spiral wrap.
The small coil in the elliptical housing can still detect a nickel at 13in and a 18K thin gold ring weighing 3gm at the same range. This coil I plan to use in cluttered areas, such as a field we own behind a watermill. The Mill goes back to the 13th century and a quick run with the standard Vallon gave a lot of signals quite close together.
Eric.
The 12in coil was a bit closer with L = 1.493mH, R = 3.3 ohms and 160.3kHz. The 5.5in coil has L = 1.446mH, R = 2.6 ohms and 172kHz. The only worry was the low R, so I added a 1.0 ohm metal oxide resistor in series with the coil to bring it up to 3.6 ohms. Running the electronics on a bench psu set to 4.5V gave 372mA draw current for the 2.6 ohms, compared tp 332mA with the additional 1 ohm, so the resistor was cemented into the end of the coil housing. The latter level of current is similar to the standard Vallon coil at this supply voltage.
To get L close as possible to 1.5mH the 15in coil requires 39 turns; 12in coil 44 turns and 5.5in coil, 70 turns. Spiral wrap on the coil, then copper fabric tape as a shield. Nickel plated copper fabric tape is to be tried on the next coil but connecting the ground wire to it requires a different method as soldering is not possible. A thin drain wire is one possibility as the sample I have has conductive adhesive. The drain wire can be between the tape and the underlying spiral wrap.
The small coil in the elliptical housing can still detect a nickel at 13in and a 18K thin gold ring weighing 3gm at the same range. This coil I plan to use in cluttered areas, such as a field we own behind a watermill. The Mill goes back to the 13th century and a quick run with the standard Vallon gave a lot of signals quite close together.
Eric.
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