Some improvements
The coil L1 is better if it consists of 2 turns and for greater sensitivity you must shield the entire box with aluminum adhesive except the cover of the input stage, as seen from the photos. The shielding must be connected to the ground of the input circuit. The handle must be at least 20 cm long with a diameter of 4 cm or 5cm, also to have maximum sensitivity you must hold the handle with both hands. The best antenna is a stylus 80cm-100cm. With 30cm it has the minimum sensitivity, the greater the length the greater the sensitivity. With these changes, it is possible to detect targets that were previously ignored, such as, for example, short time metals buried (1 month or more) in your test field.
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