WM6,
The device in posting # 64 is model APEX MKI. It is like Geonics
EM15, designed by Vaino Ronka.
Chris,
1. Target signal at metal detecting seems as modulated TX signal with frequency spectrum from 0,1Hz to maximum 6Hz. That means we need RX having bandwidth no more than 12Hz. More bandwidth means more noise. There are sensitive metal detectors operating with bandwidth 4Hz only. We need automatic digital filtering to achieve stabile narrow band receiving. Bandwidth 100Hz means 5 times more noise relative to bandwidth 4 Hz.
2. The modulation index of target signal is very small because RX receives two large signals AIR and GND having the same frequency. If you wish to detect target signal 1uV, but AIR and GND signals are 10mV, then the modulation index is 0.0001. We need suitable automation to increase the modulation index by suppressing AIR and GND signals. However if you swing search head one or two times pro seconde, the GND signal will be modulated by frequency 1 or 2Hz. This is in target spectrum.
3. We need reference signal for synchronous demodulation. For example, Vaino Ronka uses AIR signal from second RX antenna as reference signal. To demodulate weak signals,we should use GND signal as reference.
4. Your TX antenna is absolutely inefficient becase has only one small turn. Read for TX antennas and loop used in metal detecting in site
www.geonics.com