The most common detectors used today are referred to as VLF's. The technology used in this type of detector is call "Induction Balance" with Balance being the important word. Remember the phrase in Star Wars "I detect a disturbance in the Force". When a target is detected, the "Balance" of the detector is thrown off and it signals to you the user that we are now in an imbalanced condition. Hold that thought for a moment.
If you look at the typical coil, the outer ring is the transmit winding and the inner ring is the receive winding. If that's all the coil ever consisted of it would never work, because you have the transmit winding right next to the receive winding and the detector would be in constant overload. There is a third "ring" or winding that you don't see, it is another receive winding that acts to cancel out the transmit signal. This winding balances the detector so it remains quiet without a target nearby. The tuning of that winding is critical as is the physical positioning/shape/rigidity of the other windings in relation to it.
Temperature changes, mechanical shock, & flexing can all cause changes in the relationship of the length/spacings of those windings and de-tune the balance. This is a critical failure and will throw off being able to Ground Balance, correctly ID etc. Tuning a coil is like tuning a musical instrument in that the three windings are all interdependent and they should all be tuned in the same key.
Re-tuning an X-Terra coil is next to impossible. First is the coils are in a potting compound which is like an epoxy, even a minor attempt to remove it will damage the coil. Next is the IC chip in the coil, somehow they are unique to each individual coil with parameters stored in them for "that" coil. And when I say that, you can have Qty 10, 6 inch HF DD coils and the parameters for every one is different and stored in each IC chip. The minute you change anything about the coil it will no longer match the parameters in the IC chip. It's the technology in the IC chip that custom tunes each coil and keeps the performance uniform over large production runs. It also keeps the performance consistent from coil to coil.
HH
BarnacleBill
