Most accurate TID

[Mark-VA]

A "more" accurate answer for the question below depends on the following being true.
No trash in the area.
A target (coin) is laying horizontal to the top of the ground.
Let's say at an a depth of 6".

The metal detector you are using has the transmit winding's on the right side and the receiving on the left of the coil.
Now if you approach the coin from a left to right swing of the coil, with the TX side over the coin first will the target ID be more accurate?
Or is there a difference which signal is first?

[mikebg]

You mean this situation?

[Mark-VA]

Yes that is right.
Man that is a nice drawing, how did you do that?
I ponder on a lot of things and at times I find it hard to describe in words.

[Mark-VA]

Further thought, it doesn't matter.

[mikebg]

Further thought, it doesn't matter.

THIS IS NOT TRUE!
The IB search head has zones with back reading. The coin in above drawing appears in back reading zone where demodulated signal is negative. Only in central zone TID is normal. Try with a coin close to search head to see the effect.

[Carl-NC]

Now if you approach the coin from a left to right swing of the coil, with the TX side over the coin first will the target ID be more accurate?
Or is there a difference which signal is first?

"It depends."

Ideally, it does not matter. Consider Mike's drawing, and consider that we label the interior of each coil its "positive signal" region, and outside the coil is a "negative signal." So per Mike's drawing the coin is inside the TX coil (positive TX signal) but outside the RX coil (negative RX signal). As the coil sweeps directly over the coin it is inside both coils briefly (positive TX and RX signals, resulting in a TID which is in the proper quadrant). As the coil continues past the coin, it ends up still inside the RX coil (positive) but outside the TX coil (negative). Ergo, sweeping the coil in that direction gives a negative-positive-negative response. And sweeping the coil in the opposite direction gives the same response.

Except... usually TX & RX coils aren't made the same. RX coils usually have higher turns, higher inductance, and more parasitics. This can affect the transient response as the coin sweeps through the overlap region, and produce different readings depending on direction. Usually gets worse with faster sweep speeds.

- Carl

[Mark-VA]

Now I more clearly understand what actually takes place.
This is one reason why I enjoy reading this forum.
Thanks Carl
Hope to meet you at the g.....s