concentric with figure 8 Rx coil

[golfnut]

With a coil within another coil the Rx picks up the Tx coil signal - transformer action. With this a buck is required to add equivalent pick up - in opposite sense.



If the Rx coil in the middle had a half twist so it was a figure 8 shape - then the first half of the loop would pick up +x units of Tx and the second half of the loop would pick up -x units of Tx.

This then would need no buck coil? and all attempts to make this would always null first time - due to the geometry?



In use the target return would disappear if the target was boresight in the centre? Good for PP action?


I know this would have been discussed 100 years ago and thrown out, but really has anyone tried this?

S

[Qiaozhi]

This is how a "Big Foot" coil is constructed, and is described on pages 57 and 58 of Inside the METAL DETECTOR - Published September 2012

[Davor]

Exactly.
The downside to this approach is that one of the 8's loops gives you in-phase response, while the other loop gives counter-phase. It would not matter at all in case you have XOR discrimination logic to discriminate in 4-quadrant way and having both loops giving meaningful output, but since most of the rigs discriminate with AND logic, your meaningful response is limited to one of the loops. That's one of the reasons I wish my future rig to have 4-quadrant discrimination. With my pace, it is more likely that some of the big name companies will beat me to it.

[Gwil]

This type of coil, and many others, are also discussed here:

http://www.geotech1.com/pages/metdet/info/coils.pdf

"...all attempts to make this would always null first time..."

I wouldn't bet on it. Any slight asymmetry between the two loops of the receive coil would produce an unwanted signal, which would have to be adjusted out. Just like all the other types of search head, unfortunately.

G

[golfnut]

Carls pdf was pretty good I thought - thx

S