Other type of Concentric Coil ?

[Anonymous]

Typical construction of concentric coil is:
inner recieve coil + innerTX and outer TX.
I know that C.C. can be made in other way:
TX- outer coil, RX outer + RX inner coil.
What is the difference betwen those two constructions?
If someone knows please let me know!
Best regards
Andrew

 

[Anonymous]

Andrew,
Most detectors used the former type (+TX, -TX/RX) while the Shadow uses the latter (+TX/-RX, +RX). I did a bench comparison between the Shadow and a Bandido uMax which are very similar in design. The Shadow had slightly better coverage to the edge of the coil. Other than that, there's no difference. I've heard that the Shadow-type coil has also been used by other companies in the past, don't know who and don't know why it is not used more often.
- Carl

 

[Anonymous]

Carl,
I believe Garrett patented the concept you described of nulling the coil by wrapping a few turns of receive winding around the outer xmit winding. I thought I had a copy of the patent but couldn't find it. I don't remember the patent number.
Reg

 

[Anonymous]

I think that outer TX inner RX + outer RX
configuration is more effective because resistance of TX winding is lower so TX current can be higher. When resonance occures TX current is limited mostly by coil resistance. RX winding is made with thin wire so its easy to tune.
What is typical resistance of TX,RX coils ?
Andrew