Balancing DD Coils.

[Eric Foster]

Back in early March there were some questions by Rick S on the balancing of DD coils. I have used DD coils on PI detectors but have no real experience of them on IB machines. However, the same basic principles must apply. I found it was important to have a grounded screen between the coils and the soil, otherwise the coil to soil capacitance change confuses the ground balancing. The best method that I found was to individually shield the TX and RX windings. The overlap balancing is then purely due to the inductive coupling between the two coils. With unshielded windings, there is a small inter-winding capacitance which again can give errors. I have been trying to work out whether the electrical balance of the coils corresponds to the optimum ground balance if the coil was over a uniform magnetically mineralised surface. I would be interested in any thoughts on this.

Eric.

[Rick S]

Re: Balancing DD Coils.

Eric or Carl,
When assembling the DD coils on the work bench, does sheilding have to be in place for the metal detector GEB to have full scale adjustment? I either get negitive side of null or the positive side of null. I have tried everything mentioned in the March responses on this forum. I still can't seem to get the electronics of the GMVSAT to work with the coils. I have succeeded with the PI DD but not the IB DD. Any help would be greatly appreciated. Rick S

[Carl]

Re: Balancing DD Coils.

Sorry, don't know what else to tell you. You might try to contact White's, although they're probably not too interested in helping an individual build a coil. Still worth a call, though.

[Rick S]

Re: Balancing DD Coils.

Carl
On one of my DD coils, when I wave my hand over the coil it response, but to much. When compared to the stock coil, the hand can be heard up to about and 1" or so... Mine can be herd about 3" to 4" away. I use about a 3 grain nugget and I get about a max of inch away while the stock responds to approx. 4". I guess that it is to sensitive to minerals and do I need to add more capacitance in series to reduce the sensitive to minerals? Which coil would I attack the Tx or the Rx or both? I guess my other problem is still matching the impedance. I know why alot of guys don't get into IB coil construction. PI is alot more forgiving in coil building. When I figure this coil construction out I will let you know. Also I have barrowed a digital camera and will be taking pictures of my coil winder. Thanks Carl, Rick s