Benefits of a DD coil

[Gavinius]

I am planning to build a PI metal detector.
What are the benefits of using a DD coil instead of a mono one? What are the drawbacks?

[bbsailor]

DD Coil

Benefits:

Provide better electronic balance on some types of ground that might otherwise be noisy or produce signals when the coil is swept, typically in soils that have a high degree of mineralization.

Provides better performance due balancing out or reducing some types of outside electrical noise.

Isolates the RX from the TX circuit and thus eliminates the capacitance of the MOSFET that can be pretty high (several hundred pfs) on some MOSFETs. DD coils can make about 2us faster coils in some cases. Check the output capacitance on the MOSFET that you are using to see any potential benefit. Some new types of MOSFETS have a low output capacitance of about 50 to 70 pfs and would provide less benefit than the higher ones of several hundred pfs.

The RX coil can be made more sensitive by using more turns of thinner wire, typically up to 1.5 to 2 times the number or turns as is on the TX coil.


Drawbacks:

A little more difficult to make than a mono coil.

Requires a DD coil housing. Mono coils can more easily use a wide variety of improvised coil housings from frisbees to plastic can lids.

Requires some adjustment of the overlap region to obtain an optimum response on specific targets like gold nuggets. Search for recent posts by Reg about adjusting overlap.

Requires a little more expensive S-video dual-coax cable, but is commonly available.

Requires that the two crossover points of the RX and TX coils have their shields isolated from each other.

Requires a 4 pin coil connector. Grounds from RX and TX should only be joined at a common ground point on the circuit board to avoid stay currents from being amplified.

Requires a jumper being placed in mono coil plugs to connect the RX and TX coils together for mono coil use. Reg provided an illustration about how to do this on the Hammerhead a while ago.

Requires individually adjusted damping resistors on both the RX and TX coils.

Pinpointing the target requires rotating the coil 90 degrees (after a target is located) to find the actual location of the target as the entire overlap region is where the target could be located. In this case, "X marks the spot". (Remember Indiana Jones)

bbsailor

[Gavinius]

Thank you very much bbsailor, your explanation is really exhaustive.

[bigrex]

Also, DD coils don't penetrate the ground as deeply, at least when compared to the same size monocoil. This is probably since one half of the coil is for recieving and the other half for transmitting. The mono has a rounded cone shaped signal while the DD is blade shaped I believe. Someone can correct me if I'm wrong.

RCB

[Geo]

Hi. I like the mono coil. At normal ground (no heavy) there isn't problem with mono coil. Between mono and DD there is difference at sensitivity ( mono is better about 5-7% from my tests). I did not try in salt ground

Geo