Why the overlap in DD coils?

[Infamy]

Is an overlap necessary in DD coils?
Is it a case of small DD coils need an overlap, but as they increase in size the overlap is not necessary?
I'm working on a large DD coil just under 40" diameter and it would be easier to build without the overlap.

[Infamy]

I found these two posts by Reg which answer the question I raised above, but the answer raises another question, what governs the size of the gap for VLF detectors?

"Coils, Coils, and more Coils
VLF's always have a transmit coil and a separate receive coil of wire in the search head. PI coils, however, can be produced in several variations. If the same coil is used for both the transmit and the receive signal, the coil is normally called a "MONO" coil.
If two sets of coil windings are used, and those coils are basically the same size and shape with one coil used as a transmit and the other the receive, and they overlap a small amount on one side, the coil is generally called a DD coil. The name DD generally refers to the design of the coils where they are sort of like D's with one D reversed and the backs of the D's overlapping slightly. This overlap area is the main detection zone and is the area where an object is under at least part of both coils at the same time. This detection zone is most noticeable on deeper objects.
By nature, DD coils are somewhat less sensitive when compared to a mono coil of the same size. One reason for the reduction in sensitivity is the fact that the DD electrical coil windings are smaller in size than the coil windings of a mono coil even they may have the same size coil housing.
One other key factor that is important is the fact to remember about a DD coil is the main detection zone is quite narrow. This narrow detection zone, normally at or near the overlap will create a very brief or narrow signal when compared to the signal on a mono coil. This situation makes the sweep speed of the search coil much more critical. Swinging the coil too fast can easily cause a very weak object to be missed simply because the signal is so short and the circuitry filtering used to eliminate the noise will also almost eliminate such a signal.
The fact that DD coils have smaller diameter windings for the transmit and receive coil, when compared to a mono coil using the same size housing, has some advantages. Generally, the smaller receive coil is not as good of an antenna as a larger mono coil, thus less noise is detected and amplified. As a result, the detector can be much quieter when using a DD coil. In many cases the reduction in noise can outweigh the depth loss due to the size difference.
One other major asset of a DD coil is the fact the receive coil is isolated from the transmit coil. This helps in the fact that any low level noise that is generated by the transmit circuitry during the sampling time is isolated from the receive circuitry. This isolation therefore reduces the combined noise that can negatively affect a target response.
One final advantage of a DD coil is, by nature, a DD coil partially cancels the ground signal. If the coils are properly aligned or positioned, most ground signal in the receive coil is eliminated. This results in a detector that has very little ground response, yet still responds with a strong signal from a buried object.


Hi Mike,

If you have read the article on a mono coil, many of the same concepts hold true when trying to build a DD coil. Now, one of the differences is the fact that you simply build two coils rather than one.

Mono's only use a single conductor coax, but a DD requires a dual coax for the best results. Some of the better video cables work just fine for a DD coil wiring.

I have found it is best to keep the two coils isolated, meaning both coils are shielded, but insulated from each other. Since each coil has a ground connection, then the shield for that coil connects to its ground or coax shield.

Now, remember that on a DD coil, you have to have a separate damping resistor on the receive coil also. If the pc board provides for that, then fine, but I have found it to be best to simply add the damping resistor inside the coil across the receive coil.

A DD coil has a fundamental design of two elliptical or D shaped coils that overlap a small amount. Fortunately, on a PI, the overlap is not as critical as it is on a VLF coil. So, one can be quite sloppy in the design or the amount of overlap and the coil will work just fine.

A DD coil will have less ground response simply because of the coil itself and its natural ground canceling features. In other words, part of the coil(s) generate a response that effectively counters some of the ground signal, thus reducing the overall effect of the mineralization.

Now, as for depth capability, there is a difference between that of a DD and a mono but that difference really occurs when trying to detect objects over a certain size. The DD generally does better on the really small stuff such as gold nuggets in the few grain range. For beach hunters, this means real small gold earrings or gold chains will generally be detected better by using a DD coil.

When a detector such as the Hammerhead doesn't have any ground balance feature, then the DD coil provides some relief when trying to use the detector in really mineralized ground.

So, for someone getting started, it certainly doesn't hurt to build a DD coil and compare it to a mono of similar size just to see what really happens. What will be surprising will be how well the DD really works."

[Carl-NC]

The overlap is needed to achieve induction balance. If you are building it for a PI then IB is not really necessary, but can give better performance. Besides, if you don't use IB, then why make it DD? Just make it concentric.

- Carl

[Infamy]

Thanks Carl.

I'm building it of a Sovereign to detect over the following

2-3 inches air
1-3 feet salt water
0-3 feet mud

Do you modify balance by varying the overlap and testing it against ability to detect metal objects?

Is there a formula to calculate how long the batteries will last use a 40 inch dd coil?

[wam]

Why DD?

Is it not that, DD coils in PI are used because there is inductance limitation for TX so it can deliver short pulse of current (for limited supply voltage) but at the same time coil with limited number of turns has smaller ability to sense EM fields?
The RX coil is high turns type and low current and high impedance so it will be delivering higher Voltage for the same field changes.
I could be wrong but if this is the case it would be no problem with depth design.
I am using 30cm DD coil with 38 mH on RX side and it has no problem with US 25c in air from over 50cm with discrimination on. When I have done math’s it was pointing to approximately 2000 turns on RX side.
Is it possible that some one can use CRO and compare signals between TX and RX coil in HH2 design and clarify my conclusions? You can do it without CRO as well …
I notice, the older then more often wrong I am.
Regards.

[WM6]

Is it not that, DD coils in PI are used because there is inductance limitation for TX so it can deliver short pulse of current (for limited supply voltage) but at the same time coil with limited number of turns has smaller ability to sense EM fields?
The RX coil is high turns type and low current and high impedance so it will be delivering higher Voltage for the same field changes.
I could be wrong but if this is the case it would be no problem with depth design.
I am using 30cm DD coil with 38 mH on RX side and it has no problem with US 25c in air from over 50cm with discrimination on. When I have done math’s it was pointing to approximately 2000 turns on RX side.
Is it possible that some one can use CRO and compare signals between TX and RX coil in HH2 design and clarify my conclusions? You can do it without CRO as well …
I notice, the older then more often wrong I am.
Regards.

Hi wam,

do you ever use concentric two coil for PI? What is better to put inside such concentric PI coil, RX or TX coil, if CC make sense of course?

[Ism]

Is there a preferred method for which coil (Xmit/Rx) is on top or doesn't it matter?

[Rov]

In my DD designs an builds I place the Rx winding beneath the TX in the overlap,seems to work best.Ive notice this also the same in many commercial build coils.

[Ism]

Thanks Rov, I have built a very nicely functioning 10x12 DD coil based on all the info obtained on this forum. I was happy to find that my guess in the coil overlap was in agreement with you. I can now complete the project with confidence.

[IBGold]

When you get into Duel-Field DD coils you will find the overlap changes from normal overlap and they are very interesting coils.
Regards,Ian. And MERRY CHRISTMAS TO ALL.

[Rov]

Hi Ian,Still into your coil builds I see?Heard you have disposed of your Vac.moulding equip.for housings?True?
Anyway Seasons Greetings to You!The very best!Merry Xmas!
Rov