Mono Coil Versus DD Depth Difference

[WesH]

  Another coil question.  Given that a mono coils sends a signal that is cone shaped into the ground what would the field of detection be on say a 12 in mono at say 10 inches vs a DD coil that has a blade shaped signal.  Say this is in mild ground.  Just how much depth is actually lost using a DD coil in the same soil conditions.  Would a broader signal at that same 10 inches of depth have an advantage with coverage at that depth?

[Jonathan Porter]

A mono coil in my opinion does not send a cone shaped signal into the ground, the field radiates out from the windings in all directions and becomes wider and gradually less powerful the further away it goes. A DD coil does the same thing and is only constrained by its original shape which I would say infills the area around the Tx anyway. The Tx power of the Minelab machines is identical regardless of which timing is used BTW. When comparing the coil sizes relative to field strength it comes down to the actual size of the Tx, so a similar overall sized DD will have a lot smaller Tx than a Mono of the same dimensions. 

The receive on the other hand is where the magic is done, size for size a Mono has the advantage because of its surface area and lack of a null that dampens sensitivity but they are prone to ground noise and salt so require timings like Fine Gold etc to deal with ground noise, which has a trade off but the trade off is extremely variable dependent on the target etc. Generally speaking a good rule of thumb for depth comparing like for like DD and Mono coils using Normal timings is to allow for an approx 20% less depth across the board on the DD but this is highly dependent on the ground conditions. 

DD coils are used a lot in detectors because they create less ground signal response (thanks to the overlap of the windings which require a null), a lot of ground signal response kill depth, DD's also allow for things like discrimination and out of Phase cancel modes etc. 

The key to getting max depth is to pass the receive winding exactly over the center of a deep target so the coils receive points are evenly exposed to the targets field, in the case of a mono loop the whole coil is the receive with the apparent response seeming to come from the center of the coil whereas a DD manifests its response along the line of the cross over points of the windings, but once again the very center is the max signal spot for a deep target. The key is to move the coils receiver evenly and smoothly through the field of a target trying to create a good lead in signal response but more importantly transition smoothly out of the tail of the target signal so the response is recognizable from ground noise etc, generally a deep target well be quite broad relative to coil sweep.

Hope this helps

JP

[WesH]

Helps a lot JP thanks.  I was always wondering based on diagrams that showed the transmit signal from a mono coil being shaped like a funnel or cone that at depth your missing a lot of potential targets but the way you explained how the signal comes off the coil make much more sense.

[Steve Herschbach]

I was hoping you would chime in JP - great explanation, thanks!

I have yet to observe the famous DD blade as sold by marketers actually exhibit itself on a coil. Just take a DD coil and a coin and run some brief air tests (see article below under heading "Target Size"), and except for tiny objects the blade thing is largely myth. All the DD thing does is cause the electromagnetic field to elongate and become a little shallower compared to a mono coil (or concentrics on VLF detectors). The field of a mono coil as defined by a coin finding the extremity of the field is more like a basketball cut in half - a bowl. A DD coil is more like a U.S. football cut in half lengthwise - an elongated bowl.

You can only see this on a VLF detector running in all metal or pinpoint mode. Discrimination modes reduce most signals to a simple beep, and a VLF concentric will indeed beep more in the middle of the coil whereas the beep generated by a DD coil elongates along the overlap. The rest of the electromagnetic field is there however, it is simply being hidden by the single report nature of disc modes of operation.

This article below is about induction balance detectors and some of the diagrams are exagerrated but it provides a decent overview of coils, electromagnetic fields, and eddy currents.

Excerpt from "Modern Metal Detectors" by Charles Garrett

[WesH]

JP and Steve thank you very good explanation.

[Reg Wilson]

Well, what can I say? Nothing really. Jonathan and Steve have summed it up nicely. All I can add is that I believe that coil technology still has a way to go, and stand by for some new developments on the horizon.

[Nuggetmaster]

so at what point if any is a dd Better then a mono? I have a gpx 4500 with elite 14" ,a great combo, I know my machine well and have never had a problem tunning it. i recently got a claim that has very hot ground so hot that i sometimes have to turn it down below factory presets in enhanced with a mono, I have been wondering if any gains can be achieved by running a dd with higher setting and different soil timmings? thanks 

[Steve Herschbach]

After the GPX came out I quit running DD coils. A DD can aid with ferrous trash (the GPX disc function only works with DD coils) or electrical interference (the GPX cancel mode is designed for DD coils). DD coils can also help with the worst of the worst ground. For me though its mono all the way.

[schoolofhardNox]

I do a beach that has a train, that energizes it's tracks about 30 feet from it. I'm using the GPX 5000, so is the DD coil still the best option? Not sure if the mono coil will handle the salt and EMI. I run my machine fairly hot too. Deep silver at 14" is common , but sometimes you have to wait until the rails de-energize, in order to hunt. I have a 12x15 mono coil, but always thought it would not work well in that environment? Thoughts??

[Chrisco]

On 23/01/2018 at 2:31 PM, Reg Wilson said:

All I can add is that I believe that coil technology still has a way to go, and stand by for some new developments on the horizon.

Does this mean you currently have another "unfair advantage" Reg??