HELP ON PULSE DD COIL

[Anonymous]

Please could some one tell me how the dd coil works on a pulse detector,is it like a vlf detector in which the TX coil transmits into the receive coil,or is it two transmit coils over laping each other which then use common mode rejection,does the dd system give full rejection against iron.

 

[Anonymous]

Dave, try this www.kamakazi.com While you'r there, be sure to take a look at Jack Lange's terrific article "Confusion over Coils" Hope that this helps!
Regards.......Bob

 

[Anonymous]

Forgot to tell you Dave,it's under the heading-Coiltek Coils!
Bob

 

[Anonymous]

hello Dave and the forum,i make many field tests with single vds DD coils , and i can tell you ; don't waste your time with DD , REALLY the difference is too small for ground signal and you loose sensibility for small item, single coil is very better in dynamic response on a target.

 

[Anonymous]

HI Dave,
I have built both DD and mono coils for my pseudo Goldquest so I am familiar with both and the differences between them.
First, to answer your questions, my DD coils are like most in their design. One coil is the transmit and the other is the receive coil. When a pulse unit normally equipped with a mono coil has a DD coil installed, the detection of iron is about the same on the two coils. In other words, one will not see any advantage changing to a DD for that purpose.
There will be many places and types of ground where a DD coil may not have any advantage. I have found such areas and in those places the detector equipped with a mono coil displays little if any ground signal. In other words, bobbing the coil up and down above the ground produces very little signal.
However, where I hunt for gold, the DD coil has displayed some dramatic advantages, mainly the reduction of the ground response. In these areas, there will be a considerable ground response that is quite noticeable when the coil is bobbed up and down over the ground when using a mono coil. The DD coil dramatically reduces the ground signal.
One other advantage I have seen using the DD coil is the fact there is a considerable reduction of overall noise level when using the DD coil. At least this is what I have found in the places where I use my PI.
Now, I need to qualify something. I have found that if I try to perfectly balance the coils so there is minimal signal out of the receive coil with no target present, I so seem to lose a little sensitivity to small objects. However, I found that if I just approach the point of balancing the two coils, I still see a significant reduction of ground response and really do not see any obvious loss of response from various objects. I do notice the signal is a little softer from very small gold nuggets, but the depth range is about the same. As for a nickel reference, I found my 11" DD coil to exceed the depth I was able to obtain from a similar size mono coil. I attributed this mainly to the reduction of overall noise signal.
I hope this helps to answer your question.
Reg

 

[Anonymous]

Good post Reg. Have you figured out what the difference is in the ground where a DD works better as opposed to ground where a mono works just as well as the DD?
Thanks,
Fred

 

[Anonymous]

Hi Fred,
It is difficult to actually determine what ground is going to give trouble, but generally, the ground having a clay base will normally generate some ground signals.
What I have found is, in gold producing areas, there is a lot of mineralization in the soil, some of which will generate ground signals. However, if I am in a drywash where there is a lot of black sand or the base is a gravel, there will be little if any ground response.
I have also found that active creek bottoms, meaning the soil is washed on a regular basis produce almost no response. Real sandy areas free of clay seem to have a tendency to be low mineralized and produce little response.
Many of the areas where I nugget hunt have ground that appears to be a tan to light brown in color do produce some ground response which will require a person to sweep a little slower and maintain a fairly level coil height for best operation.
Other areas having deep red clay type ground and a lot of hotrocks will produce the greatest response. Not from the hotrocks but from the void they produce when they are embedded in the ground. Hunting this type of ground with no ground balance does require lifting the coil a little for best operation.
The DD coil, seems to significantly reduce the ground response in all areas and in most places, to the point that I will not use any form of ground balance or if I do, I will just use a small amount.
Reg

 

[Anonymous]

I learned that DDpulse coils can be configured into a unit that can detect metal in a wide scan, say 3 feet. I am interested in buying or building a wide scanning unit so I can drag it and cover a wide swath of ground. Reg, or anyone, can you point me in the right direction? Thanks Dennis

 

[Anonymous]

Hi Dennis,
One thing that is needed is to know is on which PI the coil is going to be used. This makes a big difference. Also, there are already some PI's that have large coils already built for them so it might be possible to purchase one already built.
As for building one yourself, I have never seen specific instructions on how to build a particular coil that seems to work for all conditions. So, I will try to give a generic answer to your question about possibly building a coil to cover or scan a 3 foot area.
First, either a DD or a mono coil can be built to scan the area you mentioned. The coil could be built in several shapes such as a rectangular, an elliptical, a round coil or some other shape to fit the specific need.
The object of building such a coil to fit a certain detector is to best duplicate the characteristics of the coil already used on that particular detector. In other words, one should try to match the inductance and resistance if possible. The coil cable should be matched also.
By doing this, the new coil will appear the same as the original coil, thus the damping will be correct which will allow all to work correctly. If there is a distinct difference, then it is quite possible the coil will not work correctly or not at all.
Unfortunately, I have not tried to build a coil the size your desire so I have no actual experience. However, I have built different sized coils and there is subtle differences between them. My guess is in building a larger coil the size you desire, you will still have to match the original smaller coil as close as possible. However, this is difficult because of the additional capacitance as the result of the larger windings, etc. So, it might need some changes to work correctly.
Other factors have to be taken into account such as the Faraday shielding and the selection of the coil cable. Both are critical and can have a significant influence on the final success or failure of the design and its intended use.
Generally speaking, larger coils are more difficult to build and still allow very short delays. On some detectors such as the Goldquest, or one of Eric's other detectors, the detector design is very forgiving meaning they seem to work without trying to maintain absolute tolerances.
Again, it may not be possible to get the coil to work at the short delays a smaller coil might be able to operate at but may work fine when the delay is advanced.
Ideally, one should have an oscilloscope to observe the signals of the new coil. This will allow you to see what is really going on and if the coil is properly damped or not. If the damping is incorrect, the coil may oscillate briefly which will cause problems until the oscillation subsides. Overdamping may cause the detector to fail to work until the delay is advanced.
Now, I really not given you a specific answer to your question but I have tried to at least point out some of the problems you might encounter.
Personally, I built several coils with different inductances and tried different shielding before I settled on my present designs. Part of the reason for this was I was trying to find the right combination that would allow for a shorter delay than the detector was originally designed to operate at.
If I were to build a larger coil the size you desired, I would expect I would have to do the same once again. I would still try to match my present inductance as much as possible and would continue to search for the ideal coil cable to minimize the capacitance. My guess is I would have a very difficult time trying to get a coil as large as you desire to operate at the same minimum delay I can operate smaller coils.
I hope this helps somewhat and at least gets you started.
Reg