DD target response

[Anonymous]

Hi,
I've been constructing a DD coil for my GQ, and while testing the overlap positions discovered that different positions will cause different target responses. I'm using flat wound coils to mimic a PCB coil I'm working on.
With 40% overlap, range and stability suffers and it's not possible to go below 10us reject.
With 25% overlap it becomes highly sensitive to very small nuggets, even more so than the mono, while maintaining a good range on larger nuggets, but it also responds positive to magnetic hot rocks that the mono nulls out.
With about 10% overlap, all the hot rocks in my collection cause a nulling of the threshold when directly under the coil centre, while still responding to medium nuggets. On two types of hot rock (Andesite and ?)it will see the nugget under the rock.
The different types of hot rock require differing levels of overlap to null them out, and as the level is changed to remove the worst offenders (ones that cause positive responses with the mono) there is a corresponding loss of sensitivity to smaller nuggets.
It appears then, that to achieve the greatest return, a coil should be tailored for each locality to deal with the worst mineralisation encountered, and the expected median gold size.
Cheers
Kev.

 

[Anonymous]

Hi,
Made a wee mistake,
"With 25% overlap it becomes highly sensitive to very small nuggets, even more so than the mono, while maintaining a good range on larger nuggets, but it also responds positive to magnetic hot rocks that the mono nulls out."
These hot rocks are not "nulled" out with the mono, they are just not seen, the threshold may only alter very slightly.
So what is happening?
Are the hot rocks changing the flux coupling between the coils, much like a ferrite core in a transformer?
Cheers
Kev

 

[Anonymous]

Hi Kev, you got it. Imagine a ferrite target - high permeability, no conductivity and your monocoil gives slight or no response to it. Just because no eddy current is induced and the monocoil integrates the spatial distortion of magnetic field. DD coil is balanced to spatial symetry of magnetic field and any distortion induces a signal in the receiving coil. Go forward, use amplifiers for both coil and you stand right at the beginnig of new class of detectors - double channel IB/PI. Congratulations, Sid.

 

[Anonymous]

Hi Sid,
I was reminded while adjusting the coil overlap, of tuning a BFO. One side of the beat note gave a null for iron, and a response for non-ferrous while the opposite side of the note gave the reverse. I think there might be a point of overlap that corresponds to the apex of the target resonance, or note, and this is unique to the target composition?
Sid, are you then saying that by sampling both the TX and RX coils, I will get differential information about the permeability of the target, and hence, metal/mineral identification?
Thanks
Kev

 

[Anonymous]

Hi Kev,
This is how I see it. Running a DD on the Goldquest gives different responses to targets purely due to the spatial relationships of the TX and RX coils. Whether they are in a state of balance or not does not matter at all, as the receiver is sampling in the TX off time.
A ferrite material or a hot rock, will alter the coupling between the coils of a DD, but the detector has no way of sensing this, as the signal being measured is only coming from the object when the TX is off. To sense the coupling change you would have to sample during the on time of the transmitter.
For an area of mineralised ground that is large compared to the coil diameter, a mono will give the greatest signal when the coil is scanned just above the ground surface. This is ground that has maghemite as its predominant iron mineral, because it has a magnetic decay that has the same effect as a conductive target. For a mono coil where the TX and RX are one, there is the shortest distance from TX

 

[Anonymous]

Hi Eric,
Thanks for your thoughts, it all sounds quite plausible.
Could determination of coupling magnitude be used for ground elimination information?
I've heard of others sampling during TX on time, but I

 

[Anonymous]

Hi Kev, that is right, the difference helps to the identification, but the problem is more complex, because there exist a class of minerals with magnetic viscosity, eg. bassalts, then use the Erics explanation. Extremely important, I think, is the first sequence of RX signal during the pulse - the rising transient magnetic field. Nice day, Sid.

 

[Anonymous]

Hi Eric,
I tried some ferrite cores last night and found that most of the larger ones would produce a positive signal when the coil overlap was adjusted to accept all hot rocks.
Tuning slugs would not produce a signal when aligned parallel to the coil face, but when set perpendicular to it, produced a solid metallic response.
If this was purely a spatial relationship, why no signal when the greatest surface area is presented to the coil, (reducing spatial displacement) and a solid metallic response when the smallest area is presented? This is the exact opposite response one would expect from articles such as coins. Although I don't know for sure what happens with on end coins with this particular double D.
I imagine that the RX winding is looking into, and down the side of the TX field, and in this way sees more of the slug when it is vertically aligned, but if this is so, why doesn't on end coins also produce a greater response with double D's in general?
I must add though that I have not critically dampened the TX winding, I'm relying on the values selected for the mono, even though this is not enough. I've been meaning to check this out on a scope and tidy things up.
Cheers
Kev.

 

[Anonymous]

Hi,
Found some very interesting information with a search on "flyback" topics. Seems, Dave Johnson has already been there.
Is there anything new under the sun? Doubt it, only refinements, combinations, specialisations, magnifications, adaptions.......Kev

 

[Anonymous]

Hi Kev,
Magnetic materials give the greatest signal when the flux from the coil is aligned along the length of the object. Same with a nail; with a mono coil, it give the most signal when perpendicular in the coil centre. For horizontal nails, you get most signal when the nail is under either side of the coil. Flux has a considerable horizontal component at these points. Hence the double beep you often get.
Ferrites differ in their response. The type of ferrite used in radio antennas and tuning slugs generally give a signal due to magnetic lag (viscosity). So called soft ferrites used in switch mode psu transformers, give no signal even though they concentrate the flux.
With a DD coil the TX field and the RX response is more complex than a mono when the target is close to the coil, but when at a distance the response simplifies.
Eric.