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Why No Concentric Coils For The Recent Multifrequency Detectors?


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On 1/23/2022 at 10:50 PM, kac said:

As for Concentrics there are standard 2 coil types Tx outer with Rx inner (reverse wound) and Co-planer where the Tx and Rx are wound flat and the Tx has a reverse wide only around the Rx creating a forced and well defined null.

I'm including some illustrations to help understand the construction (not necessarily the operation) of the different types of IB searchcoils.  First is general, from Lagal and Garrett p. 27 (1979) and note that DD (labeled a) is a specified there are a type of co-planar as is the concentric (d).  Later illustrations below go into some more detail but DD isn't covered:

coil-types.thumb.jpg.37bfda9c850f54fd4c4f97624bbb9da6.jpg

Next three are from Garrett pp. 127, 128, 130 (1985).  I've typed in the figure captions since they didn't scan well:

co-planar.thumb.jpg.47d5b5441dfa5093d3e717382a0e4e69.jpg

Figure 11-4:  ILLUSTRATION OF CO-PLANAR SEARCHCOIL WINDINGS (SAME PLANE).  This illustration of co-planar searchcoil construction shows that the windings are placed one on top of the other and lie in the same plane.  Co-planar searchoils have been in use for several decades.

(Note:  inner circular coils is labeled 'Receiver' and outer coil is 'Transmitter'.)

coaxial.thumb.jpg.9b82fbf8a6986df8280b5375be7f33b3.jpg

Figure 11-5:  ILLUSTRATION OF CO-AXIAL SEARCHCOIL WINDINGS (SAME AXIS).  Co-axial searchcoil windings are placed one above the other with one transmitter winding sancwiched in between two reciever windings.  This type winding placement gives the most uniform detection pattern of all VLF and TR detector types and is affected the least by outside electromagnetic field interference.

concentric.thumb.jpg.fe84a97429c9248c72e26ab278f3c62f.jpg

Figure 11-7:  ILLUSTRATION OF CONCENTRIC SEARCHCOIL WINDINGS (SAME PLANE AND AXIS).  Concetnric searchoil windings lie in the same plane and are centered about the same axis.  This newer type searchcoil is operationally equivelent to co-planar serachcoils.

Here are some details from the text in chapter 11 which may give some details of advantages/disadvantages of different types:

(p. 127):  Co-planar searchcoils are one of the preferred types because they produce very sharp, quick target response characteristics.  These characteristics result in excellent coin and small object metal detection capabilities.

(p 129):  ...Co-axial searchcoils give the best performance in areas of high voltage powerlines and electromagnetic interference.  (cut a sentence)  They are, however, slower response searchcoils.  That is, they produce a slower detector audio "turn on" time when targets are detected.

 

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Great read, I find DD's have a slight edge in depth but not by much and are quite often difficult to rely on in the iron trash areas I hunt. My detecting started with the tesoro machines and most came with a concentric and then a DD. Just my opinion but Tesoro's really shine with a concentric vs DD. Then came the Multi Kruzer, Which is an awesome machine. I ran the DD's for some time and had Pretty good luck with them until I bought the 9 inch concentric. It was a game changer for the MK. In all fairness the best DD I have ever used was the Nel superfly, oddly it almost seems like a hybrid of the DD and concentric configurations. As of lately I buy up concentric's at a high rate of speed, and fearing they will no longer be available. If I had to pick only one I would always choose a concentric. I wish minelab or someone else would step in and make a 9 inch concentric, I think it would make the 600/800 a machine that would be hard to beat in the performance arena. For now I run my tesoro's with concentric's for clean up in high nail infested sites.

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When it comes to favorite coils, 8x9 concentric on Tejon, 10x12 on the Tejon for deep hunting fields. The Superfly is a great coil and does incrediblly well on the Kruzer but on top of my list would be probably the Detech Ultimate 9" on the Apex. If they made a 9" for other machines I think they would do really well. It is incredibly emi resistant, very robust but not too heavy and has incredible separation.

9" round is a much better general purpose coil size, probably why most XP users stick with that. I find round coils in general easier to push around in vegitation and undergrowth making it a fantastic coil for woods hunting. In general I think 11" coils are a bit big for general purpose as they are harder to separate targets with in high trash areas, more prone to EMI and just heavier where a smaller coil can make the job more efficient and easier for the user. 11" are better suited for open fields and beaches.

I agree with Dogodog the concentrics do good on the Tesoros in iron infested areas.

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