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coplanar DD loop disclosure

A

Anonymous

Guest
This is probably prior art, but here goes anyhow.
The customary way of building a DD loop is to wind two D-shaped coils of similar shape, but usually different inductance. Then one coil is installed in the assembly overlapping the other such that a condition of induction balance is produced.
The advantages of this construction are simplicity and approximate bilateral symmetry. The disadvantages are lack of coplanarity which leads to the need for a housing thicker than would otherwise be required, difficulty in achieving a good reactive null due to the overlap of the coils, and difficulty in maintaining reactive balance due to the overlap of the coils as well as asymmetry of the assembly in the vertical plane.
An alternative construction is to wind a "feedback" coil either inside or outside the receiver coil, in the manner which is customary in concentric coplanar loop contruction. The feedback coil is wired in series with the transmitter in the manner which is customary in concentric loop construction.
If the proper amount of feedback is furnished, the coils will balance lying in the same plane without crossover. This makes thinner contruction possible, makes nulling easier, and makes it easier to produce a loop which will be stable. Another advantage is that the dipole created by the feedback winding is in phase with the transmit winding (the receiver being entirely within a negative field zone), increasing the total dipole moment of the transmit field.
One disadvantage is that the bilateral symmetry is broken. In practice, this is probably a disadvantage of no major importance.
Another disadvantage is that the response close to the loop is highly distorted, which can impair target identification and pinpointing. The problem can be partially resolved by adding additional "pinpoint" coils along the centerline either between the D's or in a plane in front of the D's, but the benefit might not warrant the additional complexity.
Some of the double-D tricks mentioned in my earlier post are difficult or impossible to do with the coplanar arrangement.
--Dave J.
 
Hi Dave,
Wow!, you are definitely a thinker. Would not have dreamed of getting all this out of a couple of coils. Keep it up, sooner or later you are going to trip over a really great idea.
Guess part of the advantage of the DD in mineralized or suseptible soil is that the receive coil is overlapping the transmit coil to achieve null and therefore sees the ground excited by both positive and negative flux (not a good way to say this I know). The ground if uniform balances. Hate to lose this advantage in whatever scheme.
I also like DD over the large transmit, small receive coil arrangement normally used because part of the transmit winding is NOT wound around the receive coil. I can't give you the scientific reasons, but my experience has been to not get the sensitivity I could with plain DD as any arrangement that had part of the transmit coil wound around it.
The DDs not being in the same plane at the overlaps does mess up the nulling, and make things fatter. The simple solution I know is to put half the receive overlap above and half below the transmit, good to first order. DDs work good.
There is a patent out there (of course) which describes doing target ranging with two receive coils one small, and one larger, concentric. Garrett references it (pays him some money) in their patent on the GTI 2000 , GTI 1500 series "imaging" detectors. These detectors are doing real DSP, have some 33 Mhz Analog Devices DSP chip in em, or something like that. Anyway only guys to actually sell a DSP detector (definition - DSP does not mean it has a microprocessor in it) which works. Heard many tried, many died when their cheapest analog detector would beat it. Or Garrett's got to get their million bucks development money back.
JC
 
Hi Dave and all,
A method of final balancing that I found successful with a coplanar coaxial coil, and which also would work with a DD, is to position a small piece of ferrite in the coil assembly. This I used in the later prototype of PPD2 which doesn
 
Thanks, Eric.
The big limitation of printed circuit coils is in getting enough copper there to achieve a high Q. If one is not looking for a high Q, of course, then that isn't a limitation. I believe the Tesoro PI units use a printed circuit coil.
One reason that most DD coils don't have a straight "back" to the "D" in the overlap region, is that it's difficult to get wire to compress and bond properly on a flat mandrel surface, because there is no tension holding the mass together.
--Dave J.
 
Hi Dave and Eric,
here is an interesting printed circuit DD coil you may not know yet (for full access your browser must have a PDF plugin installed):
http://l2.espacenet.com/dips/viewer?PN=US5245307&CY=ep&LG=en&DB=EPD
Click on "Drawing" at the top of this page, and then "Next" as the first drawing is not interesting.
Alternatively for a "standard" patent layout:
http://l2.espacenet.com/dips/bnsviewer?CY=ep&LG=en&DB=EPD&PN=US5245307&ID=US+++5245307A1+I+
This was first published here in Germany in 1990. The layout is quite unique (my printed version has a much better resolution showing all the tracks). They claim that is has a very low stray capacitance as the coil differential connection is formed separately for each winding. But read the whole description ...
Thomas
 
Hi Eric and all
I have noticed another way of final balancing of a DD dual coil system that works very well and is very practical to implement.
At the backend of the cable where you connect the detector PCB, you place a small adjustable capacitor (4-80pF) from the transmitter "hot" wire across to the receivers input.
Adjust the capacitor to approximately half.
Then balance your two coils physical as good as you can and glue them.
If you now turning the capacitor left or right you can trim the balance for in and out of the "null"
You can replace the capacitor with a solid one if you can measure or find its final value.
If you replace the capacitor with a solid one after the value is known and solder it in the connecter, you will be able to switch between different coils too.
Mark
 
Thanks, JC.
DD loops do better than concentrics on ground pickup. Here are my two preferred explanations. Others may have better explanations (Eric?). I suppose that you could come up with a whole handful of explanations that sounded different, but which were really different viewpoints of the same thing. The friendly brawl we had in this forum over the question of "does transmit matter?" in early January seems to have been of this character.
1. In a DD, the axes of the transmitter and receiver are not congruent. (The rest of this statement I am withholding as my own trade secret, but I suppose other people know it, and welcome them to post it here.)
2. Up close to a DD loop, the orientations of the lines of flux of the transmitter (and the corresponding pattern of receiver response) tend towards orthogonality in the region moderately close to the loop. A concentric loop has no such corresponding advantage in that same region, which (for a concentric coil) is where most of its dirt response comes from.
Note: Although I haven't proven this hypothesis, I have a theory that if an ideal coplanar IB loop of any type were resting on the surface of uniform ground such that the surface of the ground were coplanar with the coils themselves, the loop would be in a condition of induction balance. By playing games with naked coils half-buried in sand, I have obtained empirical evidence that the hypothesis is at least good enough to use as an aid in intuitive understanding.
Any analysis of what a searchcoil might do in real conditions must account for the fact that the surface of the ground, although it might look nearly coplanar to an uninformed person, is nowhere near coplanar as far as the coils are concerned.
--Dave J.
 
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