DD coils

[dave]

Hi everyone , I couldn't find a relevant thread so Ive started this one . My question is probably for the more knowledgeable of you who understand theory well . Having built a couple of DD coils to a specific design I have noticed an anomaly . Originally the coil halves are wound with a straight back side ( just like the letter D ) . No matter what I tried the coils were EXTREMELY difficult to set up and gave inconsistent performance , as in very poor range and no detection of gold ( 1 gram 21 ct ear ring ). So becoming frustrated , I bent the backs of the coils in a curve ( as many DD coils are ) . Presto .... really good detection on small gold ( TGSL build ) , coins etc. Discrimination is very good on iron and aluminium . The machine is much easier to adjust and is more stable also. Now for the question .... could anyone provide an explanation of what is taking place ( even a theory will help ) as I have had to do this with both coils . The straight back design will not work for me . These coils are constructed well and shielded , but would not work without the curve in the back of the D. Looking forward to any answers to this question , cheers , Dave .

[radioman]

Hi Dave,
While I have absolutely no experience with DD coils used with metal detectors, your question is an interesting one to me from the perspective of changes in inductance in coils in general. Question to clarify: So when you bent the back of the coils in a curve, the coils' shape then resembled a circle, more or less, rather than in a "D" shape?
Thanks!
R-M

[Gary Lecomte]

How about a Picture of your before and after.

"I bent the backs of the coils in a curve ( as many DD coils are )"

[dave]

Only one draw back , now I have to make a new mold for the coil covers . Before ( didn't work very well at all )............ and after ( worked better than expected ) ......and It wont fit into my original covers . About a 30 mm gap or a mm or two less between the two inside edges and it work very well .  Disc on iron is good (most important to me )although as with a lot of detectors , once pull tabs at discriminated out so is small gold .Silver and coins are still detectable while iron and al are discriminated completely out at any range ( along with gold ) . Anyway I don't profess to know much at all about coils and their behavior but I measured an increase  in inductance when the coils are bent (or was it a decrease ... have to check that )  . Another large difference is the resistance of the coil halves . They are wound from 0.5 mm enameled copper and at the correct inductance mine have 10 percent of the recommended resistance ( 2-3 ohms measured ....22-23 ohms recommended ) .  Inductance is correct at 6.5 and 6.0 uh but as I mentioned the inductance changes when the coil halves are mechanically disturbed .                        ...  It all got the better of me and I went and checked the values ....RX original config. is 6.5 ohms and recommended resistance is 18 - 25 ohms !!!!  mine is 2.4 ohms ......TX original config. is 6.0 ohms and rec. resistance is 18 - 25 ohms , mine is 2.8 ohms .....After reshaping , Rx is 6.8 uh and Tx is 6.2 uh . So I have no idea why the coil works when deformed ( a pretty much standard shape ) and doesn't work well at all in the suggested form , made to the designers specs except for the wire which is supposed to be around .3 mm ( which accounts for the difference in resistances ) . So why , if the resistance is so low , does it work so well ? Or work at all ? Nothing on the board gets hot and the machine functions as I believe it should . It will be interesting to field test this one as I think it will do moderately well out here . Ok guys that's about all the information I have for you , cheers , Dave . 

[Gary Lecomte]

INTERESTING:

In the Past, I have made Numerous DD Coils, but never tried changing the Angles like that.
Maybe I'll try it on my home made Bandido detector.

My personal opinion is Resistance does Not matter.
Just the inductance and the Associated Capacitors to get correct frequencies, relative to the Detector.
0.5mm wire is a 24 AWG. That is Fairly Large for this type of coil.
A 0.2mm (32 AWG) might be better.
And Litz Wire will probably also help for these higher frequencies.

Now I mostly just like PI Detectors.
And don't worry about discrimination.
Just because something isn't Gold, Doesn't mean it is Not Valuable.

[dave]

Thanks Gary , I will try .2mm wire. Any advice on the type of litz wire I should use as there are many types and gauges . Also, could you tell me what TPMS22 wire is as I cant find any references to it other than a mention on a video of minelab's use of it pre-dating the use of litz wire in their coils  . Cheers , Dave .

[Gary Lecomte]

Pi Detectors require a Larger wire because they put out Higher current to the coil.

Basically all the other types of detectors only need a Small gauge.
Any Gauge between 30 to 40 is OK, But working with 36 to 40 is getting pretty small and easy to break.
The Greater to number of Strands in the Litz Wire, Allows for the Highest Frequency.
But your detector is not really high in frequency, so almost any litz wire would be OK.

Sorry, I have never heard of that TMPS22.
But do a Google Search for "TPMS22 Wire". INCLUDE the Quotation Marks as Shown here and you will find some info on it.