What Are Whites Df Coils Classed As Please ??

[RickUK]

I have a question regarding Whites DF coils on the TDI Pro or 'sl' and other variations,i am aware the difference between say a DD and Mono coils etc,but how would the experts describe the DF coils as in the stock and 7.5'' DF coils are these classes as 2 Mono coils within one another or have i got this completely wrong.

As far as i can tell both the inner and outer coils within the coil case operate complete independent from one another,anyone have knowledge on how one would describe these DF coils,of course i could be totally wrong and would like some help from the electronic whizzkids 🙂

Many thanks

[Jim in Idaho]

The inner and outer coils are wired in series on both coils, Rick. Not a huge difference between that and a folded coil. Reg Sniff developed the folded coil to get around White's patent on the DF. Pretty similar in performance, but in my experience the DF's have the edge in sensitivity.....especially the 7.5 over the 5 x 9 FM. I don't know if the DF's are mono coils, or not.

Jim

[RickUK]

46 minutes ago, Jim in Idaho said:

The inner and outer coils are wired in series on both coils, Rick. Not a huge difference between that and a folded coil. Reg Sniff developed the folded coil to get around White's patent on the DF. Pretty similar in performance, but in my experience the DF's have the edge in sensitivity.....especially the 7.5 over the 5 x 9 FM. I don't know if the DF's are mono coils, or not.

Jim

Jim,many thanks for your reply,that all makes alot of sense,will probably keep the 7.5'' DF coil on the TDI and also carry the Coiltek 14x9 on a spare full stem for a quick change over when i require some additional depth and coverage.

[Steve Herschbach]

A mono coil is one winding that alternates between transmitting and receiving. A DD has two windings, one that transmits, one receives. This separation of the transmitting coil from the receiving coil is what makes a DD coil less reactive to the ground.

As far as I know the dual field is all one winding, one coil inside another, that simultaneously transmits, then alternates to receive. So a mono coil. If one coil transmitted and the other received I guess it would be called a concentric rather than a DD just based on the configuration, although concentrics are normally associated more with induction balance (VLF) detectors. The difference there is that with a PI one coil would transmit, shut off, and then the other receive, in an alternating fashion. With an induction balance both coils would be in action continuously, one transmitting and the other receiving.

If you are familiar with mono coils you know they are very sensitive close to the winding. A large mono will signal a small item twice as it passes each edge over the small item. There is in effect a hole in the middle less sensitive to small targets. Large items tend to read more in the center. The dual field attempts to fill in the hole by placing a smaller coil inside a large coil. The windings on the smaller inner coil add small item capability to the center of the coil. However, by increasing the total length of windings in direct proximity to the ground the dual field enhances the mono coil sensitivity to hot ground that a DD coil alleviates by separating the transmit and receive coils. The dual field adds to both the strength and the weakness of a mono coil at the same time.... typical trade off we see in everything you try to do with detectors.

For a 12” round coil.....

Dual Field - best small item sensitivity, most reactive to bad ground

Simple Mono - less sensitive to small items in center area than dual field, less reactive to bad ground, excellent depth on larger items under most situations

DD Coil - Designed to help deal with bad ground, a 12” DD has a smaller receive coil than a 12” mono and therefore under normal conditions less depth, less sensitivity to small items.

In my opinion for most use a simple mono coil is the best solution for a PI detector, with dual field and DD being specialty coils. Dual field for small item sensitivity in milder ground, and DD for dealing with the worst ground.

[Skull diver]

For the same reason I'm really thinking to adapt an 8"mono to my Beachunter.

Actually I want to build it by myself if possible...I've seen that this GPX/TDI specs for coils are not so identical...

Needs to know all details before to do a complete mess 😂

[Jim in Idaho]

Good explanation, Steve. The engineers are now trying to get around the need for the DF by using spiral windings. But, it seems to me they're going to have the same sensitivity to ground minerals as the DF's. I agree the DF's are probably mono coils, though I guess you could make a DF concentric, though it would be complicated.:-)

Jim

[Steve Herschbach]

I decided to go to the source - the Dual Field patent.

"A new dual field search coil for pulse induction metal detectors has multiple coplanar wire coils of different diameters connected in series with the output of a pulse source, overcoming the loss of target size resolution associated with a single coil search loop. Small objects are sensed by an internal small coil and larger objects are sensed by the larger outside coil, and the overall depth of target sensing remains similar to that of a single coil construction. The smaller coil is isolated from the pulse source by the inductance of the larger coil and will ring at a frequency determined by its own parameters. To prevent the smaller coil from ringing, a second damping resistor is connected across the terminals of the smaller coil at the junction of the large and small coils making up the modified search loop."

"FIG. 5 illustrates the placement of the physical coils, with small coil 33 inside the circumference of large coil 35 and isolated from the electrical driving source by the inductance of the large coil 35.

FIG. 6 is the associated wiring diagram for the two-coil assembly with pulse source 37 and large coil inductance 39 and resistor 41 and small coil inductance 43 and resistor 45. The coils are connected in series. The value of critical damping resistor 41 is determined by the electrical parameters for the total coil assembly, whereas the value of damping resistor 45 is calculated from the electrical parameters of isolated smaller coil 33."

 

[Rivers rat]

Interesting post and devellopment ,funny enough just bought a 12" TDI coil for my G5c

Supposed to test today but the battery has die so went for the nox

 

RR 

[RickUK]

Thanks everyone who has contributed to this thread especially to Steve H who really provided some amazing information as usual,i initially thought it was going to look like a stupid question,i was aware how Mono and DD coils work on a Pulse machine but just could not see what category the DF coils would fit in but i do have basically the answer.

For me i will probably carry on using the small 7.5'' DF coil for small targets like jewellery and for almost all my other need for a Pulse machine the Coiltek 14x9 will probably do the rest,wont part with my other larger coils as those could come in handy sometime for locating that holy grail deep hoard/s 🙂

I for one have certainly learn't something,this is why is why i still think its my favourite forum,cannot abide FaceAche and rarely post on any other forum these days,so thanks Steve for operating one of the best detecting forums around.

[Jim in Idaho]

Nice work, Steve!  I think the resistors in that circuit are to prevent the coils "ringing" when the pulse turns off. Note both coils are wired in series, so the pulse fires them together. A folded mono is the same. You just take a larger diameter coil, and squeeze it together so it makes a figure-eight with one end half the size of the other. Make a half-twist at the juncture. Then you fold the small portion over on the large portion, and you have a folded mono. Electrically, they're pretty much the same. I don't know where they put the damping resistor on the FM.

Jim