Frank,
The most common coil used on a PI is the mono coil. Almost all lower priced PI's use this type of coil. Only the higher priced PI's that have a form of ground balance use other coils and when they do, it is a DD coil. A DD coil is used to help reduce the ground effect signals mainly.
I know of no PI manufacturer that uses the coplanar type coil on a PI. I think Eric Foster experimented with one when he was trying different things on his late PPD1 PI but gave up on them as far as I know. The reason they are not used is because they don't offer any advantage for the typical PI.
Personally, unless you have some specific reason to try to build coplanar coil for a PI, I don't recommend it. You really need a scope to properly align such a coil. Also, you won't see any benefit and most likely will see a depth loss among other things.
Coplanar coils are, for all practical purposes, used on a VLF detector. On a VLF they provide a little better discrimination than a DD coil and are easier to align and keep aligned. Also, a VLF can't use a mono but needs a coplanar, a DD, or some other form of coil that has a separate TX and RX which can be adjusted for minimal signal at the output of the RX with no target in the TX field.
If a PI uses some type of a coil other than a mono, it is almost always a DD type coil. Why? Well, the DD offers some improvement in noise and most important, reduces the effect of the ground minerals. This last reason mentioned is probably the main reason why a DD might be used. Now, a DD will generally provide less depth than a mono but may have some increased sensitivity to very small objects.
So, unless you have a specific reason to try to build a coplanar, you would be better off trying to build a DD type coil and making it work.
One other note, a coplanar coil uses the bucking coil is to minimize the transmit signal as seen in the RX coil with no target within the TX field. This RX signal is quite strong on a VLF so some means of eliminating the signal is needed. That is what the bucking is designed to do. So, on PI, there will still be a relatively strong TX signal seen in the RX winding but far less because of the delay time. On a PI or a VLF the DD coil has two windings that are positioned precicely to minimize this RX signal. Fortunately, on the PI, this alignment isn't quite as critical.
So, on a PI, if there is no major need for bucking coil on a coplanar coil or the DD windings do not have to be precicely positioned and the coils will still work fine. If you don't have a scope, then this is the most logical way to go.
As for modifying your detector, well this is somewhat complex and requires you cut at least one trace on your detector pcb. You may need to cut more depending upon how the layout is done. It helps if you have the schematic and can figure out just which resistor is the input resistor to the preamp. Then you will have to cut your PC board to sever the link between the transmit transistor and this resistor while being careful that you don't sever other parts wired to the TX circuitry. Once you cut the pc board, your mono coil will no longer work until there is some changes to it or the plug.
Once the link between the TX circuitry and the input resistor, you will need to wire this input resistor to a new pin on the new plug you will have to use. In the case of the 5 pin connector mentioned in the next paragraph, you will add a wire from the input resistor to pin 2.
It makes sense if you decide to do this to use a 5 pin connector like those used on the TDI or ML detectors and use the pin wiring like those on either of those detectors. Now, you will be able to wire the TX coil to two of the pins, usually pins 4 and 5 with 5 being the shield. The RX will be wired to pins 1 and 2 with pin 1 being the pin wired to the RX shield. Pin 3 is not used. Oh yeah, you will need coil lead wire that has two shielded leads and not the simple coax used on a mono coil.
Now, once you make these changes, you will have to be able to make the mono coil such that it will work again. To do this, the mono will be wired to its plug using pins 4 and 5 but will also have a jumper between pins 2 and 4. This jumper will only be used on mono coils.
I suspect your detector is quite similar to other older PI's like the barracuda or maybe the earlie surf pi so you might be able to use their schematic to get an idea of how your detector is built. As for making the mod, you can search the Geotech forum for mods done on the Hammerhead on how to modify it to use a DD coil. I posted quite a bit of info on how to do this on that forum.
So, as you can see, this simple mod to be able to use a coplanar or a DD coil on a basic PI isn't so simple.
Reg
