? on DD coil on the Hammerhead

[southernexplorer]

Is it possible to modify the PCB to use and experiment with DD coils?
Thanks
southernexplorer

[Reg]

Hi,

Yes, the PCB can be modified so a DD coil can be used. Once modified, then a mono coil connector will have to be modified so it will work.

A plug should be used for the coils so they can be changed. Now, on a mono coil plug, a jumper has to be added to reconnect things.

To modify the pc board, the receive circuit has to be separated from the transmit. This is done by cutting a trace on the board. The logical trace to cut is the one between R 11 and TP 2. Then TP 2 can be used as the connection for the receive coil on the DD coil. If my pic comes through, you will see a red dot indicating the location where the pc board should be cut. The red dot is simply a marker to show the location and is not part of anything.

Reg

[Reg]

Hi Again,

Once the pc board is cut, then one has to connect the DD receive coil to TP 2. The receive coil does need a damping resistor that can be mounted in the coil housing or better yet, in the coil connector. This way each coil can be tuned to the PI. This allows for a wider range of experimenting.

The transmit coil still needs the original damping resistor, so the new schematic would look basically like the pic attached. I have just shown the transmit and receive coil circuitry only for simplicity. The rest of the schematic of the HH would be the same as original.

On the attached pic, L1 is the xmit coil and L2 is the receive coil.

Reg

[southernexplorer]

Thanks. I am waiting for a couple coil housings to come in so I can finish my project, and do some testing and probably some simple mods. also. I have way to many pots mounted outside the case. It can get confusing.
southernexplorer

[Reg]

Hi All,

Anyone wanting to switch coils should use a connector. This way, not only can the coils be switched quickly, but by adding a simple jumper inside the coil connector, one can go back to the mono coil design when desired.

By looking at the modified schematic, pins A and C of the connectors would connect to the transmit coil. Pins B and D connect to the receive coil. By adding a jumper between A and B, and not having a separate receive winding, one can simply use the mono coil. The connector half mounted on the control box would never have to change. Any changes would be done in the connector attached to the coil housing. By doing it this way, the change is transparent and simple to do.

Since most pins in typical plugs are numbered rather than lettered, then one would simply substitute the pin numbers for the letters in the connections. A simple and easy to get connector is the mic connector. Minelab uses a 5 pin mic connector for their PI's. I use the 4 pin since it is a little easier to work with.

Hope this makes sense.

Reg

[Largesarge]

I'm using Radio Shack P/P's 274-001 & 002 for both power and coil. The nice thing is they're separate items so you don't have unused pieces lying around.

[Reg]

Hi All,

One of the question often asked is about the inductance of the receive and transmit coils. For simplicity, one can make the transmit coil 300 uh and the receive coil 450 uh. This works very well.

I have made both the transmit and the receive coils the same inductance and the coil worked well also. At that time, I was making both coils 300 uh. However, by making the receive coil 450 uh, one gains a little on sensitivity. It isn't much but every bit helps.

One can vary the transmit and receive inductance a lot and make the PI work well. The main difference will be the minimal delay that you will be able to obtain. The larger the transmit and receive inductance, the longer the delay before sampling will be needed.

A DD design separates the transmit from the receive which makes it easier to get faster or shorter sampling times, so you can get shorter delays with a DD than with a mono with the same inductance. This is one of the advantages of the DD. Also, the ground signal is less on a DD, so there is less ground signal at shorter delays.

As for winding and aligning a DD coil for a PI, just use decent winding techniques for each set of coils. Alignment can be done to minimize the signal out of the receive winding by injecting a signal into the transmit winding and looking at the receive winding with a scope. Adjust the overlap such that there is a minimal of signal out of the receive.

If no scope is available, then one can simply overlap the two with about 1" or more of overlap and try it. Be careful of polarity. Always use a non-ferrous target and set the polarity for a positive signal when the object is passed over the overlap zone. A nickel makes a good target.

The amount of overlap is not that critical, but I would always have more overlap than less. If the overlap is not sufficient, then the target will have to drive the signal through the null point before being heard correctly. So, the overlap shouldn't be less than the null point if possible. That is why, when in doubt, have too much overlap. Even with too much overlap, there will be a large reduction in the ground signal.

Finally, make sure to shield the coils if short delays are required. I shield each winding, using a special shielding tape. I also insulate each winding from the other so the shielding of one doesn't touch the shielding of the other. In other words, each coil is fully insulated from the other. Both have their own coax and even the coax grounds do not touch in the coil housing. They can be tied together at the plug, as long as the connector isn't far from the pc board. Just do not connect the coax's together inside the coil housing, if you want to get short delays.

Reg

[Reg]

Hi All,

For those of you who decide to try to use a DD coil on the HH, you might want to experiment with a few things. First, I find that the round housings usually will produce a coil that is "quieter" as far as ground signal is concerned. I prefer the 11" ROUND housing as the best for making a coil that is less ground sensitive.

Also, you might want to experiment with the turns ratio also. I have found the 1:1 ratio seems to be quieter, ground sensitive wise, than others that I have tried. I usually build a 300 uh transmit coil and have tried receive windings that vary between 300 uh and 450 uh. For some reason, the higher uh are more ground responsive. Again, I am not sure why.

Now, more experimenting should be done in this area to find the ideal combination. Unfortunately, I do not have the time to do that, but I did want to mention some of the quirks I have found. Unfortunately, I can't explain just why certain things happen.

Overall, I think you will find the DD coil helps tremendously in reducing the ground signal. This is most obvious when the delay is very short.

Once the detector is modified to accept DD coils, then you can try other variations of coils such as the figure 8 design. This design is also known as an anti-inteference type coil, ground balancing coil, or a noise canceling coil. Such a design in coils makes the detector even less sensitive to ground responses.

From there, you can experiment with other coil formations such as a 3 coil receive I mentioned on another thread. This type of design ground balances also and is less tilt sensitive.

Reg

[6666]

Thanks to Kt315 for finding this thread, and to Reg for info.

[Kevin Heieis]

Hi I want to add the double D mod to my Hammerhead but I'm still confused on how or where to add the damping resistor for the second coil. Does the resister just need to be connected to the coil, go across both sides of the coil, or what? Any help is appreciated.

[Qiaozhi]

Hi I want to add the double D mod to my Hammerhead but I'm still confused on how or where to add the damping resistor for the second coil. Does the resister just need to be connected to the coil, go across both sides of the coil, or what? Any help is appreciated.

Both coils will need their own damping resistor. For each case you need to connect the resistor across the coil.

[kt315]

Both coils will need their own damping resistor. For each case you need to connect the resistor across the coil.

not so good decision for RX coil. you can use simplest RC filter you see in VLF type design coils. R set in serial to coil, C set after R on input of opamp.