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**Reg** · 10-19-2007, 04:25 AM

Hi Oscar,

Building a DD coil is not hard. In fact, it is quite easy since one can be sloppy even with the positioning of the windings. Fortunately, a PI will allow for less than perfect positioning of the two windings and still work fast and well.

Now, I normally use a 300 uh transmit winding and this is wound much like you would wind a mono coil. The receive winding can have a different inductance if so desired. I have used receive windings of 300 uh to 450 uh and they all have worked well.

I do recommend you install the receive winding damping resistor inside the coil, especially if you plan on using different coils on the detector. Typically, a resistor value of 1K to 1.5K can be tried as a damping resistance for initial testing. Just simply connect this resistor across the leads of the receive winding.

The windings are polarity sensitive so care has to be taken to assure the connections are correct. Now, when you are determining the correct polarity, make sure to use a non ferrous object.

So, wind your two coils, position them such that they have about an inch overlap to begin with and then connect them to the circuit. Pass a non ferrous object over the overlap to make sure you get a positive response. If you do not, then simply switch the connections on the receive winding and test again. You can switch the transmit windings also to obtain the same results. Just make sure that any winding shielding used be connected to the shield of the coax.

You will notice when testing that a ferrous object will give an opposite response than that of a non ferrous object, so if you try to set the polarity using a ferrous target, the signal will be incorrect.

If you are using a scope, you can position the receive winding such that you do get a minimum response or signal out of the coil or at the output of the preamp if you are trying to obtain the fastest coil. Fortunately, even if the windings are not "nulled" completely, a DD coil will generally be faster than a mono coil. In fact, I have deliberately built coils that are not nulled to assure a decent signal from a very small piece of gold.

Now, I have found that a round housing will generally work a little better than an elliptical if you are trying to reduce the ground signal. Also, the round housing will usually provide a little more depth than an elliptical also.
A good size is the 11" round housing such as that made by Bill Hays.

Now, as for connecting the windings, I do recommend you do try to keep the transmit and the receive separate in the coil. This includes the shielding of the windings and the coax. So, there will be no connection between the transmit and the receive except at the connector or pc board.

I also shield each winding separately and insulate the two windings from each other using something as simple as plastic electrical tape.

If you tie the shields together inside the search coil, you will find your coil will not be as fast as one where the transmit is isolated from the receive, so it is important.

Make sure each winding is wrapped such that the windings are not loose. I lace mine and use spiral wrap to assure they are very snug. If the windings are loose, they can vibrate when jarred and cause false signals.

I hope this helps a little. Don't be afraid to experiment a little to find what works best for you.

Reg

**Oscar** · 10-20-2007, 07:13 PM

Thanks Reg

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Making a DD coil for HH

Making a DD coil for HH

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