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**Reg** · 03-31-2006, 03:01 AM

Hi Malac,

Your numbers seem a little low so, yes, you probably have some type of a problem. Now, what isn't known at this time is the source of that problem. It could be the coil, or something as simple as adjustments.

First, a 470 uh coil is a little high if you plan on trying to detect small gold items. However, I would think it would work just fine when using your wedding ring or a coin as a test target. So, lets look at a couple of things that you might be able to check if you have a scope.

If you do, what is the shortest delay setting you can obtain and still have the coil work? You should try to shoot for something less than 20 usec if possible. Anything too much longer than this and you will see a loss in depth on gold objects such as your ring as well as small coins, etc.

What is the width of the sample time? You should keep this reasonably short, maybe 10 usec to 20 usec max. If this time is extremely long then there is a serious reduction in sensitivity to gold items. Now, if the minimum delay is long and this sample time is also very long, the result will be a severe loss in sensitiivty.

What is the width of your pulse? This can have a negative impact on how well you can stabilize the detector as well as depth capability. Now if gold objects are what you might be interested in, then you can reduce this pulse with to as low as 50 usec. At this setting you should be able to come close to the numbers generally given for all targets. There is little need to go over 150 usec pulse width in most cases if this detector is aimed at finding small objects.

What is the time between the main sample and the second sample? This time needs to be reasonably long. One should have at least 100 usec between the two to begin with. Longer times normally will not cause a problem.

Did you adjust R 35 through its entire range. This adjustment can cause instability at one extreme and poor sensitivity at the other.

What are you using for a battery supply? The hammerhead draws a lot of current and this can cause problems including a sensitivity loss. Ideally, one should have some form of a regulator in the power supply so things remain constant. Another alternative is to install a current limiting resistor in series with the FET and the coil. This will reduce the coil current which will help stabilize things. It will also allow one to sample sooner. As strange as it may seem, reducing the current in the pulse may allow the detector to go deeper simply because of greater stability and the ability to sample sooner.

Just as an example, I can readily beat the distances measured for different items with one detector I have that has a 39 ohm resistor in series with the FET and the coil. Now, one would really think this resistor would severely reduce the sensitivity but in reality that isn't the case. Why? Well, a low resistance coil could only allow 1/2 amp max current at 50 usec simply because of the time constants involved. The 39 ohm resistor and associated circuitry may only allow 1/5 of an amp or maybe even a little less. Ok, what does this mean in terms of depth loss? Not much, especially if the delay can be reduced. Sampling at 25 usec when using 1/2 amp will not give much more of a real target signal than sampling at 15 usec with 1/10 amp coil current. So, one shouldn't be afraid of reducing the coil current as a means of reducing the sampling time.

Finally, if you are testing in an area where there is a lot of electrical interference, then the "noise" level can be great enough that it simply is stronger than the weak signals from a distant target. In other words, the detector has the capability to detect deeper objects, but the noise level simply masks or hides those signals. A poorly shielded coil can be a cause of something like this.

These are a few things to check out at this time. Once we have these answers, we can refine the technique to find the problem.

Now, as for shielding of a coil, one can use a lot of different things. Something as simple as a silver colored ribbon can be used if the outer surface is conductive. I have used it before and it will work. I happened to find some basic silver metallic colored ribbon that had one conductive side.

There are different types of fabric tape that will work but they are much more expensive and really hard to get in small quantities. One source is Lessemf. They do seel a CuNi fabric tape for about $20 for a 25 foot roll. See the following website:

http://www.lessemf.com/fabric.html

Just scroll down to the tape mentioned. BTW, I have used some of their conductive fabrics as a shield also.

Something as simple as conductive rubber tape such as 3M scotch 13 semiconducting tape could be used. I have picked some of this up on Ebay at a very reasonable price.

Once again, this should get you started in finding a solution to your problem.

Reg

**malacoda** · 03-31-2006, 03:49 AM

Thanks Reg, I'll check the items you've mentioned

Hi Reg,

Thanks for the input. I'll check the items you mentioned but it'll be a couple of weeks before I can do so.

This coming week at work is set to be very busy. The following week I'll be at the beach w/ the wife and kids. Once I get back I'll pull out the o-scope we have at work and check the reading you mentioned.

Also, while I didn't put much weight on it earlier I'm beginning to suspect - especially after your comments - there may just be too much noise in the shop at work (very populated area, less than a mile away from Dulles Int. Airport, etc.). I have some 3M copper shielding tape on order and should have just barely enough time to re-do the coils before leaving for the beach. The outer banks of North Carolina are pretty quiet this time of year and should offer little in the way of noise and interference.

So long as coil shape doesn't really matter, I think as the first troubleshooting step - given the opportunity of the area & trip, and lack of time to do anything else before hand - I'll finish the coils (vs rewinding them in another pattern such as basket weave) and try them out in the field to see if the results are any better.

When I get back - if the results haven't improved - I'll start testing w/ the scope.

regards,
Malac

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