Using Large Detector Coils
Electromagnetic Interference (emi)
Towns, Power sources and EMI:
This is not the problem that many imagine it to be. Certainly, you will not be able to use these large metal detector coils too close to town, homes, or other power sources. In these situations you will need to allow several hundred meters, and in cases of towns etc up to 500 plus meters. High voltage power lines more. If you are metal detecting near a road there may be an occasional large vehicle that will kick up a fair bit of emi, but in my experience that is the exception. Occasionally, as with any coil, you will get a breeze or wind that is loaded with emi. Just wait it out; they seldom last more than 20 -30 seconds.
Other metal detector operators will not be able to work closer than approximately 100m from you if you are using the 35″ coil and upwards. A bit closer for the smaller coils.
Tuning and EMI:
The most important thing is to use the tuning. In my opinion the Auto Tune button is a good friend. The model I did most of my testing on was a Modded GP Xtreme, and this does not have the manual tune. Later models do. Use whatever method you can to get the best tuning. With the GPX, in most areas I could get excellent quietness for emi, and only rarely had to put up with some chatter. It was no where near the emi effects I had expected.
How to tune with large detector coils: I found the best method to get the optimum frequency is to place your coil with one edge ON the ground. Hold the coil at 90 degrees to the ground and do not move it once you have begun the auto tune process. This will give you the best frequency for a combination of emi and any soil conductivity. It is very effective.
You may not get the best results if you hold your coil in a vertical position up in the air.
Target Locating and EMI:
If you are digging a reasonably deep target it is an advantage to have other smaller coils handy. Once you are within a reasonable distance you can use whatever is suitable to pinpoint your target better. This will save you a lot of digging.
Soils and Mineralization:
From my testing to date you will be able to run up to a 40″ anywhere you can run an 18″. I was working with someone who had a 4500 using an 18″. Anywhere that combination could be used I could use the field testing 38″ coil with no problems. The 48″ field tester provided some more challenge, but perhaps 85% of the country where the 18″ could be used was also fine for the 48″ on the Modded GP.
With either a modded detector, or a later model, you will be surprised how quiet you can run your machine with a big coil, but you do need to spend the time on tuning and finding the right settings. We are in the process of getting more field results.
This is where the very large coils remind you about emi. When you hold them edge on they make a wonderful emi catcher and become bits of chatterboxes if there is much emi around. This does not effect your detecting because by the time you try to locate a target in this way it is near the surface – or should be. You will not hear any deep signal with the edge of your large mono, or even smaller mono’s for that matter.
Comfort of Use:
We recommend using a HipStick with large detector coils. When you do this you can eliminate almost all stress on your elbow and arm muscles. The main thing you need to focus on with any large, or heavy coil is the inertia as you come to the end of your swing. With large coils you should be swinging at a gentle, slow speed anyway, but it is important to ensure your coil does not roll at the end of your swing.
If you want to get really serious about giving your metal detector perfect balance you can do what we did with our field testing detector. In the image below you can see the armrest is well in front of the detector head. The shaft is also much longer. We replaced the entire Minelab shaft with a carbon fiber shaft. and then placed a carbon fiber armrest in front of that. This meant the metal detector head could be extended behind the elbow with the armrest further down the shaft without bringing the operator too close to the coil. This allows for easy stepping without kicking into the coil. On the metal detector pictured below the shaft actually extends a further 3 inches past the head, so when using a 1.2 m carbon detector coil the head can be moved back further to create a great center of gravity. The carbon fiber shaft and armrest reduced the weight significantly, and the head being further back alters the center of gravity so there is less weight to be absorbed by the bungy cord and HipStick.
The only metal on the entire detector is the head (control box), the battery, (a small one velcro fixed to the head), and the screw in the hand grip clamp.
Locating Targets (small):
With large detector coils there are differences on how large and small target respond. A small target will give two separate signals as each side of the coil passes over it, and if you are using a 50 inch coil that makes a very large area in which the target could be. Small in this case is “not large enough to maintain a response as one edge of the coil passes over and before the other edge reaches it.” Of course, a decrease in coil size means it takes a smaller object to maintain a signal from one edge to the other as the coil passes over. The exception to this is if the front or rear edge of the coil pass over the target. This will give one signal, which because it is going “along” the edge of the coil gives a wider signal and may sound more like a deep target. Illustration 1 shows the two responses that can come from a target as the coil passes over.
Illustration 1 shows the double signal from a single target as edges A and B pass over it. Arrow C shows the blank area in between target responses. The larger the actual target the narrower this area C will become. Once a target reaches a strong enough response level (big enough), the two responses will join and give the characteristic pitch variation of a deeper target as your coil passes over it. Effectively becoming one signal.
If you wish to find the “location” of a small target which has two signals as in 1 above, the first thing you can do is tilt your coil slightly so that it is closer to the ground one side and further from the ground on the other. As you pass over the area where you approximate your target to be you will receive either one signal from the side closer to the ground, or one strong signal from the close side and a weak one from the other. You can also tilt your coil back or forward in the same way to check whether the target is toward the front or rear of your coil. As you can imagine, a small signal bouncing of one any one edge of a 40″ coil means there are up to 4 square meters in which your target can be positioned. By tilting your coil and passing over this area from different directions you can create a cross hair and narrow it down to an approximate point.
Once you have determined a rough area for your small target you can hold your coil up on its edge and cross the target from directions at 90 degrees to each other. As with any mono coil, this gives you an imaginary “cross hair” over your target. One thing to note with this is that when you turn a large coil on edge you get a lot of electromagnetic interference, which can be a little distracting.
Locating Targets (large):
Locating a center for large targets is simpler than finding where a small one is because the larger target only gives off one signal as the coil passes over it. This means that the usual method of running the coil over the area from two directions at 90 degrees to each other gives a good “cross hair”. However, it is worth noting that the width and pattern of a signal will change with the depth (and size) of a target. Illustration 2 and 3 show the signal patterns for a target at medium depth and maximum depth. (for a given size)
Notice that in illustration 2 the “signal” received by each side of the coil A and B is crossed over at C. This crossover is where you get the characteristic “dip” or variation of pitch in the signal. This “dip” will get narrower as the depth of the target increases. It is also important to note how wide this signal is. Given this coil is over 1 meter wide the signal can be up to 2 meters wide. It can go even wider, depending on depth and size. As the depth increases the width decreases. See 3 below.
Note in illustration 3 that the signal is narrower than the coil, and “thinner”, or only detectable from closer to the ground. This signal is easier to miss, and can also be lost in ground noise if you are not running your machine as quietly as possible. Also note that A and B are gone, and there may be no signal beneath the edges of the coil.