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One of the most commonly asked questions by X-TERRA metal detector users is, 'which coil is best for me?' Although that may seem to be a fairly simple question to some, the coil that provides the best results in one part of the world won’t necessarily be the best choice for another. And the coil that performs best for one aspect of the hobby won’t necessarily provide the best results for another. Reaching the right conclusion can be complicated until you understand the differences in coil design and application.
We’ve all heard the saying that there is the 'right tool for the job'. In a sense, the same can be said for metal detector coils. Choosing the 'right coil for the job' may seem a bit overwhelming at first. But once you understand the differences between your coil choices, and match that information up with the sites you hunt and targets you are seeking, you’ll not only find that choosing the proper coil can and will enhance your chances of success, you’ll understand why as well.
There are several generalities that hold true for each of us, regardless of the make and model of detector we are using. One is that, in soil with high levels of magnetic mineralization, a coil of Double-D design will provide better results than a Concentric coil. Other generalities are that larger coils will detect larger targets at greater depths, smaller coils are more sensitive to smaller targets, smaller coils separate targets better than larger coils and Double-D coils separate targets better than similarly sized concentric coils. Many of us who have been detecting for years have already figured these things out, to a certain degree. But with the advent of the X-TERRA, capable of operating at one of three different frequencies, the question of 'which frequency best serves my needs' now comes into play.
X-TERRA coils are available in three frequencies 3 kHz, 7.5 kHz and 18.75 kHz. In addition to the three frequencies, X-TERRA coils available in Double-D design and Concentric. As mentioned in the previous paragraph, Double-D coils are designed to provide more operational stability in soil with high levels of magnetic mineralization, as compared to concentric coils. This is because Double-D coils are constructed with overlapping windings, one wound in reverse of the other, thus minimizing the effects of the mineralization.
The manner in which I make the determination as to whether the site I have chosen to hunt is considered highly mineralized or moderately mineralized is to simply perform a Ground Balance using my stock 9-inch concentric coil at 7.5 kHz.
When you are Ground Balancing the X-TERRA, the numbers that appear on the screen represent the phase angle of the ground. For the sake of this discussion, I’ll refer to them as the ground phase numbers. The larger the ground phase number, the weaker the influence of magnetic mineralization. A smaller ground phase number would represent a stronger influence of magnetic mineralization. In sites with moderate levels of magnetic mineralization, Concentric coils will provide a greater depth of detection than similar sized Double-D coils. And, in those moderately mineralized sites, Concentric coils will also operate with more stability and provide a more accurate Target ID. However, as easy as that sounds, there are times and places that some of you will not be able to maximize the X-TERRA’s effectiveness with a Concentric coil, regardless of which model you are using. Although there is no magic wand for determining whether a particular site is to be considered moderately mineralized or highly mineralized, I’ve developed a 'rule of thumb' that you might find useful in deciding whether or not a Double-D coil would be beneficial. I’ll explain that theory, as well as how you can implement it at any site you hunt, in Part 2.
Randy Horton (Digger)
You'll never know for sure......unless you dig it!
This is just what I was looking for! I Ordered my X-TERRA this week and want to be ready as possible the day it arrives. Mine is the "Gold pack" so, I have only the one coil. But I need to know HOW to optimize the use of it. I eagerly await part 2 of your article. Keep it comin' !
I just received my new X-TERRA 705 and have been out a few times with it finding a lot of clad and a 1952 silver dime.When I do the ground balance using auto mode it comes in at 22 if I remember right! Don't know if that is high or low ? But I hunt allot of big open areas (fields) and was considering a 15" coil.I live in the Mid western part of Illinois just across the river from St.Louis Mo. Just to give you a reference point. I don't think mineralization is a big problem for me here,Would the 15" serve me well here.I have a lot to learn about using the 705 and am fairly new to metal detecting so please excuse any UN ordinary questions I may ask.I have read your book from this site and will also be doing some air testing.But can not locate any info on the numbers for ground mineralization what is high or what is low.Like your info and still reading all I can find on the 705 thanks for all you add to the MDing info.