Coiltek Manufacturing recently invited me to help field test a new 15-inch ALL TERRAIN 7.5 kHz coil for the X-TERRA Series. Having been involved in field testing the first series of the X-TERRA Series for Minelab, and recognizing the number of X-TERRA users who had expressed their interest in a large Double-D coil, I welcomed the opportunity. I’m familiar with Coiltek products, having used several models on various detectors. So when I opened the box and saw the large, round, 15-inch Double-D coil designed for the X-TERRA, the orange color did not surprise me. Instead, I was encouraged that this new product might allow me to successfully revisit many of the sites I’d previously hunted with smaller coils.
My primary responsibility was to evaluate the coil and analyze any differences I might find in comparing it to Minelab’s 9-inch Concentric coil at 7.5 kHz and their 10.5-inch Double-D at 7.5 kHz.
The Coiltek 15-inch ALL TERRAIN coil at 7.5 kHz has an actual diameter of 37cm (14 9/16 inches). Although it looks much larger than any other coil currently available for the X-TERRA, the thin lines and Double-D design carries a weight of only 697 grams. (1lb. 8.6 oz.)
The 10.5-inch Minelab X-TERRA Double-D at 7.5 kHz weighs 572.66 grams (1 lb, 4.2 oz).
And the 9-inch Minelab Concentric at 7.5 kHz weighs 521.63 grams (1 lb, 2.4 oz.).
As I mentioned, I’ve used a variety of Coiltek coils on several other detectors. Specifically, I’ve used a similar Coiltek 15-inch round Double-D coil on both my Advantages and Sovereigns. Each of these detectors allows the user to position the housing control box under the arm-cup, providing a different fulcrum point for use with larger coils. In other words, on the Sovereign and Advantage, I can counter balance the additional weight of a large coil by moving the control housing back under the arm-cup. The design structure of the X-TERRA does not allow the user to move the control box to another location. And even if it could be mounted further up the rod, the minimal weight of the control housing would not likely ‘offset’ the additional mass of the 15-inch All Terrain coil. As such, I found the 15-inch Coiltek coil to be somewhat ‘nose heavy’ and challenging to operate for long periods of time. Especially in some of the rural places I hunt. Granted, I’m not as young or physically fit as I once was. And the additional mass of this coil may not provide a challenge to you. But for me to get the most out of my hunt time, when I use this coil I’ll need to get a body harness or bungee arrangement that will take some of the weight off of my arm and shoulder.
Depth vs. Distance
I believe there are too many variables to allow for an accurate comparison of the ‘depth of detection’ for different makes or models of detectors by the use of ‘air tests’. However, I do believe we can gain useful information by comparing different coils on the same detector, using similar settings.
With my X-TERRA 705 suspended horizontally, I placed a wooden ‘yard stick’ perpendicular to the connecting point of the lower rod. As many of you know, the first ‘setting’ I make when turning on my X-TERRA is to establish the proper Noise Cancel channel. Understanding that a detector coil can serve as an antenna for outside interference, I was not surprised when my X-TERRA 705 automatically chose a different ‘channel’ for each of these three coils (Different size, shape and design). For each ‘air test’, I set the proper Noise Cancel channel, selected the hunt mode, adjusted the Sensitivity to 30, set the headphone Volume to 30 and put the detector in Tracking. For the next several hours, I passed a variety of US coins, pieces of jewelry, modern ‘conductive’ trash and ferrous targets below the coil at varying distances. I repeated each test using various discrimination Patterns and different Sensitivity settings.
Larger coils will typically detect larger targets at a greater distance than smaller coils of the same design, shape and frequency. Conversely, larger coils are generally not as sensitive to smaller targets as smaller coils might be. With all settings being equal (other than Noise Cancel channel), I found the Coiltek 15-inch Double-D ALL TERRAIN Coil at 7.5 kHz provided a target response averaging 25 – 30% more ‘distance from the coil’ than the same target provided on each of the smaller Minelab coils (note that I did not say depth?). As expected, larger targets provided a greater difference in distance and smaller targets provided less difference in distance between the target and the respective coil.
Target Separation
To better explain my thoughts on target separation, let’s use the face of a clock as our basis... If two targets (with two different conductive values) are buried 8 inches apart, at the 3 o’clock and 9 o’clock position, then I would agree that most Double-D coils will separate those two targets in a similar manner. However, what if those same two targets were buried at 6 o’clock position and the 12 o’clock position? At that target location, any Double-D coil larger than 8 inches in diameter has the potential to ‘hit’ them both at the same time. If one is rejected or if one is much larger than the other, we risk missing the smaller target due to the masking effect of the larger or rejected target. That is precisely the reason that I recommend using a small coil when hunting areas that contain a great number of adjacent targets. Smaller coils simply have a better chance of working between targets than larger coils.
In Part 2 of this blog, I will share actual field experiences (and a few finds) I encountered with the Coiltek 15-inch ALL TERRAIN X-TERRA coil. One thing I would like to say in advance... I was pleasantly surprised with its target separation capabilities, considering its large diameter.
HH Randy
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