Coiltek 15-inch ALL TERRAIN coil for the X-TERRA Series – Part 1
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.
Coiltek 15-inch ALL TERRAIN coil for the X-TERRA Series – Part 2
In Part 1 of my blog post concerning Coiltek’s new 15-inch ALL TERRAIN 7.5 kHz coil for the X-TERRA, I described the physical differences between it and two other 7.5 kHz coils available for the X-TERRA. In this portion of my blog, I will discuss the performance of the Coiltek coil in places I typically hunt for old coins.
As mentioned in Part 1, the Noise Cancel channel capability of the X-TERRA seemed to eliminate any interference that might have been encountered with the large Coiltek coil. Ground balancing the Coiltek coil, compared to the 10.5-inch Minelab Double-D at 7.5 kHz was nearly identical. In other words, if I was able to properly Ground Balance my X-TERRA 705 with a ground phase reading of 28 on the 10.5-inch Minelab Double-D, the Coiltek coil would also properly ground balance with a ground phase of 28. Raising or lowering the Sensitivity did not have any effect on the ground balance procedure or the end result ground phase reading.
On larger targets, the Target ID (TID) was very consistent with all three coils. However, on smaller targets (such as a silver US dime), the TID would ‘bounce’ more with BOTH Double-D coils, compared to the 9-inch Concentric coil. Although I was able to overcome a portion of this variance when implementing TID Stability on the 10.5-inch Minelab Double-D, the Coiltek coil was not as responsive to TID Stability. Again, I believe this might be attributed to the greater volume of soil that is being analyzed, at any given time.
I found that to minimize false signals on the 15-inch Coiltek coil, I needed to operate with a slower sweep speed than with the Minelab X-TERRA coils. The Coiltek coil also required a lower Sensitivity setting (on the same soil) as I was able to run using either Minelab coil. Keep in mind, even though the diameter of the Coiltek coil is approximately 50% larger than the Minelab 10.5-inch Double-D coil, when you factor in the increased depth of detection, the volume of soil that must be analyzed at any one time is far greater than an additional 50%. I attempted to reduce the chatter of the Coiltek coil by leaving a high Sensitivity and implementing a negative tracking Ground Balance Offset. But doing so reduced my depth of detection more that if I simply lowered the Sensitivity and slowed down my hunt. So in my opinion, if after setting the proper Noise Cancel channel and a proper Ground Balance setting, if your detector chatters when you sweep, you need to either slow down your sweep speed, lower your Sensitivity, or both.
Living in farming country of the upper Midwestern USA, there are certain times of the year when it is virtually impossible to hunt the places I most enjoy detecting. One is when the ground is frozen and the other is when the crops are growing. As you can see in the picture below, the corn is too tall to be tromping around in. However, with the permission of the land owner, I was allowed to look along the end-rows, adjacent to where a house was located from the 1870’s until about 1915.
Within a few minutes of arriving at the site and properly setting up the X-TERRA 705 with the Coiltek coil, I got a solid audio tone with a TID that locked on at 32.
Pinpointing on the Coiltek 15-inch ALL TERRAIN coil is no different than with any Double-D coil. If you learn where the ‘sweet spot’ is on the coil, you can get a good idea of the target location by X-ing back and forth over the target area and honing in on the audio response. Another method I’ve found effective on larger coils is to slowly move your coil back and forth over the target, centering it based on the audio response. While sweeping back and forth over the target, simultaneously drag the coil closer to you. When the target response disappears, the target will be located directly under the front tip of the coil.
After Pinpointing the target, I dug into the edge of the field and pulled out some soil. At a depth of about 6 inches, I saw what I hoped to be one of my favorite coins, an old Indian Head cent...
And it was. A 1906 Indian Cent... first old coin with the new Coiltek coil.
Later, I got a high tone with a TID that had me hoping for a silver dime or quarter. Although the depth icons indicated a shallow target, that didn’t surprise me as this soil has been tilled for generations.
Digging down to a depth of 6 inches, I still hadn’t located the target. So I grabbed the shovel and dug a larger and deeper hole. Upon reaching a depth of 12 – 14 inches, something rolled off the shovel and fell back into the hole.
It was a small crotal bell. Crotal bells were used to help ‘keep track’ of livestock and are a familiar find for many of us who hunt old farm sites.
I believe the Coiltek 15-inch ALL TERRAIN coil will be a welcome addition to prospectors, relic hunters, beach hunters and those who like to hunt for coins in wide open spaces. I would not recommend it as your primary coil for coin hunting in areas with an abundance of trash or adjacent targets. The reason I say that is because I am convinced there are more old coins still hiding in parks due to adjacent targets masking the signals then because of extreme depth. As such, Minelab’s 6-inch Double-D coil is still my ‘go to’ coil for coin hunting those trashy areas.
With all that said, I have to add that I was pretty impressed with the separation characteristics of the Coiltek coil. In one of my field hunts, running zero discrimination, multiple tones, I passed over a target area that gave a blended set of tones. Carefully working the area, I managed to isolate a mid-tone signal with a TID similar to a US nickel. In the picture below, you can see the TID of 10, and the coil position in relation to the weeds and the corn stalk lying on the ground.
In sweeping back over the general vicinity, I separated another audio tone that was much lower in pitch. In this picture, notice the ‘new’ TID and the exact location of the coil, in relation to those same weeds and corn stalk. The minimal distance between these two targets demonstrates the excellent separation capabilities of this coil.
Within a few inches of each other, I recovered a piece of old iron and what appears to be part of an old lead wheel (maybe from a child’s toy wagon).
So what do I think? Although I found it to be more weight than I can comfortably handle for long periods of time without a harness, I think for those X-TERRA users who have been asking for a large, well-built coil that provides extreme depth and covers a lot of ground with each sweep, while continuing to provide very good discrimination characteristics and surprisingly good target separation, this just might be the product they’ve been waiting for.
I appreciate Minelab responding to the requests of X-TERRA users for a large coil, and working with Coiltek in providing the Double-D 15-inch ALL TERRAIN coil at 7.5 kHz.
HH Randy