Dean, I follow in the footsteps of Wolfs mate, I have tried several other coils but favour the supplied 11 mono.
In my studies on the subject of coils it is stated that the optimum size for detector loop is 16
Coil types basically fall into two categories, we refer to them as:
A).Mono : The mono coil is principally used when mineralisation is from a low to moderate level of concentration. Sensitivity and depth are noble when compared to the double D. : A mono coil is made by winding wire around a coil former, the wire that is used will have properties of resistance and when multiple strands are spun together further accurate calculations are required to compensate for an effect known as inductance which begins to take place. A coil is not simply winding wire around a bucket or coke bottle, it is an accurate science to balance the coil to the electronic transmit (Tx) and receive (Rx) circuit. The invisible search field of a mono coil is said to resembles the shape of a cone, and the maximum depth being at the apex of the cone.
B)Double D: The DD was developed after the mono as a product to over come noisy ground conditions ( moderate to high mineralisation). The DD is built like a mono, the difference is that there are 2 coils in a DD that are overlapped. Take two mono round coils squash one side in to have them look like the letter D, flip one over so it looks like you have a circle again with a line down the middle, and that is the fundamental shape of a DD. The size is governed by requirement and application. The invisible search field under a DD coil is much the same as the shape of a soup bowl which is not as deep as a mono coil field.
Other factors
SIZE of a coil: Basically there are 3 sizes to consider Small Medium and Large, there is one more size, its called You got to be kidding. One person once said to me, if you look at the coils greatest diameter then that is how deep it will go I have pondered that statement and realise I forgot to ask how would the size of nugget change that view. In a nutshell small coils for small target, shallow depth, and slow coverage. Medium coils for mid range targets and depth with increased coverage. Large coils for maximum depth and large targets and maximum surface coverage.
SHAPE has more to do with practicality and purpose, small coils can get into rough tight areas between rocks and into crevices, whereas an elliptical shaped coil can get into even tighter confinements. When deeper penetration is required or coverage desired a larger coil would be preferred, but the environmental terrain will impact on your selection, use a spoked coil frame and get ready to curse every tree and twig that wants to make it their new home. So the type of coil you use is a balancing act for soil types, type of terrain, desired target size and surface coverage.
Conclusion : there are so many coils on the market with differences in shape, size, spoked, solid, water proofed, different operational frequencies, composition etc etc,
It can provoke doubt when a person performs poorly ( cant find gold ), it is easier and safer (ego) to find fault with their equipment than fault in their use of the equipment, so they blame the equipment, go buy another coil(s) then spend more time in the field and bingo, success, it has to be the new coil, nothing to do with spending more time in the field or luck or improving their search techniques.
When testing is done in a controller environment with skilled unbiased technicians then I might consider performance reports as being a guide, but while it is left to people waving inconsistently random object in from of a loop or guessing distances and depth it is all meaningless.
Pete
