He recently touched on a coil selection factor that is not taken into account in the coil selection table in the FAQFAQ. And that is that size matters.
It does, but the intent of the chart was to give a rule of thumb for equally sized targets. When looking at the table, strict attention should be paid to the use of the word "favored" as was chosen carefully and meant to convey a "bias" or "trend". It is not meant to convey "absolutely deterministic". As high conductivity items that are small or thin, can sometimes be better found with a high frequency coil.
Aside from what is in the table, there are several other factors that should be taken into account, and their weighting is determined by each individuals circumstances. But trying to include all of that information would make for a very complex and difficult to read table, and possibly be more confusing.
1. How mineralized is the ground, as a lower frequency will go deeper.
2. Am I hunting in iron, as lower frequencies and concentric coils tend to disc out iron better.
3. Am I hunting in stubble, as spoked coils tend to catch and hang up.
4. How is my health, as even though not optimized for a particular target, a lighter coil may allow me to hunt longer. So I trade off a little less target bias for a longer time in the field.
There is a lot of gray area, even though it's much easier when the choices are black & white.
In order to try to address the point brought up by Jackpine, which I have kind of avoided for fear of causing more confusion, I have taken a stab at trying to present some useful information. This may not work as intended, since once again it is meant to show a "trend" or "bias". I can tell you right now that it is no where near being mathematically nor geometrically correct. First looking at the diagram below the black diagonal line is the "trend" line and will be the focal point in the examples presented. A vertically oriented line is drawn between coil size & frequency. Then a horizontal line at the point the trend & vertical lines meet, to show the size & conductivity bias.
First a static version of the nomograph for your use if you desire. Then the examples are animated GIF's that you need to click on and watch like a short slide show that repeats itself.
Example 1:
Example 2:
HH
BarnacleBill
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