Some search coil questions from a beginner

[Punchy71]

Hello,
I'm new to metal detectors and have some search coil questions.
1) What size would a small, medium and large sized coil be?
2)Are there only three different types of search coils: A)concentric coils B)monoloop coils and C)Double D coils? Or are there more types than that?
3)Are all search coils interchangeable only within the same brand of detector or is there some kind universal standard on search coils where any coil will work on any detector?
4)Is there anything else that a beginner should know about search coils that you can think of, like for example: some brands of search coils are made better than others or maybe work better or have greater depth or whatever?
Thank you for any help

 

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[Carl-NC]

1. I generally consider medium to be 8-10". It's also a good size range for all-round hunting.

2. Mono, concentric, DD, OO, 00, Figure-8, coaxial, orthogonal, DOD, 4-over-1, prolly more.

3. Not even within the same brand. And certainly not across brands.

4. Most well-made search coils are pretty comparable in performance. Aftermarket coils used to claim "20% Depth Increase!!" or some such nonsense, tain't so.

 

[Loco-Digger]

The 2 most popular coil types are the concentric and the DD. I began with a machine with an 8" concentric coil. The machine was pretty basic but did have 3 tones, the tones were the only info you could evaluate to decide if I was digging the target or not. The general rule of thumb is the diameter of the coil directly correlates to it's depth. Target size and conductivity are the variables. An 8" coil should be able to hit on a quarters at about 7.5 to 8 inches, a dime or penny at 6 to 6.5 inches depending on soil conditions. A larger coin like a silver Dollar might be hit on at 9 inches. Large targets can be detected at greater depths (i.e. an aluminum can should be detected at about a foot deep using the 8" concentric The Fisher 5"DD breaks this rule since it can go much deeper than 5" on a quarter. Just know that VLF (very low frequency) technology pretty much maxed out over a decade ago, so now you are seeing the new machines offer multiple frequencies and more tweaks on setting to eke out more performance.

After a couple months into the hobby I had dreams of more depth than the 8" concentric could reach and the cheap detector I had did not have any manufacturer nor aftermarket coils available so I upgraded to a slightly used Fisher F4 which came with the stock 9" x 11" DD. This began my love affair with the DD coils. I then found a great deal on a used F75LTD with Boost a few month later. In my experience the higher end machines have better electronics and give tighter target IDs at greater depth and have fast processors which allow for quicker target separation As a general rule 10 inches deep is max depth for most machines to give a good target ID, if the target is deeper than that the target ID numbers will drop.

I have 4 coils for my machine. The site dictates which coil to use. The more dense the targets are the smaller the coil I use. In a target sparse area such as a field I use my large coil the Detech 12" x 15" Butterfly (DD) coil which allows for more coverage per swing and more depth than the stock DD. Just know that larger coils add weight and to use the large coil for more than an hour I use a sling which neutralizes the additional weight. My main coil used the 5" x 8" nel sharpshooter. It provides better target separation then the stock DD and gives up a little depth, but people have been detecting in my area since the 1970's. Most yards, schools and parks have been pounded over the decades so the only good (keeper) targets remaining are the ones in the trashier areas which are being masked by trash or were too deep for the older machines to hit on. As I stated earlier when I first started I wanted depth but have evolved to want better target separation and mainly use smaller coils (smaller than the stock coil). In super trashy sites I use the 5"DD and the fastest process my machine can run (FA Process).

Below is a comparison of how both the concentric and DD reach into the soil and detect. Notice that the Concentric coil has a greater chance of having multiple targets in the detecting field which makes it harder for the machine to give you a good tight target ID. You also need to overlap your coil sweeps more with a concentric than a DD to get max depth coverage.

DD Coil


Concentric Coil



Coils are designed to operate most effective at a certain frequency (khz). If you have multiple detectors from a manufacturer you might be able to swap a 13khz coil on another machine running 5.9 khz but will it not work optimally It would be like installing 14 inch tires on a car designed for 15 inch tires. The car will go down the road as designed but your speedometer will not give you a good MPH reading due to the tires being smaller than the car was designed for.