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VLF Concentric Vs DD Coils


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VLF detectors can come with DD or concentric coils. This is similar to but not quite the same thing as DD versus mono on a pulse induction detector. There are three basic coil types in wide use, Concentric, Double D (DD) and Mono. A VLF detector transmits and receives simultaneously and so there must be both a transmit coil and a receive coil. A pulse induction (PI) detector can alternate between transmit and receive and so one coil can be employed as both transmit and receive in an alternating fashion. There is confusion on this but simply put VLF (induction balance or continuous wave) detectors do not employ mono coils. A VLF can only employ the concentric or DD options. A pulse induction coil can  work with all three types.

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Metal detector coil types illustrated - concentric vs DD vs mono

VLF detectors are more alike than different these days. One thing that can make a real difference is what coil the detector comes with. Two very similar models will be quite different if one comes with a 12" round concentric, and another an 11" x 7" DD.

In this case the very same detector can be turned into two different detectors by changing coils. Here are a couple rarely seen on the F75 - the 10" x 5" DD and 10" elliptical concentric. A big difference between the Teknetics T2 and Fisher F75 was that the F75 came later and the ability to use concentric coils was added. The T2 can only use DD coils.

Why DD or concentric? DD coils are all the rage these days. However, concentrics have more consistent detection patterns with less dead spots up close. Every notice how that DD coil goes wacky on shallow targets? And concentric coils are much better at identifying flat ferrous targets like bottle caps. Here it is from the guy that should know best, Dave Johnson, lead designer on the F75.

About Search Coils by Dave Johnson

Search Coil Field Shape by Dave Johnson

Coil Basics by Carl Moreland

I like the 10" DD for the solid design, better for working in stubble or other locations where an open coil might hang up. The concentric just for being better behaved. The DD is the better prospecting coil for bad ground. Concentrics do well on milder ground.

A note on coils. The blade like knife edged detecting pattern depicted in DD marketing ads, and even the diagram above, is largely myth. Electromagnetic fields radiate and simple coils cannot focus them into beams. A round concentric coil has a search field shaped like a soccer ball cut in half. A similar size DD coil will have a search field more like a football cut in half lengthwise. If you ever doubt this, just fire up your detector with a DD coil, flip it over, and run a coin under the coil noting where the signal fades at different locations under the coil. This is best seen via true all metal modes, as disc modes suppress the edge signal to a large degree. But it’s there, and targets under the edge of a DD coil will mask centerline targets to a degree that would surprise most people.

Finally, depth is more related to coil width than coil length. Think of a 5" x 10" coil as a stretched out 5" coil, not as a skinny 10" coil. That is why when listing elliptical coil sizes you will often see me putting the small number first. Marketers do it the other way around so you tend to think you are getting more than you are. A truer picture can be had by comparing the total area of a coil, one to the other, but for quick and dirty comparisons, comparing by width will give you a more conservative idea of comparative coil performance, than comparing by length.

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Fisher F75 with 10" Elliptical DD Coil

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Fisher F75 with 10" Elliptical Concentric Coil

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This is my opinion of DD vs Consentrics from field use.

People have the impression that DD coils eliminate masking. They do not. Often times they are vulnerable more so to masking then a concentric.

Another misconception of DD coils is that they are better in hot ground. This is a maybe. If you are using a small DD it can help. But in the same ground, using a small concentric will work as good if not better.

In order to compensate in the depth loss of a DD vs a concentric, you have to go to a larger DD coil. When you do this you are now reading more ground increasing the problem with hot ground. By going to a larger coil, you also increase the problem of masking.

Larger coils also have the tendency of ignoring smaller objects.

Using a DD also decreases your ability to pin point and discriminate.

Most VLF detectors derive their best performance from a 6" to 7" coil on coin and jewelry type targets, in most hunting situations, at moderate depths. Smaller targets, such as small gold need smaller coils to see it.

All other size and type coils, larger or smaller, are designed for nitch hunting situations. These are special applications for your hunting style or area conditions. Use of these coils create trade offs in performance in many different ways.

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back with my first Garrett 1980 deep seeker, I knew if a target was iron or nonferrous just from the way the coil responded...Iron was picked by inches or feet away from the actual target and anything nonferrous only responded as the coil came over the target-if my memory is not faulty.

 

fred

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I like concentric coils on VLF's because they produce a hotter signal response but also allow you to whip the coil a lot more aggressively compared to a DD, which helps no end in identifying small shallow targets in hot ground.

I had a concentric 10" elliptical coil on my XT 17000 years ago and loved it, problem was they had major issues with them out of the factory so a lot of them were very noisy even in mild ground which then killed them in the market place. Found a lot of gold with the one I had when it was working properly.

To my mind if your using a VLF to chase gold then you may as well use a concentric coil and then just target areas with average to low mineralisation rather than go down the dull muffly signal response of DD coils on tiny surface targets, if the ground hots up too much then by all means put on  a DD or even better just grab an SDC 2300. 

JP

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 The big problem now days, the MFG's don't want to build many concentric coils so in order to get different size coils, you are forced into buying DD's. It really cuts down on how you can utilize your detector.

Even with aftermarket coils, the choices are very limited. 

The right coil for the right situation, can make or break a detector. Coils can make a good detector, a great detector. 

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Manufacturers are afraid of negative feedback from customers complaining about noise so opt for the soft solution of providing DD coils only!! Case of not understanding what is required to be effective when searching for gold or worse trying to do too much at once.

If the GB II, an outdated design, can do it effectively and IMHO MUCH quieter than the GB PRO with its DD coil and lower FREQ, why can't the more modern offerings, especially with the benefit of reasonable auto tracking features, be able to do it? 

How bout giving me a GB II in a modern GB PRO housing with a concentric coil and Minelab auto ground tracking with three frequencies say 19Khz, 32Khz and 70Khz? 

JP

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Great article! Thanks for posting the link. Timely because I just got back from speaking to the local GPAA chapter about metal detecting and that was one thing I said - use small coils!

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When I get serious about finding value targets, or I am hunting a place for the first time, I grab my 6 1/2" concentric to hunt with. Nothing I have owned or used has yet to out perform it. 

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