Gold Detector Coils Explained - For Minelab’s GPX and GPZ Series Detectors

In the ever growing market of accessory items available for your detector, the one product that has experienced somewhat of a revival is the humble search coil. There is a myriad of coils available on the market for your gold detector these days, which has lead to endless questions on various forums, blogs and social media groups begging the question: “Which coil should I buy?” So my aim for this article is to inform you about coils in general, what makes them different, what various types of coils are good at and why having a wide selection of coils isn’t a bad idea.

To start with, here is a quick look at some of the things that make coils different:

1. Configuration – Monoloop, Double-D, Concentric, Figure-8, and the new Super-D GPZ coils.
2. Size – larger coils detect deeper than smaller coils.
3. Shape – round, elliptical, butterfly, semi-elliptical.
4. Design – solid, open-web, semi-open web.
5. Waterproof rating – will it handle a light rain shower, can it be submersed?
6. Construction features – type of material used, type of wire, quality of plugs and cable, etc.
7. Colour – more a personal preference or a brand identifier.

Coil Basics

A search coil’s basic function is to transmit an electromagnetic field into the ground, and to then receive the signal back, with the information then processed by the detector. The complicated timing of events and processing varies depending on the specific technology which we won’t go into here. (NOTE: There’s some great information on Minelab’s website www.minelab.com if you wish to delve a bit deeper!)

Some people like to think of the coil as their detector’s antenna, and in many ways that is exactly what it is. A big antenna will pick up signals further away.

Coil Configuration - Double-D vs. Monoloop

Double-D coils use two back-to-back “D” shaped windings, which overlap down the centre of the coil. The left winding is generally the Transmit winding, and the right winding is the Receive winding. In a Monoloop coil, a single winding is used to Transmit and Receive, which creates an inverted cone shaped detection field. Be aware that in a Double-D coil, the width of the transmit winding is half the width of the coil plus a small amount for the overlap. So, in an 18” round Double-D coil, the actual dimensions of the transmit winding are approx. 18 inches long, and approx. 10 inches wide, compared to an 18” Mono whose transmit winding is virtually 18” wide. This is one reason why a Monoloop coil can achieve better depth compared to a Double-D of the same size, but Double-D coils still have their place.

What are some advantages of a Double-D coil?

1. Wider ground coverage at depth, less overlap required.
2. Less effected by Electromagnetic Interference (EMI) and Atmospherics.
3. Can utilise the Iron Reject function (basic ferrous discrimination).
4. Smoother running in saturated soils.
5. Allows the use of the Cancel mode (GP & GPX series), to operate near power lines and    electric fences.
6. Pin-point is along the centre of the coil.
7. Large deep targets have a more defined (narrower) response.
8. Reduced false signalling in heavily mineralised soils.
9. Less likely to signal on buried charcoal. 
10. Reduced risk of tracking out a faint target when using Tracking ground balance

Then why do most prospectors use Monoloop coils?

1. Greater depth compared to same sized Double-D coil.
2. Increased sensitivity to small targets – particularly at depth.
3. Sharper target signal response.
4. Can employ a faster sweep speed – great for patch hunting.
5. Work well with “smooth” timings: Fine Gold, Enhance, Sensitive Smooth.
6. Cone shaped detection field is of uniform shape, regardless of sweep direction – excellent for detecting around obstacles and in thick scrub.
7. High sensitivity near the rim of the coil, allows you to use the edge of the coil to pin-point shallow targets.
8. Elliptical Mono coils start to share some characteristics of Double-D coils, particularly greater ground coverage at depth, and a sharper response on borderline targets.

So which option should you use – tips for GPX Series users

A Monoloop coil will generally provide the best outright performance in very mild ground in either: Sensitive Extra, Normal or Sharp timings. In highly mineralised soils, Monoloop coils can be run successfully in Fine Gold, Enhance or Sensitive Smooth timings, and even with a reduced Rx Gain or Quiet Audio Type, very good sensitivity to smaller targets is retained, but a loss of depth on larger targets is evident. However, high EMI areas, as well as saturated soils and highly magnetic soil types can still be difficult to work, and in these instances taking advantage of a Double-D coil’s smoother operation may be the answer. Many seasoned prospectors will continue to use Monoloop coils in less than perfect conditions, with varying results, as it will come down to their skill in identifying ground signals, and those that are worth investigating. 

For those chasing larger gold at depth, a large Double-D coil, such as the 18” Commander in Normal timings, may be a better proposition; however one must be prepared for a loss of sensitivity, which may mean a lot of swinging between target digs. I have used my old 18” GP extreme Double-D coil to great effect, running it in the Mono Coil/Rx position – this provides a great compromise between stability, depth and sensitivity.

Coil Size

For me, coils are like fishing rods, you can never have too many. So what are the differences between various sized coils? 

Small Coils

I generally call small coils those that are 11” round or smaller, such as 8” round, 10”x5” elliptical, and 6” round, etc. Small coils are nice and light, and great for navigating through thick scrub and very rocky terrain. Performance wise, small coils put out a very strong field close to the coil, so offer fantastic sensitivity to small shallow objects. Small coils can be used for both testing out the potential of a new area, as well as cleaning out a productive patch. The other benefit of small coils is that they will pick up less EMI, which can improve your threshold stability. The only two real drawbacks of small coils is they don’t cover a lot of ground per sweep and they won’t punch very deep on larger nuggets.

Mid-sized Coils

Coils that fall in to the mid-sized coil category are those between 11” and 17”. These include 14x9” elliptical, 15x12” semi-elliptical, 14” round, 17” elliptical, 16”round and the GPZ 14 coil. Mid-sized coils are great general purpose coils as they offer the best compromise between depth, sensitivity, weight, and ground coverage. Mid-sized ellipticals in particular have been a popular choice for patch hunting (i.e. scouting larger open areas for new nugget deposits), as they are lightweight, have decent depth and offer excellent sensitivity to smaller nuggets. Mid-sized round coils are my coil of choice for going over old workings, as they give you very good depth, but still retain good sensitivity if you sweep nice and slow and overlap to maximize your coverage at depth. They are also light enough to swing for extended periods.

Large Coils

Basically anything larger than a 16” round, which include 18” rounds, 20” round, 22” round, 24” ellipticals, etc. Due to their physical size, large coils are generally the heaviest coils, particularly solid designs, and difficult to work in cluttered terrains, so are best put to use in more open ground. Their main purpose is to achieve maximum depth on larger nuggets. As the detection field is less dense close to the coil, but spread out over a bigger area, this results in a slight loss of near surface sensitivity, which can actually help to reduce the signal response you may get from hot rocks and hot pockets of ground. The other benefit of large coils is that they usually make you improve your technique, by sweeping the coil slower and more methodically, and moving large obstacles out of the way, which can lead to more gold finds.

Coil Shape – Round vs Elliptical

One of the main differing factors in coils is shape. So how does the shape of a coil change its performance? As a general rule, a round coil has the capability of more depth compared to an equivalently sized elliptical coil. This is due to the transmit winding being physically wider, and the way the electromagnetic field is transmitted into the ground. A round coil handles ground mineralisation well, has good depth, and is easy to achieve good coil control when swinging. However, the larger sizes can be difficult to manoeuvre in heavy vegetation, and give a reduced signal response to smaller shallow targets. The main appeal of a round coil is that they will always achieve better depth than an equivalent sized elliptical coil, assuming they have identical electronic properties.

However, elliptical coils come with their own advantages, mainly very good sensitivity, a sharper (less broad) signal to deeply buried targets, and better ground coverage at depth. All these benefits are the reasons why elliptical coils are often the preferred choice when it comes to patch hunting. An elliptical coil is also better in thick scrub, and will give a great signal response on smaller targets, being especially sensitive at the toe and heel of the coil. However, this can work against you at times as it also gives an increased response to hot rocks and mineralisation pockets. As always, it is all about compromises, and elliptical coils do have a couple of negatives. For a Monoloop winding in an elliptical shape, the response on the side of the coils is poor compared to a round coil, which the user needs to be aware of when detecting around obstacles. A large elliptical coil isn’t the perfect coil for use in disturbed ground, such as on old shallow diggings, because the length of the coil prevents you from following the contours of the ground accurately, resulting in a loss of depth, so a smaller elliptical coil will work much better in difficult terrain.

Standard Coils

The standard coils included with most gold detectors will be a coil size and shape that will be a good all-round size to complement the technology used and the performance characteristics of the detector. For example, a high frequency VLF detector (like the Eureka Gold) will often come standard with a 10”x5” coil, as that will enhance the ability of that detector’s ability on smaller gold.

Minelab’s MPS technology Pulse Induction machines excel at finding gold deeper in more mineralised soils, so the standard coil is often a little larger to complement that ability. The 11” round coils included with most Minelab GPX detectors are well renowned for their sensitivity, good depth and stable running. “If all else fails, go back to the stock 11” coil” is a common phrase for good reason.

So what about the GPZ 7000’s Super-D coil?

The GPZ is a special beast in many ways, with its unique design (only shares some common mechanical parts with the CTX 3030), breakthrough ZVT technology (sort of hybrid between VLF Continuous Wave and MPS Pulse Induction), and new Super-D coil configuration. Due to the requirements of the ZVT technology, the GPZ doesn’t work with either a Double-D or Monoloop coil configuration. The windings used consist of a rectangle/oval shaped transmit in the centre of the coil, and then a pair of “D” shaped received windings on the left and right of the coil. The end result is two blade like search patterns, which converge into one for deeper targets - sort of like using two small Double-D’s for surface targets, and a bigger Mono for deeper targets. And funnily enough, this is what it feels like when using the GPZ in the field. Small shallow targets will give you a double beep signal, whereas larger deeply buried targets sound the same as they do with a big Monoloop coil – a broader often dipping signal response.

The GPZ 14 coil is neither a round nor an elliptical – it doesn’t even qualify as a semi-elliptical (a term I used when developing the 15”x12” Commander coils). The GPZ 14 is what you could call an “out of round” shape, like a soccer ball that’s been put in a vice and squeezed slightly. The GPZ 14 is also quite a unique design in other ways, with how the skid plate clips on to the rim of the coil, to the cable entry point being between the shaft clamp, which protects it from damage. But just when we GPZ users were getting to fully understand the GPZ with its 14” Super-D coil, Minelab bring out another radically different coil, the GPZ 19!! Unlike the GPZ 14 coil, the new 19” coil is a different design, being an open-web “butterfly” style coil. 

With the GPZ 19, a Sensitivity reduction of 1 or 2 levels may be required when using High Yield. When using Normal Ground Type (in milder soils) I’ve actually been able to run a higher Sensitivity setting. I just couldn’t believe how smooth it was running for such a big coil, and I blooded the coil with its first piece after a short time, a very small flat piece of about 0.1g which proved that the big coil hasn’t given up much sensitivity. As I carry out more testing it will be interesting to see exactly how much more depth the GPZ 19 gives over the GPZ 14, and if there is any drop off in sensitivity to small gold below 0.1g. Stay tuned for my next blog where I compare the GPZ 14 and GPZ 19 coils!

To find out more about Nenad Lonic and Phase Technical visit www.phasetechnical.com.au. (The full article on which this blog is based has previously been published in Gold Prospecting magazines.)