Coils - Mono vs DDs

One of the most interesting questions we are asked - when we start out, so we use a mono or a DD, how big and how small, what is the difference between them. What do you reckon?

Gday

There are advantages and disadvantages to the way both dd and mono coils work and I will give you some of my observations.

MONO COILS Vs DD COILS

MONO advantages

1. they will usually get better depth over a dd coil
2. they are more sensitive than a dd coil
3. they are sensitive around the entire perimeter of the coil
4. the bigger mono coils are usually much lighter than the dd coils of the same size
5. Work better on the later gp series and gpx series detectors
6. Easier to pinpoint a target with
7. better for probing under scrub or around trees due to being sensitive around the perimeter

MONO disadvantages

1. They are harder to learn to use
2. In some ground they cant handled the mineralisation and become too noisy to use (depending on detector)
3. More sensitive to ground noise, hot rocks, and ironstone
4. descrimination mode does not work

There are probably some advantages and disadvantages that I havent covered here and some people may not agree entirely with me, but what I have found is that the mono coil is better for general use if the ground conditions allow, as there is no depth advantage over a dd coil if the ground is that bad that the mono coil is moaning and groaning and you cant distinguish the target signals from the background noise, where the dd coil will run quietly enough for you to hear a target signal.

With a mono coil you have sensitivity all around the perimeter of the coil, so when you are probing about under bushes or around trees you will pick up any target that is near any part of the coil, with a dd coil you have to remember that it is more sensitive on the left side to the centre line of the coil than it is on the right side to the centre line of the coil, and if you push the coil forward under a bush and the target happens to be under the centre line of the coil then it will be very faint or you may not hear it at all, to get a better idea or why this happens you need to look at how the coil is configured and the shape of the signal it puts into the ground.


DD advantages

1. dd coils are quieter in highly mineralised ground
2. they can be switched from dd to psuedo mono as required
3. they better suit the earlier model sd and gp detectors
4. they can be used in some areas that mono coils are quite useless
5. can be used in descrimination mode
6. descrimination mode still works on pseudo mono setting

DD disadvantages

1. generally they are heavier than mono coils
2. harder to pinpoint with
3. less sensitive on the right than the left
4. less sensitive to smaller pieces
5. less sensitive around the perimeter of the coil



Once again there are probably advantages and disadvantages that I have not mentioned about dd coils, but in some conditions they will be all you can use unless you are operating one of the latest gpx series detectors, its always worthwhile to have some in your kit as you will find that conditions on the ground can change dramatically from area to area and having a dd coil may be the difference to getting or not getting gold.

Lastly it is generally accepted than round coils, whether they be dd or mono coils have a depth advantage over eliptical coils, and once again it is because the shape of the coil dictates the shape of the signal that is transmitted into the ground as does the size of the coil, so these factors need to be considered when selecting coils.

cheers

stayyerAU

Hi stayerAU

You have pointed out some good comparisons between the Mono & DD coils, and how the Mono will work a lot better on the later machines 4000 & 4500 with their bigger array of settings to choose from.

There is one point relating to DD coils that you have is not quite right, ( "less sensitive on the right than the left" ) this is only the case when a DD coil is used in pesudo mono mode. When a DD coil is used in DD mode it is more of a blade like pattern from front to rear directly under the center line. This makes getting close to trees etc a little more difficult with a DD coil in DD mode as the sensitive part is not closest to the tree. Using a DD in pesudo mono mode overcomes this by making the left hand edge very sensitive.

cheers dave

Morning StayerAU/ Dave & members,
Very informative wrap up around DD & Mono's.
Have been using a 20" SL Nuggetfinder for a few years and rarely fit any other.
Has anyone experienced that the RH circumference is more sensitive when pinpointing or checking ground noise with the coil tipped on it's side?
Doesn't effect the coils sensitivity over all, I am just curious whether any others have noticed this and why?

Happy hunting.
Peter

Most accessory DD coils will work fine in the Mono (Enhance) mode. You don't get as much depth as a mono coil of the same size, but stability and sensitivity is improved. I like to use the Mono mode with a DD coil to pinpoint small targets.

You can also run a Mono coil in DD (normal) mode, but it is usually a bit noisier than the Mono mode.

Think of Deep and Sensitive as seperate functions, and you can't get confused. Sensitive will boost the response to small, irregular, flaky or specimen gold as the detectors electronics react faster. Depending on the ground conditions, this can improve or worsen the stability of your threshold. The disadvantage is that on solid (generally larger) nuggets, you depth potential is sacrificed. Running in Deep does not counteract this, it will only boost the signals already picked up in Sensitive mode.

I rarely search in Deep as it often causes the threshold to become wavery. Jonathan Porter and others say your brain adjusts to this after a while, but mine won't so I leave it off whenever it upsets the threshold. I like using Deep in conjuction with Salt mode when looking in deeper noisy ground.

Sensitive works well in conjuction with the Mono mode with a DD coil, for improved stability and sensitivity (compared to a standard mono coil), when looking for small stuff in very mineralised ground.

All this applies to all the GP series.

"Lastly it is generally accepted than round coils, whether they be dd or mono coils have a depth advantage over eliptical coils, and once again it is because the shape of the coil dictates the shape of the signal that is transmitted into the ground as does the size of the coil, so these factors need to be considered when selecting coils.
cheers
stayyerAU"


You wrote a great post overall and no offense but I think that particular line is a little too general and not explained quite enough. Then by that what is said a round 6" GoldStalker has a depth advantage over an elliptical 11x17" NuggetFinder. See what I mean, the statement at the top is too general. It should be more specific on sizes.

A NF(SL mono) or CT(UFO mono or Big Red DD) 12x24". 12 + 24 = 36 divided by 2 = 18" round coil equivalent. For an elliptical coil, some people add (length + width) divide by 2 = performance equivalent to a round coil of that size. I disagree and think an elliptical coil is generally only as good as its width. Meaning a NF 11x17" mono is about equivalent to a 11" round CT or Commander mono. (That is why I say about and not exactly.) It is true the shape of the coil dictates the shape of the signal that is transmitted into the ground so the shape of the signal of a 11x17" will be different than a 11" round. Why don't the coil manufacturers show us a color diagram of the force field of each coil??

Chosing a coil to use is very hard sometimes because no one has made public a coil chart plot graph to tell which coil is best for what sized gold, how much bigger gold depth is lost by using smaller coils, and how much small gold depth is lost on using bigger coils, etc. For an example, yes an 18" round mono can detect a 1 gram nugget(let's say 1/4" round surface area) very easily BUT how much more depth does a 7x12", 9x14", 12x15", 12x18", 12x24", 8", 10", 11" or 14" round mono have on a 1 gram nugget over an 18" round mono?? What I mean is, will the 7x12", 9x14", 12x15", 12x18", 12x24", 8", 10", 11", or 14" round mono detect a 1 gram nugget at over twice the depth that a 18" round mono can?? (The same test can be made on a 1/8", 1/2", 3/4", 1", etc., etc. round surface area of gold for the different coil sizes.) That is the type of thing I want to know?? This fuzzy logic and not made clear by coil manufacturers, not even one bit and we need this kind of info. Yes there are different types of ground but make the test bed on the same ground type for all coils and let us know the info. I know it will not be exact and totally perfect but it is better than the information we have now which is ZERO!

A test done my a manufacturer would only be between its own different sizes coils of course. But a private independent test can be done if someone has the time and all the sizes of coils of all manufacturers. Also maybe one or more of the coil manufacturers would not want such a private test out because this would ultimately mean testing one coil manufacturer against another coil manufacturer and in the same coil sizes we would find that one has a certain % of depth advantage over the other.

For example I would like to know which mono has the best (depth) performance, an 18" Commander, 18" GoldStalker, or 18" NuggetFinder SL??

Also which DD has the best performance, an 18" Commander, 18" CoilTek DD Pro, or a 18" NuggetFinder??

To compare round coils with an elliptical equivalent you have to work out surface area, 18x12 is about the same as a 16 ins round [approx] people say elliptical don`t go as deep as round they are right, so if you want the same depth you have to go for a larger elliptical for a fair comparison, and all calculations should be done in metric as all oz. coils are built to metric spec`s.
cj

Coils are a lot like camera lenses: There's never one single lens (or coil) which does everything.

Several things need to be factored in when assessing a coil and it's capabilities:

Larger Coil Diameter = Sensitivity to Deep Nuggets - but will Ignore many smaller Nuggets altogether, especially at depth.
Medium Coil Diameter = Sensitivity to Smaller and Mid-sized Nuggets - and MAY detect deeper Nuggets IF large enough.
Small Coil Diameter = VERY sensitive to VERY small Nuggets - but unable to penetrate as deeply for larger Nuggets.
The small coil is the ONLY way to maximize small gold target recovery in certain areas. It's especially good on fine specimens in quartz where the gold might not be tightly clustered. Even rice-grain sized 'nuggets' are easily heard with this coil yet might be ignored by many others.

Double-D coils are shallower reaching than comparative Mono Coils but Double-D's offer Discrimination. Also, Mono Coils penetrate deeper than Double-D Coils yet Mono coils require tighter sweep overlapping and (normally) don't work so well in mineralized ground.
There are several thousand smaller gram-sized nuggets to every Ounce+ nugget found, so therefore (statistically speaking), the smaller coils (for smaller nuggets) will be more lucrative for the serious detector. Those hunting deeper, larger nuggets have to use a larger coil. However, they will not find as many of the numerous smaller nuggets and only rarely will they find a large nugget.

As for depth, I think every one of us would like to be able to see a chart that shows the depth comparison of each brand and size of coil. But that's not in the interest of any of the manufacturers to do so. And no manufacturer wants a chart showing that their brand of coil is severely or even marginally inferior in performance to another. Quality varies between the various brands as well. It probably pays to research your needs before seeking out the right coil for the type of ground and targets that you're looking for. The number ONE reason why people buy a particular coil is because they've seen someone else using it in a picture in the magazines or, (most commonly) because it's the coil their mate told them to get. The arguments over coils eventually boil down to a single issue and it's not always about performance.... it's whether or not you've insulted your mate - who advised you to buy the same coil he is using. So the weakest point on a coil appears to be the ego of the person using or recommending it and the desire by users to be seen to be using the "best" coil out there. That's why actual performance isn't often bought up in discussion online. Nobody is going to say that X brand/size is better than Y brand/size because the users of those brand/sizes will want to defend their choice of coil brand/size.

In my opinion, most people (most) require just the coil that comes with their detector. An example is the new GPX series detectors which come with the ideal all-use coil. For a serious hobbyist, they might need a second and possibly even a third coil if they're hitting regular Gold patches. But each coil needs to be specialized. Yet MOST users would be hard pressed to explain why they need more than one or even two coils. An obvious exception to the rule is going to be the Pro-Nuggeter who Prospects for a living and that user's needs will vary considerably so they might own four or five coils for the same detector.

I met a guy today who has 12 coils. All for the one detector. I suspect that he was given one or two by a mate. But that number of coils is unnecessary for the casual or serious Hobbyist. Even the Pros rarely use more than 4 different coils in a year. For those working in quartz reefs, areas of smaller nuggets near the surface or in creeks, a VERY sensitive mini-coil is bound to offer spectacular results. Those seeking only larger nuggets in proven and virgin ground would cash in nicely with a super-large coil if the ground allowed it.

As the OP has already noted, the Mono's can only now be used to good effect on fairly mineralized ground due to the new timings of the GPX-4500.

I made this image to show the difference between the field of two high frequency coils that can be used on the X-Terra 70 for Prospecting. It might show the difference in the shape of the field emissions between a larger Elliptical coil and a much smaller 6" coil. Both are Double-D coils.



You'll notice that the smaller coil has a much stronger secondary field (green) - due to the "Skin effect" - than the larger coil. This is where the coil wires are wound more thickly on smaller coils, producing a larger and more intense primary field (red) yet the secondary field (green) clings closer to the wires than it does on the larger coil where the windings are slightly sparser. The field on the larger coil is larger and yet weaker. This is why the tiny DD coils are indispensable for some.

If you want a large, 18" coil (regardless of brand), you'll only go as deep as the soil conditions allow you to. But an extra inch in emission field depth can easily mean you could hear nuggets missed by everyone else. However, your odds of finding medium and smaller nuggets at depth are considerably diminished with the largest coils. If you are REALLY lucky, you might just find a larger nugget for your digging efforts. But since you're wandering around with a coil that could in theory reach down to about 1.5 meters in depth, you might be in for a serious workout with little or no reward. I'm sticking to the Commander coils myself and this is only personal preference. As I've said before: I could be persuaded to use other brands including Coiltek & Nugget Finder because I think they're very good coils and there's a lot of success associated with their use. But I'm of the opinion that I will personally get the best of results (depth, construction and performance) from my Commander Coils. I like the enclosed design (doesn't catch of twigs etc) and the depth is considered to be slightly superior (as noted by other some members here). I also believe the slight weight difference is counterbalanced by the support of the bungee cable. So if a little added weight is offset by better depth, it gives me an advantage, especially on seriously flogged ground where pickings might be slim yet nuggets deeper down may reside.

Thanks for posting Nero
Be good to see picks like the one you posted for all the different coils.
Could even include different size targets and depths they can be detected.

thanks nero very well explained.. there is a radiation pattern for antanas just look in a radio amature mag. carnt see why the same thing can,t be done for coils.. do they react to a specturmanliser or a field strenth meter or what.. the makers must do tests ect..never got to think about it till now...interesting...ex ham radio.Bob..