Technical Question about Multi-Freq. on a salt-water beach

[Vagabond87]

I'm hoping this hive-mind can help me answer some questions about detecting on a beach with a VLF. I've been hearing that Multi-Freq detectors are "far superior" to single freq. on a salt-water beach, but I can't quite wrap my head around WHY this would be the case. I would have assumed it would be more a function of ground balance than multiple operating Khz, but I've heard so many different users hail this multi-freq. stuff. Part II to my question is- Which coil style is best for heavily mineralized ground-again, I've heard both sides, but can't find any actual technical info to back anything up. What does everyone think?

 

[Nauti]

I can't help you with the technicalities of why a multi frequency machine is better but after hunting beaches exclusively for the last 7 years,i can say multi frequency machines are far better for searching wet salty sand than single frequency machines.They are deeper and far more stable at higher sensitivities which is what you need for the deep jewellry.
That is'nt to say you can't search the wet sand with a single frequency machine........i used a whites m6 for ages on the wet sand with good results....it's one of the best single frequency machines you can get for the wet sand,nice and stable and fairly deep too.There are other single frequency machines that will do a good job too.
Ultimately,if you want maximum depth and stability though,multi frequency machines are the way to go.

As for coil choice,i tend to disagree with the main train of thought which is that dd coils are better than concentric coils.My favourite beach machine ever was a whites beach hunter 300 which had a 12" concentric coil,this combination punched very deep into the salt sand.....deeper than my minelabs which are often considered the best beach machines.Also,overall,even in land hunting,i've had just as much success with concentric designs as i have with dd designs.,even in so called mineralised soils.











6

 

[Vagabond87]

Neil- Thanks for the reply, I appreciate that very much. I've found the same thing; I've never had an issue with concentric coils until someone TOLD me I had a problem with them! This is why I have trust issues! How nice of them to solve problems I didn't even know I had!

 

[Nauti]

The problem is that people just go with the flow with whatever is popular at the time.When dd coils became widely available the marketing men with their glossy adverts convinced the detectorist that dd coils were the way to go.It's the same with detectors.....a new model comes out with a load of new features and the adverts promise better performance,faster speeds and more finds........so they must be better right?
In reality,in actual performance terms......and these are only my own thoughts.......detectors have not come on in the leaps and bounds that the marketing men like to make us believe and this includes dd coils v dd coils.I think the only person you can really trust is yourself and the detecting experience you have gained over your time detecting.

 

[Tom_in_CA]

I believe it's because on the wet sand inter-tidal zone, anywhere on the wet-salt beach, is: The mineral content changes every few steps. If you take a step closer to, or further away from the water's edge, the mineral and salt and moisture content change. So the better a machine ignores these changes, the better it is for those conditions.

 

[vlad]

(short answer-more ground balance points)

Posted April 19, 2016

You have two issues on most beaches. A detector can see salt as a conductive signal, exactly the same signal as small gold or large but very deep gold. A detector also sees the ground which with most beaches means magnetite.

A single frequency detector can ground balance to a single point, either the salt signal or the magnetite signal, but not both at once. Some single frequency detectors can't actually ground balance to the salt range at all so having an expanded ground balance range is one way to deal with the issue. On low mineral beaches this works ok but with more mineralized beaches you have a problem as you can ground balance to salt or magnetite but not both at once.

To get around this many single frequency machines have a "salt" mode that allows the salt range to be eliminated by simply discriminating it out while the unit is ground balanced to the magnetite content.

Multi frequency machines can ground balance two channels, one salt, one magnetite. Better for most situations.
(((a question is raised; is if a 2 freq is better than a 1, a three better than a two.........why not a 6 frequency or..............))) vlad

In either case you eliminate the salt signal, and any gold that reads in the salt range, like most micro jewelry (thin chains, ear rings, etc) or weak signals from items at borderline depths. PI detectors do the same automatically by the nature of the way they work. This problem is essentially unsolvable using metal detectors based on electromagnetics.

The simple answer therefore is any mid frequency machine with a "salt" or "beach" mode will do the job, as will some machines that have an expanded ground balance range. They generally work very well on drier sand and get more problematic in the water. Salt content actually varies widely at different beach locations and so what works well for one person in one location may not work well in another, especially as magnetite gets tossed into the equation.

A properly designed single frequency machine on a clean white non-magnetic beach can do just as well if not better than a multi frequency machine for depth. But as you add magnetite to the beach the inability to deal with two issues at once gives the multi frequency machines the edge.

Personally I would never use a single frequency detector actually in salt water so can't help you there. Conversely, I would be happy to use most any of them out of water on the beach itself. It would just depend on what I owned at the time.

 

[lytle78]

Good description of the problem. For VLF IB detectors the issue is balancing for the salt signal - ground balance.

The way PI detectors work is that they don’t take their “sample” of the return signal until some microseconds after the transmission pulse is switched and the current in the coil collapses. This is called the pulse delay,. Eddy currents induced in targets decay at a rate related to the characteristics of the target. Salt water has an extremely quick decay rate (although as the volume of water increases, the rate goes up. Existing beach PI machines have pulse delays of 15 or so Msec and diving detectors have even longer ones due to the much greater water volume.

Small gold nuggets and small or low carat or white gold have a pretty quick pulse delay, so a detector with a minimum pulse delay of >10 msec is going to be deaf to them.

Building detectors and coils with short pulse delays requires extreme good engineering and attention to details. Very low noise electronics to ensure weak returns are heard, good shielding and extremely careful coil construction. So far, no production beach detectors have done it.

Eric Foster hand-built detectors like the Aquastar and Deepstar with minimum pulse delays which touched or slightly bettered 10 msec pulse delay and they worked in salt water.

The Manta team claims usable pulse delays of less than 8 msec in running salt water. These are prototypes - building these kind of machines in a factory setting hasn’t been done yet.

 

[Vagabond87]

Vlad/Lytle78 That was exactly the type of answer I was hoping for. NOW I feel as if I can comprehensively wrap my brain around this.

Any technical insight into coils and mineralization? Garrett has a coil chart on their site that seems to favor DD in heavily mineralized environments, while giving concentric coils the edge in "typical" ground conditions.

Thanks in advance!

 

[vlad]

(Reprint from JBirds Treasure Baron Page: loop designs and characteristics)
There is a difference between the standard round and DD loops. On the coils I designed for Discovery I don’t recall a problem with excessive phase shift between the two coil configurations. When I first started designing coils for Discovery I decided to pick a particular frequency of operation and inductance for the Transmit and Receive coils. In addition the “Q” of the Transmit must be control within a certain range. For those who don’t know......the coil’s Q is the ratio of the coil’s inductance to its resistance at a given frequency. Whenever the coils change in size the turns are modified to return the inductance to the standard value. This tends to maintain winding resistance and more importantly, the Q of both the Transmit and Receive coils. So, its important to have standard coil values to target the design to. Normally this would be the coils, inductance, resistance and effective Q. It these values are maintained the resultant coil phase will be maintained over all coil designs regardless of the coils size and shape.

The DD coils can get you if you are not careful. As you know the Receive is generally the same size as the Transmit on these coils. Coupling that with the tendency to keep the Receive turns constant can result in a serious change in the coils output phase. Therefore, the Receive turns must be reduced considerably to lower the inductance back to the standard value. From a practical standpoint the inductance does not have to be exactly equal to the target inductances. As I said the tendency is to keep the turns the same as you change from one coil design to another. This tends to keep the sensitivity the same across many designs. However, that should not be the consideration. In this case its more important to control the phase across many designs. It’s better to look at it this way. For example, suppose that we build two Receive coils where one coil has twice the diameter of the other. But we keep the turns the same in both coils. For this example the larger coil would have an inductance that was twice the smaller coil. These coils would not have the same output phase. The larger Receive would easily have more sensitivity than the smaller coil because of the greater turns and coil area. However, this would not be a good design. The turns on the larger coil must be cut by .707 times. This would make both coils have the same inductance. Ideally we would also need to change the Receive wire size to keep the Receive resistance constant. Remember the coil Q is the ratio of its inductance to resistance at a given frequency. If we keep the inductance and resistance constant then the Q would also be constant. However, I don’t generally change the wire size on the Receive because if you maintain the inductance constant the resistance tends to not change as well. As I said, math calculations show that the wire size should be changed and to what size. But from a practical standpoint the Receive wire size can be left the same. When we reduce the Receive turns on the larger coil the coils characteristics approach the characteristics of the smaller coil. However, the larger coil will still has more sensitivity than the smaller one because of its greater area. The key here is not to get so concerned about the coil’s sensitivity that you forget about the overall design.

All that being said the DD coils do have the worst phase shift away from the target value. However, it can be control within acceptable limits as outline above. I don’t recall the exact phase tolerance on the Discovery DD coils but I think it’s below 0.5 degrees. We always calibrate the fixed ground phase trimmer to be +0.5 degrees. The phase of most soils do not go below -0.5 degrees. Therefore, we have a total difference here of 1 degree. This 1 degree differential is well above the 0.5 degree tolerance on the DD coil. As a result we don’t have any serious phase problems with the DD coils.

One question that might seem important here is.......Why be concerned about the output phase on coils if you have a detector with a manual ground balance? After all, any phase shift between coils can be compensated for with the manual ground balance. Well that’s true. But there is more to this consideration that must be understood. You may have many detector designs some with fixed and some with a manual ground balance. Also, I have produced many designs that had a fixed ground balance for the motion mode but a manual ground balance for the GB mode. The bottom line is this. You must decide on a standard and stick with it across all coils designs. This keeps everything interchangeable.

Yes, it is important to have good quality caps for the Transmit tank capacitor. The main reason to use polypropylene is because their capacitance is very stable over time. Much better than most caps. Polystyrene caps are better but they don’t come in the larger capacitances like those needed for the tank Transmit coil. If the capacitance is stable over time then the loop frequency is also stable over time. That’s very important. If the frequency changes the Receive signal phase changes for a whole bunch of reasons all related to the frequency change. So it’s important to keep the frequency constant. The other capacitor characteristic like it Dissipation Factor (called DA) and leakage are not that important in this application.

You would see no difference in sensitivity between coils with different tank capacitors (polypropylene vs. polyester) if the capacitors had exactly the same capacitance. However, you might see a slight improvement in drive efficiency. The polypropylene cap would probably take less current to drive than the polyester cap. The Transmit wire size has very little bearing upon the coils overall sensitivity. However, it will greatly effect how much current(or power) is required to drive the Transmit coil. The designer could make the Transmit wire size very small and reduce the weight of the coil. That would be very impressive. But you would not we impressed with the battery life. The coil would draw huge currents and drain the batteries quickly.

One last point. The internal fixed ground adjustment is calibrated using a dummy coil. Not a real live coil at all. So of like a dummy load on a ham transmitter. The ones that consume the power but do not radiate into the air.

Litz is not use by most manufacturers because of the following reasons: Litz is more expensive than standard solid wire and it is harder to work with. Soldering it is more difficult and as far as I know it’s not available with self-supporting coatings. Also, it is probably difficult to quantify the improvement in using Litz. I have been using Litz wire in Transmit coils since 1989 but not for the reasons mentioned by Minelab. The applications where I have used Litz wire were only in industrial metal detectors where there is no ground considerations.

There are several effects to consider when discussing the ability for a detector to reject the ground mineral signal. The first is frequency. Some of the original mine detectors operated at 1000 Hz. At that frequency the reflective phenomena is almost nonexistent. At least it is not a design consideration.. At higher operating frequencies the reflective ground effect can be broken up into two main effects, static and dynamic.
Static effects refer to the fact that the ground is not balanced out when the loop is at various distances from the ground. This is the effect you mentioned. Using Litz wire for the coils in the loop will reduce the static effect problem.

The second or dynamic effect has not been address by anyone as far as I know. This phenomenon is due to the motion of the coil across mineralized ground and prohibits you from obtaining a true balance. It has nothing to do with the reflective ground effect but it appears to be related to it. But it’s not. I have been interested in this effect for many years because it directly effects the performance of Motion detectors. A special circuit design can eliminate the problem. Some of the detectors that used this circuit were the Teknetics 9000, 8500 and Mark I. To some degree it was incorporated into the Discovery Treasure Baron.

Although not fully exploited. You can not tell what this particular component design arrangement is, just by looking at the schematic of the detector.

Ordinary hook-up wire is not the same as Litz. It very important that the individual Litz wires be insulated. This distributes the total current evenly in all the wires. If they are not insulated then you might as well as not have individual strands. Also, the individual strands are wound in a very particular fashion that minimizes the skin effect in each strand. The end effect is that the resistance of a properly designed Litz is (almost) completely flat from DC to RF frequencies.

I have built and tested Litz wire loops for consumer detectors. However, they never went into production. The improvements gain by reducing the dynamic effect mentioned above and the use of AGB circuits were enough to satisfy our design requirements. Also, the use of a preset ground balance makes the static effect phenomena almost irrelevant. This is not to say that the elimination of static effects are unimportant.

Here are some general design parameters for a coil:

Transmit Coil -- 25 to 30 turns of 22 gauge wire. Diameter 7 to 8 inches.
Receive Coil -- 200 to 300 turns of 31 gauge wire. Diameter 3 to 4 inches
Feedback Coil -- 6 to 10 turns of 22 gauge wire. Diameter same as receive.
Tank Capacitor -- 0.47uF

Generally the Receive is about half the diameter of the Transmit. So if you choose an 8 inch Transmit use a 4 inch Receive. Wind the Feedback Coil on top of the Receive coil. The wires are insulated so it ok to have the Receive and Feedback touching. The Transmit and Feedback coils can have the same wire gauge. Here is a very very important point! The end of the Receive wire nearest the Feedback Coil must be connected to ground. In other word it must be connect to the loop shield and to the ground in the circuit. If you don’t do this the completed coil will not operate correctly. The R null component of the coil will be excessive and may overdrive the detector. This has to do with the high capacitive coupling between the Receive and Feedback windings. Connecting the coils as I have outline above will solve the problem.

The Transmit and Feedback coils must be connected series opposing. If they are connected incorrectly you will not be able to obtain a null signal from the Receive coil. This is generally not a problem since it only works one way and not the other. Basically what is required here is that the magnetic field produced by the Larger Transmit coil must be in the opposite direction to that of the Feedback coil. This is the key to obtaining a magnetic null for the Receive coil.

One final important point. The ratio of the Transmit turns to the Feedback turns is about 3.3 times. This holds only for this size coil, 7 to 8 inch diameter. In other words to determine the Feedback turns divide the Transmit turns by 3.3.

About the connection of the Receive in relation to the Feedback wind. It’s seems to be a very minor thing and not important. The only reason this is a problem is due to the closeness (touching) of the windings. If they were an inch or two apart it would not be a problem. There is one other little point that I forgot to mention. Again this is very important. The Feedback winding must be connected to the un-driven side of the Transmit tank circuit. For the same reasons I mentioned before this reduces the capacitive coupling between the Transmit and Receive which will allow the resultant Receive null to be small.

The dynamic effect I refer to is a small second or third order effect. Most designers are not aware of this effect since it is so small. Other than the detectors I mentioned no one that I know of have ever used this circuit.
This dynamic effect has very little to do with the earth’s magnetic field. It has to do with the interaction of the metal detector’s magnetic field and the magnetic material in the ground when the loop is moved in relation to the ground. Unless it’s eliminated it is impossible to obtain a true balance if the coil is in motion. As you can see this is an important concept for the motion mode. It helps for the GB mode too. But, it is not as apparent in this mode since the loop does not have to be in motion all the time.

Sometimes there are very simple answers why some manufacturers use paint and others use foil or paper for the loop faraday shield. Usually it is related to ease of construction or cost, not necessarily to performance. Each technique has its advantages and disadvantages. And, you may only determine what those factors are by trial and error. My background has been in using paint shields. This process can be a little tricky if you don’t know what to avoid. For example. It’s best to have a very smooth surface on which to place the paint. Painting a shield on an irregular surface can cause excessive noise in the detector. If fact early Discovery loops had only the bottom cover painted in order to avoid too many surfaces that were not smooth. However, after some problems were worked out all their loops were 100%­shielded. Making an electrical connection to a paint shield is difficult too. When I was with Teknetics we experimented with many correction methods before settling on one process that produced consistent results. A poor connection was prone to breaking loose or producing very high detector noise.

Generally its best to have some distance between the shield and the coils. However, I have seen many designs were the shield is place directly on the wire. A foil shield for example. The conductivity of the shield must be low enough to not interfere with the detector operation. My personal experience has suggested that resistances around 10K ohms per square or optimum. However, the resistance can vary quite a bit without effecting detector operation. When this value drops below 1k then you can have pickup problems.



The standard loop size is the best size to use on a fixed GB detector since it was the coil most likely to which the detector was designed. If you read my post to Reg you will see that I took extra care to try to insure that the other loop sizes meet the same characteristics as that of the standard coil. It is also true that larger coils pickup the ground more than smaller coils. So any phase errors due to a detector-coil mismatch will make this problem worse. The only sure way to get around this is in using a detector with a GB mode and a manual ground balance.

If the ground is very heavily mineralized due to natural mineralization or pollution, a fixed GB machine would probably be of no value. An error of several degrees, as I point out above, will translate into a negative offset totally masking all targets. To make matters worse, most detectors are designed to work in moderately mineralized ground. Where the ground strength is not excessive. High mineralization will overdrive the front-end circuits of most detectors making them useless. Raising the coil above the ground will eliminate front-end saturation. However, as an operator you may never know just how high to raise the coil in order to avoid saturation.

Loop fold-over was a term I came up with to describe the non-linear characteristics of a loop. Generally, as you lower the loop to the ground the output increase as a function of the mineral in the ground getting closer to the coil. However, I have noticed that some coil configurations will reverse signal polarity if the ground is close enough to the loop. This characteristic is loop design dependent. In some ways its good since it tends to balance the mineral out. Its very difficult to observe in an actual “real” ground condition since its simply a small change in the amplitude of the loop signal. In most cases the loop output does not change polarity. This characteristic is most easily observed using a point source ground, a piece of ferrite

 

[Vagabond87]

Them's a lot of fancy words I suppose I set myself up for that one...

All kidding aside, thank you very much!! This is why I'm loving this forum! Going to take a few weeks and read this, and I'll be back with more crazy questions! Thanks Vlad!

 

[Tom_in_CA]

N/T

 

[vlad]

George's innovation was legendary; @ one time he was 'stand alone.' Dave Johnson once said George used a propriety system in TID that maybe no one else might have come up with.
But Dave takes a back seat to no one; the number of designs he worked on are astonishing. (His CZ came out about '92, and 26 years later is still in production and one of the deepest.)
Anyone interested George is still around: hamby@cyberis.net