While experimenting with my Gold Racer, I think I saw more stable VIDs by raising the coil and scanning perpendicular to magnetic north. There seemed to be a height that was most stable, and scanning perpendicular to magnetic north tended to eliminate oscillation between very low and very high values. It wasn't perfect, but it was suggestive. I'm wondering if anyone else has anything to offer about this.
By Mxt Sniper
I wonder if Minelab will build a dedicated nugget detector in the style of the equinox, but with gpz type technology. This seems to be the way the CZX by Fisher is headed. I think the Equinox will open up many more machines in the same style eventually. I would hope for concentric and dd coils and more power in a lightweight dedicated nugget hunting package.
By Steve Herschbach
A nice summary quote from Tom Dankowski about why Simultaneous Multi Frequency (SMF) is worth consideration over single frequency options...
“SMF's punch through bad dirt better. Hold on to accurate ID's at depth....and in bad dirt....better. Handle EMI better. Genuinely handle wet-salt better..... to include more accurate ID at depth.,.,.,.,.,.,.,., and a host of other rationale/justifications.”
Fisher Intelligence (5th Edition) by Thomas J. Dankowski
By Steve Herschbach
When I saw a video showing the Makro Gold Racer recovery speed using two nails and a gold ring, it caused me to reflect on the various internet nail tests. Nearly all employ modern round nails, when these items rarely present issues.
The common VDI (visual discrimination scale) puts ferrous items at the low end of the scale, and items with progressively increasing conductivity higher on the scale. The problem is the size of items also matters. Small gold is low on the scale, and the larger the gold, the higher it reads on the scale. A silver quarter reads higher than a silver dime, etc.
All manner of ferrous trash including medium and smaller nails fall where they should when using discrimination and are easily tuned out. The problem is large iron and steel items, and ferrous but non-magnetic materials like stainless steel. Steel plates, large bolts, broken large square nails, axe heads, hammer heads, broken pry bar and pick tips, etc. all tend to read as high conductive targets. Usually it is just the sheer size pushing it higher up the scale.
Detectors also love things with holes, which makes for a perfect target by enabling and enhancing near perfect eddy currents, making items appear larger than they really are. Steel washers and nuts are a big problem in this regard, often reading as non-ferrous targets.
Oddball shapes cause problems, particularly in flat sheet steel. Old rusted cans often separate into irregular shaped flat pieces, and roofing tin (plated steel) and other sheet steel items are my number one nemesis around old camp sites. Bottle caps present a similar issue in modern areas. These items produce complex "sparky" eddy currents with both ferrous and non-ferrous indications. Many thin flat steel items produce remarkably good gold nugget type signals in old camp areas.
Two general tips. Concentric coils often handle ferrous trash better than DD coils. A DD coil is often the culprit when dealing with bottle caps where a concentric coil often makes them easy to identify. Another thing is to use full tones. Many ferrous items are producing both ferrous and non-ferrous tones. Blocking ferrous tones allows only the non-ferrous tone to be heard, giving a clear "dig me" signal. This was the real bane of single tone machines with a simple disc knob to eliminate ferrous objects. You still heard the non-ferrous portion of the signal. Multi tones allows you to hear the dual ferrous/non-ferrous reports from these troublesome items, helping eliminate most of them.
Certain detectors can also show multiple target responses on screen at once, like the White's models featuring the SignaGraph (XLT, DFX, etc.) and CTX with target trace. These displays show target "smearing" that stands out differently from the clean VDI responses produced by most good items. A machine with a simple VDI numeric readout can only show you one number at a time and the only indication you might get is "dancing" numbers that refuse to lock on. Usually though the predominate response overrides and fakes you out. This is where a good high end visual display capable of putting all VDI response on screen simultaneously can really help out.
I have been collecting these odd iron and steel items to practice with and to help me evaluate which machines might do best in ferrous trash. The main thing I wanted to note here is contrived internet videos with common round nails often present a misleading picture. Many machines do very well on nails yet fail miserably on flat steel.
By Steve Herschbach
The latest issue of the ICMJ is out, and I have an article in it titled Selectable Frequency vs Multi Frequency Detectors. Those of you with a digital subscription can read it online.
The ICMJ has a policy against mentioning brand names in articles so I wanted to post this as a supplement to the article.
Most metal detectors run at a single frequency. Low frequencies, that is single digit frequencies under 10 kHz, react well to high conductive targets, like coins, or large items, even if those items are of low conductivity. If you look at this typical metal detector target scale below you will note that non-ferrous items read higher not just based on conductivity but size also.
Low frequency detectors also do not "light up" the ground or hot rocks as much as detectors operating at higher frequencies. Many do not even offer ground balance controls because a factory preset level works well enough for some uses. Low frequency machines under 10 khz therefore tend to be aimed at the coin detecting market. There are too many models to list but most people have heard of the 6.5 khz Garrett Ace 250 as a perfect example.
High frequencies 30 khz and over have extreme sensitivity to low conductive and small items, but also struggle more with ground penetration and hot rocks. Their extreme sensitivity to tiny trash items like aluminum bits do not make them very practical for any detecting except gold prospecting. Machines 30 khz and higher tend to be dedicated prospecting machines. Examples would be the 48 khz White's GMT, 71 kHz Fisher Gold Bug 2, 56 kHz Makro Gold Racer, and 45 khz Minelab Gold Monster 1000.
In 2002 White's introduced the White's MXT at 14 kHz, and it is a perfect example of how detectors running in the "teens" make excellent "do-it-all" detectors. Since then everyone and their brother has jumped on that bandwagon, and there are too many machines running in the 10 kHz - 20 khz region to mention. Prospectors in particular would recognize the 19 khz Fisher Gold Bug Pro, but few know it is also sold in slightly different versions as the Teknetics G2, Fisher F19, and Teknetics G2+, all 19 kHz detectors sold to the general coin and relic market. Garrett has the 15 kHz AT Pro and 18 khz AT Gold to name a couple more popular metal detectors.
Well if low frequencies are good for coins and high frequencies good for gold, why not make machines that can do both? Or both at once?
Selectable frequency refers to machines that can select from one of several possible frequencies, but analyze the signal from only one frequency at a time. These may also be referred to as switchable frequency detectors. Multiple or multi frequency detectors analyze the signal from two or more frequencies at once. In theory this multifrequency analysis can be done simultaneously or sequentially at a very high speed. The end resultant is the same - the results from two or more frequencies are compared to derive information that cannot be had by analyzing a single frequency alone.
Multiple frequency detectors usually have a fundamental frequency, and then other "harmonic" or secondary frequencies they also use, but the power (amplitude) fades with distance from the primary frequency. From page 9 of Minelab's Metal Detecting Terminology:
You can find more information on harmonic frequencies at http://www.ni.com/white-paper/3359/en/ and here also.
Coils normally must be wound specifically to make use of any given frequency or set of harmonic frequencies. A coil will usually work best at the given fundamental frequency making it difficult to get the best possible performance at all frequencies using one coil. The Minelab X-Terra series specifically requires a coil change to achieve a frequency change for this very reason. People who own them know 3 kHz coils weigh more than 18.75 kHz coils. Why? Because heavier windings are used at 3 khz for optimum performance at that frequency.
Here is what is probably an incomplete list of selectable frequency detectors and year of release:
1989 Minelab Eureka Ace Dual 8 kHz 19.5 kHz
1993 Minelab XT 17000 6.4 kHz 32 kHz
1994 Compass X-200 6 kHz 14 khz
1997 Minelab XT 18000 6.4 kHz 20 kHz 60 kHz
1999 Minelab Golden Hawk 6.4 kHz 20 kHz 60 kHz
2002 Minelab Eureka Gold 6.4 kHz 20 kHz 60 kHz
2005 Minelab X-TERRA 50 7.5 kHz 18.75 kHz
2006 Minelab X-TERRA 70 3 kHz 7.5 kHz 18.75 kHz
2009 Minelab X-TERRA 305 7.5 kHz 18.75 kHz
2009 Minelab X-TERRA 505 3 kHz 7.5 kHz 18.75 kHz
2009 Minelab X-TERRA 705 3 kHz 7.5 kHz 18.75 kHz
2009 XP DEUS 4 kHz 8 kHz 12 kHz 18 kHz
2016 Rutus Alter 71 Variable 4 - 18 kHz
2017 XP DEUS Additional 14 kHz 30 khz 55 khz 80 khz options
2017 Nokta Impact 5 kHz 14 kHz 20 kHz
2017 Makro Multi Kruzer 5 kHz 14 kHz 19 kHz
2018 Nokta Anfibio 5 kHz 14 kHz 20 kHz
Multiple frequency or multi frequency machines have become very confusing, as a lot of marketing material has focused on the number of frequencies transmitted. What really matters is what frequencies a detector receives, and how the information is compared and processed for results. Some commentary here. Many people look at the marketing material and assume that a machine processing multiple frequencies is somehow working across the board to deliver the best possible results at all frequencies. However, the two issues outlined above do apply. The machines are employing harmonic frequencies, and so cannot compete with a machine optimized at a single frequency as opposed to one of the distant harmonics running at less amplitude. Second, making one coil run perfectly at all frequencies is extremely difficult, again giving the dedicated machine an edge.
I highly recommend people not go down the technical rabbit hole but instead focus on what the machines do, on how they act. Two things are very apparent.
First, the big market for a long time was coin detectors, and the goal always was to identify coins as deep as possible while ignoring trash as well as possible. Processing two or more frequencies simultaneously gives the detector engineer more information to work with. All the focus was on developing great coin detectors and guess what, the multi frequency machines for all intents and purposes act just like very good lower frequency coin detecting machines. Good ground rejection, and great discrimination on coins for as deep as it can be achieved. The multi frequency machines don't really go deeper than single frequency coin detectors, they just do a better job delivering clean discrimination results to depth.
Here is a list of introductory models of multi frequency detectors and year of introduction. I am not listing all the derivative models to reduce clutter. I will post that later.
1991 Fisher CZ-6 5 & 15 kHz
1991 Minelab Sovereign BBS
1999 Minelab Explorer S/XS FBS
2001 White's DFX 3 kHz & 15 kHz (Simulates single frequency by ignoring half the dual frequency signal)
2012 Minelab CTX 3030 FBS2
Second, single frequency detectors have a ground balance problem. They can ground balance to mineralized soil, OR they can ground balance to salt water. Multi frequency machines can reduce signals from both mineralized beaches and salt water simultaneously, making them ideal for saltwater use.
1993 Minelab Excalibur BBS (Sovereign in waterproof housing)
1995 Fisher CZ-20 5 & 15 kHz (CZ-6 in waterproof housing)
2001 White's Beach Hunter ID 3 & 15 kHz (DFX in waterproof housing)
There is a third class of machine that can run either as selectable frequency OR multi frequency detectors. Quite rare at this time.
2009 White's Spectra Vision 2.5 Khz or 7.5 kHz or 22.5 kHz or all three at once
2018 Minelab Equinox 5 kHz or 10 kHz or 15 kHz or 20 kHz or 40 kHz plus multi frequency options
In my opinion multi frequency has delivered well on its promise. The Minelab BBS and FBS machines are renowned for their ability to discriminate trash and detect coins due to their sophisticated processing. Again, focus on what they do. Not even Minelab in their marketing tells anyone these are prospecting detectors. Second, the Fisher CZ-20/21 and various Minelab Excalibur models are without a doubt the most popular and successful non-PI saltwater beach detectors made.
I have a DFX and I think it is a fantastic jewelry machine in particular. A good coin machine but lacks a bit of punch. The Vision/V3i upped the ante but while amazing on paper suffers from interface overload. The Minelab units are simple by comparison and a lesson on how people in general just want the detector to get the job done. Feature overload is not a plus. However, I think White's has the right idea. The ability to run either separate frequencies or multiple frequencies at once is very compelling. I just think nobody has really done it right yet in a properly configured package. The V3i has the ingredients, but needs to be stuffed in something like an MX Sport with a simplified interface and improved ground balance system. It really never did beat the MXT in some ways and many people when "upgrading" to the V3i end up going back to the MXT.
Selectable frequency has yet to really deliver on its promise in my opinion. So far it has been difficult to produce a selectable frequency machine that truly performs at all frequencies on par with a dedicated single frequency machine. The Minelab Eureka Gold at 60 kHz just never gets mentioned in the same breath as the White's Goldmasters/GMT or Fisher Gold Bug 2. Also, most selectable frequency machines in the past have been very feature limited prospecting machines, restricting their overall market appeal.
I personally think we have seen enough variations of single frequency detectors. I do not believe much can be done to exceed the performance of the dedicated single frequency VLF type machines we currently have. What can obviously be done is a better job of packaging machines that deliver true punch at different frequencies, or multi frequency machines that bring across the board performance closer to what is expected of PI detectors. I do think we are seeing this happen now. The new Nokta Impact and the new DEUS V4 update are expanding the available options in selectable frequency in more usable packages. The Minelab GPZ and other hybrid platforms blur the line between what is traditionally considered PI and VLF and simply need the addition of discrimination to go to the next level. There is still a lot of potential to deliver machines that might reduce the number of machines many of us feel compelled to own by delivering more across the board performance in a single machine that would now take several detectors. Exciting days ahead.
For those who want to try and get their head around selectable frequency and multi frequency technology, Minelab and White's have a gold mine of information in a few of their references. Dig into the following for some great explanations and diagrams.
Minelab - Metal Detector Basics and Theory
Minelab - Understanding Your X-Terra
White's - Spectra V3i Owners Guide
White's - V3i Advanced Users Guide
Better yet are the last three parts of the DFX instructional video by White's featuring engineer Mark Rowan explaining frequency and multi frequency methods: