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:
By Steve Herschbach
Steve's Law of Target Depletion - All good locations with high value targets will be detected with progressively aggressive means until no metal can be found. When any location contains items of great perceived value, detector technology will normally be applied in reverse order of aggressiveness. First will be VLF discrimination "cherry picking". This will be followed by varying degrees of "turning down the discrimination" to dig iffy targets and by using the barest of ferrous/non-ferrous discrimination. This will finally be followed by "all metal" detecting to remove masking effects by either VLF or PI detectors. If the location is considered good enough all targets will eventually over time be completely removed until no detector is able to acquire a target. At this point a site may be considered "hunted out" until a new technology arrives allowing for more depth or ground separation capability, when a few more remaining metal items will be removed. The key concept is that since discrimination is unreliable, all metal items must be removed from high value locations in order to rest assured nothing has been overlooked.
Nugget hunters and beach hunters get right with the program. If a nugget "patch" is located it will be relentlessly pounded until no metal remains. Beaches survive by being a renewable resource. Good relic locations can and will be subjected to the same attention given to nugget patches. The rule is that as long as you can find a piece of metal hope remains that good items can be found. If not you, somebody else can and will return until no metal remains. I have promoted PI detectors for all uses for this very reason for over 10 years now - see that last few paragraphs at www.losttreasure.com from 2005.
Most people consider depth to be problem number one, but for many areas target masking is by far the more serious issue. Until detectors can actually see through trash instead of blocking it out, even the smallest surface trash can and will block deeper adjacent items from being detected. Superb discrimination only gets you so far and ultimately the only solution is to remove the surface trash to see what lurks below. The only real limitation we face in this regard is in areas sensitive to digging holes of any sort, like a well groomed park. Even there, slow careful extraction of surface trash over time can reveal old coins missed by others for decades.
Beneath The Mask by Thomas Dankowski
By ☠ Cipher
Apparently there's a hidden analysis screen in the FBS machines that lead one to a spectrograph of sorts. Much like the V3i which has features hidden away from the end user, supposedly the analysis screen can be accessed via a button sequence that is yet to be discovered.
If you read through the thread, I'm iDetectorX in the last post. If this is a spectrograph it does tend to confirm what Geotech has been telling us all along, that FBS is actually a 2 frequency system in terms of what is actually demodulated and processed, which would be a better explanation in my view as to why it never made it to the end user. In any event, I think it would be an interesting challenge to try to access these hidden item. I'm actually in the process now of doing some experiments with 2 V3s and a V3i, but FBS machines interest me as well.
....ah, forbidden fruit.
By Steve Herschbach
The World's First Smart Detector & Imaging System that can display the shape, depth and dimensions of underground metals in real time. Ideal for Deep Treasure Hunters, Archaeologists, Municipalities, Utility Companies, CSI and Law Enforcement Agencies.