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
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 V5 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 White's 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. (2018 note - Minelab Equinox released). It really never did beat the White's 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:
While looking around on the Minelab site I came across this article by Bruce Candy. It will certainly be a re-read for some but for me it was a first.
There is much more than just Minelab in the article. It included ground balancing, discrimination, gold detectors, coin detectors and a host of other related issues with knowing some of the technology about target detecting.
It doesn't yet include ZED technology but does explain why it is so hard to have a gold detector that discriminates. (When you discriminate you lose targets!)
Metal Detector Basics & Theory by Bruce Candy
By Steve Herschbach
I was looking at some old metal detector catalogs and got a chuckle out of these charts from the 1973 Garrett catalog. People get up in arms about advertising claims these days but get a look at these. To their credit they say "large metal objects" and do not define what that is (dump truck?) but we are talking 1973 BFO detectors here. I need to ditch my new detectors and get one of those old machines! Unfortunately depths were measured in inches then, not feet, on normal targets.
The irony is the page is addressing "misleading advertising".
Garrett approximate coil depths on large metal objects
By Steve Herschbach
I have used many metal detectors over the years, and right now I have to say that the new Makro Racer 2 has perhaps the easiest to understand, best laid out, most practical display and menu system I have ever seen in a top end detector. Now, you can sure say you hunt by ear and do not need a screen and I get that, but if we are going to put a screen on a detector, then let's do it right.
Simple detectors with few functions are easy to make screens for - there is not much you need. But even then just the basics are often wrong. Machines that feature target id numbers, what is the thing you will most look at on screen? The target id numbers! Yet these are often way too small or off to the side as if an afterthought.
The Makro Racer 2 id numbers are huge, much larger than on the original Racer and Gold Racer, which are already good sized. The number 88 display in the diagram above is fully 1.5" x 1.5" in size in real life. Other machines have some pretty big numbers but I think this sets a record as I can't think of any machine with larger id numbers on screen though some are close.
Makro Racer 2 LCD display and controls
Makro Racer 2 screen layout
Makro Racer 2 screen and control descriptions
The number can be the ground balance number, target id, or depth reading. You get a text display just above the number confirming which it is. Below the numbers are three zone references, Fe, Gold/Non-FE, and Non-Fe, that are used to set tone breaks and audio for the three main zones or bins as they are sometimes called.
Another basic feature lacking on a lot of machines - the meter backlight. With the Racer 2 you get off, intermittent, or full time backlighting, and it includes the translucent red control buttons. The control ranges between 0-5 and C1-C5. At 0 level, the keypad and display backlight are off. When set between 1-5, they light up only for a short period of time when a target is detected or while navigating the menu and then it goes off. At C1-C5 levels, the keypad and display will light up constantly. I do not know of anyone doing a better backlight.
The right side of the meter is informational - ground phase (ground balance number), mineral % (ground magnetite content), coil warning notices, and a six segment battery meter.
Across the top below the 0 - 99 reference sticker, is a series of 50 "bullets" each of which covers 2 target id numbers. Open bullets (which appear gray in the diagram but are invisible in real life - see top photo) indicate accepted target id numbers. Blacked out segments show what discrimination and notch setting you have programmed in a single quick glance. When a target is detected, the big number on the display will be mirrored by one or more of the bullets flashing dark.
The four control buttons are simple as can be - up and down takes you through the left hand menu area. Right or left lets you set each function selected by going up and down. The menu is basically the entire feature list just laid out right there for you to see. You want to know what this machine can do, just look at the screen. Most other machines you have no clue without reading the owners manual or at least pushing buttons to see what functions appear.
Some settings like the backlight are system wide for all modes. All other settings like Gain are independent in each mode, and can be saved independently in each mode. This means you can play neat tricks like setting up a couple modes with dramatically different settings and then flip back and forth easily between two modes for target checking.
You even get to decide what mode is the default start up mode. The Racer 2 starts up in the last mode where the save function was performed. If you always want to start in Beach mode, just modify and save something in Beach mode. Next time you start the detector, you will be in Beach mode.
It is simple. It makes sense. No cryptic abbreviations or acronyms. No sub menus. It is, in metal detector terms, a work of art. Whoever designed this should sign it so I can frame it and hang it on my wall.
By Ridge Runner
I was just wondering if you had the opportunity to test a new detector before it came on the market would you be willing to do it ?
Most of the time it’s given to people you could say that their name is in lights . We have so many people that has been swinging a detector for years that are more qualified than the so call star .
Let me hear your view point on this subject . If you ever have the pleasure or opportunity to do it in the pass let us hear that too .