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Steve Herschbach

Why Consider Multifrequency?

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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

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Pretty impressive that he made those observations 16 years ago (maybe even earlier with his expertise with the Fisher CZ family).  Those of us who have just moved over to simultaneous multifrequency are learning (and appreciating) those advantages.

The "handle EMI better" part has me scratching my head a bit.  What I've found with the Minelab Equinox (and I experience this a majority of the time in my local urban hunting) is that I have to run at a gain of 18 +/- to quiet down the Equinox.  The frequency micro-adjust feature when in multifrequency doesn't really help if there is significant EMI.  I usually try and manually do the micro-adjustment but even that has marginal improvement.  Of course a gain of 18 is pretty darn sensitive, so if that will quiet things down I'm fine.  Once I couldn't even get a gain of 15 to ignore EMI.

However, at worst I can choose a single frequency (effectively taking advantage of the detector being a selectable frequency machine as well) and find a fixed frequency (almost always 15 kHz and 20 kHz and sometimes 10 kHz) that lets me run at gains in the 20's.  I don't know if this is what Tom was getting at, but it's a huge plus.  With a single frequency detector, even though you might not experience EMI problems as often, when it happens your options are limited to turning down the gain, and in extreme cases you have to go so far down that it renders the detector nearly worthless.

 

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My T2 hates powerlines and urban areas, It's not uncommon I can't run it at all or have to run it wound right back to 50% gain to get it stable, it runs better with the Mars Tiger coil than any other I have for it but still has the same problems..  In these same places my T2 can't run, I can run my Nox in multi frequency with a gain of 24.  My Garrett Euroace (8.25kgz) can't run within 20 meters of my house, I assume my home wifi is messing with it, it just goes crazy, I can fire my Nox up in my house with a gain of 21 and in multi and it be stable.

My Go-Find 40 is 7.8khz and doesn't get affected by any EMI around my house or anywhere I've taken it, my Gold Bug Pro at 19khz is the same, rarely bothered by EMI.

The T2 is 13khz, the Nox has a 15 khz frequency in it's array, however it must just be far enough away from the 13khz to make it stable, unless of course it's something to do with being Multi frequency, I've never had to switch to a single frequency to settle mine down.  The lowest I've had to go on the gain is 21 to get it stable.

It seems to me 13khz is the worst for EMI.

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I don't post very often but I peruse this and Tom D's forum daily so if this sounds redundant it's because I picked most, if not all, of it up here and there. Having said that I have also seen it proven time again with my own detectors.

Keep in mind that while SMF detectors may handle EMI better than SF detectors, they are not immune to it. Many modern detectors including the Equinox and T2 are "sparky" by design.  While this increased sensitivity offers improved depth when conditions allow the use of the extra horsepower, it also makes them much more susceptible to EMI than others that have had their performance, for lack of a better way to put it, "governed" by the manufacturer.

The F75 is often used as a yardstick when it comes to this type of thing so I'll use it as such here. When you lower the sensitivity on an F75 from 99 to let's say 90 you haven't lost depth as much as you've lowered the audio gain. Depending on your hearing ability you should still hear targets at ROUGHLY the same depth as when running the sensitivity at 99, the detector's audio report will simply not be as loud. The upshot to this is reduced audible and visually apparent EMI. 

The downside to all of this is that all EMI is not audible nor is it visually noticeable via jumpy VDI numbers. I first saw Tom D mention silent EMI in a thread on his forum detailing his experience with the F75 Ltd. prototype. This silent EMI can be caused by many factors such as weather that vary from day to day. I have more than once recovered deep targets that I know I've ran the same coil on the same detector from the same direction over previously without hearing a peep (at least not a "dig me" peep) out of them.

Again, most of this info has all been posted previously. It's just something to keep in mind when comparing the susceptibility to EMI between detectors.

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17 minutes ago, Chris Woods said:

I don't post very often but I peruse this and Tom D's forum daily so if this sounds redundant it's because I picked most, if not all, of it up here and there. Having said that I have also seen it proven time again with my own detectors.

 

You don't post near often enough, There is some good advice/information in your post.  Thanks.

 

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If SMF detectors seem better at rejecting EMI, it's probably just because in many situations they don't need to run as hot to get the same results. But, I'm not even sure about that... Also, as GB_Amateur points out, a non-SMF detector with selectable frequencies may have the advantage, as you can select a frequency to minimize the particular EMI.

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If the multifrequency is sufficient to shift the work channels, it can well eliminate the EMI, and if it's not so good enough to use a smaller coil, which is mostly EMI-resistant, the other option is to use a program that is less disturbed, and and then degrade sensitivity ...
 

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