GrievousAngel

“What you will find is there is no singular answer to the question you are seeking because the variables in play are simply too complex.  I get the impression you are looking for the optimal frequency to detect . . . “

No not that at all. I found the science interesting, and I guess I was looking for a 'meaningful understanding' that made sense to me. I understood what -3db half-power was about in electrical power transfer and impedance matching, so I set out to see if my intuitive feel was accurate as applied metal detectors. It helped me understand why a specific frequency is better for a specific target, as opposed to accepting and putting to memory. Like why a dime reacts better to 2.7kHz than say 15kHz? Other than the manual says so.

I'm kind'a weird in that way. I like understanding as opposed to running on 'blind faith' one may say. As you have mentioned, be careful to not get trapped in the technology thing. And you are right. The bottom line is to get outside and go detecting!

I appreciate your comments. I need those 'checks and balances' to stay focused.  Later, Billy

BTW . . . I would really enjoy traveling to Nevada and detecting with you and others in that area. I've lived in CO's Eagle River Valley (I70) for over a year in 2013-14. I love the Western states. I'm hard to pinpoint at times, as I enjoy outdoor adventures and yet I'm into technical details, etc. I could learn so much out there! I live in SE Georgia, and raised my family on the Ogeechee River near Statesboro.

I got a few articles to read now. Like I said, this is a great site to be part of.  You and others have done an excellent job with it.

Steve and the rest of us . . . 

Without any doubt, this site is the 'Top-Drawer'  source of accurate information, in all aspects of metal detecting - technical to use in the field! I'm totally impressed. There are so many knowledgeable detectorist hanging out on this site, who are eager to share information. 

I have an OK engineering background, but I'm short on actual use in the field. 

Thanks to all,

Billy

10 hours ago, Chase Goldman said:

Billy,

Here's what you do.  Search this forum for any post by "Geotech". He actually posted in one of your threads. That's Carl Moreland.  He is a design engineer who has worked for Whites and now works for First Texas.  Suck up any nuggets you can from his posts..

Then hop on over to his site.  If you ever want to build your own detector or just want to nerd out on the various technologies and scientific principles used for metal detecting (Faraday's law, induction balance, pulse induction) you can find the subject matter discussed in excruciating detail on the references pages and forum there. 

Finally, grab a paper or electronic copy of Carl's book, "Inside the Metal Detector", co-authored with George Overton.  Among a host of other topics about metal detector principles and technology, they describe how operating frequency affects target detectability in depth.  What you will find is there is no singular answer to the question you are seeking because the variables in play are simply too complex.  I get the impression you are looking for the optimal frequency to detect a silver Roosevelt dime vs. the optimal frequency to detect an 1849 $1 Gold piece vs. the optimal frequency to detect a King George III Copper.  The answer is a less than satisfying, "It depends".  And the best you can really do is speak in generalities.  For example, if you want to detect high conductors (e.g. silver coins) at depth go low in frequency (say 4khz), you want gold jewelry then 20 khz is a good neighborhood.  Then you need to consider ground effects that tend to attenuate higher frequencies and EMI which tends to be more prevalent at lower frequencies and you find that metal detecting is not about finding the -3dB half-power reactive component sweet spot but is really about balancing and trading off all these competing effects to give you the best chance to detect your particular target of interest.  HTH

Ahh- Carl beat me to it.  Thank you!  I was wondering when you were going to chime in.

Thanks for leads. I will knock on their doors soon. It is fortunate to be able to connect with others that truly have a 'meaningful understanding' of a given topic. Top-Drawer Experts!

Later, Billy

8 hours ago, Geotech said:

The use of the term "-3dB frequency" is misleading if you know what that term usually means in electronics. It's not a -3db frequency at all, rather it's just the frequency where you get a maximum R-response. A picture is worth 900 words so...

Suppose we're talking about a copper coin of some kind. At 1kHz it has a small response magnitude at a phase just past 90°. Increase the frequency to 2kHz and the magnitude and phase both increase. Again for 5kHz, 10kHz, and 20kHz. The magnitude and phase always increase with increasing frequency. Seems like the target will be easier to detect at 20kHz, that's a big response.

Except... we don't look at the target magnitude. Instead, we only look at the R-response. The reason is that ground is phase-adjusted to lie along the 0° axis which means the X-response may have a lot of ground signal in it while the R-response has none. This means that only the R-channel is clean and useful for initial target indication. (When you listen to a threshold-based detector you are listening to the R-channel. If you hear ground noise then it's because the ground phase isn't properly adjusted and some of it is getting into the R-channel.)

In this example the peak R-response is at 5kHz. This also corresponds to one-half of an eventual maximum X-response the target could possibly generate. Payne calls it the half-power/-3dB frequency which I don't care for because it doesn't correspond with the traditional use of those terms. Some people call it the resonant frequency which is even worse, there is no resonance at all. It's just a result of only using one-half of quadrature demodulation.

In any case, it is true that (everything else being equal) a 5kHz detector will detect this particular coin better than at other frequencies.

I agree. Yes it can be confusing for sure, as the terms are often used loosely. Due to time and avoiding over-stating an accepted law in a more casual setting, you have to be careful how one might explain a concept.

Yep, it is the 'special off-set' frequency at which the R-response component is at maximum level (magnitude) in which the detector receives and processes. Conceptionally I think of it as shifting the center frequency intended to be transmitted, up or down so it's at maximum magnitude when received. In terms of electrical power, -3db frequency point is called the half-power level, in which maximum power transfer is achieved via  impedance matching. This law is also critical in microwave waveguide design, i.e., used in many engineering fields.

My definitions and concepts were taken directly from the articles below. I should have referenced the sources in more detail.

The article Signal Frequency And Multiple Frequency as found on this site, https://detectorprospector.com/forums/topic/3193-selectable-frequency-and-multiple-frequency/ was the entry point for my thread. This article also referenced George Payne from his article found at http://jb-ms.com/Baron/payne.htm (2002).

I would suggest reading these articles if not already done so. I certainly improved my understanding of the science behind VLF frequency selection, single or multiple.

Cool stuff! Billy

 One of the experts in this arena is George Payne as referenced in

9 hours ago, GB_Amateur said:

I'm sure some parts have been detected.  You still have all the typical difficulties of permissions, protections, destructive modernization,...

Sad it is. I think about concrete covering up important finds more or less for ever! But they can surface 100's of years later.

9 hours ago, Tom_in_CA said:

 

I do not think it's worthwhile to just wander any old "historic wagon train trail".   Instead, you want to hunt where they stopped at nights.   Eg.:  The recurring stopping spots (since each passing wagon/group would tend to re-use the same spots, over and over).   Eg.:  Like stage stops, springs, trading posts, etc......

 

Right. You must research and plan.

I would love to spend a month + checking it all out. I have been out that many times, but Time Won't Let Me!

Have you gathered many finds in that area?

Billy

23 hours ago, Steve Herschbach said:

The problem is the secret sauce is the processing, and nobody that knows for sure is going to tell you what that is. Everyone can speculate all we want, but Minelab is not going to release proprietary information in any way, nor will anyone else. They often are not even doing patents now as being too revealing, relying instead on hard encoded algorithms.

See this post for reference

 

Ok Steve, I guess I'll go down Dimitar's Tarsacci rabbit hole . . . just knock on the door to keep me from getting too lost! 

I was not aware of his work in any detail. I've noticed his name about but that's about it.

Billy

Absolutely right Steve, it's held very close.

Although, I wasn't chasing the received signal processing, e.g., electronic circuitry or the coding, I was more interested in understanding the science behind determining the optimum frequency (single or multi) to be transmitted for a given target.

As I mentioned, I ran into the reference to George Payne's paper discussing the significance the -3db frequency shift up or down from the center transmitted frequency (per -3db half-power law ), where R magnitude is maximum and X magnitude is halved. Per Payne it turns out that the returned signal is maximum when at this -3db special frequency point. Therefore the VHF processing unit receives this optimum signal, in which improves it's performance substantially. (The amplitude(s) of these wave forms are easier seen in the frequency domain.)

Thanks to your article Selected Frequency vs. Multiple Frequency, I have a much better understanding of the details surrounding the selection of a specific frequency for a specific target. That's the short version of why certain model detectors are better at certain targets ranges. The -3db Frequency point as a huge impact on the detectors design.

A few days back, I did not have as clear of a understanding as I do now, but it ain't that clear . . . Steve your articles great. Thanks for sharing your knowledge of detectors, from use in the field to the technical aspects. 

I apologize for repeating some of my comments and references, but it's difficult to drop into a related thread without some premise.

Later, Billy

9 hours ago, Steve Herschbach said:

No worries, Andy is the smart one, I’m the pretty one! 

I will leave the deep dive into frequency tech to others. I have a good layman’s grasp of it all, but I’m no wizard. I do know there is way more to things than just frequency, and focusing too much on it now does not necessarily serve anyone but engineers all that well.

For example, you can study specs all day long on, for instance, Apex vs Vanquish or Legend, or even my old DFX. Yet it is only actually using the detectors side by side that tells you anything real, and what you learn fast is that what looks perfect on paper, may not be so in actual use. Apex is a good example of a detector that was created just to say “we have a multi”, but which in actual use will not have many people ditching their Garrett AT models. Other than possibly saltwater, it would be a step down, not up, as is evidenced by Garrett placing it as an Ace, rather than at a higher price than the AT models.

Long story short, for regular folks like me, things under the hood are getting way more complicated than appears on the surface, with hybrid processes moving us past the world of simple explanations. And at the end of the day, while I told myself I needed those single frequency options on Equinox to get best performance options under all conditions, reality is I am in multi 99% of the time. Single frequency is only rarely helpful, maybe for a certain hot rock or target, or an EMI situation.

Anyway, I did my best to put some basic information out there for the laypeople amongst us, but from my perspective I have little interest in delving into the subject any deeper, and will leave it to brighter minds than mine. I’m pretty satisfied right now to just grab what we have got and go use it, and trust that the squirrel running on the wheel under the hood is doing it’s job. 

I emailed Minelab referring to my interest in the -3db frequency half-power level and it's impact on receiving the maximum returned signal for processing. I also referred to the George Payne article that was referenced in the article named Single Frequency and Multiple Frequency.

I just got a reply from Minelab engineering stating that information is not available for public consumption. So maybe it is deeply woven into Minelab's Multi-IQ weighting algorithms. From my limited point of view, it would seem logical.

I've taken this about as far as reasonable plus I learned a little along the way. Thanks for your advice. Moving on.

Billy

Me too!

That's it. I would recommend detectorists to watch the series. Traces history throw there finds at certain locations.

Thanks for the info. Billy

. . . where two guys wandered around the USA detecting relics and coins? They used Garrett's AT detectors I think back. They would locate/research historical homes and other locations of interest, then ask for permission to hunt. There was a different story every week I think.

I always thought that historic wagon train trails would cough-up great finds. I would assume one could travel the same pathway, either by road or off-round, with proper research and planning.

I would like watch those shows again.

Billy

What was the last meteorite found in the USA Mainland? How 'bout worldwide? 'Falling Stars' are visible every evening somewhere on this planet I would think, but finding them is something else of course.

I used to watch Meteorite Men on the Science Channel years back. What were those cats names? Gregory maybe and one other guy. It always interest me . . . how meteorites were located and hopefully found. I remember one they found that was huge like maybe 3' in diameter. It wasn't from a recent fireball that someone saw falling. I always wondered why they were looking on this particular large farm & pasture land? It was like 6' deep. I think they were using a custom 2-box detector on wheels. It lay along the side of a long fence line.

Did anyone see that particular Meteorite Men?

Anyways meteorites are cool!

Later, Billy

I gave my Garrett L&S PI to my grandson.

I found it was basically a ‘dig everything’ detector that worked Well on saltwater beaches and most soils at good depth.

Are there any other advice or tips I can pass on?

i stopped using it has it a very heavy detector. Made like a tank but heavy!

That is what led me to purchase the Equinox 800.

Later, Billy 
 

What profile is typically better for wading or diving in freshwater? I am assuming no presence of black sand or other strong mineralization in soil or the water.

I venture to say, the same settings that works on surrounding land! However I haven’t spent enough detector time to form a strong opinion.

What have other Equinox users found to work well?

Billy
 

What is a good Equinox compatible  headphone for wading in saltwater surf knee deep? I can easily see a larger wave splashing and covering detector and me for a second or so! So maybe the term water-resistant is applicable. 

Plus has good sensitivity to fast weak signals. I just posted a similar thread where I mentioned  the advantage of a lower mass speaker component.

Suggestions?

Billy

I’m looking an Equinox compatible water resistant headphones. I said ‘resistant’ because I don’t plan to dive, just wade in the ocean surf like knee deep.

They need to be sensitive enough to hear fast dim signals. The more mass the speaker component has, the less sensitive they are. Larger mass speaker diaphragms are not as responsive to fast changing signals with weaker signal amplitudes.

Wired or fast Bluetooth OK as long as they deal with saltwater and sand!

The ML-80 headphone don’t appear water-resistant, so I am looking  

Suggestions?

Billy

16 hours ago, Steve Herschbach said:

Well, for starters, 4kKz was added after all that. You are pulling up an old thread. And really it’s just a marketing tag, not an explanation of the facts of the tech at work. It was a simplistic was of counting frequencies and modes on 800 versus frequencies and modes on 600. That’s it, nothing more.

And I’m not Andy. 

I’m sorry Steve. For some reason I thought you where Andy S. My apologies Steve  

I understand it is a  marketing thing and not to be referred to as technically correct.

i was more interested in the -3db point of a selected single frequency as discussed on a different thread.

Billy
 

On 1/16/2022 at 3:51 PM, Chase Goldman said:

The terminology you are using implies to me that you seem to be looking at this like radar.  RF being reflected off metal.  That's not what is going on here.  It is induction balance.  You need to be thinking transformer.  In very simplistic terms, the transmit coil magnetic field  induces eddy currents in the target  (and ground) that are then magnetically coupled to the receive coil.  The resulting phase shift is indicative of the electromagnetic properties of the target.  Not sure why you are repeatedly referencing the half power (-3db) "cutoff" frequency.  I must be missing something.  HTH

Actually it is similar to radar, but the transmitted magnetic waves cutting throw the target's eddy currents is what produces the returned 'radar-like' signals. I think I said that correctly. Billy

On 1/16/2022 at 3:24 PM, PimentoUK said:

Getting complicated, now.

Detectors don't just measure the reactive component of a target. If they did, a 13kHz machine ( decent all-rounder Fisher F75, for example ) would be hopeless at finding 1 kHz targets ( US silver dollars / half-dollars ), as the phase lag is about 86 degrees, with the reactive component way less ( 7% ) than the resistive component.

"I would tend to think this has a lot to do with Multi-IQ design and resulting algorithms"

Now you're bringing multiple freqs into it ... and it sounds like you don't really understand the why's of multi-freq.

Using multiple frequencies has very little to do with "hitting the target with a range of freqs hoping that one will hit the spot". It's primarily about working out the ground signal, so it can be largely eliminated, thus making the target more visible.

Detectors don't just measure the reactive component of a target. If they did, a 13kHz machine ( decent all-rounder Fisher F75, for example ) would be hopeless at finding 1 kHz targets ( US silver dollars / half-dollars ), as the phase lag is about 86 degrees, with the reactive component way less ( 7% ) than the resistive component.

Of course they don't. Returned data is in the form of X (inductance-like current momentum) and R (inversely related to conductivity). Of course that is a simplified analogy.

I am not disagreeing with all you have said. You are correct. Billy

On 1/16/2022 at 3:24 PM, PimentoUK said:

Getting complicated, now.

Detectors don't just measure the reactive component of a target. If they did, a 13kHz machine ( decent all-rounder Fisher F75, for example ) would be hopeless at finding 1 kHz targets ( US silver dollars / half-dollars ), as the phase lag is about 86 degrees, with the reactive component way less ( 7% ) than the resistive component.

"I would tend to think this has a lot to do with Multi-IQ design and resulting algorithms"

Now you're bringing multiple freqs into it ... and it sounds like you don't really understand the why's of multi-freq.

Using multiple frequencies has very little to do with "hitting the target with a range of freqs hoping that one will hit the spot". It's primarily about working out the ground signal, so it can be largely eliminated, thus making the target more visible.

"Now you're bringing multiple freqs into it ... and it sounds like you don't really understand the why's of multi-freq." by PimentoUK.

Maybe so, but please refer to this article: Selectable Frequency and Multiple Frequency from this site. In which part is taken from an article by George Payne. See this link: http://jb-ms.com/Baron/payne.htm (2002).

My comments was taken directly from these two articles. The -3db point yields max returned signal of a given center frequency. I then became interested and wondering if the -3db half-power data was somehow used in the different weighting algorisms used in Minelabs Multi-IQ. I would guess, very likely.

I am hoping to get comments on this specific concept. Maybe I put this thread in the wrong forum.

Thanks, Billy

21 hours ago, FloridaSon said:

Is Beach 1 & 2 then a different range or does the salt ‘balance’ make the only difference? Thanks 

I think we are saying the same thing. In a previous thread I clarified what I was saying.

Please refer to latest Equinox Manual. Frequency, p. 29. I was relating to this page. it's just a marketing thing. With v3.x and the addition of 4kHz (3,7kHz) as a single frequency (even though not included in Multi-IQ algorisms, it does expand the range or ratio to ~ 11, therefore the marketing graphic could/would 6Fx11 (800) and 4Fx4 (600). 

BTW, that was not the main idea I was interested in. Further into my comments I referred to -3db point of a given single  frequency. I was relating [it] to the half-power level of the transmitted alternating wave and the resulting received alternating wave prior to being processed into a maximum magnitude. I think it is the R component. I was hoping someone would expand on the -3db point of a given frequency, as I understand it, that would yield the strongest returned signal for processing.

Please refer to the article Selectable Frequency and Multiple Frequency, p. 1-2. This part of the article was taken from a previous work by George Payne at http://jb-ms.com/Baron/payne.htm. 

Thanks, Billy

On 1/16/2022 at 3:22 PM, Chase Goldman said:

Billy,

Yes, pretty much.  All I'm saying is that the individual selectable frequencies and the marketing term 5Fx8 are separate and distinct from Multi-IQ.  About the only thing you can infer is that the various Multi-IQ modes in total, roughly cover the 4 to 40 khz range.

Thanks helping me clarify. Personally, I don't want to share information that is actually inaccurate, so I often ask if someone agrees or disagrees. I enjoy an awareness of technically correct background knowledge, but at times I go out on a weak limb which usually falls.

Billy

30 minutes ago, Chase Goldman said:

No.  Minelab has already stated the individually selectable single frequencies (or their range ratios) have no correlation to the base frequencies used in Multi IQ.  Take 4 khz, it was introduced 2 years after the release of the Equinox.  That update that introduced it did not appreciably change Multi IQ mode behavior.  At the end of the day, the actual base frequencies and number of discrete frequencies that comprise any Simultaneous Multifrequency Frequency implementation matters little to the end user if the manufacturer is not willing to be explicit about it and as long as the mode behaves generally as described (Park 1/Field 1 favoring high conductors, larger targets>implying low frequency weighting; Park 2/Field 2 favoring mid conductors, smaller targets>implying high frequency weighting) and has good signal processing algorithms.

I think we are saying about the same thing. The 4kHz (3.7kHz) signal is available for use as a single frequency, but is not used the detectors algorisms for weighing in each mode/profile. Using 3.7 the ratio (range dependent) works out with proper rounding but is just a marketing thing, eg., Nox800 with 3.7kHz addition, the 5Fx8 could/would be considered 6Fx11.

Billy