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Rcpt Ack of F75 Ltd prototype

Posted by NASA-Tom 
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 06:07PM
Bryannagirl,
I think as long as one is cognizant of the fact, they will be fine. If one has areas say in the spring/summer that are a bit overgrown, one may consider redecting them in the winter and such. Also, I've never ran a T-2, but from reading overall performance seems very close. The T-2s were not quite as sparky as the f75s. And relic hunters favored the T2s, while coin hunters favored F75s. With the DST upgrade, you will in most places using discrimination have more useable depth-inground vs a T-2 without the upgrade. All Metal on my F75 was quieter than disc, I would think T-2 it also was the case. So the AM depth issue may be more of an issue with both the upgraded units (F75,T2). But to be fair, I haven't any data from say Tom D to support. I think he plans to obtain and publish eventually. An airtest using your T-2 set to Bp in AM with max sensitivity over a quarter may give a clue. Most folks are seeing around 18". Keith had an upgraded F75 that was giving a whopping 22" I believe on a nickel. I think he has since sold that particular unit. It was hotter it seems than most I've read about.
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 06:12PM
Quote:"that is odd you would think detecting distance is detecting distance"
No, no,no.
The depth you can find a target at is determined by by how strong the target signal is relative to that of the ground, as you will recall:
[www.dankowskidetectors.com]
If you raise your coil 5 inches (as per the manufacturers suggestions) your ground signal drops off, maybe 4-fold. But your target signal drops off, maybe a similar amount (but it depends on the target and how deep it is). So the net result is they are both reduced by a similar amount. So if you can increase the gain of your detector (by a factor of 5-ish, say) you can detect 'as normal'. I think the actual 'extra gain' of these machines is 8-fold.
What's not obvious, is that ground signal falls off at a different rate to that of a target signal, and combine this with other variables like the unknown nature of the target, and swinging 5 inches clear of the ground is going to have some drawbacks.



Edited 1 time(s). Last edit at 02/25/2015 06:55PM by Pimento.
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 07:34PM
I may of said that wrong or maybe right. So if in mild ground your detector can hit a nickel at 10 inches at max gain and coil on ground. Now place that nickel at 5 inches in the same mild ground and you would think you could raise the coil no more then 5 inches (or pretty close to that) and still get the same reading as nickel at 10 inches coil on ground. Now in "Bad Ground" maybe you can read that same nickel at 8 inches. So placing nickel at 5 inches I would only expect to be able to raise coil 3 inches (5 inches of bad ground plus three inches of air) I could maybe see gaining a little from the fact the air attenuation of the signal would be less then bad ground plus you have less bad ground at 5 inches to deal with then at 8 inches. So maybe you can raise coil 4 inches (for illustration purpose only).

So let's go back to mild ground. If I understood correctly in BP mode on F75 in my example you could raise your coil 3 inches above ground and still see the dime at 10 inches deep. With coil on ground you may see to 10.2 inches but no really gain in depth from lowering your coil. At 5 inches that same coin could be seen with coil 8 inches above ground. That does not seem intuitive is what I was trying to say..

Thanks

Wrote:
-------------------------------------------------------
> Quote:"that is odd you would think detecting
> distance is detecting distance"
> No, no,no.
> The depth you can find a target at is determined
> by by how strong the target signal is relative to
> that of the ground, as you will recall:
> [www.dankowskidetectors.com]
> .php?2,73496
> If you raise your coil 5 inches (as per the
> manufacturers suggestions) your ground signal
> drops off, maybe 4-fold. But your target signal
> drops off, maybe a similar amount (but it depends
> on the target and how deep it is). So the net
> result is they are both reduced by a similar
> amount. So if you can increase the gain of your
> detector (by a factor of 5-ish, say) you can
> detect 'as normal'. I think the actual 'extra
> gain' of these machines is 8-fold.
> What's not obvious, is that ground signal falls
> off at a different rate to that of a target
> signal, and combine this with other variables like
> the unknown nature of the target, and swinging 5
> inches clear of the ground is going to have some
> drawbacks.

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 08:20PM
Phew. Complicated pile of questions.
I don't think anything's intuitive with this ground / target business. I've made scientific measurements of some of these signals, and it's still hard to sort out the facts. Add in things like different coil sizes / shapes, and 'what works best' changes again.

OK, I'll tackle the last paragraph:
"So let's go back to mild ground. If I understood correctly in BP mode on F75 in my example you could raise your coil 3 inches above ground and still see the dime at 10 inches deep. With coil on ground you may see to 10.2 inches but no really gain in depth from lowering your coil. At 5 inches that same coin could be seen with coil 8 inches above ground."

*"With coil on ground", ground signal = 20 elephants. 10" coin = 1 elephant. Coin is just detectable. 1 elephant is just enough signal level, and 1/20th of the ground's signal is just enough to make it distinguishable. Result = beep.
*"coil is 3" above the ground", ground signal = 10 elephants. (10+3)" coin = 0.5 elephants. Not enough signal. No beep.
*Now engage BP boost mode: ground signal = 80 elephants. (10+3)" coin = 4 elephants. Coin signal is big enough to trigger the detector? Yes. Coin signal is big enough to distinguish from ground signal? Yes 1/20th of it, so just good enough. Result = Beep.

*Now, put coin at 5 inches: With coil low: Ground signal = 20 elephants. 5 inch coin = 20 elephants.
*Coil at 8 inches above ground: Ground signal = 3 elephants. (5+8)" coin = 0.5 elephants. Not enough signal, no beep.
*Engage BP mode: Ground signal = 24 elephants. (5+8)" coin = 4 elephants. Signal is big enough, and at 1/8th the ground signal, is also good. Result = Beep.

That was hard work....
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 08:49PM
Pimento, it's obvious your knowledge of the inner workings of metal detectors far exceeds mine. I'm trying to follow your post. And I don't want to hijack this thread, buy maybe you can shed light on this as it could be related. I've read where some folks have reported digging coins, say a dime i.e. and report the depth dug was more than their detector would air test on said coin. Is this possible? If so, what are the contributing factors? And to follow up, I've read in numerous places where folks say minelabs don't airtest as much distance as they can detect say a coin in the ground (depth). Is this true?? And if so, again, what are the contributing factors??



Edited 1 time(s). Last edit at 02/25/2015 08:58PM by tnsharpshooter.
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 08:59PM
It's too off-topic. And I'm sure those questions have been asked and answered before, on here and other forums.
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 09:02PM
Understand, thanks anyway
Re: Rcpt Ack of F75 Ltd prototype
February 25, 2015 09:03PM
As you can tell I love analogies - love the explination - thanks

Pimento Wrote:
-------------------------------------------------------
> Phew. Complicated pile of questions.
> I don't think anything's intuitive with this
> ground / target business. I've made scientific
> measurements of some of these signals, and it's
> still hard to sort out the facts. Add in things
> like different coil sizes / shapes, and 'what
> works best' changes again.
>
> OK, I'll tackle the last paragraph:
> "So let's go back to mild ground. If I understood
> correctly in BP mode on F75 in my example you
> could raise your coil 3 inches above ground and
> still see the dime at 10 inches deep. With coil on
> ground you may see to 10.2 inches but no really
> gain in depth from lowering your coil. At 5 inches
> that same coin could be seen with coil 8 inches
> above ground."
>
> *"With coil on ground", ground signal = 20
> elephants. 10" coin = 1 elephant. Coin is just
> detectable. 1 elephant is just enough signal
> level, and 1/20th of the ground's signal is just
> enough to make it distinguishable. Result = beep.
> *"coil is 3" above the ground", ground signal = 10
> elephants. (10+3)" coin = 0.5 elephants. Not
> enough signal. No beep.
> *Now engage BP boost mode: ground signal = 80
> elephants. (10+3)" coin = 4 elephants. Coin signal
> is big enough to trigger the detector? Yes. Coin
> signal is big enough to distinguish from ground
> signal? Yes 1/20th of it, so just good enough.
> Result = Beep.
>
> *Now, put coin at 5 inches: With coil low: Ground
> signal = 20 elephants. 5 inch coin = 20 elephants.
>
> *Coil at 8 inches above ground: Ground signal = 3
> elephants. (5+8)" coin = 0.5 elephants. Not enough
> signal, no beep.
> *Engage BP mode: Ground signal = 24 elephants.
> (5+8)" coin = 4 elephants. Signal is big enough,
> and at 1/8th the ground signal, is also good.
> Result = Beep.
>
> That was hard work....

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!
Re: Rcpt Ack of F75 Ltd prototype
February 26, 2015 03:13PM
Very interesting thread; lots of info.

BUT I think the elephant in the room re pre-upgrade units vs post-upgrade units, is this:
Many are concerned about differences in obtained depths between post-upgrade units, i.e., do I have a good one or a bad one?
Here's the elephant: Did I/you have a good one or a bad one before it was upgraded?

Who really knew? I've not had the luxury of running my detector over the same target with another F75 for comparison.
Air tests that others published? Method? Environmental conditions? Sensory/experience factors?

With the upgrades, we have warranty! Part of that warranty is fitness for purpose, is it not?
If we question the depth performance of the upgrade (or any machine while under warranty), why not send it back under warranty for Fisher to check it out and see if they can boost depth performance for us?

Pleasant Garden, NC
AT Max, Nokta Impact, MX Sport, Nokta FORS Relic, GPX 4800, Infinium, Racer, Deus, F75SE, Nautilus DMC II (order of acquisition, last to first)

Does an archeologist argue with a plow? A bureaucrat with a bulldozer?
Re: Rcpt Ack of F75 Ltd prototype
February 26, 2015 11:01PM
All-Metal Mode. This is a scientifically difficult mode to report...... as threshold/ears/re-tune can dictate final resultant; yet, some good data can be extrapolated. First, , , , for all testing, I used 11" DD coil, 9.0, Sens 99, Ground Balance of '90' and 'Motion' mode. Buried clad dime in Florida dirt. I was always seeking to hear a slight, but audibly intelligible threshold change.....enough to be categorized as a positive target response. Also, the process modes that you can select whilst in ID mode...... carry over to the all-metal mode. (With several unique observations I will discuss later).

bp = 11.6"
cl = 11.6" (Elongated audio response)
je = 11.0"
pf = 11.1"
de = 11.0"
bc = 10.1"
FA = 10.4"

Notice how there is not as much linearity carried over from the ID mode results. And notice the All Metal depth was slightly less than ID mode. Largest contributor for this phenomenon/resultant is due to Florida's low mineral dirt. As a qualifier...... I must say this: Some of the process modes required changing the 'Threshold' a small amount..... so as to maximize detectors audible intelligence.

* Ironically.......... performing only air-tests........... and depth/performance is nearly always at maximum performance/depth....... even when switching through (and testing) all of the different process modes. The only change/difference is...... the 'length' of the audio response. Invoking 'cl' mode ... (then switching back to All Metal mode).......... and the targets audio response is elongated.

* Testing the 'Static' mode is too interpretive ...... with largely varying resultants. When you reset the SAT/threshold audio...... by pulling the trigger (and releasing)..... THIS ALONE can have a dramatic effect on depth/performance; hence, too many variables preventing solid/repeatable data.

Okay. 9.1 performance/results:

bp = 11.5"
cl = 11.6" (Elongated audio response)
je = 10.9"
pf = 11.0"
de = 11.0"
bc = 10.0"
FA = 10.3"

There is minimal differential/delta in performance/depth whilst comparing 9.0 to 9.1........... All Metal -to- All Metal.

* There is a slight EMI mitigation enhancement whilst in 9.1....... and All Metal.

* Whilst in All Metal......there is a large performance difference ........ in regards to depth........ whilst comparing air-tests with "in real dirt" results. Especially with low conductors..... like a nickel. I can acquire a nickel at 19"++ in an air-test. In the ground....... at best..... is 12.8". Interestingly........... if the nickel is 12.6" deep.......... you can sweep the coil 4.0" above the ground (nickel is a total of 16.6" distance from the coil) and still clearly detect the nickel. This is to say..... the All Metal mode is more air-gap forgiving (when compared to ID mode).

* Another ironic observation is............ when FA mode is invoked........... I can acquire a dime at ... just over 10" ... in real dirt....... IN A TEST GARDEN ENVIRONMENT ONLY! In the real world....... on live hunts........ it is rare to acquire a clad dime beyond 7" depths. This seems to only hold true for the FA mode. This is the same phenomenon the XP GMP encounters. I can acquire a 12.0" clad dime in a test-garden environment ONLY. In the real world........ the nominal/median average of the GMP (with stock 9" DD coil)..... is in the 7" depth range,,, on a clad dime.

* In air-tests............... switching through any/all of the process modes (then invoking All Metal mode)........ and there is a very minimal gain/loss. Nearly the same air-test distance performance. In the ground...... the resultant differential is dramatic.

* Even with NO audible EMI............ switching through the F1 - F7 noise-cancelling/EMI mitigation Freq's...... there is certainly notable performance changes. One "silent EMI" freq..... as compared to another "silent EMI" freq. THIS is where a large amount of my reporting/documenting 'delay' emanated/stemmed from. WHAT do you choose. HOW do you 'average'. Then.............. will tomorrow's atmospheric EMI level/status change the results........... from day-to-day? Yes! Some days.......... the delta/differential/change........... is minimal. Other days...... and it's fairly dramatic. (The woe's of utilizing a ElectroMagnetic detection device......... in a ElectroMagnetic atmosphere....... on a magnetic/ElectroMagnetic Core Earth!!! )))

Because audible EMI is quite minimal on this new LTD2 platform...... I have had minimum interests in testing/data collection for Sensitivities of anything less than '99'...... as long as EMI is silent.

My test-garden dirt is 1-bar dirt. (0.1).
Re: Rcpt Ack of F75 Ltd prototype
February 26, 2015 11:51PM
Great info Tom. I think it will change my detecting strategy when relic hunting where targets are sparse; especially in areas where it's hard to get the coil close to the ground.
Re: Rcpt Ack of F75 Ltd prototype
February 27, 2015 06:45AM
Hi Tom,,thanks again, boy this is some weird unexplainable data....It's like backwards from what you think it would be....JJ


* Another ironic observation is............ when FA mode is invoked........... I can acquire a dime at ... just over 10" ... in real dirt....... IN A TEST GARDEN ENVIRONMENT ONLY! In the real world....... on live hunts........ it is rare to acquire a clad dime beyond 7" depths. This seems to only hold true for the FA mode. This is the same phenomenon the XP GMP encounters. I can acquire a 12.0" clad dime in a test-garden environment ONLY. In the real world........ the nominal/median average of the GMP (with stock 9" DD coil)..... is in the 7" depth range,,, on a clad dime.
Re: Rcpt Ack of F75 Ltd prototype
February 27, 2015 11:05AM
Might this difference be largely caused by the fact that the dime in the
test garden is typically placed parallel to the ground, while the
dime in the wild is seldom so cooperative and therefore presents the
detector with a much smaller surface to detect?
Re: Rcpt Ack of F75 Ltd prototype
February 27, 2015 03:11PM
I have often thought about that. I even considered tilting my coil so the transmitted field is parallel to the coin. I have not tested this but I guess that the affect on a off angle coin would be minimal - reasoning is I think it is much less the transmitted field as it is the direction of the field created by the target.

Kind of off topic but not completely - is how hard it is to get consistent and repeatable results. Scintific testing as a rule ignores any testing that does not produce repeatable results. If the results vary that means there is some aspect of the test that is not controlled. As Tom alluded EMI is a big variable that is hard to control. I often wondered if building a Faraday Cage would be the correct way to deal with the EMI issue. It would not need to be huge. Big enough for coil plus enough distance that metal on outside of cage would not be detected.

I also wonder if the reason ground tests are some times better is that the ground itself when coil is placed close to ground acts like a EMI shield. But not really sure - I know just enough to be dangerous smiling smiley

TallTom Wrote:
-------------------------------------------------------
> Might this difference be largely caused by the
> fact that the dime in the
> test garden is typically placed parallel to the
> ground, while the
> dime in the wild is seldom so cooperative and
> therefore presents the
> detector with a much smaller surface to detect?

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!
Re: Rcpt Ack of F75 Ltd prototype
February 27, 2015 03:50PM
Hi Bryanna/all ,,,,good luck with the Faraday cage....LoL....I built an EMI detector years ago....There's enough electromagnetic waves in the air on any given day to light a small/miniature light at any time seven days a week 24 hours a day...Even a storm/lightning miles & miles away can be detected....When the humidity etc. is just right on certain days EMI can take alternate routes to wreak havoc on your poor little EMI sucking coil...LoL......JJ
Re: Rcpt Ack of F75 Ltd prototype
February 27, 2015 06:38PM
Thank you Tom for the huge amount of work you put into this. Extremely helpful knowing what the machine is supposed to do, with all of the inconsistencies out there between machines.

Truly helpful for me because I am just learning the F75. This is my first.

Kenny
Re: Rcpt Ack of F75 Ltd prototype
March 01, 2015 02:17PM
I have had access....... and have utilized a Tempest Booth (Faraday Cage) on many occasions. Off the clock....... as long as it was tuitionally educational/beneficial...... I could test metal detectors. Nearly every metal detector tested...... were 'per expectations' as far as my expected resultant(s). Nothing surprised me. The only metal detector platform that broke all rules......... and far exceeded my expectations............ was the 'boost process' detectors. In my case..... the F75. When the F75 was not in any form of a 'boost process' mode......... it is a fairly normal performing 'per expectations' unit. When placed in 'bp' or 'cl' mode............. the results were nothing short of stunning. This is the only environ whereby Dave Johnson's 64X claim............ could be fully witnessed/realized..... in all of it's splendid 'magic' glory. The test target (test-standard) was a U.S. $1 Type-1 gold coin.
Since then, I have made minimal inferences to these tests (however, have made a few posts implicating 'some' of my testings/experiences). The reality is....... it is NOT "Real-World" applicable. BUT: It has prompted me to find a real-world location whereby EMI is minimal..... but real. . . . ie ....... "best" Real-World conditions.
What does all of this lead to: "Silent EMI". Something I started to share..... here on this forum...... many years ago. It is (unsuspectingly) more prevalent than could be imagined. In the Tempest Booth........... one phenomenon that was eye-opening...... the F1 - F7 EMI freq mitigation tool...... on the F75. In this specific environment/application....... F1 - F7 virtually had zero effect on performance. However, what could be measured/witnessed was..... solely/only/exclusively...... internal circuit noise (cross-talk between electronic components) whilst in the Faraday Cage. Switching through F1 to F7 had such little effect........ on depth/range. Now............. the overall depth/range on the very small gold coin....... was unsuspectingly stunning.

In the real world.............. "silent" EMI is what can be disheartening. For example: If F2, F3, F5 and F6 are perfectly quiet channels............... than any one of them should be great. Truth of the matter is................ ONE of them will outperform all of the others. Sometimes..... by a small margin.......... sometimes by a large margin. And THIS is exactly 'why' my intended scientific testing of this new F75LTD/2 has been exceptionally difficult to test (and subsequently 'report'). Removing variables is quite difficult...... especially 'unknown' variables. Yep: How do you know what you are missing...... If you do not know it even 'exists'. You don't know...... what you don't know. Which............. in turn............ can unsuspectingly skew results....... 'seemingly' valid data. Sooooo....... I always seek to find (and report) nominal-median-average resultants.

EMI is a 'known' performance-reducer.
SILENT EMI is a 'UNknown' performance-reducer.

There are some things in life/(destiny) that you can 'control'. There are other things that........... until technological advancement(s).......... you can NOT control.
Re: Rcpt Ack of F75 Ltd prototype
March 01, 2015 04:27PM
Hi Tom,

I am not sure "silent EMI" is the best term - but maybe again it is. I have read several posts related to Minelab FBS and to lesser extent BBS units about using the units multiple freq channels to improve performance on various targets. In many of those units and some X-terra units Minelab uses automatic noise canceling - fancy words for having a built in method to find the one frequency with the least amount of noise.

When you say silent you mean via the speaker - and possibly by observing VDI - you hear no noise. So even thought there is noise, at least by the measuring methods you have, the noise is "Silient" or undetectable. I wonder if Minelabs method has the ability to detect noise that would other wise be "Silient" to us? Not sure, but I would assume so, but maybe not.

Tom, just curious when testing to find the quietest channel do you first turn off DST - I assume yes because what ever DST does it seems to do it pretty well. But from what I am reading not without the cost of some performance. But that seems to be offset by the ability to run the units at full sensitivity.

It is funny all metal detectors are affected by noise. EMI is present In many places we hunt especially the urban variation, but more and more even rural locations are affected. Makes me wonder!

*In the old days of Cell phones they used Analog technology - with these older phones as you reached the limits of distance (and EMI) you could hear the signal degrade but often the noise would be greater then the words of the person you where talking with but still you could make out - although with some difficulty - what they where saying. With new digital phones you can here them clear until you reach the "Cliff" (a technical term used for when the electronics determines the signal to noise ratio is to high and drops the communication) and you loose the call. The truth is you could communicate at greater distances with the older technology since there was no "Cliff"and you decided when the signal to noise was too high. *

Wonder if any of this is creaping in to the world of detecting - better save those old analog winking smiley


> Tom wrote:

> In the real world.............. "silent" EMI
> is what can be disheartening. For example: If F2,
> F3, F5 and F6 are perfectly quiet
> channels............... than any one of them
> should be great. Truth of the matter
> is................ ONE of them will outperform all
> of the others. Sometimes..... by a small
> margin.......... sometimes by a large margin. And
> THIS is exactly 'why' my intended scientific
> testing of this new F75LTD/2 has been
> exceptionally difficult to test (and subsequently
> 'report'). Removing variables is quite
> difficult...... especially 'unknown' variables.
> Yep: How do you know what you are missing...... If
> you do not know it even 'exists'. You don't
> know...... what you don't know. Which.............
> in turn............ can unsuspectingly skew
> results....... 'seemingly' valid data.
> Sooooo....... I always seek to find (and report)
> nominal-median-average resultants.
>
> EMI is a 'known' performance-reducer.
> SILENT EMI is a 'UNknown' performance-reducer.
>
> There are some things in life/(destiny) that you
> can 'control'. There are other things
> that........... until technological
> advancement(s).......... you can NOT control.

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!



Edited 1 time(s). Last edit at 03/01/2015 05:00PM by Bryannagirl.
Re: Rcpt Ack of F75 Ltd prototype
March 01, 2015 04:38PM
Tom
What is the 64x claim? Sounds interesting.
Re: Rcpt Ack of F75 Ltd prototype
March 01, 2015 06:10PM
I work in RF engineering (WiFi) and we have two large RF chambers at work. This would certainly be a fun test to try with the BP mode.

On the flip side, a portable RF Analyzer/Spectrum Analysis device could provide the data required for the best usable channel on the F75 in the field.

~Brian


NASA-Tom Wrote:
-------------------------------------------------------
> I have had access....... and have utilized a
> Tempest Booth (Faraday Cage) on many occasions.
> Off the clock....... as long as it was tuitionally
> educational/beneficial...... I could test metal
> detectors. Nearly every metal detector
> tested...... were 'per expectations' as far as my
> expected resultant(s). Nothing surprised me. The
> only metal detector platform that broke all
> rules......... and far exceeded my
> expectations............ was the 'boost process'
> detectors. In my case..... the F75. When the F75
> was not in any form of a 'boost process'
> mode......... it is a fairly normal performing
> 'per expectations' unit. When placed in 'bp' or
> 'cl' mode............. the results were nothing
> short of stunning. This is the only environ
> whereby Dave Johnson's 64X claim............ could
> be fully witnessed/realized..... in all of it's
> splendid 'magic' glory. The test target
> (test-standard) was a U.S. $1 Type-1 gold coin.
> Since then, I have made minimal inferences to
> these tests (however, have made a few posts
> implicating 'some' of my testings/experiences).
> The reality is....... it is NOT "Real-World"
> applicable. BUT: It has prompted me to find a
> real-world location whereby EMI is minimal.....
> but real. . . . ie ....... "best" Real-World
> conditions.
> What does all of this lead to: "Silent EMI".
> Something I started to share..... here on this
> forum...... many years ago. It is (unsuspectingly)
> more prevalent than could be imagined. In the
> Tempest Booth........... one phenomenon that was
> eye-opening...... the F1 - F7 EMI freq mitigation
> tool...... on the F75. In this specific
> environment/application....... F1 - F7 virtually
> had zero effect on performance. However, what
> could be measured/witnessed was.....
> solely/only/exclusively...... internal circuit
> noise (cross-talk between electronic components)
> whilst in the Faraday Cage. Switching through F1
> to F7 had such little effect........ on
> depth/range. Now............. the overall
> depth/range on the very small gold coin....... was
> unsuspectingly stunning.
>
> In the real world.............. "silent" EMI
> is what can be disheartening. For example: If F2,
> F3, F5 and F6 are perfectly quiet
> channels............... than any one of them
> should be great. Truth of the matter
> is................ ONE of them will outperform all
> of the others. Sometimes..... by a small
> margin.......... sometimes by a large margin. And
> THIS is exactly 'why' my intended scientific
> testing of this new F75LTD/2 has been
> exceptionally difficult to test (and subsequently
> 'report'). Removing variables is quite
> difficult...... especially 'unknown' variables.
> Yep: How do you know what you are missing...... If
> you do not know it even 'exists'. You don't
> know...... what you don't know. Which.............
> in turn............ can unsuspectingly skew
> results....... 'seemingly' valid data.
> Sooooo....... I always seek to find (and report)
> nominal-median-average resultants.
>
> EMI is a 'known' performance-reducer.
> SILENT EMI is a 'UNknown' performance-reducer.
>
> There are some things in life/(destiny) that you
> can 'control'. There are other things
> that........... until technological
> advancement(s).......... you can NOT control.
Re: Rcpt Ack of F75 Ltd prototype
March 01, 2015 10:03PM
Bflo Wrote:
-------------------------------------------------------
> Tom
> What is the 64x claim? Sounds interesting.

I suspect this is what he is referring to;

Dave: "Getting extra depth out of a VLF, multifrequency, or PI machine is very difficult, because these machines follow an inverse 6th power law relationship between signal voltage and depth. If everything else is maintained equal, doubling the depth requires 64 times as much signal. If this is done by increasing transmitter power, doubling depth requires 4,096 times as much battery drain. That’s the basic reason why depth increases come so slowly in this industry.

The biggest impediment to getting usable depth in the ground, is interference from magnetic and electrically conductive minerals in the ground, which can produce signals hundreds of times as strong as that of the metal target you’re trying to detect and hopefully identify. There are several approaches to extracting the metal signal from the ground mineral signal, but they all have their limitations. That’s why you see several different technologies coexisting in the market."
Re: Rcpt Ack of F75 Ltd prototype
March 01, 2015 10:14PM
Ok thanks for the post.
Re: Rcpt Ack of F75 Ltd prototype
March 01, 2015 10:22PM
I wonder if Fisher's engineers thought of this problem when they made DST 'always part-on'. If you're checking for EMI, it's the time when you would want less filtering,so you can hear and evaluate the interference. In principle, they could turn it off just after a frequency-shift has occurred, with a short 3-second time-out, say. It may also be possible to 'measure' the EMI during this period, and display a figure on the 88 display. Probably just over-complicated fluff, it's your ears that are the best tool for this job.
Re: Rcpt Ack of F75 Ltd prototype
March 02, 2015 01:59AM
SILENT EMI = Any EMI that you can not hear audibly...... nor see with any VDI indication; yet, is still present. . . . . . . and hinders overall performance. This is to include the inability to hear/see EMI in any mode..... including unfiltered all-metal mode.

Minelab (and other Mfr's) may have a/any form of 'auto-freq-selection // channel selection' for best performance; yet, still comes with consequences. It is not uncommon for local EMI conditions to change at moments notice.

As far as the F75LTD/2...... there have been many tests (at differing locations)...... whereby, F1 - F7 expressed zero audible (or visual) EMI...... nor subsequent delta/differentiation. Yet, each differing freq selection would produce different performance (depth/ID) resultants....... both in air-tests...... and 'in-dirt' tests. Then......... the next day.............. F1 - F7 would produce a completely different set of performance resultant. And still............ with no audible (or VDI) EMI indications. Just this 'variable' alone ..... invalidated (and lengthened) my testing/qualification/validity....... and for several weeks. How do you capture/harness a 'unknown' variable. . . . . then 'control' the unknown variable. . . . . with the intent/theory of making the 'variable' into a 'controlled constant'.

With the F75LTD/2 .... You can not turn off DST. This makes 'measuring' and testing quite difficult ..... especially in the removal of 'variables'. On the older generation SE's/LTD's....... EMI was 'known'. With the DST units..... EMI ..... for the most part...... is silent/unknown. . . . . . even with differing F1 - F7 freq selections.

In local summational closure .......... when all is said-n-done .................. it sounds like the DST units 'quietly' operate at a (possible) reduced performance level; When.............. the reality is............... they operate/function in a much more usable/functional/less-fatiguing fashion. Yes.... there may be 'silent EMI' reducing overall performance; yet, with the non-DST earlier generation LTD's/SE's........ you may not be able to audibly silence the EMI....... and would hunt at the same reduced performance level............. just a bit audibly noisy/obvious.
Re: Rcpt Ack of F75 Ltd prototype
March 02, 2015 04:32AM
ATraditional detector technology requires users to "work into the noise" at high gain settings to achieve maximum possible depth. The F75's latest technological advance,
Digital Shielding Technology (DST®), permits the use of maximum sensitivity settings with minimum background noise. This noise reduction reduces user fatigue and enhances the detecting experience.
The user may choose to operate with DST or without DST. The default setting, at power-on, employs DST
To turn DST OFF, perform the following:
1. Turn detector off.
2. Press-and-hold the grey MENU button and
push-forward-and-hold the TOGGLE SWITCH.
3. Turn the detector on, while you are still holding the controls. 4. Continue to hold the controls.
5. See the "90" displayed.
6. Rotate SETTINGS KNOB clockwise.
7. See "91" displayed.
8. Release the MENU button and TOGGLE SWITCH.
Upon release of the controls, the detector will operate WITHOUT DST
DST status is saved to memory after the user performs this procedure.
Each time the detector is powered back on, it will continue in the programmed setting.
The same procedure is used to turn DST back on, except that the SETTING KNOB is turned counterclockwise to change "91" to "90".
DST can be turned on or off as desired, but each change requires the user to first power the detector off.

Tom,
The above is direct from the manual.
This makes no sense it says 91 is off but everything else says it is a higher level of DST. So like you said there appears to be no off just medium and high DST. I will look at my T2 SE2 see if it says the same thing.

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!
Re: Rcpt Ack of F75 Ltd prototype
March 02, 2015 09:23AM
Re: this, Tom:"SILENT EMI = Any EMI that you can not hear audibly...... nor see with any VDI indication"
I think that this EMI likely could be measured, and presented on the display, if not to the ears. It's not showing on the display in normal detecting mode, because that is derived from raw data that has been ground-filtered, differentiated, compared to a threshold etc etc. But the microprocessor brains has access to fairly raw signals, so the ability to process that info in a different way, and then quantify it, seems achievable.
Re: Rcpt Ack of F75 Ltd prototype
March 02, 2015 09:55AM
Bryanna ....... From all of the different LTD2's that I have tested......... and have measured 9.0 and 9.1 (with repeatable consistency); my aggregate collected data is completely incongruent with that portion of the manual (ie claiming 9.1 has DST disabled). If DST is completely disabled in 9.1.......... and EMI is at it's most reduced status (greatest mitigation)............... why even incorporate DST into the unit............... if the EMI issue has been resolved this well.

Pimento....... Yes, there could be a software tool employed that would allow the user to select the ability to audibly/visually witness and measure instant/current EMI conditions; yet, (except on this forum) most detectorists may not know what/how to utilize this tool..... without getting into trouble........ or lacking a good understanding of EMI, the causes, , , , and the 'know-how' to approach reduction/mitigation.
Re: Rcpt Ack of F75 Ltd prototype
March 02, 2015 11:32AM
It would be difficult to use, I agree, you can understand why ML automated the frequency-selection process, to spare the user the trouble.
On the subject of intelligent automation of a process, if it were possible to quantify the level of interference, then it would be possible to automatically select the degree of DST/other methods to use for optimum results.
Re: Rcpt Ack of F75 Ltd prototype
March 02, 2015 01:44PM
Tom, I agree with you there is only strong and stronger EMI mitigation now if there were a third choice maybe finding a quiet channel would be a better solution. But like you said hour to hour day to day EMI changes so what is good now is not good later.

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!
Re: Rcpt Ack of F75 Ltd prototype
March 02, 2015 02:44PM
Tom does the BP and CL processes have the SAME dramatic effect on the 75LTD2 as on the 75LTD? and What did FT do to weaken the AM side on the 75LTD2? Thanks for all your input.
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