This is a thread that I have wanted to start.... about a Decade ago.

There are things that I do in the field..... that have been questioned: 'Why'. It made me think: What may be common sense to one; may not be common sense to another. (Common sense is rather subjective). I had to question my methodology rationale. . . . . then attempt to put into thoughts/words..... so as to be able to convey/impart the knowledge/skillset to others.

I have heavily covered EMI and Silent EMI plenty of times on this forum; yet, I have never explained certain/specific sequences that I perform..... for mitigation or go/no-go decision making..... due in part to EMI. Some encounters Decades ago..... has molded the way I think/perform today.

At the age of 52 (4 years ago)….. I had a (distressful) epiphany. With zero health problems, I suddenly realized:
TOMORROW MAY NEVER COME.
THERE MAY NEVER BE A TOMORROW.
It is exactly this "Come to Jesus" thought...… that has redirected my processes.
Life is short. I choose to do things with maximum efficiency.

Sooooooo……. back to EMI. Let's say I have a particular field that presents serious potential.... for my intended quest. It may be a several-hour round-trip to this special site. I want the best equipment that is the most technologically advanced.... that will present maximum potential for success.

I arrive.
Turn F75LTD 'on'. Individually run through all the noise-cancel channels..... and find the one that presents the least EMI. I learn that 3 of the noise-cancel channels present no audible EMI. Channel 2, 3 and 5. Even though these three channels present no audible EMI...… I air-test the unit on a clad dime. Channel 2 allows detection of clad dime to a air-test distance of 9". Channel 3 allows for a air-test distance of 8". Channel 5 expresses 9.5". Even though I can hear no audible EMI...… the "silent EMI" restricts my best performance to a maximum of 9.5". Knowing the F75 ….. under normal circumstances...… will air-test a clad dime to 12"; now expresses reduced performance to 9.5". My "no-go" decision is implemented. . . . . due to my (minimum) 10" requirement.

I then swap detectors to the Equinox. Perform Auto noise-cancel. Then verify the auto noise cancel selection..... by individually testing each one of the noise-cancel channels...… manually. I find 2 channels that are better than the auto noise-cancel selection. Then...………… I perform the required/standardized clad dime air-test..... utilizing the three best noise-cancel channels. One channel presents 7" air-test. Another channel presents 9". And the 3rd channel presents 9.5". In an EMI-free area.... the EQX will air-test a clad dime to 13". Again...…. "no-go" decision is implemented.

Now I grab the Anfibio. Only one channel presents no audible EMI. All other channels express varying degrees of EMI. But/yet; clad dime air-tests to 8.5". No-go again!

I switch to the CZ. There are no noise-cancel channels. Unit is exceptionally quiet/stable. CZ performs on-site air-test to 11" on a clad dime. (It'll do 12.3" air-test in EMI free areas). Go/no-go decision passes...… and I choose to hunt with a slight handicap in depth...… due to silent EMI; yet, the CZ presents the best depth potential. The moment I start to sweep the coil, I realize the area is littered with carpets of square nails...…. rendering the CZ 'useless'.

The final decision; the site is EMI inaccessible. Unhuntable. I must then leave...…. and find a different site.

Site 2.
This time...… the EQX air-tests a clad dime to 10.5" …… expressing minimal 'audible' EMI. . . . on it's best noise-cancel channel. Interestingly (educationally): one of the other noise-cancel channels presents no audible EMI also; yet, air-test performance is only 7". Silent EMI is crippling.

Site 3.
The Anfibio presents the best clad dime air-test performance; whilst the other units do not pass the go/no-go process.

This is a very common occurrence. It is easy to not realize just how bad 'silent' EMI is. One detector may work 'ok' in a particular area, whilst other detectors may hardly perform. This can also explain why: some days.... you have really good success.,.,.,.,.,.,., yet, other days you have so-called: bad luck. (((There's science behind this))). Having the ability to perform the simple clad dime air-test on any particular site...…. and with multiple different detectors...…. can dictate your so-called: luck.

THOUGHT ONE: COMPLETE

Noise cancel doesn't actually cancel noise. It very slightly adjusts the operating frequency (by maybe 1%) so that the noise falls outside the post-demodulator bandwidth. It's still there, in the RX coil, in the preamp, and in the demod circuit. It's very possible that in the demod the noise is modulating the target signal and spreading the energy out, effectively reducing the signal strength. If that is the case, I have an idea on how to fix it. Thank you.

Carl...…. that would be a technological-advancement-of-the-century...….. in regards to metal detection equipment.

The other Silent KIller Tom!

good info.

Keith

“I don't care that they stole my idea . . I care that they don't have any of their own”
-Nikola Tesla

Great googa mooga.

Good information. Something a lot of us don't think about. Great you are thinking outside the box. Your thinking may have led to something new making detectors work better.

Thank you,
Rick

Some good info.

Look forward to part 2 or as you call it thought 2.



Edited 1 time(s). Last edit at 12/11/2018 12:12AM by tnsharpshooter.

Great info!

HMMM makes me think.... I like that.

Thanks, Tom. HH jim tn

Thanks Tom!
Eye opener for sure.
Some quick testing inside my house showed that my Equinox did NOT choose the best
channel on its own when factoring in the air test.
I was able to find two other channels that netted me an additional 1/2” + using Multi.

I’m now picturing a new detector that constantly scans and evaluates the EMI and adjusts on the fly.
There could be a little EMI gauge/channel that you can see on your screen just like your battery indicator.

Bryan

NASA-Tom Wrote:
-------------------------------------------------------
> Carl...…. that would be a technological-advancemen
> t-of-the-century...….. in regards to metal detecti
> on equipment.


Carl is not to far off base with this. In the live audio world we've been using feedback ferrets for many years to remove harmonic feedback from audio signals.
This has gone from analog and now into the digital world a few years ago with the advancement of DPS "Digital Signal Processing" technology.
It is completely possible to use the same type of technology to sample any range of frequency, pinpoint an undesired wavelength and then terminate it and then (Notch) from the audio response.
When I say terminate I am referring to the RX side which also impacts the audio out put.

Again its already been done, someone simply needs to apply it to detecting.

Up to my ____ in Pulltabs, Grant



Edited 1 time(s). Last edit at 12/11/2018 11:58AM by Up to my ____ in Pulltabs, Grant.

So why those detectors Tom? You must have more, are they the ones you pack when not knowing what the are is like or are they your top of the line units. The Nox and Anfibio they are fairly new on the market? No deus or impact?

LowBoy

TAKE A LITTLE TIME KICKBACK AND WATCH SOME OF MY DETECTING VIDEO'S BELOW ON YouTube

[www.youtube.com]

If you don’t dig it, then how are you going to know what you’re missing!
How can you have your pudding if you don’t eat your meat!

Tom. i don't know if you have ever run the Deus but if so how do you noise cancel one? As far as I know when the Deus is turned on and held up in the air it goes through all it's start up procedure and that's it. Once that is done I don't know if there is any way to tweak the noise cancelling. Any help on that would be great as after reading your post I see how much your process is. Thanks , gene

Silent or not EMI can definitely be a killer on depth, and also being one that has realized that my hunting time is limited by the time I'll be on the right side of the grass, I would never spend the time air testing a dime at a new site, especially using multiple detectors. A site that I've already hammered or that I'm convinced holds targets beyond reach may convince me to try and scratch out every extra tenth of an inch but then again many sites I hunt just don't have targets that deep. Being in Arizona and doing most of my detecting in the western states, I find it rare that coins seldom get deeper than about 6 inches. The problem I encounter is depth being hindered by mineralization. Anxiously awaiting your tips on mineral impact abatement.
I really appreciate the time, and effort you spend enlightening us on many aspects of detecting.
Thanks
Tom

Per NASA Tom's "silent masking" scenario,

I recently tested a number of detectors, FTP CZ-3D & F75 75 year edition, (no DSP), Whites DFX & TDI SL, , Garrett GTI 2500 & AT Max, all were tested for air distance audio using sample targets glued to paint stir sticks as follows; a silver Canadian dime 92.5% silver, Canadian 5 cent piece,92.5% silver, (same composition as dime), and a man's 10 Karat gold ring.
All the units went silent, i.e. failed to report any audible signal through the detector speaker after 6-6.5" as measured on a plastic ruler placed at 90 degrees to the coil on the counter top.
All units tested at zero discrimination setting; I found it curious that each unit went radio silent within a half inch of each other; various sensitivity settings starting from maximum, and various GB settings were tried.
N.B. Units were not tested in AM modes.
Thoughts, comments?
This test was conducted inside a local detector dealer's shop where there are overhead fluorescent lighting fixtures approximately 8' above the glass counter top.

Drew.

Thanks for the info Tom. I’ve never went that far in dealing with emi. I usually take 2 or 3 units and pick the one that makes 0 to the least amount of noise to hunt with! I’ll hafta give your method a try though

Tom, typically noise canceling is performed with the coil in a horizontal plane but air testing is done typically with the coil in a vertical plane ... does polarized 'noise' introduce variations that skew or invalidate the air test (since most VLF noise is potentially vertically polarized) for determining the depth when the detecting will actually be in a horizontal plane? Just wondering. Have you tried noise canceling and air testing in both planes to see if there is a significant difference?

Fascinating.

Not something I've given a ton of thought to (checking depth loss due to SILENT EMI at a given site, with an air test at the start of a hunt).

As always, NASA-Tom, when you are talking, it's a good idea for me to pull up a chair and listen...

Thanks for taking the time to share your thoughts.

Steve

My biggest Emi problem seeems is overhead flying aircraft. This link here is interesting. Some night hunting may indeed be what is required to get the goods in some places due to air traffic.
[maps.unomaha.edu]

Thanks Tom...... as always i have some dumb comments, but ill set back and learn.

Although I have played with the noise cancel little bit on the Nox (among others) I never thought you needed to tweak it with that much detail. Thanks Tom, you’ve given us all something to think about. Though for me, once I’m out with all my equipment after a long drive, I’m not sure I've ever considered any spot as “unhuntable”. I’d stubbornly try it anyway. Guess that’s the reason for those ‘bad luck’ days.

tnsharpshooter Wrote:
-------------------------------------------------------
> My biggest Emi problem seeems is overhead flying a
> ircraft. This link here is interesting. Some nig
> ht hunting may indeed be what is required to get t
> he goods in some places due to air traffic.
> [maps.unomaha.edu]
> TrafficAnimation.html

TNSS what an interesting flight map--as an ole pilot I am a plane watcher and have always been intrigued by the number of high flying prop planes that pass over our place and your map shows it.Thanks.

Tom Slick Wrote:
-------------------------------------------------------
> Silent or not EMI can definitely be a killer on de
> pth, and also being one that has realized that my
> hunting time is limited by the time I'll be on the
> right side of the grass, I would never spend the t
> ime air testing a dime at a new site, especially u
> sing multiple detectors. A site that I've already
> hammered or that I'm convinced holds targets beyon
> d reach may convince me to try and scratch out eve
> ry extra tenth of an inch but then again many site
> s I hunt just don't have targets that deep. Being
> in Arizona and doing most of my detecting in the w
> estern states, I find it rare that coins seldom ge
> t deeper than about 6 inches. The problem I encoun
> ter is depth being hindered by mineralization. Anx
> iously awaiting your tips on mineral impact abatem
> ent.
> I really appreciate the time, and effort you spend
> enlightening us on many aspects of detecting.
> Thanks
> Tom


I'm with Tom on this ( I live in AZ, too). Coil to the soil time (for me) is the most important aspect of making recoveries. As Tom said, mineralization is the major depth killer here.
Thank you for the education. Always appreciated.

Dean

EMI is no stranger to the beaches around the Grand Strand. Some areas I just move on. It's almost always in the same spots though some have been quite from time to time. So it appears that the circuits used on the machines are not going to be reliable and that nothing is better than manual frequency adjustment but could be very time consuming. If I can not get the desired results at a known spot then go manual to see if you can get a better result channel by channel. Something to try.

Ok I have to ask.....if you aren’t getting the full depth from your dectector you leave? 9” seems pretty good if you know the beach and made a decision based on that knowledge and the days conditions. Why would you not see what targets are there after reading the beach before leaving?

I have received a couple of PM's with questions..... that would be more suited for en-masse sharing.

* Approx 95% of hunted sites.... have measurable (inhibiting) EMI to contend with. (It is rare to find EMI-free sites).

* When a site presents fairly bad EMI on one detector...…,,,,,,, it is nearly a guarantee this particular site-specific EMI will affect most other detectors in the same fashion.... (((slightly greater than))) 70% of the time. The remainder 30% of the time..... one of the other detectors may not be affected with this site-specific EMI. . . . . . . or. . . . . . . affected at a substantially reduced amount. Hence; the criticality of taking-the-time to find WHICH detector will work...….. and ………. just as critically important: find/learn which detector(s) will NOT work (and would waste your time). To carry a coin in your pocket..... so as to perform a quick air-test.,.,.,.,.,.,., only takes seconds...….. and can make a night-and-day difference in real-world success (or hours of no success).

* It is common to have a predetermined (steadfast/stubborn) notion of 'what' detector will perform best at a particular site. Then...…………… the epiphany ensues after you test a few detectors;...……. only to learn that: the detector of which will perform the best..... is the detector that you want to utilize the least.

* John; spot on! Vertically polarized EMI is nearly always the worst. Horizontal polarized EMI is usually least. Over 90% of the time ….. Vertical and Horizontal EMI are identical in frequency/type/content. Only the amplitude differs. , . , . , . , with the substantially higher amplitude going to the Vertically polarized plane. In most cases...… I prefer to hold coil vertical...… then perform noise cancel. One caveat would be..... if the EMI is emanating from the ground.... due to buried electric cables. Then..... holding the coil parallel to the ground.... is best practice ...for noise cancel functions. Yes...…………. we detect on a horizontal plane; so 'why' hold the coil in any other 3D X-Y-Z fashion? …… Because it is a tool/aid for best noise-abatement procedures. And...………….. in vertical fashion...…………. "pointing" the (bi-directional) coil …. can help find the most amount of EMI (for you to mitigate)…….. and...…… can sometimes find the emanating source/culprit..... by 'pointing' to the radiating source/element.

There's a tremendous amount of performance to be gained..... with the mitigation of EMI; both from a end-user standpoint...… and …… especially from an engineering-controls standpoint (Mfr/D.E.).
I still speculate most EMI is 50 & 60Hz basised. . . . . derivative & harmonics.

That is where I live. EMI first, ground second.

Good post.

HH
MIke

If I could get 9" with the EMI at some spots I would hunt them. With silent EMI an issue no telling what I've missed. I used an X Terra 705 around some power lines that no other machine I or others tried would work around. Exception but found tokens and coins from a turn of century cable car track



Edited 1 time(s). Last edit at 12/11/2018 03:09PM by ShovelNose.

MichiganRelicHunter Wrote:
-------------------------------------------------------
> Thanks for the info Tom. I’ve never went that far
> in dealing with emi. I usually take 2 or 3 units a
> nd pick the one that makes 0 to the least amount o
> f noise to hunt with! I’ll hafta give your method
> a try though

I do the same thing. I pull out a detector and if the EMI makes it start to shake, rattle and roll, I switch to another detector. In most cases, but not all, the higher frequency detectors seem to fair better at handling EMI at sites that I detect. Using a smaller heavily shielded search coil like the NEL Sharpshooter helps a lot. I've also noticed that 'noise cancel' does not help while in the pinpoint mode when using the CTX. Now, Tom, has me wanting to pull out a frigging dime before I start detecting. The only EMI free site that I've ever been at was in a cave where no TV, radio, or cell signals could reach.

tabman