Performance Verses Emi

[Mark Gillespie]

Years ago, when I learned how EMI affects a detectors performance I did a simple study. I took my detector to many different locations to determine the maximum air distance of a US dime from the coil, using the most stable settings at the time. I used the greatest distance as a starting point to near zero EMI affect on my detector (my opinion). I then marked the maximum distance I could obtain on the lower shaft (12" for the Equinox). So now, when I go hunting, I can get a good guess by air testing the machine (machine health check), if something is wrong with either the machine or maybe even silent EMI (not audible chatter).
Looking at the picture, I have a mark at about 12" which is the maximum distance I can get using a US dime and the Equinox after the best settings and noise cancel. It is surprising, just how many times I can't get a repeatable audio at 12" regardless of my settings (effects of EMI). But moving only 20', things might change, better or worse. This works good on all the detectors I've ever used. The F75 tends to be affected a little more by EMI but the maximum air test is quite a bit more than the Equinox. Both machines have places where they outperform the other.

[dsb]

Do you have sites that are unpredictable EMI wise?  I had many but mostly urban.  I have had some luck running N/C with coil on the ground.  Was hoping the 900 would have an edge over the 800 but have not heard . And then when i get a to hunt a old yard but need to hurry i run 4-10 single KHz and cherry pick silver i notice less EMI.

[schoolofhardNox]

EMI is a killer no matter what machine you have. Even a pulse machine is affected. And EMI can change even if you stand still. You never know when more EMI is produced, whether it be from from military, aviation, or just an increase in house hold electric usage. I've even notice that sometimes are better to hunt that others depending on the sun. I love late afternoon hunts vs early morning (sunrise) hunts. Maybe just coincidence, but I do better in the late afternoon. I wonder if guys that hunt at night can tell the difference from a day hunt?

[phrunt]

JW and I often gold hunt at a spot that has high voltage power lines run smack through it from a hydro dam, it's amazing how different times of day affect the EMI in the area, it's rather remote apart from those power lines, no houses or anything going to cause EMI, not on a flight path or anything like that.    We both agree on the characteristics of EMI at this place and how different times of day or even weather affect it.

During heavy fog which happens quite a bit at that spot being a gully at the bottom of mountains the EMI drops right down, detectors go so calm, even the old model GPX purrs along.  Wind direction seems to affect EMI from the lines too, depending on which direction the wind blows it seems to change the EMI, so weird.  Times of day mean a lot, perhaps its demand on the lines but certain times of day have higher EMI, towards the end of the day usually the detectors enjoy a more stable operation right when you're getting tired and hungry the detector starts to purr 🙂

At dusk it's usually a really good time for stability which is odd as that would be a high power load time with everyone at home cooking dinner, so I don't know if it has anything to do with power draw.   As it gets dark EMI goes right down.

You can be hunting in one spot in the morning (a spot where I not too long ago found a 4 gram nugget) and the detector be frustrating to use, noise cancels all the time and then as the day goes on suddenly the EMI calms right down and things improve dramatically. 

The GPZ is by far the best detector to use at this spot, it really handles the EMI exceptionally and can detect right up near these massive power lines, especially when outfitted with the concentric coil, even though it's 15" it handles EMI much better than smaller coils than it is, perhaps due to it's small receive winding.

EMI is a depth killer so doing all you can to have a detector running without being affected by EMI is important, I think you're better off running a lower sensitivity and not getting bad EMI than running unstable with EMI with higher sensitivity.  

EMI is also a big reason detectors get unstable or bouncy target ID's, EMI is acting like a target, the detector is detecting something (EMI) so even if you hit a good target but have EMI at the same time it can change your Target ID's undesirably.

[Badger-NH]

23 hours ago, Mark Gillespie said:

Years ago, when I learned how EMI affects a detectors performance I did a simple study. I took my detector to many different locations to determine the maximum air distance of a US dime from the coil, using the most stable settings at the time. I used the greatest distance as a starting point to near zero EMI affect on my detector (my opinion). I then marked the maximum distance I could obtain on the lower shaft (12" for the Equinox). So now, when I go hunting, I can get a good guess by air testing the machine (machine health check), if something is wrong with either the machine or maybe even silent EMI (not audible chatter).
Looking at the picture, I have a mark at about 12" which is the maximum distance I can get using a US dime and the Equinox after the best settings and noise cancel. It is surprising, just how many times I can't get a repeatable audio at 12" regardless of my settings (effects of EMI). But moving only 20', things might change, better or worse. This works good on all the detectors I've ever used. The F75 tends to be affected a little more by EMI but the maximum air test is quite a bit more than the Equinox. Both machines have places where they outperform the other.

Have you compared multi-frequency with single-frequency when dealing with EMI? I have sites near powerlines where I can't run Multi without lowering the Sensitivity way down but 15 kHz is unaffected.

I generally run single frequency most of the time anyway since my ground is very mild. I only use multi in mineralized ground or around iron.

 

 

[Digalicious]

23 minutes ago, Badger-NH said:

Have you compared multi-frequency with single-frequency when dealing with EMI? I have sites near powerlines where I can't run Multi without lowering the Sensitivity way down but 15 kHz is unaffected.

 

 

 

I experience that at just about every one of my inner city sites. Granted, on most of sites, even 5 khz is a lot more quite than any SMF mode. But on a couple of sites, I have to use 20 khz or 40 khz. Since EMI is only getting worse, I expect more and more sites will be like that in the future.

There's also something inherent with SMF that makes it much noisier in EMI. For example, take a SMF detector that also has MF, and in EMI, compare the noise between a SF and it's equivalently weighted SMF. The SF will be much more quite, despite the frequency being the same. I mentioned all that to an engineer, and he suggested that it was likely caused be SMF adding and combining more than 1 frequency, thus "amplifying" the EMI.

[JCR]

@Mark Gillespie has brought up a very good "detector health check". NASA Tom has also recommended this. I should try to do it but forget in my impatience to get busy hunting.  I wonder to what degree solar activity changes manmade EMI?

[dsb]

On 4/5/2023 at 1:07 PM, schoolofhardNox said:

Maybe just coincidence, but I do better in the late afternoon. I wonder if guys that hunt at night can tell the difference from a day hunt?

this is interesting... https://prostockdetectorsmi.com/files/2017/03/better-finds-after-sunset.pdf

[EL NINO77]

On 4/6/2023 at 2:30 PM, Digalicious said:

I experience that at just about every one of my inner city sites. Granted, on most of sites, even 5 khz is a lot more quite than any SMF mode. But on a couple of sites, I have to use 20 khz or 40 khz. Since EMI is only getting worse, I expect more and more sites will be like that in the future.

There's also something inherent with SMF that makes it much noisier in EMI. For example, take a SMF detector that also has MF, and in EMI, compare the noise between a SF and it's equivalently weighted SMF. The SF will be much more quite, despite the frequency being the same. I mentioned all that to an engineer, and he suggested that it was likely caused be SMF adding and combining more than 1 frequency, thus "amplifying" the EMI.

Mark...

the explanation is very simple.....With multi-frequency, the total *TX of the detector is divided into 2-3 frequencies that use the multi-frequency program....and thus the TX is divided in this way... thus the overall Signal/noise EMI coefficient is reduced multi-frequency program,,

Add to that a greater possibility of interference with one of the 2-3 frequencies used in a given multi-frequency program.. and we immediately have a significantly higher probability that a multi-frequency program can be more often affected by EMI..

One frequency of the tor uses all the detector's TX concentrated into 1 frequency... in such a situation it can provide an advantage in a better signal/noise Emi distance... and it is also easier to choose 1F undisturbed working frequency of the detector....,..which will be work stably....as a group of 2-3 frequencies used by Multifrequency...

In order to have an overview of how Emi affects the given detector....if you have your own test field....you can try to set the detector to the most affected frequency... and try the detector in a deep test,,,to see if the given detector can affect visible EMI or Silent Emi,,,
 
For example, in my test, I set the detector to the most disturbed EMI..IN 16.4 khz..... graphic scan Emi helped me to select the most disturbed frequency in this case 16.4 khz...!!!So that it works as unstable as possible at a sensitivity of 90....!!!

.....Then I reduced the sensitivity of the detector from 90 to a stable sensitivity of 55... so it was also a test for EMI silence...and I tested the detector set up like this on my 3 deep targets... on my Test field  "4X4"...

As you can see, the detector was able to detect deep targets at a disturbed frequency when it was set to a fairly low sensitivity... 55 out of 99 max...
The large 15" coil, which has excellent deep detection properties even at the low sensitivity of the detector at 55, also contributed to someone...

another result of this test is also the knowledge that even a large coil can work effectively in EMI,,,if you sufficiently reduce the sensitivity of the detector to an acceptable level....

...the detector range results agree with previous depth tests of various coils on this test field...

 Picture:The value is the minimum sensitivity value for each of the tested coils
  needed to reliably detect 3 deep targets on my test field... :