Fisher F75 Coil Comparison

[GB_Amateur]

I did some testing in my back yard to compare relative depths achievable in my soils with various coils on the Fisher F75.

Caution:  these are relative measurements specific to my soil conditions (including ferromagnetic mineral content and current moisture content).  The purpose is merely to compare coil sensitivity for particular settings which push the detection depth limit under these conditions but otherwise should not be used as an absolute indication of how this detector with these coils mounted will perform under different conditions by other detectorists.  My ground is of moderate mineralization, measuring 2-3 bars on the F75 depending upon exact location in my backyard and coil choice (higher reading for largest coils).

Detector setup:  Fisher F75 Limited (black) operating in motion 'all metal' (USA nomenclature, not Minelab's!), DST on, gain=99 (max), ground balanced, noise cancelled, threshold=0.  I plugged Garrett GS2 headphones directly into the socket, i.e. 'wired' headphones.

Test targets:  5" deep 95% copper Memorial USA 1 cent, 6" deep 25% nickel, 75% copper USA 5 cent ('nickel').  (These have been in the ground ~ 2 years.)

Measurement meaning:  because the coins are at fixed depth, in each measurement I raise the coil until I lose the signal in either left or right swing directions.  That is, I require the target to sound off clearly as the coil moves from left to right and from right to left.  I use wood blocks as shims with a least significant thickness increment of 1/2" (~1.3 cm).  I estimate half that or 1/4" as the measurement's systematic uncertainty.  Note that these measurements are intermediate ('hybrid') between full ground and an (full) air test.

Coils tested:

Fisher stock 7"x11" DD and 5" round DD (both included in the Limited/black model package), Coiltek 6" round DD prototype (built for Fisher F5 and operated successfully on my Fisher Gold Bug Pro), Mars 6"x10" DD Sniper, and Detech 13" round DD Ultimate.  (I forgot to test the only concentric I have for this detector -- Fisher 5.5"x10" which is the stock coil on their F70.  I'll do that later this week and add it to the table below.)

Results:

Summary/Conclusions:  With the exception of the yellow-highlighted Coiltek prototype (which was not designed for the F75), the order of depth does reflect coil size.  The Mars 6"x10" DD performs almost as well as the stock Fisher 7"x11" DD.  The largest coil, the Detech Ultimate, is only a bit better than those two which likely reflects the considerable increase in ground it 'sees'.

[tvr]

I've got the original F75LTD with the DSP update. I have not done depth tests with known targets, but have a liking for the little 6.5" Elliptical Concentric coil in trashy areas. Separates very well and when working tot lots can get close to the poles and edges of side walks and fences and just find targets.

Good info in your post. Thanks for the info.

[kac]

That is a really good test, I agree that Mars coil really looks good. Wonder if the Ultimate 13 is getting smothered? I have that issue with the Nel Big in some areas where the larger size becomes counter productive and depth gain minimal. Larger concentrics seem to behave same way with smothering from ground mineralization. Also noticed this with the Superfly 12x11 vs the 7x11 on the MK at the beach. The Superfly did exceptionally well on dry and damp sand with incredible depth and target clarity but as it approached the salt the larger size wasn't as good as you would expect.

[palzynski]

15 hours ago, GB_Amateur said:

...

Results:

  The largest coil, the Detech Ultimate, is only a bit better than those two which likely reflects the considerable increase in ground it 'sees'.

From 13.5 to 14.5 this means 7% depth improvement between the Detech 13round vs the stock 7X11 on the 5cents . On the other hand the coil weight rises from  420g for the stock 7X11 to  540g for the Detech 13 , which means 28% increasement . 

So to summarize only 7% depth improvement for 28% weight/balance loss with the 13 ultimate... If you add to this the important iron masking of a big coil like the Detech 13,, it looks like from this teststhat the 7X11 T2 stock coil is a very good  compromise ....  

For info I have tested a 13ultimate on a Detech Chaser 3 or 4 years ago and it had good results for big coins at depth in very clean/sandy areas (  low iron trashed ) .  Now I have a cheaper and lighter solution with the Vanquish plus the V12 coil which goes as deep as the 13 ultimate.

[EL NINO77]

You did a very instructive test ... GB_Amateur ....👍

First of all, you can notice one fact, that with a 13khz detector, a larger coil significantly increases the range on a highly conductive 1c coin, than the increase in range on a low-conductivity nickel 5-cent coin ...

I mean, the use of larger coils has its ingenuity to increase the range, and it helps with many brands of detectors ... here it is necessary to keep in mind, what the range of conductivity such an upgrade can help ...

One of the important factors here is the frequency of the detector, where with high-frequency detectors 13-30 and more khz 11-13 "larger coil can significantly improve the range of highly conductive coins .. compared to smaller coils ..
I think that in the discrimination we get even significantly larger differences in range between different coils ...

The ground filter used for a given detector also plays an important role here, where the high value of the ground filter used for a small coil strongly limits the range of the detector, especially on high conductors and larger objects.

[GB_Amateur]

1 hour ago, EL NINO77 said:

First of all, you can notice one fact, that with a 13khz detector, a larger coil significantly increases the range on a highly conductive 1c coin, than the increase in range on a low-conductivity nickel 5-cent coin ...

I'm confuse with this statement.  (Maybe something got lost in translation?)  It does not appear to me that the higher conductive (but smaller) 95% copper 1 cent coin saw more improvement as coil size increased compared to the lower conductive (and larger) 25% nickel, 75% copper 5 cent 'nickel'.  Yes, starting with the 5" round DD coil and going to the intermediate sized coils it did better but after that there was only a small improvement.  The nickel didn't improve as much from small to medium but continued to improve as coil size increased from there.

Is the size of the target the big effect here rather than the (pure) conductivity?  My data along cannot distinguish between those two effects.  A comparison using a very early small cent (Flying Eagle and 1859-63 Indian Head) which is similar in size to the modern Memorial but intermediate in composition (12% Ni, 88% Cu) might sort this out (or not...).  I do have one of those IH's (not a find) so could maybe use it in my variable depth teststand (or even in a pure air test).

[EL NINO77]

We can ask the question, if the mutual height of the detector in combination with certain coil sizes on coil sizes on different large and different conductive objects has a mutual influence on the range ... the results can be a bit surprising .... because they definitely break the usual detection rules which arose even at a time when all detectors used mostly low frequencies of 3-8 khz ....., and concentric coils.

  There was also a myth of myth ... that coils larger than 8-10 "inches no longer bring any significant improvements to detection..in the depth and separation ..., or in strong mineralization.

The development of well-adjustable detectors in the last 10-15 years, and the use of DD coils, however, shows that detectors can successfully use even slightly larger 11-13 "coils where coil 11" has become practically a standard coil for many top detectors ...

 The use of higher frequencies of 12-30khz maintained excellent sensitivity, good separation, even with 11 "coils, and increased the maximum reach for searching for the required objects in the ground.

Higher frequencies have a certain lower physical sensitivity to highly conductive targets ... and one of the ways to eliminate this shortcoming is the possibility of using a larger coil - which then significantly increases the range of all objects, but the most achievable will increase the range of high conductors.

 

Now, for example, we already know the answer to the question: why did the 5.7 "coil not be able to detect 50% of the objects in the picture .. especially large high-conductivity silver coins?..or any signal for large silver coins..?

...but the range was also missing on some very small low-conductivity coins ..

The standard 11 "coil was detected through a depth of 11 cm ... in 7Bar Fe3o4 Black sand ... completely all objects ... The detector works at 30 khz ...

[Mike Hillis]

That was an interesting test.  I like it.

Is there a reason you don't have anything deeper in your test bed?

You didn't say if you had Boost On or Off?

Your conclusion was "the order of depth does reflect coil size".     Actually the statement your test has shown is that the "order of air gap reflects coil size".     I just caution that  'Air gap' is air gap and 'in ground' depth is in ground depth.   The two different attributes don't actually reflect a total when added together.  

HH
Mike  

 

 

 

 

 

[GB_Amateur]

8 minutes ago, Mike Hillis said:

Is there a reason you don't have anything deeper in your test bed?

My main test area was built with a variable depth double barrel (ala shotgun) angled PVC tubing to allow both continuously variable depth and nearby target interference (one target in one barrel, another target in the other barrel, depth of the two can be the same or different, as can the separation, but depth difference and separation are intimately tied together).

I buried these two coins for calibration purposes.  However my shotgun setup has a problem in that there is a (so far) hard-to-find piece of iron trash that makes it difficult to listen for good signals as the targets get deeper.  I need to remove all the soil and run a magnet through it before replacing.  Just haven't gotten around to that...  😕

10 minutes ago, Mike Hillis said:

You didn't say if you had Boost On or Off?

Pretty sure it's off, but I need to check.  It's such a weird feature (IMO) to have a detector where the discrimination settings in memory affect the all-metal side's performance.  But you've reminded me of that feature covered in the user manual.  Another variable is DST on/off which some say has no effect.  I've always left mine on (and I did include that detail in the earlier post).

13 minutes ago, Mike Hillis said:

Your conclusion was "the order of depth does reflect coil size".     Actually the statement your test has shown is that the "order of air gap reflects coil size".     I just caution that  'Air gap' is air gap and 'in ground' depth is in ground depth.   The two different attributes don't actually reflect a total when added together.

Yes, good point.

I did consider trying to figure the depth limit (in my ground) at which the VDI becomes unreliable for these two buried targets.  That's a tough distinction to make, though, and even more aribtrary than determining the depth at which signal is lost.  The other thing I'd like to measure, though, is depth vs. gain/sensitivity setting for at least a couple different coils (not 4 or 5 of them).  That along with the higher priority of adding the concentric coil to the table.

[EL NINO77]

Very interesting is the weaker increment of the range of the coil 13 "ultimate in the test ... on Allmetall .. I would expect more ..

Could you repeat this test with Fisher F75 on another program? .. I have the impression that the differences in the range of the coils will be more pronounced ..