why

[golfnut]

with something like a GMP can you get 10" on a hammered silver?

BUT

with say IDX, ace250, classic, tesoro, this style of kit 7" would be the limit.


I know to get 3" more is a massive improvemnt --

Is the delta due to


overall system gain

preamp type

detectors - full wave ?

filters

interrogators/correllators in the baseband (14,16, or more bits in a/d 's )

coil contstruction

cable quality

Tx, free running or clock derived


etc etc any clues guys..

S

[Skippy]

I'd be surprised if an ACE250 could easily find a hammered coin at 7 inches (and not call it iron).

The 'best detected at frequency' for a typical hammered is 10 - 20 KHz, say 15K, so the GMP at 18K is in the right range (and the ACE at 6.6K is just too low).
It has a tone-break disc, so it can be set up to "non-ferrous-tone" on large iron, which may be a non-iron target for weaker/deeper targets. Doing this is possible on other machines, but requires careful watching of VDI values etc. And of course the downside is the digging of iron.

Because it runs in an all-metal type of audio, it will alert you to the presence of a target that is deeper than a typical disc mode machine. That opens up the possibility of investigating the target further (swinging over it from all directions, different swing speeds, smoother swing etc etc) and maybe coming to a 'dig it' decision. Some other machines (Tek T2 / Fisher F75 for example) are pretty much as deep in disc mode (with the right settings) as in A.M mode, so this technique can also be used on them, and other detectors. Plus, machines like the T2 / Fisher F75 can do all-metal audio with disc VDI display.

Bear in mind some owners of high-end machines love to tell everyone how super-deep their machine is, so don't go thinking 10 inch hammered coins are routine finds.

Depth of course is heavily dependant on ground signal strength. Get on a mild patch of soil, and a couple of extra inches could be obtained.

User skill is pretty important, too. I'm sure a newcomer to a GMP would conclude they aren't anything special depth-wise.

[Koala]

Not a GMP but a good test of how difficult a thin hammered is and that's knowing where it is so the swing is directly over the center of the coin. https://www.youtube.com/watch?v=TOFZQ25aL3U


Modern coins are a different kettle of fish

[Skippy]

And regarding the innards, etc: I'm sure the search-coil is well-made, and it is well-matched to the pre-amp (for noise / gain trade-offs etc), and the pre-amp will be a ultra-low noise type (I will try and find out what it is). I don't think full-wave synch detectors are actually essential - the Fisher F70 has half-wave, one of the distinctions between it and the F75 (and Tek T2), and it is a pretty competent machine.
GMP's run off 8 x AA cells = 12 Volts, It's tempting to think that the search-coil is driven with a decent signal, which would help the noise side of things...but of course the Deus coil only runs off a single Lithium polymer cell, so big TX drive isn't essential.

[Henrikras]

I think a silver hammered got lower ideal search Freq. that you you say Skippy (10-20 khz), closer to 10 khz that to 20 khz.
Another thing I learned and this got much to say, is the soils humidity ! Allso said like this - how long time since last rain. A Dry summer day many days since last rain give me lower depht than a day after a good long rain.

Henrik.

[Skippy]

I'm sure Danish coins are different to English ones. English coins were considered the "standard" in Medieval times, and many Continental European countries copied the design / size / weight / silver quality of them. These imitations were known as Esterlings, here are some UK-found examples:
http://www.ukdfd.co.uk/ukdfddata/showcat.php?cat=38
However, I quoted 10 to 20KHz because they cover a wide range. The smallest of them, such as cut-quarter farthings, typically have 'corner frequencies' of 24KHz.
http://www.ukdfd.co.uk/ukdfddata/sho...rthing&cat=152
Whole pennies are typically 12 KHz. However, I have a Scottish penny that's near to 8kHz (may be pure silver, not Sterling Silver) and an English one that is a high 35 KHz (it may be debased silver). We also have quite common hammered copper coins, which are usually small, and ID above 15KHz.
Factor in variable corrosion, plough-damage (bends, cracks), and all the other problems, and a range of 7 to 30 KHz might be more realistic, but 10 - 20 would encompass the majority, I suspect.

[golfnut]

I agree with all of what said.

Damp soil works best -
frequencies match target volume
Full wave det isnt essential.
GMP coil made well - for sure.
good preamp,,,,

We bundle our turns and bind up.
Commercial winds are side by side and layered and fixed. This may be a bit of the advantage??

[Skippy]

I'm pretty certain the GMP pre-amp op-amp is a ST-Micro TS461CLT, which is fairly modest, noise-wise, with 4 nV / root-hertz. there are plenty of parts about with 1 nV/rt-Hz should you need them.

[Koala]

Interesting the g-gmp and gmp one at 4.6khz the other at 18khz had the same depth ish and both had lost the signal by 7" so kinda give up at the same depth as the eurotek

I have some sixpences will have to dig one out and try my IDX on it.

quote from http://www.garysdetecting.co.uk/goldmax.htm

I buried a 1922 sixpence at 8 inches until the Gold Maxx just lost the signal, the G Maxx picked this target up with a clean tone, raising the coil 1 inch the target was lost.
Conclusion
The G Maxx and Gold Maxx have virtually the same depth on a sixpence size target in clean soil, however the Gold Maxx will have an advantage when looking for smaller targets or searching mineralised ground. The G Maxx will be better for bigger targets buried deep.

[muddy fingers]

Here are two tiny English hammered coins that I found fairly recently- surface finds on damp soil. The round Charles Ist halfpenny is 9mm diameter. They are both made from good quality silver. I actually found them with my Blisstool which is 8Kz but it does seem to find tiny surface (2" down) coins quite easily as well as much deeper larger coins. I'd like to know how it can do this as my old 12Khz Tesoros wouldn't have signalled on them so easily.




I keep wondering whether to use my GMP (18Khz) on some sites or the Blisstool. The GMP is quieter overall and generally easier to use. I took both machines out once and tested them on bad ground on a very tiny (about 8-9mm diameter) thin gold Indian coin laid on the surface. I actually got a slightly better signal from the Blisstool so I am still confused about the frequency issue, i.e. which is the best for tiny hammered.. The Blisstool has a built in Lithium battery so perhaps that is making it more sensitive/powerful than the GMP.

[Skippy]

Quote from the Gary test: "a 1922 sixpence at 8 inches"
This is a 50% silver coin, modern milled type, with a 'corner frequency' of about 6KHz. It's not at all a good representation of a typical hammered coin. It is a reasonably tough target, because it's small (similar to a US dime) about 2.8 grams. So at 8 inches it's not going to be an easy target. I recall Gary trying a sixpence at 10 inches test, but it was too tough, hardly any machine tested gave a 'dig' signal.

[Henrikras]

Skippy or another; what Freq. would you think a Golden stater or one size bigger gold coin is best?
Ive been testing a little with a 8 gram Gold coin size 22-24 mm with those different detectors I got, and to me it seems that 8-12khz did it best, but there wasn't much different in depht with 8/12khz, 17/18 khz or 25 khz.. I think the round form helped the lower freq.
The reason I tested this and used some time to find out which Detector I got there would do it best, is that I got a field were 3 gold coins (6 gram) was found some years ago.. This field is just been sown and are ready to search, flat like a pancake.
One week of rain and the soil is allso friendly this way.(damp I think you call it).
Only sad thing is that this field is a club field, so I'm not the only one trying, and that has been going on since last coin was found in 2011. The good thing is that I live near, only 5 km to this field.
Henrik.

[Skippy]

I guess such a coin would be in the 6KHz to 12KHz window, so I would personally choose 8K or 12K on the Deus, but I think the difference would be small.

[golfnut]

Hi Henrik, I saw a vid with a larger coin about 28mm with 18kHz and it wasnt very good.
So it works both ways. high freq on big targets isnt so good.


Thick section mid size coins can be illuminated by a good range of signals ~8 to 20 like roman solidus,,, say 14k?

small dia like celtic 1/4 stater gold and thin small silver like 14 to 20,,,, say 16k?

[Koala]

I am interested to learn but don't understand this frequency thing (is there any properly tested proof and not marketing hype)

solid silver cut quarter test
http://www.staffsmetaldetectors.co.u...ered_penny.htm

3" depth range I can see a 6.5KHz and a 18KHz machine so I doubt frequency (within reason) has that much affect

It's almost impossible to field test with any certainty as there are two many variables to contend with. Even two identical machines with identical coils don't always have the same air test and that does not include different ground conditions.