66cm coils (2 feet)

[Funfinder]

After the idea and imagination the creation and realization can start,
that's why I wanna suggest to you such a real great sized coil.

Sorrily so far no single MD-producer (besides some homebrew P.I. coil makers)
had the idea or was motivated enough to create and offer such coils
even they have their high and special values and attributes.

Of course they have to be built extremly lightweight like a circle
with a small stripe from up to down - max. weight 1kg! DD technology.

Coils of this size are very important because they close the far too high
and expensive gap to all those 1 meter coil detectors compared with
the not big enough 45cm coils for real deep finds.

In other words: Either persons will be able to find real deep treasures
with their usual detectors and with a 66cm coil or they will find it NEVER at all,
because they will never buy a high expensive 1 meter coil detector!


45cm coils are often just not deep enough or better said:

A 66cm coil is up to 50% deeper as a 44cm coil!
And for 50% more depth everyone should be happy to sweep a little bit heavier coil!


btw. I know exactly what I'm talking about because I have two 1 meter coils -
one for the Jeohunter and another for the DeepHunter. And an usual sweepable 66cm coil
would be a perfect compromise between 44cm and 1meter coil and THE solution to
reach very deep treasures most uncomplicated and without very high costs!

Seen from the todays plastic-stability its still within the green range and up to
medium mineralized soil the penetration power will be still very good for motion circuits.


So I'm convinced we need it, and we will have it sooner or later anyway, because it's
a fantastic opportunity and possibility with highly lucrative potentials!

Especially with modern, just 500 gram outoput electronics incl. cables and batteries, its no longer
a weight problem but just a barrier in some minds who think it's just not possible to handle.
Wrong - its easy to handle because the weight is within the limits - and we even can make them
hand-holdable but much more important is:



The world with all the treasure-promising areas is huge!


A 25cm coil many times is like peeking with a needle into a bed here and there, 5mm deep.


Really, we need such coil-size for huge areas and for deeper treasures or the whole efforts can be useless!

[Skippy]

I assume the deep targets you are searching for are large; larger than coin-size?
Most of the coins and relics I search for are small, and coils in the 7 - 13 inch range are optimal. I was always curious what items a very large coil would find, and was planning to build a 2-box conversion for my machine (similar to that Garrett arrangement). Most of the large items in U.K soils are iron, unfortunately. I have experimented with a sensitive non-motion mode (Fisher F75), and holding the coil approximately 30 cm above the ground, but almost all targets were iron (plough parts, horseshoes).

I think mechanical rigidity, and stability, will be one of your problems. I like the Teknetics T2 style coil housing, with the stiff spine, and the use of reinforcing ribs out to the loop. (I don't like the poor symmetry of it, though, I would put the stem-fixing lugs in the centre)
It's worth pointing out that Teknetics made a 15 inch coil for the T2, but users though it was not that deep.

[Skippy]

I'm also concerned about the extra EMI pickup that the larger receive coil will inevitably have. Would you be modifying your detector to have less receive sensitivity? And maybe more transmit voltage, to compensate?
I was considering dropping my RX gain when I use my 2-box conversion.

[Nupi]

Hi Funfinder,

An interesting project.
Can you tel me with what objects detects the coil of 66 cm 50% more than the 44 cm coil?

regards nupi.

[Koala]

Think it depends where you are and what you are looking for. Been an plenty of fields where a 8" CC preforms much better than a 10" DD even though the 10" DD has much more depth. Not much use if the detector is being overloaded ever sweep by iron slag. Here in the center of the UK a very large coil looses more than it gains.

Probable why people with both the 11 and 13 deus coils round here normally prefer the 11"

[Skippy]

Another obvious question - What detector operating frequency are you intending using this coil on? I would think that these larger targets would be easier to find with a 5KHz or below machine, maybe even 1 or 2KHz. One reason I'm not expecting miracles from my 2-box project is the limitations of a 13KHz operating frequency.

[eclipse]

I think you just inspired me to build a large coil for my Pi project

[Skippy]

Is this thread dead already?

[Koala]

I would not building one even if the project moved forward. See you are in the UK Skippy. Near me every swing would pick up a horseshoe or plow shear.

What sort of land would you be using this on.

[Skippy]

Koala: My 2-box conversion would be used on ploughed farmland, with history back to the Roman / Iron Age period. Searching for larger non-ferrous targets, 40cm+ depth. My only interest in this large DD idea is because it is an alternative to a 2-box, if used in the same way, ie. holding it clear of the ground 30 - 50 cm. My Fisher F75 has motion-discrimination target ID (on the display) in non-motion all-metal mode, so when you've found a target, you can then have a go at sweeping to see if it's iron / non-ferrous. The sweeping aspect would be easier with a big DD than with a 2-box.
I've tried out the non-motion AM mode, using the stock 11" x 7" DD coil, held clear of the ground. It does work, but it's obvious that the small coil size is a limitation, and something about 60% to 100% bigger in size would likely work better.

It would be nice if Mr FunFinder took an interest in his own thread, and maybe answered the questions we've asked, etc.

[Funfinder]

Sorry for the answer delay, I was focused on totally other things in the last time.


Thank you Skippy and others for your interest, I will try to answer the questions and I drew a coil-sketch (see pic).


As you see, the coil is pretty narrow and therefore more stable and less heavy.
Neverthless the size-wave-lenght effect should guarantee higher depths.

The frequency will be not especially important, we can still use 3-25 kHz and I doubt
that 3kHz will be much deeper.

What mainly counts ist the coils inductivity to make it compatible enough with usual detectors
which are built for much smaller coils.

I'm already thinking about the adaption for all kind of other MDs - perhaps made by a company like
Detech that already has experience with larger coils for usual MDs like Garrett.

It is simple, no question - its just about how to reduce sensititity loss as best as possible and
reach far higher depths. More windings with thinner wires and additional adapter-circuit? Something like this.


@ Skippy
The find size depends on the depth:
With the DeepHunters 1m coil you still can detect 5cm things if they are just 25cm nearby.
But of course the main goal is to reach larger and much deeper stuff.
With an IB DD coil which means enough "covering the area and depth" power by its EM-field.

The advantage of a 2box is that the coils are pretty far apart of each other but its not good
if this stuff is 30cm above ground-level. 30cm more or less depth is alot thats why its much
more meaningful to sweep a directly connected coil as close as possible over the search fields.

Stability will be no problem, not with our new high-stability compound-plastics.
The problem is to find out first what would be the most hard and flexible stuff to built it.
Glass fiber also works but better is a hard plastic that is not enough bendeble but still doesn't breaks
even if the temperatures are cold outside.

iron - disc - allmetal - noble metal only:

Iron and steel always gets better detected but seen overall this is a question of the detectors circuit.
If the coil is good adjusted it should disc out all the unwanted stuff the same good as a small coil.

btw. such a coil also can be used together with some sort of bungee-cord attachment so it will "fly" like
a bird through the air because its attached at the shoulder or somewhere.
But strong treasure-hunters won't need such gimmicks, especially if the coil is light enough.


The EMI pickup:
A very good point: Of course the detectors electronic must be good enough so it doesn't catch all kind of interference
already with its small 26cm coil like the Garrett Ace does. But this depends also on how close are e-smog sources.
Most of modern motion detectors are already emi-stable enough to work with 66cm coils.
Its not that huge gain of EMI with a larger coil, it merely depends on how strong is the EMI in the surrounding area.
Noise might be 15%-25% higher compared with a 40cm coil.


@ Nupi
The main gain with a 66cm coil will start already with 10cm large objects (at over 70cm depth)
The medium or main targets will be 30cm in size at 1,2m to 1,8m.

But of course what also counts is that stuff like treasure-chests or real huge stuff, 50-80cm in size
will become reachable far deeper.

Per instance you can reach some tools-alu-case (30x45cm) with a 1m coil at ca. 2m depth with a 1m coil.
This is not possible with a 45cm coil! But with a 66cm coil the depth for targets like this
will reach never before seen depths and with almost "no" additional effort.

The whole thing also has to do with "dont let yourself get halted by small unwanted stuff"
and cover real huge fast areas within shortest possible search times.
Its also "discrimination by big size and real depth".

Especially for areas where the info or intel says: the targets are buried or hidden real deep.



@ Koala
Small coils are easier to use and already can give super depth if the circuit is stable and
the contrast factor is strong.
But depth really needs larger coils, in the end there is so far no way around it, at least as long
we don't use rows of small high sensitive coils.
And hunting through huge areas also demands for larger coils.


I guess if we're lucky then Makro from Istanbul will built such a coil first and then we will test its depth
and performance. Such coil is not usable for all sites but it can be priceless to find new promising sites
very fast or to discover already know places with huge walls and rocks or other stuff where stuff could
be found, that is beyond the depth of a 45cm coil.

Anyway, we are here in a technical forum and if someone has some spare meters of copper-wire and
knows a bit about inductivity and coil-specifications it should be no problem to homebrew such a coil.
Pioneer-works!

Bending some small plastic pipes by heat could aready work good enough - melting them together
with the soldering iron.

[Skippy]

Quote:"Its not that huge gain of EMI with a larger coil. Noise might be 15%-25% higher compared with a 40cm coil."
In my estimate, a 66cm coil will pick up 85% more EMI voltage than a 40cm coil.

Quote:"its not good if this coil is 30cm above ground-level, that's why it's more meaningful to sweep a directly connected coil as close as possible over the search fields."
The idea of using a coil some distance above the ground is 1) It won't pick up as much ground signal (maybe 10 - 20% of the 'close' level) and 2) it won't give triggers on small shallow (<20cm) targets. Having less ground pickup means you can proportionally increase sensitivity/gain.

[Funfinder]

The EMI pickup is a question of circuit stability and contrast factor
while the ground depends on how high is its mineralization.
(How you got your 85%-plus -calculation? Higher antenna gain?)

Detectors that still will give good find signals with low background noise output
should be able to handle a 66cm coil, at least if its used 15cm above ground.

btw. 30-50cm above ground will raise the e-smog because the closer the coil is
to the ground the less e-smog it will catch and also the depth will be reduce
far too much if the coil is too high above.


If we take a look per instance at the high-stable non-motion Jeohunter or DeepHunter
circuit there is absolutly no difference concerning e-smog or mineralization-trouble
no matter if using the 45cm or 1m coil.

But I also can give you the compare values of the Garrett GTI 1500 - standard 30cm
coil vs. 45cm Detech: This is motion circuit with low e-smog immunity and
if the soil is not too heavy mineralized and if the usual e-smog sources like power-
or high voltage-lines are far enough away it is almost no difference - just of course
the depth with the 45cm Detech coil is much much better.
And you still can sweep the 45cm coil as good as directly over the floor.


Its clear that a 66cm coil must not be used with the cheapest available detectors!
It also may drain more power. But outside of villages and towns and at not that
heavy mineralized sites I'm shure it will do a great and highly efficient job to reach
the most deep and real far away from each other spreaded targets.


And for persons who don't wanna filter out by their own ears the amplified EMI and mineralization
signals there is still the depth reducing possibility of walking with such coil in their hand while
holding it 30cm over the ground. Or at high mineralized ground and with no EMI source nearby.


thx for your interest.
Do you have the possibility, motivation, location, fiting detector and knowhow to experiment with such a coil?

[Skippy]

Quote:"How did you get your 85% plus calculation? Higher antenna gain?"
I admit, it was an estimate, based on memory of some calculations I made when considering my 2-box project.
So I decided to calculate things properly, to explain my thoughts:
You could use approximations, such as L is proportional to coil diameter, and L is proportional to Turns-squared. But... it's probably easier to use the Qoil Qalculator on this Forum.
I ran through some realistic numbers, here they are:
Assume the "40cm" coil has RX coil radius = 150mm. Assume wire diameter = 0.25mm. Winding 103 Turns gives L = 10.0 mH.
Now scale the dimension up by 66cm/40cm, giving new Radius = 247.5mm. Increase wire diameter to 0.28mm (to keep R unchanged). You now need turns T = 76 to give L = 10.0 mH.
So coil dimensions are increased by 66/40 = 1.65 , and new number of turns is 76/103 = 0.74.

Now consider the loop antenna. Received signal is proportional to the coil area, and is proportional to the number of turns. (easy to find this info online)
So our new 66cm coil will pick up (1.65 squared), multiplied by 0.74 = 2.00 x the interference voltage. That is +100% above that of the 40cm coil. So my +85% guess wasn't far out.
This pickup increase might be OK, it depends on the machine, and the interferece level. But it does suggest that perhaps reducing RX gain down to 50% may be necessary to combat EMI. This would lose you 15% depth, worst case.

Quote:"Do you have the possibility, motivation, location, fitting detector and know-how to experiment with such a coil?"
Know-how? Probably enough.
Good Detector? Yes, Fisher F75. But EMI-pickup is a problem for this machine.
Possibility? Not this year. Ideally in time for the crop harvest in Autumn 2016, so it's probably a winter 2015 / spring 2016 project.

[Funfinder]

Hi skippy, thx for the reply.

I found now a nice analogy:
Using a small coil is like eating a soup with a tea-spoon!

A real man doesn't use a spoon for the soup anyway - he takes the dish to the mouth
and drinks the fluid stuff until the real food is available - now a spoon or fork is useful.

Anyway, I always must laugh even when I'm walking with my 40cm coil through the pampa
when I compare the size of this coil to the area which I wanna search through.
There is still no logical meaningful relation between coil-size and search-ground!

And 25-30cm coils are for children sand-boxes or for very restricted areas - 50x50 meters per day
which is just a 1/400 of a km2. And this world is huge and good search areas can be really big.

Or for the experienced ones: 70x70m = 4900 square-meters per day which is ca. 1/200 of a km2.
Would still take the treasure hunter 200 days for just one square kilometer.


The main important thing often is to get a clue first:
Per instance first they found a roman horse-shoe (sandal) in a german forest and afterwards they
looked closer and found out about a huge battle that was going on there - with 1000s of antique finds!

Such easy to handle large coils are a must to gather new info or to "brute force scan" large areas.
I guess such a 60cm coil even will find still bigger coins and especially stuff around 3cm in size.


Of course the EMI interference must not "delete" the sensitivity.
One method would be using higher EM-field-strenght for the coil so
the detector mostly hears its own energy-signals and not foreign ones.

btw. a non-motion circuit is pretty immune against e-smog and has
absolutly no problems with even 1 meter coils, per instance the Jeohunter or DeepHunter.
But a motion circuit would be easier to use if it can handle the auto ground balance good enough.