Coil size questions?

[Nova]

Could someone care to elaborate for us uneducated fools this statement: "Double the coil size reduces the signal by 1/8", and let`s say it`s for a pulse induction detector and were talking about going from a 20in to 40in coil.
Being from Australia i own an SD2200d and i have done some tests.
I went to the Castlemaine goldfields of Victoria and i found an old timers hole which was around 4ft deep, i jumped in and with my tape measure ,measured 1m deep from the top and then dug a hole at 1m deep into the side of the wall. I then put into the hole a top cut off a coke can flat into the hole.
I then connected my 24in dd coil put my machine into it`s deepest setting and managed to easily hit the target. Then out of curiosity i connected up my 11in dd and to my shock easily got the target also.
Castlemaine is not the most mineralised ground that i`ve hit, i would say it would have to be Wedderburn, intense mineralisation here, if i had done the test in this ground the result probably would of been different.
Going by the tests that i`ve done, to me it does not make any sense to use a 40in coil, if i can hit the coke can top at a metre easily with a 11in dd, then how deep could i hit a 100oz gold nugget at?
The reason i`m mentioning these large coils is the fact of a current metal detector company is making these available to the public at the price of $4k- 5k Australian.
For that price the benefit would have to be significant, but the question to all you who know better is ; is it ?

[Qiaozhi]

Could someone care to elaborate for us uneducated fools this statement: "Double the coil size reduces the signal by 1/8" ...

The quote is wrong.
Doubling the coil size reduces the sensitivity to one eighth. Also, reducing the target size by a half reduces the sensitivity to one eighth.

[Nova]

G`day Qiaozhi; yes, that was the statement i was referring, i think username "Green" posted it somewhere but my memory of it was not 100% clear. So; are you saying sensitivity reduces by 12.50% , or it reduces all the way down to 12.50%, if the latter is the case, then that is a significant drop, particularly in respect to double the coil size, it would also confirm my suspicions regarding very large coils,(are not worth it).

[Koala]

is it not the same as in a VLF

I have a 5" and 10" coil. Very small coins are identical in depth.

there is a cut off point around a pre decimal half penny where the larger coil starts to take over

then the bigger the target the bigger the advantage. But you have to step over the edge before you see any benefit.

you can go the other way where below hammered penny size the 5" coil is deeper.

[green]

Chart I posted awhile back. Each coil is 1.5 times the next smaller, 1.5 cubed=3.375. Signal strength for the larger coil should be 3.375 times the next smaller. Close at detection distances greater than 2 times the coil diameter. At shorter detection distances less increase and like Koala replied the smaller coil has a higher signal at short detection distances.

[godigit1]

Im not a engineer but I am a experianced detector prospector.
I have a 2200V2 and same experiance. These are super deep machines Biggest coil I use is a the wallaby DD. Its very deep and still hits Small targets pretty impressively . Going to such a big coil could cause you to miss a patch of smaller stuff.
The other thing is you are seeing a much larger portion of the ground causing you to loose even more sensativity.( The science is exact the ground is not) as far as mineralization and target masking by hot rocks or Mining trash you will never find anything but old wagonwheels or that PIPE dream nugget.
I would not hunt with such a big coil but use it after you have cleaned a patch to see if something is deeper maybe if the mineralization is low. I cant imagine any one digging two targets at max depth in one day. The gold areas I hunt are mostly old compacted or cemented river channel.
Unless you going to drag it behind a ATV in low mineralized clean soil IMO anything over 18 inches the trade offs are a loss. think of how sensative your machine is to EMi EfI and adding a bigger antenna. If you look at it this way One peice of iron with a 18 inch coil masks 3 square feet from the edge of your coil in a radius. If your punching 3 feet deep thats a yard of material And a half yard below the coil at any given time.
Proven method of going deeper is to scrape off layers.
Dankowski forum is a good place to reacearch the effects of soil and masking I woud recommend anyone who wants to swing a mondo coil to examine the dynamics first.
Just saying

[Nova]

Thanks Everyone for your replies, Godigit1 exactly what i`ve been thinking "Proven method of going deeper is to scrape off layers". This is something i will get around to doing eventually.
Just a question with your SD, have you tried mono coils on your machine and how did it handle if you did?, i recently bought a detect ultra mono 14in, depending what area in victoria i was in, was fairly quite but did dig a few false signals.
So: is soil masking related to the very top ( surface) only, i know red clay is deflective more so when wet, and i`m not an engineer either, but i am by nature very curious and dont like to be doing anything blind, must know as much as i can, within reason.

[godigit1]

Hi Nova,
What I mean by soil masking and maybe not the total proper term. But when the ground becomes hot enough that it has a larger response than the targets. Ill use a VLF as example.
At tahoe on a really windy stormy day I had to get out get my mind off things.
So I dicide to torture myself and go for a fall hunt. By the time I got to the beach it was snowing sideways.
Big waves for tahoe were coming in.
Well there I went down the beach almost getting blown over I was skirting the waves. Not really finding anything.
Now here comes the lesson for me, I get close to a rock and while my back is turned a huge wave shoots up the back side of the rock. It was like a couple ten gallon buckets being droped on me. Soooo I start back down the beach to the car and the whole shore had turned black all the top sand had washed away to almost pure black sand. And As walked I picked up quarters nickels dimes bottle caps off the surface I could not belive what I was missing in the sand with my VLF I was Using A G2 that day. it would barely hit the targets on the surface and it hit the bottle caps the hardest Of coarse.
As far as The wet red Clay yes More conductive when wet If the ground has low mineral this could help depth High mineral and or salty and alkalyn soils are harder when wet beacuse the ground becomes more responsive. Pi machines are of coars less effected.
I have a paltapus mono for the Sd I bought it used and at the same time A TDI. The TDI is less effected by noise so I started hunting it more. My Sd Has sat For a couple years now in storage. I havent been able to get out in the desert for a while..
I did some testing but beacuse its a salt coil its shielded more and was less sensitive. On My Sd I Liked the WallabyDD and 6X9 joey DD. they pretty much cover the field for me. The wallaby for finding patches and the joey for cleaning up. Though I have never found a patch with a PI but Just ocasionally a leftover. Last Time I went out there at least 5 guys out there jumping claims all with the newest 7000 dollar machines. Getting harder to find anything without a VLF and just dealing with the hot soil.
Ive taken to porspecting the parks and beaches sled and sports fields.
I hunt parks with Gb Pro Or G2 and truthfully way more easier gold there. I have collected 7 machines over the years I cant bring myself to let go of. The Gb Pro I have has paid for all of them in the parks or for itself many times over.
Ok Long winded Sorry.
Hope this helps.